Active Volcanism on Io as Seen by Galileo SSI

ICARUS 135, 181–219 (1998)ARTICLE NO. IS985972

Active Volcanism on Io as Seen by Galileo SSI

Alfred S. McEwen, Laszlo Keszthelyi, and Paul Geissler

Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721E-mail: [email protected]

Damon P. Simonelli

Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853

Michael H. Carr

U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025

Torrence V. Johnson, Kenneth P. Klaasen, H. Herbert Breneman, Todd J. Jones, James M. Kaufman,Kari P. Magee, and David A. Senske

Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109

Michael J. S. Belton

National Optical Astronomy Observatories, P.O. Box 26732, Tucson, Arizona 85726

and

Gerald Schubert

Department of Earth and Space Sciences, University of California, Los Angeles, 2810 Geology Building, Los Angeles, California 90095

Received November 24, 1997; revised April 3, 1998

The plume monitoring has revealed 10 active plumes, compa-rable to the 9 plumes observed by Voyager. One of these plumesActive volcanism on Io has been monitored during the nomi-was visible only in the first orbit and three became active in thenal Galileo satellite tour from mid 1996 through late 1997. Thelater orbits. Only the Prometheus plume has been consistentlySolid State Imaging (SSI) experiment was able to observe manyactive and easy to detect. Observations of the Pele plume havemanifestations of this active volcanism, including (1) changesbeen particularly intriguing since it was detected only once byin the color and albedo of the surface, (2) active airborne plumes,SSI, despite repeated attempts, but has been detected severaland (3) glowing vents seen in eclipse.times by the Hubble Space Telescope at 255 nm. Pele’s plumeAbout 30 large-scale (tens of kilometers) surface changes areis much taller (460 km) than during Voyager 1 (300 km) andobvious from comparison of the SSI images to those acquiredmuch fainter at visible wavelengths. Prometheus-type plumesby Voyager in 1979. These include new pyroclastic deposits of(50–150 km high, long-lived, associated with high-temperatureseveral colors, bright and dark flows, and caldera-floor materi-hot spots) may result from silicate lava flows or shallow intru-als. There have also been significant surface changes on Iosions interacting with near-surface SO2 .during the Galileo mission itself, such as a new 400-km-diame-

A major and surprising result is that p30 of Io’s volcanicter dark pyroclastic deposit around Pillan Patera. While thesevents glow in the dark at the short wavelengths of SSI. Thesesurface changes are impressive, the number of large-scaleare probably due to thermal emission from surfaces hotter thanchanges observed in the four months between the Voyager 1700 K (with most hotter than 1000 K), well above the tempera-and Voyager 2 flybys in 1979 suggested that over 17 years theture of pure sulfur volcanism. Active silicate volcanism appearscumulative changes would have been much more impressive.ubiquitous. There are also widespread diffuse glows seen inThere are two reasons why this was not actually the case.eclipse, related to the interaction of energetic particles with theFirst, it appears that the most widespread plume deposits areatmosphere. These diffuse glows are closely associated with theephemeral and seem to disappear within a few years. Second,most active volcanic vents, supporting suggestions that Io’sit appears that a large fraction of the volcanic activity is con-atmopshere is dominated by volcanic outgassing.fined to repeated resurfacing of dark calderas and flow fields

that cover only a few percent of Io’s surface. Globally, volcanic centers are rather evenly distributed. How-

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182 MCEWEN ET AL.

ever, 14 of the 15 active plumes seen by Voyager and/or Galileo include a ‘‘subsurface ocean’’ of liquid sulfur (Sagan 1979,are within 308 of the equator, and there are concentrations of Smith et al. 1979b). The idea of a sulfur crust was under-glows seen in eclipse at both the sub- and antijovian points. mined by the abundance of calderas with steep slopes thatThese patterns might be related to asthenospheric tidal heating cannot be supported by pure sulfur (Clow and Carr 1980,or tidal stresses. Io will continue to be observed during the Schaber 1982). The presence of temperatures exceedingGalileo Europa Mission, which will climax with two close flybys

650 K over the caldera of Pele provided especially convinc-of Io in late 1999. 1998 Academic Pressing evidence that the caldera walls cannot be composedKey Words: Io; volcanism; infrared observations; satellites ofof sulfur, whose melting point is 393 K (Pearl and SintonJupiter; spacecraft.1982). In spite of this evidence for a nonsulfur crust, mostearly post-Voyager models for Io’s active volcanism fa-vored low-temperature sulfur volcanism, perhaps drivenINTRODUCTION AND BACKGROUNDby silicate magmatism at depth, and only rare outbursts

Io is a volcanic wonderland. Following discovery of a of silicate volcanism (McEwen et al. 1989 and referencesthermal outburst (Witteborn et al. 1979) and the prediction therein). This paradigm was supported by the fact thatof intense tidal heating (Peale et al. 1979), the Voyager sulfur is abundant on Io’s surface and in the corona, neutralspacecraft revealed a world covered by volcanoes, many clouds, and plasma torus, while silicates had not been di-of them active (Smith et al. 1979a). Voyager discovered rectly detected on Io’s surface or in its surroundings. Re-a total of 9 active volcanic plumes (Strom et al. 1981). cent results indicate that the Si/Na ratio in Io’s neutralObservations of about 30% of Io’s surface by Voyager’s clouds is less than 1023 relative to the cosmogonic ratioInfrared Interferometric Spectrometer (IRIS) identified a (Na et al. 1998).total of 22 hot spots (Pearl and Sinton 1982, McEwen In contrast, Carr (1986) advocated ubiquitous active sili-et al. 1992a, 1992b). Telescopic monitoring of hot spots cate volcanism. Although hot-spot temperature modelshas shown continuous volcanic activity in the years since from Voyager did not exceed p650 K, Carr’s thermal mod-Voyager (Spencer and Schneider 1996, and references eling showed that the emission at Loki was consistent withtherein). The global average heat flow is probably at least long-lived eruption of p1400 K silicate lavas. Short-lived2.5 W m22, comparable to the most volcanically active events with high temperatures (1225–1550 K) were seenregions on Earth (Veeder et al. 1994). Many surface in 1986 (Johnson et al. 1988) and in 1990 (Veeder et al.changes were apparent between the images acquired by 1994), which were interpreted as evidence for large silicateVoyager 1 and those collected four months later by Voy- eruptions (Johnson et al. 1988, Blaney et al. 1995, Daviesager 2, including two very large new plume deposits 1996). In recent years, telescopic imaging of Io at shorter(McEwen and Soderblom 1983). The many changes in the IR wavelengths (1 to 5 em) has revealed many more high-color and albedo of the surface over four months led to temperature hot spots, with temperatures typically be-expectations that Io would look very different in Galileo tween 650 and 750 K but sometimes much hotter (Spencerimages 17 years later. However, images from the Hubble et al. 1997b, Stansberry et al. 1997a).Space Telescope (HST) showed few very large-scale Resolving these contrasting views of Io’s volcanism waschanges (Sartoretti et al. 1995, Spencer et al. 1997a). one of the goals of the Gallileo Mission (Carr et al. 1995).

The Voyager data and the various Earth-based observa- While the Near Infrared Mapping Spectrometer (NIMS) istions provided only tantalizing clues to some of the most better designed for determining temperatures and materialfundamental questions about volcanism on Io. In particu- compositions, SSI was expected to make important contri-lar, we did not have a global picture of the distribution of butions with its high spatial resolution and color data. Earlydifferent types of volcanic activity in time or space. Such NIMS and joint NIMS-SSI results are detailed elsewherea global view is the key to understanding the generation (Carlson et al. 1997, Lopes-Gautier et al. 1997a, 1997b,and distribution of magma and heat within Io. Frequent Davies et al. 1997a). Other scientific objectives of SSI atmonitoring of Io by the Solid State Imaging (SSI) experi- Io are to (1) complete the global inventory of topographicment during the Galileo Mission was planned to address landforms (Carr et al. 1998), (2) determine Io’s large-scalethis question. shape (Thomas et al. 1998, M. Davies et al. 1998), (3) better

Another fundamental question regards the composition understand regolith properties (Simonelli et al. 1997), andof the volcanic eruptions on Io. The bulk density of Io (4) study volatile and atmospheric processes (Belton et al.(3.57 g cm23) and our basic assumptions about the origin 1996, Simonelli et al. 1998).of the Galilean satellites (Pollack and Fanale 1982, Lunine In this paper we focus on the SSI observations of activeand Stevenson 1982) suggest that Io is dominantly a silicate volcanism on Io during the first 10 of 11 orbits in thebody. However, the surface of Io is covered by SO2 and nominal satellite tour (June 1996 through December 1997).other sulfurous materials (Nash et al. 1986) and early mod- We begin by describing the observations, including a dis-

cussion of the evolution of observational strategies beforeels suggested that the sulfur layer could be quite thick and

ACTIVE VOLCANISM ON IO 183

TABLE I et al. 1995). However, failure of the high-gain antenna toSSI Spectral Bandpasses (Effective Wavelengths fully deploy eliminated the capability for extensive moni-

for Average Io Color) toring and a tape recorder anomaly resulted in a decisionto not acquire the high-resolution Io images during JOIFilter designation Effective wavelength (nm) Bandpass width (nm)a

(O’Neil et al. 1997). Furthermore, p17% of the tape wasVIO 418 45 off limits during the satellite tour as protection against aGRN 560 65 potential failure. (Note that there is usually only one tape-RED 664 60 recorder load per orbit.) This tape reduction impacted727 731 10

Io observations most severely in orbit E6, when a tape-756 757 19intensive plume inventory was eliminated.889 888 16

1MC 990 50 Obtaining the crucial SSI data has been a challengeCLR 644 440 met with a variety of techniques (Klaasen et al. 1997).

In particular, most of the Io images returned during thea Full width at half maximum.nominal mission have undergone ‘‘lossy’’ Integer CosineTransfer (ICT) compression (Cheung and Tong 1993). On-chip mosaics (OCMs) have been used to acquire several

and during the satellite tour. Major sections describe color full-disk images on a single SSI frame (Fig. 1). Jupiter’svariations, plumes, surface changes, and hot spots. We intense radiation environment is a significant additionalthen move on to our preliminary interpretations of high- complication. The radiation-induced noise in the imagestemperature hot spots, individual volcanic centers, erup- increases with decreasing range to Jupiter and is a functiontion types, and the global patterns and tidal heating mecha- of residence time on the CCD. Range to Io generally corre-nisms. We end with a brief discussion of the planned Io sponds to range to Jupiter, so the highest resolution Ioobservations during the Galileo Europa Mission (GEM). images are the noisiest. Noisy images are difficult to com-

The eight color bandpasses of SSI are listed in Table I, press so they are more expensive in bits returned. Thisand the characteristics of the nominal-mission orbits are problem is minimized by using the fastest frame times,summarized in Table II. The letter E, G, or C in each including a fast summation mode (resulting in 400 3 400-orbit designation indicates targeted flybys of Europa, pixel images rather than the full-frame 800 3 800 format).Ganymede, or Callisto, respectively. The few images allocated to Io in each orbit have been

carefully planned to achieve the highest priority scienceSSI OBSERVATIONS results. The effort to optimize the data return has required

attempts to predict Io’s volcanic activity and appearance,Strategyoften unsuccessfully. Monitoring of active plumes has

Galileo was expected to revolutionize our understanding proven to be especially difficult.of Io by providing observations over an extended spectral The appearance of Io varies significantly as a functionrange and with frequent monitoring, and with very high of wavelength, location, photometric geometry, and time,spatial resolution imaging as the spacecraft flew by Io dur- so the choice of images must be made carefully in ordering Jupiter Orbit Insertion (JOI) (Carr et al. 1995, Smythe to extract the maximum amount of desired information.

Images at low solar phase angles are best for multispectralmapping and for comparison with images from Voyagerand HST to detect changes. High-phase images best revealTABLE II

Nominal-Mission Orbits forward-scattering surface and airborne materials. Obser-vations of the bright limb are best for detecting active

Closest approach Date of closest volcanic plumes, but many images are required to provideOrbit to Io (km) approach to Io

complete longitudinal coverage. Topographic shading isbest seen near the terminator, at high illumination anglesG1 697,000 6/28/96

G2 441,000 9/06/96 and low-to-moderate viewing angles. Io is a very activeC3 244,000 11/06/96 world so repetition of observations is necessary to searchE4 321,000 12/18/96

for changes. Because of Io’s changing appearance as aE6 401,000 2/20/97function of phase angle, repeat observations at similarG7 531,000 4/03/97

G8 956,000 5/07/97 phase angles are needed to confidently detect resurfacingC9 607,000 6/27/97 events. All sunlit images of Io (except the G2 plume inven-C10 319,000 9/18/97 tory) through orbit C10 are listed in Table III, and eclipseE11 780,000 11/07/97

images are listed in Table IV.

184 MCEWEN ET AL.

FIG. 1. ‘‘Raw’’ SSI frame 394505145. Frame is an on-chip mosaic (OCM) with exposures through VLT, RED, and GRN filters, respectivelyfrom left to right. North is down. Prometheus is seen on bright limb. Small bright spots and streaks are ‘‘radiation noise.’’

Observation Categories used for surface monitoring plus the 889 and 1MC filters,which might reveal absorption features due to iron-rich

Observations have been generalized into the followingsilicates. The 889 and 1MC bandpasses require longer ex-

categories.posures and/or more sensitive gain states for adequate

Topographic mapping. Imaging optimized for the signal, which in turn increases smear and/or noise, so it ishighest resolution global coverage (accumulated over difficult to achieve a high signal-to-noise ratio (SNR) inwhole tour) at illumination angles that accentuate topo- these colors. Global coverage was planned via one colorgraphic shading. Useful stereo pairs have also been ac- set in E4 and three additional color sets in C10. The E4quired from the overlap regions (Schuster et al. 1997). data actually returned excluded the RED image and in-Seven full or partial frames were acquired in C3, plus two cluded only one-third of the 889 image (Table III), but weframes in G7 and single frames in E6, C9, C10, and E11. have reimaged much of this hemisphere in six colors andAbout 80% of global coverage from 60–908 illumination at much higher spatial resolution (3 km/pixel) during orbitwas achieved. A global mosaic from these frames (except E14 of GEM.the E11 frame, and including other frames to fill gaps) is

Plumes. Bright-limb observations for a systematicshown in Fig. 2.plume inventory (in orbit G2) or targeted to suspectedSurface monitoring. Full-disk images in four colors atplume locations. The best plume observations are shownlow-to-moderate phase angles, for monitoring spatial andin Fig. 3 and a sample from the G2 sequence is shown in Fig.temporal variations in color and albedo. We acquired4. Color sets (typically VLT, GRN, RED) were targeted toglobal coverage three times during the nominal tour: nearsuspected plume locations in the later orbits (E6–E11) ofthe beginning (G1 1 G2), the middle (E6 1 G7), and thethe nominal mission (e.g., Fig. 1).end (C9 1 C10). A global color mosaic from low-phase SSI

images is shown in Plate 1. The four bandpasses normally Eclipse. Long-exposure images acquired when Io is inutilized are VLT, GRN, RED, and 756. Io shows consider- Jupiter’s shadow to search for high-temperature hot spotsable diversity over these wavelengths, whereas the spectra and diffuse atmospheric/plume glows. Eclipse observa-are flat and featureless from 0.76 to 1.0 micrometers, at tions acquired through orbit C10 are listed in Table IVleast at large scales (Spencer et al. 1995, 1997a). Surface and the images are shown in Plate 2 and Fig. 5. The CLRmonitoring frames are also useful for detecting plumes on filter has proven to be the most useful as such imagesthe bright limb. reveal both hot spots and diffuse glows. Use of the 1MC

filter along with the CLR enables estimation of hot spot6-Color mapping. Full-disk images in six colors forcompositional mapping. The color filters include those temperatures (McEwen et al. 1998).


TABLE IIISSI Observations of Io, Orbits G1–C10 (Excluding Eclipse Images and G2 Plume Inventory)

Frame Scale Phase S/C Sun Limb Terminator ICT Notes, active volcanismnumber PICNO Filtersa (km/px) angle Longitude Longitude Longitude Longitude Qb RJ detected (and key nondetections)

349542200 G1I0001-4 V-G-R-7 23 48 69 21 339 111 3 30 (No plume over Acala, Surt)349674000 G1I0005-8 V-G-R-7 14 55 264 210 174 300 3 22 V-filter smeared, Zamama plume, Ra

Patera glow on nightside in R filter,(no plume over Volund)

349746339 G1I0010-13 V-G-R-7 15 25 338 313 248 43 4 17 Hint of Pele plume in G-filter,(No plume over Pillan)

350013900 G1I0020-22 V-G-R 9 122 212 335 302 245 5 16 (no plume over Loki, Amaterasu,Sengen)

350024300 G1I0025-27 V-G-R 10 122 227 350 317 260 5 17 Ra plume, (no plume over Aten P.)359986578 G2I0073 G 5 4 175 178 265 88 3 13 Pillan darkened since G1359986600 G2I0074 R 10 4 175 178 265 88 5 13 Partly saturated359986604 G2I0075 V 10 4 175 178 265 88 5 13 (no plumes seen against Jupiter over

Pele, Babbar, Svarog, Zal,Hi’iaka, Gish Bar)

359986607 G2I0076 7 10 4 175 178 265 88 4 13368558239 C3I0001 C 3.5 31 149 181 239 91 4 10 TOPO, Prometheus368558252 C3I0002 C 3.5 31 149 181 239 91 4 10 TOPO368578900 C3I0010 C 2.6 35 174 210 264 120 4 9 TOPO, only top of frame returned368599800 C3I0020 C 2.5 39 201 240 291 150 4 9 TOPO368599813 C3I0021 C 2.5 39 201 240 291 150 4 9 TOPO, Prometheus368620000 C3I0030 C 3.0 47 222 269 312 179 3 9 TOPO, Pillan P. brighter than in G2368641300 C3I0040 C 4.1 60 239 299 329 209 3 10 TOPO, (no plume over Ra or Acala)368981522 C3I0050-51 C-G 23 169 256 65 346 335 10 32 Na-cloud, 12.8-sec exposures,

Prometheus/Culann plumeand Pele hot spot

374493545 E4I0010 C 16 34 51 17 321 107 7 19374493645 E4I0013 V 16 34 51 17 321 107 6 19 (no Ra plume)374575800 E4I0015 G 6 0.5 135 135 45 225 4 12 Prometheus, new N. Polar

plume deposit?374575845 E4I0016 G 6 0.5 135 135 45 225 4 12 2 half frames to cover disk374575922 E4I0018 V 12 0.5 135 135 45 225 6 12 (no plume over Kanehekili,

Masubi, Janus, Mulungu,Kurdalagon)

374575945 E4I0019 7 12 0.5 135 135 45 225 6 12 Surface changes at Culann?374575968 E4I0020 1MC 12 0.5 135 135 45 225 7 12374576000 E4I0021 889 12 0.5 135 135 45 255 7 12 1/3 frame only, S. Polar region374849545-6 E4I0030-31 C-G 18 153 13 166 103 76 7 20 Amirani or Monan plumes,

(Pele not visible)374850045-6 E4I0032-33 V-R 18 153 13 166 103 76 7 20 V-filter: Pele (460 km high),

Amirani or Monan, (no plumeover Sigurd, Altjirra)

383490345 E6I0001-3 V-G-R 21 78 345 267 255 357 4* 24 long exposures, (no Pele plume)383540100 E6I0007-8 V-7 18 57 34 338 304 68 5* 20 (No plume over Loki, Amaterasu)383563739-41 E6I0020-23 V-G-R-7 16 46 57 11 327 101 3-7 19 Kanehekili surface changes, clipped

images, (no plume over Ra, Acala)383600839-40 E6I0030-33 V-G-R-7 11 31 95 64 5 154 5, 8 16 R-filter not returned, Prometheus,

(no plumes over sub-Jupiter region)383655100 E6I0040 G 10 23 165 142 75 232 7 13 Limb longitude of radio occultation383655104 E6I0041 R 10 23 165 142 75 232 7 13383655107 E6I0042 V 10 23 165 142 75 232 7 13 (No plume over Zal, Hi’iaka)383655111 E6I0043 7 10 23 165 142 75 232 7 13383694100 E6I0050 C 4.1 31 229 198 139 288 8 11 TOPO, (No plume over Shamash)383758500 E6I0060 G 11 14 276 290 6 200 8 9 Fading around Ra P., darkening

of Amaterasu P., Pillan P.brighter than G2

383758504 E6I0061 R 11 14 276 290 6 200 7 9383758507 E6I0062 V 11 14 276 290 6 200 8 9 (No plume over sub-Jupiter region)383758511 E6I0063 7 11 14 276 290 6 200 7 9389654039 G7I0023 V-G-R-7 10 42 111 70 21 160 7 17 Surface changes near Amirani, Zal P.?,

(No plume over Ukko, Karei)389752400 G7I0030 C 5.7 47 258 210 168 300 7 11 TOPO, Prometheus, Ra glowing

on nightside, (no plume overZamama, Culann)

continued

186 MCEWEN ET AL.

TABLE III—Continued

Frame Scale Phase S/C Sun Limb Terminator ICT Notes, active volcanismnumber PICNO Filtersa (km/px) angle Longitude Longitude Longitude Longitude Qb RJ detected (and key nondetections)

389771978 G7I0040 C 6.1 37 275 238 185 328 7 10 TOPO, (no surface changes at Loki,Pillan P. albedo similar to C3)

389772000 G7I0050 G 12 37 275 238 185 328 7 10389772004 G7I0051 R 12 37 275 238 185 328 7 10389772007 G7I0052 V 12 37 275 238 185 328 7 10 (No plume over Aidne, Fo, Sethlaus)389772011 G7I0053 7 12 37 275 238 185 328 7 10394435100-1 G8I0004-6 V-G-1MC 13 71 129 58 39 148 5 22 Kanehekili plume, Prometheus,

(3.2-sec 1MC for nightsidehot spots—none detected,no plume over Janus)

394478145-6 G8I0007-9 V-G-R 10 81 200 120 110 210 5 19 Amirani or Monan plume,(no plume over Altjirra)

394488745 G8I0010-12 V-G-R 10 84 218 135 128 225 5 18 (no plume over Malik, Maui)394505145 G8I0013-15 V-G-R 10 86 244 158 154 248 5 17 Prometheus plume, (no plume

over Shamash, Culann, G1E).394519145-6 G8I0016-18 V-G-R 10 85 263 178 173 268 5 16 Zamama plume, (no plume

over Volund, Aidne)394552445 G8I0019 G 11 74 299 226 209 316 5 14394552500 G8I0020 R 11 74 299 226 209 316 5 14394552545-6 G8I0021 V 11 74 299 226 209 316 5 14 Marduk plume, (no plume over Isum,

Lei-Kung)394552600 G8I0022 V 11 74 299 226 209 316 5 14 Second violet image with long expo-

sure394552645 G8I0023 1MC 11 74 299 226 209 316 4 14 0.53-sec exposure; no hot spot seen

at Loki394552700-1 G8I0024 C 11 74 299 226 209 316 5 14 Bleeding; Ra nightside glow401740700 C9I0005 G 21 4.5 48 52 138 322 4 11401740704 C9I0006 R 21 4.5 48 52 138 322 4 11401740707 C9I0007 V 21 4.5 48 52 138 322 4 11 Hint of plumes over Malik/Shamash?,

(no plume over Tupan)401740711 C9I0008 7 21 4.5 48 52 138 322 4 11401785378 C9I0010 C 8.3 47 69 116 159 26 4 11 TOPO, Prometheus plume401785400 C9I0011 G 17 47 69 116 159 26 4 11401785403 C9I0012 R 17 47 69 116 159 26 4 11401785407 C9I0013 V 17 47 69 116 159 26 4 11 Prometheus plume (no plume

over Culann, Shamash)401863178 C9I0015 G 6 81 146 227 236 137 4 14 Prometheus shadow, (no surface

changes at Shamash)401863200 C9I0016 R 12 81 146 227 236 137 4 14401863203 C9I0017 V 12 81 146 227 236 137 4 14 Large new plume, probably over

Pillan.401863207 C9I0018 7 12 81 146 227 236 137 4 14401876300 C9I0020 V 13 78 168 246 258 156 3c 15 Long exposure (1.6 sec), (Pele plume

not detected)413570400 C10I0010-11 G-R 11 36 64 28 334 118 5 13 new Masubi plume deposits413570900 C10I0012-13 V-7 11 36 64 28 334 118 5 13 No plume over Acala413571400 C10I0014-15 1MC-889 11 36 64 28 334 118 5 13413659700 C10I0020 C 3.8 45 110 155 200 65 6 9 TOPO413744178 C10I0025 G 6 61 215 275 305 185 5 13 dark Pillan plume deposits413744200 C10I0027 R 12 61 215 275 305 185 5 13413744203 C10I0028 V 12 61 215 275 305 185 5 13 No Loki plume413744207 C10I0029 7 12 61 215 275 305 185 5 13413744668 C10I0030 1MC 12 61 215 275 305 186 5 13413791000 C10I0035-37 V-1MC-889 10 75 269 343 359 253 5 16 Pillan plume: faint glow on nightside;

no visible plume over subjovianregion

413791545 C10I0038-40 G-R-7 10 75 269 343 359 253 5 16416101500 C10I0114 C-R-V- 64 178 225 42 135 315 9 93 Disk of Io detected only in CLR filter.

1MC-889

a Color filters: C 5 CLR, V 5 VLT, G 5 GRN, R 5 RED, 7 5 756 (see Table I).b Higher Q leads to greater compression.c VG3 (strongly nonuniform) ICT Q-matrix utilized.


TABLE IVSSI Eclipse Observations of Io

Frame SCET Scale Exposure ICT S/Cnumber PICNO Date (hr:mn) (km/px) (sec) Q longitude Filter RJ

350029700 G1I0030 6/29/96 03:47 10.5 2.13 3 235 CLR 18350029742 G1I0031 6/29/96 03:47 10.5 0.27 3 235 CLR 18374478045-6 E4I0002 12/17/96 19:46 17.6 8.53 7 36 CLR 20374478201 E4I0004 12/17/96 19:48 35.2 6.4 35 36 GRN 20374478223 E4I0005 12/17/96 19:48 35.2 6.4 35 36 RED 20374478246 E4I0006 12/17/96 19:48 35.2 6.4 24 36 VIO 20374478269 E4I0007 12/17/96 19:48 35.2 6.4 24 36 756 20383809200 E6I0070 2/21/97 8:14 9.2 6.4 8 296 CLR 9389608268 G7I0001 4/03/97 01:29 33.3 6.4 5 61 CLR 21389608300 G7I0002 4/03/97 01:29 33.3 8.36a 5 61 GRN 21389608322 G7I0003 4/03/97 01:30 33.3 8.53a 5 61 RED 21389608345 G7I0004 4/03/97 01:30 33.3 12.8a 5 61 VLT 21389608768 G7I0010 4/03/97 01:34 33.2 6.4 5 61 756 21389608800 G7I0011 4/03/97 01:34 33.2 6.4 5 61 1MC 21389608822 G7I0012 4/03/97 01:35 33.2 6.4 5 61 889 21389608845 G7I0013 4/03/97 01:35 33.2 2.13 5 61 CLR 21394394100 G8I0002 5/06/97 16:00 18.6 6.4 5 75 1MC 25394394145 G8I0002 5/06/97 16:00 18.6 6.4 5 75 889 25394394200 G8I0003 5/06/97 16:00 18.6 6.4 5 75 CLR 25401957700 C9I0025 6/28/97 18:36 14.6 13.85b 6 280 1MC 20401957745 C9I0026 6/28/97 18:36 14.6 6.4b 6 280 CLR 20413546765 C10I0001 9/18/97 03:34 13.2 6.4 5 47 CLR 15413799100 C10I0045 9/19/97 22:05 11.4 25.6 5 276 1MC 16413799100 C10I0046 9/19/97 22:06 11.4 6.4 5 276 CLR 16416072400 C10I0100 10/05/97 21:11 63 6.4 0 181 CLR 93

a Sequence error resulted in exposures during slews and filter contaminations (0.44 sec GRN added to RED image and 0.27 sec RED added to VLT).b Sequence error resulted in exposure of 1MC during slew; CLR image includes 7.75 sec of 1MC filter.c Coordinates within Jupiter’s magnetosphere.

Photometry. These include (1) images with Io illumi- Orbit G1 provided our first look at Io at 10–20 km/pixel since 1979, and the first images at red to near-IRnated primarily by Jupitershine, to search for nightside

brightenings and color changes associated with condensing wavelengths at this resolution (Belton et al. 1996). Therewere several surprises. Perhaps the most puzzling resultfrosts or other phenomena (Buratti et al. 1995, Simonelli

et al. 1994, 1998), and (2) very low phase angle observations was that several regions that changed between Voyagers1 and 2 had reverted to their appearance at the Voyagerto map regolith porosity variations. In addition, the combi-

nation of surface monitoring images at low phase angles 1 flyby! HST had shown the same result (Sartoretti et al.1995, Spencer et al. 1997a), but we had suspected (incor-with plume observations at high phase angles enables map-

ping of variations in phase function and regolith properties rectly) that the low resolution was misleading. We werealso surprised by the lack of apparent changes near Loki(Simonelli et al. 1997).Patera, in spite of the fact that Loki has been the mostenergetic hot spot on Io for at least two decades (Spencer

IO’S VOLCANISM: DISCOVERIES AND RESULTS and Schneider 1996). About a dozen large-scale surfacechanges compared with Voyager were seen; the most pro-

Chronological Summarynounced changes surrounded Ra Patera. An active plumewas seen over Ra in both sunlit and eclipse images. Discov-Io results during orbits G1–C10 are summarized in Table

V. Here we describe the more surprising or puzzling discovery that portions of the Ra deposits are unusually bright onIo’s nightside in the RED bandpass remains unexplained.eries in chronological order and place them into the context

of other Io discoveries by NIMS, HST, and ground-based The other major surprise of G1 was the eclipse imageshowing seven high-temperature hot spots and diffuseobservers. This narrative helps to explain why we made

certain observation and playback decisions. Every orbit plume and atmospheric glows. This result led us to expandthe eclipse observations in later orbits.has brought surprises.

188 MCEWEN ET AL.

Orbit G2 included two sets of Io observations: a plume change was due to an unusually forward-scattering phasefunction rather than volcanic activity. A long-exposureinventory and a surface monitoring color set. The plume

inventory consisted of 30 frames, covering the bright limb high-phase image (frame no. 368981522) for detection ofthe Na cloud also revealed continuation of the hot spot atevery p108 of rotation (but with several gaps due to Jupiter

occultation, transit, or eclipse), but these images had to be Pele and plume activity at Prometheus.very highly compressed and are of poor quality. The images Orbit E4 was severely restricted in data return, but pro-nevertheless revealed the presence of an active plume at vided unique photometry opportunities. In a high-phasePrometheus and there is a hint of a plume over Culann VLT image acquired to image Io illuminated by Jupi-Patera (Fig. 4). No other plumes could be detected. Since tershine we discovered a faint enhancement over the limb,nine plumes were active during Voyager and two or three similar in apparent size and shape to the Pele plume seenplumes were seen from just a few images acquired in G1, by Voyager 1 (Fig. 3). It was also at the right latitude forit seemed at the time that the combination of low resolution Pele, but the limb longitude was 2838 and the vent of Peleand low SNR, exacerbated by the high compression, had was on the far side, 278 beyond the limb. If this was theeliminated unambiguous detection of all but the especially plume of Pele it had to be 460 km high, much larger thanbright plume of Prometheus. the 300 km plume imaged by Voyager 1. Meanwhile, Pele’s

The surface monitoring set provided our best look ever plume was discovered in images acquired by HST in Julyat the antijovian hemisphere and revealed many changes of 1996, with a height of p400 km (Spencer et al. 1997c),since 1979. A major surprise was the discovery that the thus confirming that Pele was much larger and fainter thanvent of Prometheus was located p70 km west of its position during Voyager 1. We acquired the first 1MC image resolv-in 1979, although the plume and surrounding deposits ap- ing low-albedo volcanic centers and found no evidence forpeared very similar to the Voyager observations. Also very an absorption typical of Fe-bearing silicate minerals thatintriguing was Pillan Patera, whose floor darkened mark- are abundant in mafic volcanics in the inner Solar System.edly between G1 and G2. A hot spot at Pillan was detected

Orbit E6 included a collection of images for surfaceby NIMS in G2 (Lopes-Gautier et al. 1997a). A regionmonitoring and plume, topographic, and eclipse imaging.seen in G1 and G2 at very different phase angles revealedThere were several surprises, as usual. The bright materialsmarked differences and contrast reversals (Simonelli et al.seen around Ra Patera (orbit G1) had faded significantly1997). This led us to modify future observations to betterand pronounced surface changes occurred around Kanehe-match the phase angles of some observations (for changekili. We saw that Pillan Patera was still bright but this timedetection) and to acquire global coverage at both low andin a low-phase image, so its darkening in G2 was due tomoderate phase angles to map the phase-function varia-volcanic activity after all. E6 images covered six differenttions.bright-limb longitudes but did not show a single visibleOrbit C3 provided the closest pass to Io of the nominalplume (although we saw Prometheus near the terminator).tour (Table II), but we were restricted to only 8 frames. WeIt now seemed apparent that Io had far fewer bright visiblechose to concentrate on a series of CLR-filter topographicplumes than during the Voyager flybys, but this was puz-mapping images. These images revealed that mountains,zling because there were many hot spots, at least as manycalderas, plateaus, and other landforms imaged by Voyageras in 1979 (Lopes-Gautier et al. 1997a).1 on the Jupiter-facing hemisphere were also ubiquitous

and the antijovian hemisphere (Carr et al. 1998; see Fig. Orbit G7 provided another set of images for surface,plume, topographic, and eclipse imaging. A major infrared2). The Io images compressed much less than predicted

with the result that we could return less data from other brightening of Loki began just after the E6 encounter(Spencer et al. 1997b), so we were looking for a reappear-targets (Europa and rings) that came after Io in the play-

back sequence. The plume over Ra Patera should have ance of plume activity and surface changes near Loki. TheG7 eclipse image (Plate 2) did indeed reveal a plume-been well resolved in one of the images but had apparently

ceased activity or become much smaller or fainter. We like feature near Loki, although longitude determinationis ambiguous, but there were no obvious surface changes.began to suspect that the paucity of plumes detected in

the G2 plume inventory could have been due to a real Eclipse images acquired through color filters revealed dif-fuse plume/atmospheric glows in the VIO, GRN, and REDpaucity of bright plumes. The floor of Pillan Patera had

returned to the brightness level seen in G1 and Voyager, bandpasses but not in the 756, 889, or 1MC bandpasses.The glow in the RED is consistent with [OI] emissionsso we suspected (erroneously) that the apparent G1–G2

FIG. 2. Global mosaic of highest resolution SSI images of Io, Simple Cylindrical map projection. Names of features discussed in the text areindicated; arrows indicate unnamed features mentioned in text.


PLATE 1. Color mosaic of Io in Simple Cylindrical format, 756, GRN, and VLT mosaics displayed as red, green, and blue, respectively. Colorsare slightly exaggerated compared to natural color. Mosaic constructed from low-phase (,158) images.

seen by HST (Trauger et al. 1997), but the emissions in based observers (Howell et al. 1997) also detected a veryintense hot spot near Pillan Patera, the likely vent regionthe VLT and GRN have not been explained.for the plume. The eclipse image also showed a hot spotOrbit G8 included high-phase color observations ac-at Acala Fluctus, surrounded by diffuse glows, and furtherquired when volcanically active regions were located ondiffuse glows around the Pele/Pillan and Marduk regions.the bright limb, to attempt to image plumes. Given theThese observations support suggestions that Io’s atmo-paucity of plumes seen on Io outside of eclipse throughsphere is strongly influenced by volcanic outgassing (e.g.,orbit G7, we had the sinking feeling that the only plumeIngersoll 1989, Johnson et al. 1995, Lellouch 1996).we would succeed in imaging would be Prometheus. We

did see Prometheus, but also plumes over Kanehekili, Za- Orbit C10 provided five- or six-color coverage over muchof Io as well as an additional topographic frame and 3mama, Marduk, and Amirani. The eclipse image also re-

vealed a plume-like feature very similar to that seen in eclipse images (Plate 2). Analysis of the six-color datahas revealed an absorption band near 900 nm in the darkG7, but the longitude was clearly too far west and it now

appeared likely that the plume seen in both orbits was materials (Geissler et al., manuscript in preparation). Thefive-color set revealed a new 400-km diameter dark spotemanating from the vicinity of Acala Fluctus rather than

Loki. The eclipse image also revealed a remarkable field around Pillan Patera, similar to the deposits around BabbarPatera. An eclipse image showed the intensity of the Pillanof 14 bright spots near the subjovian region (Plate 2).

Furthermore, this image showed that the diffuse glow over and other hot spots to be much reduced from C9. Anothereclipse image provided our best view yet of the subjovianthe limb region of Prometheus (antijovian region) ex-

tended to an altitude of p700 km. field of bright spots (about 26 spots resolved) and showedthat the entire region contains a diffuse glow. The thirdOrbit C9 consisted of another mix of images for surface,eclipse image, although of low resolution (60 km/pixel),plume, topographic, and eclipse imaging. Two days beforeprovided our best look at the antijovian region and re-playback of frames C9I0015-18 (Table III), John Spencervealed a diffuse glow extending about 308 east and west(personal communication) reported HST imaging of a newfrom the antijovian point, similar to the subjovian glow.bright plume at either Reiden Patera or Pillan Patera. The

SSI frames that were subsequently played back had been Orbit E11 returned eclipse observations designed tomeasure temperature (McEwen et al. 1998), three-colortargeted to Reiden Patera on the bright limb and confirmed

a large new plume (Fig. 3). SSI eclipse images and ground- plume images at high phase angles, a topographic mapping

192 MCEWEN ET AL.

FIG. 3. Plumes on Io seen by SSI in reflected light. The best images of each plume are shown, to the same scale. Actual plume sizes may belarger than they appear when the vent is significantly beyond the limb (especially true for Pele, Ra, Pillan, and Amirani). Images are from orbitsG1 (Ra, Zamama), E4 (Pele, Amirani), G8 (Kanehekili, Amirani, Prometheus, Marduk), and C9 (Pillan).

frame, and a low-phase (0.58) color set. The E11 data have ration). The association between low-albedo features andnot been analyzed in detail for this paper, but obvious hot spots (Pearl and Sinton 1982, McEwen et al. 1985) hasresults can be listed. Plumes were seen at Zamama (much been confirmed; the eclipse images show that the hot spotslarger and brighter than previously), Prometheus, Marduk, fall directly on the low-albedo areas (McEwen et al. 1997).Pillan, and Kanehekili. Hot spots were detected at Pele, However, the lowest albedos do not necessarily correspondPillan (2 spots), Marduk, Lei-Kung, Isum (2 spots), Kaneh- to the highest temperatures, and the low-albedo areas areekili (2 spots), Janus, Zamama, Amirani, Prometheus, Cu- much more extensive than the high-temperature (.700 K)lann, and unnamed volcanic centers at 22 N, 238 W and 3 areas. In addition, diffuse red areas are closely associatedN, 76 W. with active volcanos. Examples (see Plate 1 and Fig. 2)

include Pele, Marduk, Culann, Isum, Zamama, Zal, Eu-Color and Compositional Variations boea, Ulgen, Tupan, Prometheus, and Amirani. Reddish

areas that are not near currently active volcanic centersThe most distinctive color units occur in association withare relatively muted and less intensely red. Unusual brightthe most volcanically active regions on Io, except for mostyellow units surrounded Ra Patera in 1995–1996 (Spencerbright white areas (Plate 1). These relations are described

in more detail by Geissler et al. (1997; manuscript in prepa- et al. 1997a, Belton et al. 1996). Although some of the


FIG. 4. Portion of the G2 plume sequence showing Prometheus (top middle) and Culann (bottom) rotate onto the limb.

brightest white areas occur adjacent to active volcanic cen- the regions that remain bright at moderate to high phaseangles occur around active volcanic centers such as Mardukters (e.g., Amirani), many are far removed from any known

active volcanism (e.g., Bosphorus Regio; Smythe et al. and Zamama. If this is also a grain-size effect of SO2 , itsuggests that freshly condensed SO2 frost grains anneal1997).

Little is known about compositions on Io other than the and grow over time (Clark et al. 1983).The spectral properties of the red regions are consistentpresence of SO2 . The bright white regions seen by Voyager

were thought to be rich in SO2 frost or ice (McEwen et al. with metastable short-chain S3/S4 molecules mixed with avariety of possible sulfurous materials such as S2O, poly-1988). NIMS spectral mapping (Carlson et al. 1997) has

shown that the areas that are bright white at low phase sulfur oxide, or elemental sulfur (Nelson and Hapke 1978,Hapke and Graham 1989, Nelson et al. 1990, Spencer et al.angles are characterized by coarse-grained SO2 and that

finer grained SO2 is abundant elsewhere, especially at high 1997a). S3/S4 produces a strong increase in reflectance long-ward of p0.6 micrometers, barely detected by the reddestlatitudes. This helps explain why Io appears very different

at moderate-to-high phase angles (Fig. 1) than it does at Voyager filter (0.59 6 0.03 micrometers). These materialsare truly red in color, as the reflectance increases suddenlylow phase angles (see corresponding portion of Plate 1).

Ratios of images acquired at high and low phase angles between green and red wavelengths rather than increasinggradually with wavelength as do many planetary surfacesmay map out grain-size variations (Simonelli et al. 1997),

in agreement with the NIMS result. In addition, some of that are called ‘‘spectrally red.’’ Quenched short-chain sul-

194 MCEWEN ET AL.

FIG. 5. Concatenation of eclipse images like Plate 2, but with labels of major features: Ka, Kanehekili: Za, Zamama; Pe, Pele: Ma, Marduk;Zl, Zal; Ac, Acala; Lo, Loki; Am, Amirani; Pi, Pillan; Pr, Prometheus.

fur was invoked by Sagan (1979) to explain reddish flows scales of years to decades (as described below) could beexplained by transformation of S3/S4 to S8 , which in turnon Io, but Young (1984) criticized this idea on the grounds

that flows cool slowly and S3/S4 would rapidly revert to should be modified by solar ultraviolet light to produce anS8–Sy mixture (Steudel et al. 1986).stable S8 . With the red sensitivity of CCD detectors we

now can see that the truly red areas are predominantly What is the origin of Io’s reddish polar regions? If thefading of red material is due to transformation of S3/S4 ,associated with diffuse deposits near active vents, so they

appear to have resulted from airborne pyroclastics. Fine then this transformation might proceed more slowly atthe colder temperatures of Io’s polar regions. The reddishdroplets in airborne ejecta are plausible candidates for

very rapid cooling and quenching (Moses and Nash 1991). polar regions are patchy rather than continuous, so perhapsthese are also volcanic pyroclastics, but more than a fewThe fact that these red deposits appear to fade over time


PLATE 2. Color-coded eclipse images of Io. The left-hand column is aligned at the longitude of Kanehekili and the middle column is alignedat Pele. See Fig. 5 for labels of key volcanic centers. Intensity levels increase through the color sequence black–blue–green–yellow–red. The middleC10 image received reflected light from Europa on the antijovian hemisphere (right half as seen here).

decades are required for the color to fade at the polar deposits (McEwen et al. 1985, Rothery et al. 1996). Tele-scopic spectra and HST images have shown that Io (attemperatures. Alternatively, the reddish color at the poles

could have an entirely different origin, such as alteration large scales) does not exhibit a 1 micrometer absorptioncharacteristic of Fe-rich silicates (Spencer et al. 1995,by radiation (Johnson 1997).

The composition of the low-albedo regions is a key un- 1997a). The SSI 1MC/756 ratio images show that the low-albedo features down to p10-km scale also lack 1-microm-known. This is clearly the material most closely related to

the erupting lava, for which the temperatures are consistent eter absorptions, but the 889-nm bandpass is indeed darkerthan the 756-nm or 1MC reflectances (P. E. Geissler, manu-with silicate compositions. Hapke (1989) proposed that the

dark material is basalt. There is no reason to expect basalt script in preparation). NIMS spectra show a broad1.25-um absorption (Carlson et al. 1997), but lack the spa-to be adundant on Io (Keszthelyi and McEwen 1997b) but

a variety of mafic silicates are possible. The visible color tial resolution (from the prime mission) needed to see mostlow-albedo volcanic centers. Fresh basalts on Earth oftenof most of the dark materials is reddish, unlike pure basalt,

but can be approximated by basalt with sulfurous fumarolic lack a 1-micrometer band due to abundant glass and/or

196 MCEWEN ET AL.

magnetite (Hunt et al. 1974), so basalts or other Fe-rich are shown in Fig. 3 and plume-like features seen in eclipsevolcanics on Io cannot be ruled out. Another possibility can be seen in Plate 2 and Fig. 5.is that they are Fe-poor silicates, as intense magmatic dif- We have detected plumes in illuminated images lessferentiation may have segregated iron into the lower man- often than expected from Voyager experience, except intle (Keszthelyi and McEwen 1997b). Some of the dark images specifically targeted for suspected plumes. Frommaterials may be coatings of metal sulfides from reaction G1 images we determined that there were no plumes atbetween hot lava and sulfur (Johnson and Burnett 1990). Loki and discovered a new plume at Ra Patera (p75 kmSulfur can also be very dark depending on cooling history high) and a small plume (p60 km high) over a new darkor in the presence of contaminants (Moses and Nash 1991). volcanic center named Zamama (p58 south of Voyager-Although the highest temperatures are too high for sulfur era plume Volund). A G1 color image with Pele at theto be stable, such high temperatures cover very small areas bright limb revealed no evidence for a plume except forand most of the dark areas are much cooler. a 1-DN enhancement in the green-filter image, which is

not a convincing detection by itself. The G1 eclipse imageActive Volcanic Plumes and G2 plume-inventory images marginally detect a new

small plume (,50 km high) at Culann Patera. G2 and C3Voyager Observations and Galileo Expectationsimages showed that Ra’s plume was no longer visible, but

The 2 Voyager flybys in 1979 revealed 9 plumes that are we saw a diffuse glow above the limb region of Ra in thelarge (.50 km) and bright (visible at uv to blue wave- E4 eclipse image. The E6 eclipse image showed a similarlengths in low-phase images exposed for imaging of surface plume-like enhancement over Marduk. A high-phase E4features) (Strom et al. 1981, McEwen et al. 1989). The image revealed a plume p75 km high over Amirani. Thisplume of Pele was the faintest and was seen only during same E4 image (violet filter, 1538 phase angle) reveals theVoyager 1. All of these plumes could be seen in images top of Pele’s plume. Eclipse images showed a faint diffusewith resolutions from 10 to 30 km/pixel. Voyager plumes glow over Masubi, but no visible plume has been detected.were brightest (especially compared with the surface) in However, C10 images revealed new circular plume depos-the uv bandpass (p340 nm), but were also easily seen in its at Masubi, so a plume must have been active betweenthe violet bandpass (p413 nm). The Loki plumes appeared C9 and C10. A visible plume was detected at Marduk insignificantly larger in the uv than at longer wavelengths. orbit G8, as well as a new plume at Kanehekili, and a newThe plumes are best seen at high phase angles, as they plume was discovered near Pillan Patera in C9. The Pillanare more forward scattering than the surface. Voyager plume should have been seen previously if active (e.g., inacquired more than 100 full-disk images at better than 30 PICNO G1I0010 or E6I0001) and thus was probably a newkm/pixel during each flyby, but many of these are highly (or much bigger and brighter) plume. The Acala plumeredundant. One or more plumes are detectable in p50% has only been seen in eclipse, but there have been noof the Voyager full-disk color images, either at the bright illuminated observations with the proper bright-limb ge-limb, near or beyond the terminator, or against the disk. ometry except in orbits G1 and C10. Acala was not seenSSI’s shortest bandpass is violet (p418 nm, comparable in the G1 eclipse, in spite of favorable geometry. Throughto the violet bandpass of Voyager), and the camera has orbit C10 there were no signs of the Voyager-era plumesimproved sensitivity and geometric characteristics. Gal- Loki-west, Loki-east, Volund, or Maui, in spite of observa-ileo’s tour provided multiple opportunities to image Io at tions that should have revealed these plumes if similar in10–30 km/pixel during every orbit, and often at higher size and brightness to the Voyager-era plumes.phase angles than the Voyager approach images. Although To summarize the plume activity, we can say that (1)SSI can acquire and return far fewer images per pass than there were fewer bright visible plumes in the first half ofdid Voyager, they are less redundant and the improved the Galileo tour than in early 1979, and (2) three newsensitivity of SSI aids the detection of plumes. However, plumes (or four counting Masubi) appeared (or bright-SSI lacks a uv bandpass and the ICT compression is espe- ened) by the time of orbits G7 through C10. However, duecially damaging to features near the limits of detection. to the poor quality of the G2 plume sequence and theGiven these pluses and minuses compared with Voyager, marginal detection of several small or faint plumes, it re-we expected to see plumes in about half of the SSI images mains unclear whether or not we have detected all of theof Io, assuming comparable volcanic activity. plumes. The volcanic centers with plumes are discussed in

more detail in the Discussion section.SSI Observations of Plumes

Two plume geometries were modeled by Kieffer (1982):(1) balanced or umbrella-shaped plumes which producePlume observations by SSI, Voyager, and HST are sum-

marized in Table VI, and plume detections and key nonde- circular deposits, and (2) overpressured (underexpanded)plumes which produce irregular deposits. The largest Gali-tections are tied to specific SSI frames in Table III. The

best visible SSI images of each plume through orbit C10 leo-era plumes appear to be umbrella-shaped and produce


TABLE VSummary of Io Discoveries and Results, Orbits G1–C10

Orbit Major new results

G1 Ra Patera: plume (visible and eclipse), surface changes, red bandpass glow on nightside.VGR–GLL surface changes at Euboea, Sengen, Acala, Fjorgynn, Amaterasu, Ukko.Fading of Surt and Aten plume deposits since VGR-2.Zamama: new plume, hot spot, diffuse glows in eclipse.Pele: intense hot spot, no visible plume.Additional hot spots at Isum (2), Mulungu, Marduk, Reiden, Lei-Kung.Atmospheric and plume limb glows seen in eclipse.Correlation of low-albedo and red deposits with recently active volcanoes.Red polar deposits seen to be patchy.Bright patches at high latitudes seen to surround mountains.

G2 Prometheus: plume active, vent 70 km west of VGR-era location, new lava flows.Zamama: complex surface changes.Absence of large, bright plumes except Prometheus.VGR–GLL surface changes at Culann, Marduk, Mycenae Regio, Shamash, Amirani.G1–G2 darkening of Pillan Patera.G1–G2 comparison reveals major variations in regolith properties.Improved shape of Io from limb fits.

C3 G2–C3 brightening of Pillan Patera.Discovery of new large mountains: Dorian, Rata, Tohil, Euxine, Skythia, Gish-Bar.Layered terrain common on antijovian hemisphere.Imaging of Na cloud around Io.

E4 Pele plume detected, 460 km high.Plume detected at Amirani.Hot spots seen at Kanehekili (2), Janus, Gish-Bar.Plume-like feature near S. Pole seen in eclipse.Surface changes in N. Polar region.No evidence for SO2 condensation on nightside.

E5 Solar conjunction, no dataE6 Kanehekili surface changes.

Darkening of Amaterasu Patera.Fading of new deposits around Ra Patera.Hot spots at Loki, Pele, Acala?, Fuchi?Plume-like glows over Marduk and subjovian region, in eclipse.Surface changes in S. Polar region.

G7 Plume-like feature seen in eclipse, probably over Acala Fluctus.Atmospheric/plume glows in eclipse seen in VLT, GRN, RED.Hot spots at Kanehekili (2), Janus, Zal, 65N, 140W.Loki region seen to have little topographic relief.

G8 Visible plumes seen at Kanehekili, Zamama, Marduk, Amirani, Prometheus.Plume-like feature in eclipse over Acala Fluctus.Hot spots seem at Kanehekili (2), Janus, Zal, Amirani.Evidence for very high-temperature hot spots at Kanehekili and Amirani.Field of 14 faint bright spots in eclipse seen near sub-Jupiter region.Atmospheric limb glow extends 700 km from limb over Prometheus region.

C9 Pillan Patera: new 120-km high plume; very intense and high-temperature hot spot.Additional hot spots at Pele, Loki, Acala, Svarog, Marduk.Diffuse glows in eclipse around Acala, Pillan, and Marduk.Additional mountains seen; some appear to be tilted crustal layers.

C10 New 400-km diameter dark deposits around Pillan Patera.New plume deposit at Masubi.Detection of absorption band near 900 nm in dark areas.Hot spots seen at Pele, Marduk, Pillan (2), Acala, Kanehekili (2), Janus, Amirani, NE of Hi’iakaSubjovian and antijovian bright diffuse regions seen in eclipse.p26 faint bright spots seen within subjovian region.Plume-like feature seen near N. Pole in eclipse.

198 MCEWEN ET AL.

PLATE 3. Key volcanic centers. (a)–(g) are color images merged with highest resolution mosaic (Fig. 2) in Simple Cylindrical projections.(a)–(g) are at the same scale; width of (a) is 575 km. The merged images show color units in relation to topography and fine albedo markings suchas flows. (h) shows the Pele and Pillan region in the Spring of 1997 (left) and Fall of 1997 (right), in Orthographic projections. New dark materialaround Pillan is p400 km diameter. Enhanced color from 756, GRN, and VLT bandpasses.


TABLE VIActive Plumes on Io

Plume Latitude, Height Nondetections by SSIname longitude When seen (km) on bright limb Notes

Kanehekili 217, 36 5/97 50–100 12/96 new plume deposits seen 2/97, persistentSSI/NIMS hot spot

Masubi 244, 54 3/79, 7/79 50–100 12/96 new plume deposits seen 10/97Amirani 124–128, 114 3/79, 7/79, 12/96, 5/97 50–100 SSI/NIMS hot spot, plume vent at latitude

129 in 1979, 124 in 1996–7.Maui 119, 122 3/79, 7/79 50–150 5/97 NIMS hot spotPrometheus 22, 152-4 3/79, 7/79, all GLL orbits 50–100 none NIMS hot spot, VGR–GLL change in

from G2–C10 vent longitudeCulann 220, 160 6/96?, 9/96? 0–50 4/97, 5/97, 6/97 NIMS hot spotZamama 118, 173 6/96, 5/97 50–100 4/97 SSI/NIMS hot spotVolund 123, 177 3/79, 7/79 50–100 6/96, 5/97 NIMS hot spotMarduk 227, 210 3/79, 7/79, 2/97, 5/97 50–100 SSI/NIMS hot spotPillan 212, 242 6/97, 9/97 100–150 6/96 Intense SSI/NIMS hot spot 6/97,

persistent NIMS hot spotPele 218, 256 3/79, 7/95?, 7/96, 12/96, 300–460 6/96, 2/97, 6/97 HST observations 7/95, 7/96, 7/97,

7/97? persistent SSI/NIMS hot spotLoki-W 119, 305 3/79, 7/79 150–400 6/96, 2/97, 9/97 North of Loki Patera.Loki-E 117, 301 3/79, 7/79 20–150 6/96, 2/97, 9/97 NE of Loki Patera.Ra 28, 325 6/96, 12/96? 50–100 9/96, 11/96, 12/96, 2/97 12/96 plume in eclipse, no hot spot

detectedAcala 111, 334 4/97, 5/97, 6/97 200–300 6/96, 11/96, 2/97, 9/97 Plume seen in eclipse only, SSI hot spot

circular (or elliptical) deposits. Kieffer (1982) suggested Apparent changes between Voyager and Galileo imagescan be caused by differences in (1) the two imaging systemsthat the Loki plumes were overpressured. Several of the

smaller plumes (Zamama, Marduk, and Culann) may also and their respective color filters, (2) viewing and illumina-tion geometry which introduces photometric effectsbe overpressured. Although we cannot directly resolve the

plume structure, there are no circular plume deposits in and/or shadows, (3) spatial resolution, or (4) changes onIo. The ability of SSI to detect longer wavelengths thanthese latter areas. In some cases (especially Marduk) it

appears that the irregular diffuse red materials may be the the Voyager camera is responsible for many of the strikingdifferences in the color images from the two spacecraft.plume deposits.The green bandpasses of the Voyager imaging system

Surface Changes (0.54 6 0.04 em) and SSI (0.56 6 0.03 em) are mostdirectly comparable, although the violet bandpasses areSurface Changes since Voyageralso close. We have primarily relied on comparisons be-

There have been many major changes to the volcanically tween images taken in the green, using the violet imagesactive surface of Io during the 17 years between the Voy- for corroboration. However, both Voyager 1 and 2 haveager flybys and the first Galileo images. All detected gaps in green-filter coverage in the hemisphere that in-changes appear to be due to color and albedo ‘‘painting’’ cludes Loki and Ra, and to cover these gaps we have usedof the surface rather than major topographic changes, al- the Voyager orange filter (0.59 6 0.03 em). A series ofthough the Voyager–Galileo overlapping images are only Voyager–Galileo comparisons in color synthesized fromsufficient to show kilometer-scale differences. The de- green/orange and violet bandpasses are available from thetected changes include what appear to be new plume de- Planetary Photojournal at http://photojournal.jpl.nasa.posits and lava flows and the fading of previously distinc- gov/.tive surface units. Perhaps more surprising is that few Photometric effects must also be considered. The surfacesurface changes are obvious around some of the most ther- of Io has extremely varied and unusual photometric behav-mally active areas such as Loki. There are also cases where ior, including strong contrast reversals as a function ofthe changes between Voyager 1 and Voyager 2 disappeared phase angle (Simonelli et al. 1997). This led to some tempo-over the intervening years. In this section we first describe rary confusion when lava flows would disappear and reap-the steps we took to ascertain that variations between pear in subsequent images. These problems are minimizedimages are indeed the result of new surface features and by only comparing images with similar phase angles. Un-

fortunately, this greatly diminishes the volume of datathen describe the major changes.

200 MCEWEN ET AL.

available to search for surface changes. The Loki hemi- fallout onto hot surface areas may have been remobilized,thus producing the small bright halos around dark (andsphere (longitudes p260–3608) was mapped by Voyager 1

at 108 phase and by Voyager 2 at 208 phase. Comparable probably hot) features west and southeast of Surt.Fading of plume deposits can be seen at other locationsimages from Galileo were obtained in the G1 and E6 orbits

(frame numbers 349746339 at 258 phase and 383758500 at (e.g., Volund, Fig. 10a). Apparent fading of plume depositsat Maui (Fig. 11a) and NW of Loki (Fig. 8b) are at least148, respectively). The leading and antijovian hemispheres

(longitudes p0–2608) were imaged by Voyagers 1 and 2 partly due to the absence of airborne particles, as darkplume material can be seen against the disk in Voyagerat 2–88 phase and by Galileo at 48 in orbit G2 (frame

number 359986604), 0.58 in orbit E4 (frame number images of these plumes.New plume deposits larger than 200 km diameter are374575845), and 48 in orbit C9 (frame number 401740700).

To date we have only searched for large, clearly resolved apparent at Ukko Patera (Fig. 11b), Ra Patera (Fig. 7),and Euboea Fluctus (Fig. 8c). This is a surprisingly shortsurface changes. We have not yet registered the images to

each other accurately enough to search for 1–2 pixel scale list of new large plume deposits over 17 years, consideringthat two large new plume deposits appeared between Voy-(much less subpixel) changes. Also, the images are not

photometrically corrected to the level that would give us ager encounters and at least three new plume deposits(Kanehekili, Pillan, Masubi, and possibly others near theconfidence in interpreting the more subtle brightness

changes, and we have made no attempt to match the image north and south poles) have formed during 16 months ofthe Galileo tour. It seems likely that most plume depositsresolutions. Future work with these processing steps should

reveal many additional small or subtle changes. formed between 1979 and 1996 have faded away; this ideais supported by HST observations of Ra Patera (SpencerThe surface changes are broken into appearance and

disappearance of several broad categories of surface units et al. 1997a). Perhaps some additional, muted plume depos-its will be recognized following more careful processingin Table VII: (a) circular plume deposits, (b) diffuse red

deposits (which appear dark and diffuse to red-blind Voy- and differencing of the images.The semi-circular plume deposit at Euboea (Fig. 8c) isager), (c) bright white or yellow patches, (d) dark flow-

like deposits, and (e) dark materials on caldera floors. Note an interesting puzzle. The deposit is circular and has ayellow color to the northwest, but to the southeast it isthat some of these categories probably overlap: diffuse red

deposits or bright patches may also be plume deposits and irregular and red. It is possible that the two ends of EuboeaFluctus erupted different magmas, or the vent geometrydark caldera floor materials may consist of coalesced dark

flows. Table VII lists these changes; in most cases several may have caused an unusual pattern in the expansion ofthe plumes, balanced to the northwest and overpressureddifferent types of new deposits occur at a single volcanic

center. The major changes are shown in Figs. 6–11 to the southeast. If the latter suggestion is correct, thenthe red material forms as a function of eruption conditions(grouped by geographic regions on Io), and some of the

most active centers are shown in Plate 3. rather than different initial compositions.

Diffuse red deposits. Some of the most obvious newCircular plume deposits. Figure 6 shows the large-scalechanges around Surt, Pele, and Aten between Voyager 1, deposits are diffuse red materials; all occur near active

volcanic centers. These centers include Marduk (Fig. 9d),Voyager 2, and Galileo. Note how the deposits from theinferred short-lived plumes at Surt and Aten, which Zamama (Fig. 10a), Culann (Fig. 10d), and others (see also

Plate 3). The red materials appear dark at wavelengthserupted between the Voyager 1 and Voyager 2 flybys (andperhaps at later times), seem to have disappeared in Gali- shorter than 0.6 em, including all of the Voyager color

filters, but are bright in the SSI RED and near-IR band-leo images, reversing most of the changes. In contrast, thedeposits around Pele, which has certainly been active in passes. They are associated with active plumes in many

cases, but do not form circular rings around the vent exceptrecent years, are relatively unchanged. This suggests thatthe plume deposits fade and seem to disappear in less at Pele. In some cases they appear to be associated with

linear fissure-like features, such as that extending SE fromthan two decades. Several processes may contribute. Brightareas surrounding mountains may rebrighten due to down- Marduk. Their diffuse appearance suggests a pyroclastic

origin, perhaps from overpressured plumes.slope movement of plume deposits or fresh condensationof SO2 frost. Reddish plume deposits rich in metastable The red coloration must be short-lived, fading within

time periods of years to decades in the equatorial region.sulfur may revert to stable forms which blends in withmuch of Io’s surface. Dark areas such as the caldera floors All of the major red deposits in Io’s equatorial region are

new, not present in 1979. (Pele appears unchanged sinceof Surt and Aten as seen by Voyager 2 seem to brightenover time in the absence of continued active volcanism. Voyager 2 but has been very recently or currently active,

so the surface layer is new.) It is highly unlikely that suchNote that several features covered by Surt and Aten depos-its did not completely reappear in Galileo images. Plume red material was generally absent in 1979, and there are


TABLE VIIVoyager-to-Galileo Surface Changes

New dark New dark BrighteningLocation New plume Fading plume (red) diffuse Fading of dark New bright Fading of flow-like of dark

name deposit deposit deposit diffuse deposit deposits bright deposits deposit deposit

Acala Fl. X X XAidne P. XAltjirra P. XAmaterasu P. XAmirani/Maui ? X X X X XAten P. X X X XCulann P. X X X X X XDaedalus P. X XDahzbog P. XEuboea Fl. X X X ? ?Fjorgynn Fl. XHi’iaka XLei Kung Fl. X X XLoki X XMarduk ? ? X X X XMasubi XMycenea R. X X X XPillan P.Prometheus X X X X XRa P. X X X (G1 toE6) X X (G1 to E6)Sengen P. ? ? X X XShamash P. XSurt X X X XUkko P. X X XZal P. X XZamama X X X X30 N, 150 W X X46 S, 107 W X31 S, 73 W X2 S, 218 W X

several examples of ‘‘dark’’ (to Voyager) diffuse deposits are pyroclastics or surficial coatings on the flows such asfumarolic sulfur.that have faded before 1996, which we suspect were for-

Bright deposits that are flow-like are relatively rare.merly red. Examples include dark diffuse materials at Cu-Figures 11a and 7 show the two clearest cases, at Amiranilann Patera (Fig. 10d), Acala Fluctus (Fig. 8a), Sengenand Ra Patera. In the case of Amirani the new brightPatera (north of Aten in Fig. 6c), and Marduk. The darkmaterial appears to have flowed west from the dark linearmaterial that appeared near Zal Patera between Voyagersfeature. Simultaneously, the bright flow-like feature to the1 and 2 (McEwen 1988), and faded away before Galileo,south of Amirani has faded (although it remains prominentis another example.at higher phase angles as in Fig. 2). Relations at Ra Patera

Bright deposits. New diffuse bright white or yellow de- are also complex. The entire region surrounding Ra Pateraposits are common, usually associated with other signifi- has been blanketed by a relatively bright diffuse deposit,cant surface changes (Table VII). The ubiquitous nature probably the fallout from the plume seen in G1. Withinof new bright deposits suggests that almost any volcanic and around the plume deposit are a series of bright flow-activity on Io will remobilize near-surface or crustal vola- like features and a broad dark flow. It appears that thetiles. bright flows issue from both the central vent of Ra Patera

There are some bright diffuse deposits that do not have and from along the length of the dark flow. This suggestsa circular planform as expected from balanced plumes. On that the dark flow is producing brighter secondary flows,the northwest portion of Lei Kung Fluctus the dark flows except that this would require that the bright materialappear to have been largely covered by new bright yellow flowed uphill in places according to the topographic data

of Schenk et al. (1997). The changes at Ra Patera betweenmaterials (Fig. 9a and Plate 3b). It is unclear whether these

202 MCEWEN ET AL.

FIG. 6. Voyager-to-Galileo comparisons for (a) Surt, (b) Pele, and (c) Aten. Projections here and in Figs. 7–11 are Simple Cylindrical. SeePlate 1 and Fig. 2 for scales. Surt and Aten Galileo (GLL) images from orbit G1; Pele image from E6.

orbits G1 and E6 (Fig. 7) show that the bright flow-like flow at Ra Patera, which appears to have flowed aroundthe margin of a plateau to the east and north (Schenk et al.materials have completely faded (darkened) into the back-1997; see Fig. 2). Perhaps the most striking new flow is theground, and the dark flow is also fading (brightening).one at Prometheus (Fig. 10c and Plate 3c). The sinuousThis fading has been verified by HST images at constantdark band extends between the Voyager-era center of Pro-photometric geometry (Spencer et al. 1997d). However,metheus and the current eruptive center, about 70 km tothe bright flows remain prominent in high-phase imagesthe west. It is not clear if this dark feature is (a) a flowacquired in orbit C10, so the transformation process mayfrom the current eruptive center flowing back into theresult in a change in photometric function.Voyager era vent or (b) a flow from the Voyager era

Dark flows. The formation of new, dark, flow-like de- vent that is moving to the west. The latter possibility isconsistent with the presence of a smaller flow, extendingposits is common (Table VII). We have already noted the


FIG. 7. Voyager-to-Galileo comparisons for Ra Patera.

north from the Voyager-era vent. Hence, it is possible that past 17 years (e.g., Fig. 11c). The topography of the actualcaldera has probably not changed substantially, just thethe current Prometheus plume is being produced at thealbedo of the floor materials. Altjirra (Fig. 11c) and Ukkoflow front as it advances through the SO2-laden bright frostPaterae (Fig. 11b) are newly-named calderas and there aredeposits. Other prominent dark flow-like features can bethree small new dark spots in Colchis Regio (Fig. 9c).seen at Culann Patera (Fig. 10d), Fjorgynn Fluctus (Fig.There are also a number of calderas that have become8a), Shamash Patera (Fig. 10e), Zamama (Fig. 10a), Mar-much darker between Voyager and Galileo (e.g., Arushaduk (Fig. 9d), and Acala Fluctus (Fig. 8a).Patera in Fig. 11c, Hi’iaka Patera in Fig. 11d, and Kurdala-Dark flows seen by Voyager have become bright at sev-gon Patera in Fig. 9e). Loki Patera may be another exam-eral locations, such as at Mycenea Regio and regions tople, but the active plumes obscured our view of the surfacethe east (Fig. 10e).in 1979. Other calderas have faded (brightened), some

Dark calderas. A number of new dark calderas have almost completely, though most can still be faintly dis-cerned (e.g., an unnamed caldera near the center of Fig.appeared and a similar number have disappeared in the

204 MCEWEN ET AL.

FIG. 8. Voyager-to-Galileo comparisons for (a) Fjorgynn and Acala, (b) Loki, and (c) Euboea.

11c). At this point in time it is not clear that any of the enhancements (Geissler et al. 1997). There are a few excep-tions, but even these centers may have been active only anew dark calderas represent new eruptive vents as opposedfew years ago (and may well show new activity in theto renewed activity at well-established volcanic centers.next two years during GEM). More careful processing andMany areas that are volcanically active did not exhibitanalysis of the imaging data may reveal changes within theobvious surface changes. These include many dark calderasdark areas.that are persistent hot spots (Lopes-Gautier et al. 1997a).

It seems likely that the resurfacing of these regions is domi-Comparison with HST

nated by dark erupted materials that are confined to pre-viously dark regions. The dark surfaces would likely Eight surface changes between Voyager and HST images

acquired in 1994 were described by Spencer et al. (1997a).brighten over time in the absence of continued volcanicactivity. Nearly every dark unit (green filter normal albedo None of these changes are obvious from side-by-side image

comparisons. (Note that the resolution of HST is about 160less than p0.4) seems to exhibit recent or ongoing thermal


FIG. 9. Voyager-to-Galileo comparisons for (a) Le-Kung Fluctus, (b) Daedalus, (c) dark spots in Colchis Regio, (d) Marduk, and (e) Kurdalagon.

km/pixel.) The changes are discernible only after careful the general success of Voyager–HST change detectionlends confidence in the differencing techniques, which canprocessing and differencing or blinking of the images. All

of the changes are verified by Galileo–Voyager compari- be used for more detailed Voyager–Galileo, HST–Galileo,and Galileo–Galileo comparisons.sons except for uv-violet darkening around Kanehekili.

However, SSI has seen changes around Kanehekili be-Changes during the Galileo Tourtween G1 and E6, so it is plausible that HST detected new

red materials that faded away between 1994 and 1996. Pronounced surface changes during the Galileo tourSome of the initial interpretations of the HST images have occurred at Ra Patera (Fig. 7), Kanehekili, Pillanshould be reconsidered. For example, Galileo shows no (Plate 3h), Amaterasu, Masubi, and in the polar regions.evidence for an eruption at Huo Shen Patera (158 S, 3308 These and more subtle Galileo-era changes will be de-W), and we instead suggest that the apparent differences scribed in a future publication.seen at low resolution are due to several surroundingchanges. Although Spencer et al. stated that Voyager–HST Hot Spotschanges near 348 N, 1128 W and 408 N, 1308 W were not

Observationsapparent in Galileo images, changes can be seen in Voy-ager–Galileo comparisons. Many changes seen by Galileo SSI acquired images of Io while eclipsed by Jupiter in

orbits G1, E4, E6, G7, G8, C9, and C10 (Table IV, Platewere missed by HST due to resolution limits. Nevertheless,

206 MCEWEN ET AL.

FIG. 10. Voyager-to-Galileo comparisons for (a) Zamama, (b) a region centered at 288N, 1498W, (c) Prometheus, (d) Culann, and (e) MycenaeRegio, Shamash, and Malik.

2, and Fig. 5). The eclipses provide the best opportunities at 12 RJ or more. Although smear during long exposuretimes reduces the resolution to p50 km, this is neverthelessfor SSI to see hot spots as all direct solar illumination of

Io is blocked by Jupiter and there is very little Jupitershine significantly better than the imaging resolution currentlyavailable from Earth-based telescopes or from NIMS. Theillumination as the Sun–Jupiter–Io angle is near 1808.

There was secondary illumination from Europa on the most prominent noise spikes were removed from theeclipse images via techniques described by Eliason andantijovian hemisphere during the second C10 eclipse. Gali-

leo provides several advantages over Earth-based eclipse McEwen (1990), combined with hand editing of elongatedradiation noise streaks. The fact that the hot spot signalobservations: (1) Galileo can potentially see all hemi-

spheres of Io in eclipse, whereas Earth-based observers is smeared over many pixels in a distinctive pattern for eachimage greatly aids discrimination of hot spots from noise.can see only the Jupiter-facing hemisphere, (2) we can

observe eclipses at larger angular separations between Io Eleven useful clear-filter eclipse images have been ac-quired through orbit C10 (Table IV). There have also beenand Jupiter, minimizing scattered light, and (3) Galileo is

more sensitive to faint point sources because it is much eclipse observations through visible color filters, which pro-vide information on the airglow, and images acquiredcloser to Io.

SSI is very sensitive to Jupiter’s energetic radiation envi- through the 1MC, 889, and 756 filters for constraints ontemperatures. The Pele hot spot was also seen on Io’sronment, so images acquired when relatively close to Jupi-

ter contain many noise spikes. The ‘‘radiation noise’’ is nightside in a long-exposure clear-filter image acquired ata high phase angle in orbit C3 (PICNO C3I0030). Oneespecially severe because of the longer frame times needed

for extended exposures. Range to Jupiter (measured in hundred percent of Io’s surface is covered by these obser-vations, but hot spots are difficult to detect at high emissionJupiter radii or RJ) correlates with range to Io, but SSI’s

telescope nevertheless enables imaging at p10 km/pixel angles so the polar regions are poorly covered, and the


FIG. 11. Voyager-to-Galileo comparisons for (a) Maui and Amirani, (b) Ukko Patera, (c) Altjira, Arusha, and Catha, and (d) Hi’iaka.

region from about longitude 1408 to 1708 is covered only spots are smeared, we can estimate the center of the bright-ness distribution to about the nearest pixel. The two meth-at very low spatial resolution. A relatively long-exposure

(0.53 sec) 1MC image of Io’s nightside from longitude 1488 ods of geometric control give nearly identical results, withuncertainties of p18. For Table VIII all images were con-to 2198 (PICNO G8I0006) did not reveal any hot spots,

which places only weak constraints on their intensities. trolled by fixing the coordinates of either the Pele or Janushot spots. This approach leads to acceptable limb fits andA total of 30 distinct hot spots have been confidently

detected in this set of images (Table VIII). All but a few consistent positions for the hot spots.of these have been detected in more than one Galileo

Hot Spot Temperature and Area Constraintsobservation (SSI or NIMS). Many smaller and fainterbright spots may be additional hot spots, patches of airglow The SSI clear bandpass is very broad and has a complexover local vents, or noise. Especially notable is a field of shape (Klaasen et al. 1997). It is not meaningful to convertp26 eclipse bright spots seen near the sub-Jupiter point brightnesses to radiance values, as the effective wavelength(left side of Plate 2). Most of the sub-Jupiter bright spots changes significantly with temperature. We analyzed theare probably real features, not noise, as they have been data by first computing, as a function of temperature, theseen in more than one eclipse image. However, most are number of electrons/pixel/sec expected for a blackbodycomparable in brightness to the airglow so its is unclear if source that fills an entire pixel (see Fig. 2 of McEwen et al.these are hot spots, local gas vents, or some other phe- 1997). Next, we sum the DN values within a box containingnomenon. the smear ellipsoid from the apparent hot spot and subtract

the contribution from the surrounding background in orderGeometric Control

to measure the hot-spot signal. Most of the images usedhere (Table VIII) were acquired at gain setting 4, in whichThe presence of several hot spots and a diffuse glow

marking Io’s limb enables correction of eclipse-image cam- 1 DN equals 38.7 electrons. Thus, (box sum-background)3 38.7/exposure time gives the number of electrons/secera angles sufficient to locate the hot spots to an accuracy

of about 18. Camera angles were initially updated in two (see Table VIII). The G7 images were acquired in summa-tion mode with 31 electrons/DN. Electrons/sec can be con-independent manners: (1) by positioning Io so that the

model limb best matches the diffuse limb glow, and (2) by verted into a model temperature if we assume an area forthe hot material, or we can assume a temperature andassuming that we know the position of at least one hot

spot and using it as a control point. Although the bright compute the area. We consider an area equal to that of a

208 MCEWEN ET AL.

TABLE VIIISSI Hot Spot Locations and Clear-Filter Intensities

Area atHot spot Emission Electrons/sec 1000 K

name Latitude Longitude angle PICNO (3103) (km2)

Ruwa Patera 1.4 N (60.3) 1.2 W (60.4) 35.8 (60.3) E4I0002 0.76 6 0.4 0.13 6 0.070.7 N (60.4) 1.4 W (60.6) 45.8 (60.3) C10I0001 2.4 6 0.3 0.28 6 0.04

— 15.3 N (60.3) 4.7 W (60.5) 45.0 (60.2) C10I0001 2.2 6 0.3 0.25 6 0.04

— 11.5 N (60.6) 13.3 W (60.6) 35.7 (60.7) C10I0001 0.62 6 0.2 0.06 6 0.02

— 13.1 S (60.3) 22.8 W (60.5) 54.0 (60.4) G8I0003 0.58 6 0.4 0.16 6 0.1

— 5.5 N (60.5) 23.3 W (60.5) 52.8 (60.5) G8I0003 1.5 6 0.9 0.4 6 0.2

— 5.2 N (60.1) 24.1 W (60.3) 23.7 (60.5) C10I0001 2.8 6 0.4 0.25 6 0.03

— 16.5 S (60.4) 27.9 W (60.3) 25.0 (60.2) C10I0001 1.1 6 0.2 0.10 6 0.01

Kanehekili-N 14.5 S (60.3) 33.4 W (60.3) 14.6 (60.1) E4I0002 14 6 6 2.0 1 0.813.6 S (60.7) 32.6 W (60.7) 31.6 (60.5) G7I0001 .4.8 6 2 .2.8 6 114.8 S (60.7) 32.7 W (60.9) 29.7 (60.5) G7I0013 .11 6 3 .6.4 6 214.5 S (60.2) 33.4 W (60.3) 44.2 (60.1) G8I0003 6.9 6 1 1.5 6 0.314.3 S (60.5) 32.9 W (60.4) 20.1 (60.2) C10I0001 9.8 6 0.6 0.84 6 0.05

Kanehekili-S 17.2 S (60.3) 35.5 W (60.3) 17.0 (60.3) E4I0002 .23 6 2 .3.8 6 0.417.1 S (60.8) 37.0 W (60.5) 29.2 (61.0) G7I0001 .6.0 6 3 .3.4 6 217.2 S (60.6) 36.0 W (60.8) 30.5 (60.5) G7I0013 6.9 6 2 4.0 6 117.3 S (60.5) 35.9 W (60.4) 42.6 (60.5) G8I0003 4.9 6 0.6 1.0 6 0.117.1 S (60.4) 35.7 W (60.3) 20.5 (60.2) C10I0001 11 6 1 0.97 6 0.1

Janus 4 S (60.3) 39 W (60.4) 4.0 (60.1) E4I0002 4.2 6 0.2 0.59 6 0.034 S (60.8) 39 W (60.7) 22.9 (60.3) G7I0001 .6.6 6 0.2 3.6 6 0.14 S (60.5) 39 W (60.8) 23.2 (60.5) G7I0013 4.5 6 1 2.5 6 0.64 S (60.5) 39 W (60.6) 37.1 (60.5) G8I0003 3.6 6 0.5 0.71 6 0.084 S (60.4) 39 W (60.4) 9.1 (60.2) C10I0001 3.4 6 0.6 0.28 6 0.05

3.8 N (60.4) 76.1 W (60.5) 29.9 (60.4) C10I0001 2.2 6 0.2 0.20 6 0.02

Zal 37.0 N (61.0) 76.3 W (61.0) 39.7 (60.3) G7I0001 3.9 6 0.6 2.5 6 0.436.3 N (60.6) 76.1 W (60.3) 37.2 (60.5) G8I0003 2.6 6 0.2 0.51 6 0.03

Gish Bar 16.3 N (60.2) 91.0 (60.3) 56.2 (60.5) E4I0002 .22 6 0.10 .5.5 6 0.03

Amirani 23.4 N (60.5) 116.4 W (60.5) 46.3 (60.4) G8I0003 3.0 6 0.2 0.69 6 0.0323.2 N (60.2) 116.3 W (60.5) 71.6 (60.8) C10I0001 0.30 6 0.1 0.077 6 0.03

— 65.2 N (61.6) 132 W (66.0) 82 (66.0) G7I0001 .7.2 6 0.3 .26 6 4

— 66.0 N (61.3) 144 W (68.0) 87 (63.0) G7I0013 5.0 6 0.3 48 6 80

Zamama 18.9 N (60.1) 172.0 W (60.8) 63.4 (60.2) G1I0030 4.6 6 1 0.51 6 0.2

— 28.5 N (60.3) 189.4 W (60.6) 50.4 (60.4) G1I0030 1.8 6 0.2 0.14 6 0.02

Fo 40.1 N (60.3) 200.8 W (60.5) 23.1 (61.0) G1I0030 3.7 6 0.5 0.20 6 0.03

Isum-N 32.9 N (60.1) 204.7 W (60.4) 41.4 (60.2) G1I0030 11 6 1 0.73 6 0.1

Isum-S 30.3 N (60.5) 206.8 W (60.6) 38.4 (60.5) G1I0030 7.2 6 1 0.46 6 0.07

Marduk 26.8/27.8 S 208.5/209.5 W 39.0 (60.2) G1I0030 16 6 1.7 1.0 6 0.127.1 S (60.5) 209.1 W (60.4) 73.6 (60.8) C9I0026 1.9 6 0.5 0.64 6 0.2

continued


TABLE VIII—Continued

Area atHot spot Emission Electrons/sec 1000 K

name Latitude Longitude angle PICNO (3103) (km2)

Mulungu 17.6 N (61.0) 216.8 W (60.7) 23.1 (61.0) G1I0030 3.7 6 0.5 0.20 6 0.03

Reiden 12.5 S (60.1) 234.9 W (60.3) 15.2 (60.2) G1I0030 11 6 0.4 0.59 6 0.0212.6 S (60.2) 235.5 W (60.9) 15.4 (60.2) G1I0031 7.0 6 0.6 0.36 6 0.03

Pillan 9.9 S (60.7) 242.3 W (60.9) 39.2 (60.8) C9I0026 .380 .47

Pillan N&S 9.5/11.4 S 242.7/242.2 W 35 (61.5) C10I0045 77 6 1 5.6 6 0.08

Pele 18 S (60.5) 256 W (61.5) 29.5 (61.5) G1I0030 .19 6 0.9 .11 6 0.0518 S (60.3) 256 W (60.4) 29.7 (60.2) G1I0031 .230 6 3 .13 6 0.2

18 S (61) 256 W (61.5) 18 (62) C3I0050 .130 .2918 S (60.4) 256 W (60.8) 43.4 (60.6) E6I0070 .96 6 0.4 .5.0 6 0.0218 S (60.5) 256 W (60.5) 30.5 (60.5) C9I0026 .49 6 0.8 .5.5 6 0.118 S (60.5) 256 W (61.0) 26.9 (60.7) C10I0045 .150 6 2 .9.9 6 0.1

Svarog 53.9 S (60.4) 270 W (60.4) 54.7 (60.3) C9I0026 .20 6 1 .3.3 6 0.2

Loki 10.8 N (60.3) 310.5 W (60.3) 17.9 (60.2) E6I0070 6.7 6 0.6 0.27 6 0.02

— 17.1 N (60.5) 332.9 W (60.5) 54.4 (60.4) C9I0026 1.1 6 0.2 0.18 6 0.03

Acala 10.7 N (60.5) 333.2 W (60.8) 53.7 (60.3) C9I0026 6.8 6 2 1.1 6 0.3

— 4.8 N (60.3) 356.1 W (60.8) 41.1 (60.8) E4I0002 0.80 6 0.1 0.15 6 0.02

— 4.4 N (60.6) 355.1 W (60.7) 51.6 (60.7) C10I0001 0.97 6 0.09 0.12 6 0.01

Note. Coordinates relative to assumed positions of Janus or Pele. Uncertainties only reflect the level of noise in the data. Marduk G1 and PillanC10 observations show two hot spots too close together to analyze separately. . indicates parts of the hot spot were saturated. Pele C3 and PillanC9 utilized the degree of bleeding to help constrain the minimum flux in the saturated hot spot. A significant error in the Electrons/sec valuesreported by McEwen et al. (1997) for the E4 eclipse has been corrected here.

pixel to represent the maximum likely area (and the mini- K result from two-temperature fits to the NIMS data forZamama (Davies et al. 1997a). Also, this calculated areamum likely temperature) for two reasons. First, most of

the hot spots on any particular image produce smear ellip- provides a normalized metric for comparing changes inintensity. The 1000 K areas lead to a predicted flux fromsoids of similar size and shape, consistent with unresolved

(subpixel) point sources. Second, if the full-pixel the Jupiter-facing hemisphere of up to 5 GW em-1 str-1at 2.27 um, less than half of typical observed fluxes (Spencertemperature/area model were correct, then we would esti-

mate a full-disk flux at longer wavelengths (2–5 em) from et al. 1997b). This is a reasonable result because there mustbe significant contributions at 2.27 em from temperaturesthese high-temperature areas alone that would exceed typi-

cal full-disk hot spot fluxes (Veeder et al. 1994, Spencer below 1000 K. Of course, there is a continuum of possibletemperature/area models based on the SSI clear-filter dataet al. 1997b).

Since the full-pixel temperature/area model must usually alone. The peak temperatures are better constrained by theratio of CLR and 1MC eclipse observations; a preliminaryoverestimate the area and underestimate the temperature,

we include in Table VIII models in which we assume a analysis of such data gives temperatures ranging from 800to more than 1500 K (McEwen et al. 1998).temperature of 1000 K, and then compute the area needed

to match the SSI data. We corrected for projected area asa function of emission angle, but note that intensity may

Hot Spot Variabilityvary in a more complex manner with emission angle if thereare deep cracks or fire fountains. 1000 K is a reasonable The hot spots seen by SSI display a range of temporal

variability (Table VIII). In general the higher temperaturebrightness temperature for active basaltic eruptions (Kesz-thelyi and McEwen 1997a) and is consistent with the 1100 component is more variable than the lower temperature

210 MCEWEN ET AL.

emissions seen by NIMS. The temperatures seen by SSI see many more if not limited by spatial resolution. Largeareas at warm temperatures are expected from cooling(.700 K) are consistent with silicate magmas that were

exposed on the surface for less than about 30 min (Kesz- silicate flows, so most of Io’s heat flow and resurfacingmay be accomplished by silicate volcanism (Carr 1986,thelyi and McEwen 1997a). Most of the longer wavelength

emission seen by NIMS can be explained by lava that has Blaney et al. 1995, 1997). Mobilization of sulfurous materi-als in plumes and flows are probably also important pro-cooled for hours to days (Davies et al. 1997a). Kanehekili-

N, Kanehekili-S, and Janus were all significantly brighter cesses, but are driven by the movement of silicate magma.A number of recent observations indicate magma tem-during E4 and G7 than during G8 and C10. Some spots

are seen only intermittently by SSI but consistently by peratures on Io that are higher than terrestrial basalticvolcanism. Three short-lived events observed from theNIMS. For example, the hot spot at Gish Bar Patera was

seen just once by SSI despite four observations covering ground require temperatures of p1350 K or higher(Veeder et al. 1994, Spencer et al. 1997b, Stansberry et al.that region, but it has been seen during several orbits by

NIMS (Lopes-Gautier et al. 1997a). The hot spot at 1658, 1997a). SSI eclipse observations in orbits G7 through C10were acquired through the 1MC filter as well as the broad-p1408 W was seen only during G7 (but in two different

images) at a very high emission angle (p858) and has not band clear. Preliminary analyses of the CLR/1MC ratiosindicate that temperatures commonly exceed 1300 K, andbeen seen by NIMS. These characteristics are similar to

the north polar event described by Stansberry et al. (1997a), are sometimes as high as 1500 K or more (McEwen et al.1998). Analysis of Galileo NIMS data also shows an espe-for which they suggested that fire fountaining could explain

the visibility at very high emission angles. Once the foun- cially high-temperature component in many hot spots (Da-vies et al. 1997b). These temperature models are based ontaining has subsided the hot material may be largely hidden

from view. observations acquired at wavelengths from the visible to5.0 em, so they are difficult to dismiss as due to unusualThere does not appear to be much evidence for lateral

migration of hot spots during the Galileo tour. The south- emissivities or the electronic excitation of gases in thevisible. Because of rapid radiative cooling, even from aern hot spot near Pillan in C10 may be the best candidate,

but could easily have been hidden within the large C9 vigorous fire fountain, remote temperature observationsare usually at least 200 K lower than the temperature ofbright spot (which included bleeding of columns to the

south). SSI detected hot components from Loki at two the liquid lava (Keszthelyi and McEwen 1997a). Whenthe rapid radiative cooling is considered, these hot-spotvery different locations in E6 and C9, but the Loki hot

spot is known to encompass a large area (Goguen et al. observations suggest liquid temperatures hotter than 1700K. Preliminary interpretations are that these very high-1997). All other hot spots have the same positions in differ-

ent orbits, within measurement errors. Positions of hot temperature lavas are superheated or composed of ul-tramafic compositions (McEwen et al. 1998).spots were too poorly determined prior to Galileo to mea-

sure possible changes in positions over longer periods oftime. Individual Volcanic Centers

In this section we discuss in more detail the most activeDISCUSSIONvolcanic centers or regions, including all of those with

High-Temperature Hot Spots observed airborne plumes (Table VI). These centers usu-ally include hot spot activity and surface changes as wellHigh-temperature hot spots are abundant on Io andas plumes, so it is helpful to summarize the complex obser-silicate volcanism appears ubiquitous. There are p20 tovations for each location. NIMS has detected hot spots at30 easily detected spots (areas larger than 0.1 km2 ateach of these centers except Ra, Masubi, Acala, and thep1000 K) at any given time. Sulfur boiling in a vacuumsubjovian field of vents (Lopes-Gautier et al. manuscriptcannot get much hotter than about 500 K (Lunine andin preparation).Stevenson 1985), so these hot spots are not due to pure

sulfur volcanism. Metal sulfides cannot be ruled out (Nash Kanehekili is a persistent high-temperature hot spot firstdescribed by Spencer et al. (1990) and seen in many subse-1993), but are extremely dense and unlikely to reach the

surface in large quantities. We are probably seeing active quent telescopic observations (Spencer and Schneider1996). This region was observed by the Voyager 1 IRISeruptions of silicate magma, although not necessarily of

basaltic composition. A brightness temperature of 1000 K instrument only at a very high emission angle, so it is notclear whether or not it was a hot spot in 1979. Its locationsuggests a lava exposure time scale (if basaltic) of order

100 sec, consistent with active lava lake convection, open has been uncertain from telescopic observations, but SSIdata show that Kanehekili contains two separate hot spots,channel flow, or pahoehoe flows (Davies 1996, Keszthelyi

and McEwen 1997a). NIMS has detected about 20 addi- and determines their positions to within a degree (TableVIII). The hot spots fall on a large dark region, perhapstional hot spots, too cold for SSI to detect, and would likely


consisting of coalesced lava flows. The high-temperature (Fig. 11a) can be interpreted as due to cessation of plumeactivity. Dark lobate patterns near the vent region arecomponent seen by SSI in both spots was more intense

during E4 and G7 than during G8 and C10. The two hot probably flows.spots might mark two separate flows or vents, or could Prometheus is the most faithful plume on Io, active dur-mark the vent and toe of a single eruption (in which case ing every observation of Voyager and Galileo with thethe flow extends more than 100 km from the vent). Color appropriate geometry (see Figs. 3 and 4 and Plate 3c).and albedo changes at Kanehekili have been detected by Orbit G2 provided our first look at Prometheus since Voy-HST and SSI, and a bright plume was seen in orbit G8 (Fig. ager and revealed an active plume and a new dark lava3). Surface changes show that the plume began erupting flow. It is by far the brightest Galileo-era plume, and it is(or increased in activity) sometime between G1 and E6. the only Galileo-era plume that has been clearly seenEclipse images from E4 and subsequent orbits show a against Io’s sunlit disk. NIMS data indicate that the hotdiffuse glow around Kanehekili (Plate 2), which suggests spot includes a component hotter than 1000 K (Daviesthat plume activity began before E4. Sunlit E4 images with et al. 1997b). From image C3I0021 (2.4 km/pixel) with thethe bright limb at longitude 45 W did not show a plume, terminator at longitude 1508 W, it is apparent that anybut Kanehekili was 98 behind the limb, sufficient to hide topographic relief near the vent is less than a few hundreda 50-km high plume. We suspect that the plume initiation meters. As previously described, the plume is eruptingwas coincident with an increase in high-temperature emis- from a position about 70 km west of the 1979 vent andsion and that the plume activity began between G1 and may be erupting from near the front of the new dark flow.E4. The plume appears to be erupting from the southern We have not seen a distinct plume at Prometheus inhot spot, which is at the margin of the dark region. eclipse, in spite of several observations with the appro-

priate geometry (Table IV). However, there is an extendedMasubi was the site of a Voyager-era plume at 448 S, 548W, associated with dark flows. This is the highest latitude of glow over this region in several eclipse images (Plate 2),

including a faint enhancement 700 km above the limb inany observed active plume. There are many other diffusecircular deposits at high latitudes, which suggest that high- G8. We suspect that this extended glow is due to gases from

Prometheus and other vents near the antijovian region,latitude plumes occur but tend to be short-lived. Masubiwas active during Voyagers 1 and 2, but its plume has extended downstream by the plasma flow from Jupiter’s

corotating magnetosphere. Galileo may have directly de-not been observed during Galileo. HST images from 1994showed a general brightening around Masubi compared tected this material (O11, O1, S11, S1, and SO1

2 ions) duringthe Io flyby in December of 1995 (Frank et al. 1996).to Voyager (Spencer et al. 1997a) and SSI showed several

new bright patches in this region (Belton et al. 1996). Faint Culann. The G1 eclipse image (Plate 2) and G2 plumediffuse glows were seen around Masubi in all eclipse images inventory (Fig. 4) both marginally suggest a small (,50of this hemisphere (Plate 2), and C10 images show a new km high) plume over Culann Patera (Plate 3c). From thecircular plume deposit. No hot spot has been seen at G2 plume inventory there appears to be a plume fromMasubi by NIMS or SSI through orbit C10. time-lapse views of Culann seen against the disk. We have

no detection of this plume on the bright limb in spite ofAmirani is a 300-km long, north–south trending darkmarking, perhaps from fissure-fed flows (Plate 3f). During several good observations in orbits G7, G8, and C9 (Table

III), so the plume activity may have diminished since 1996Voyager 1 an p100-km high plume was erupting from nearthe northern end of the fissure (298 N), and a small, faint or perhaps a plume was never detected here. Galileo im-

ages reveal extensive surface changes compared to Voy-plume was erupting from the southern end (208 N; seeVoyager frame 16372.50). A plume near Amirani was seen ager, verifying a change first detected from HST (Spencer

et al. 1997a). Both low-albedo flows and diffuse bright redin E4 and G8 at latitude 24 6 28, between the locationsof the Voyager plumes. The location of the 1996–1997 materials are present, as is the case near many current

high-temperature hot spots. We will compare GEM imagesplume vent is not obvious from the surface albedo patterns.Bright flow-like patches have appeared or faded from 1979 of Culann to the earlier images to search for evidence of

fading of the red material.to 1996, and the dark feature appears broader than in 1979(Fig. 11a). Amirani and Maui together were a prominent Zamama (Plate 3e). G1 and G8 images revealed aVoyager 1 thermal anomaly (Pearl and Sinton 1982). A small plume (p60 km high) erupting from a position (1738high-temperature hot spot has been seen here by SSI dur- W, 188 N) that is a few degrees south of the Voyager-ing G8 and C10 and possibly G7. era plume Volund. The G1 eclipse image also showed an

intense hot spot at this location, surrounded by an extendedMaui (Plate 3f) is located just southwest of Amirani andwas the site of a bright plume seen by Voyager. There has diffuse glow (Plate 2). NIMS also detected this hot spot,

and detailed modeling of its emission is consistent withbeen no sign of plume activity here during the Galileotour, but it remains a hot spot. Voyager-to-Galileo changes basaltic volcanism (Davies et al. 1997a, 1997b). G2 images

212 MCEWEN ET AL.

revealed dramatic surface changes in this region (Fig. 10a). the southwest of Pele (Plate 3h). The dark deposit has thesame diameter and location as the plume so it undoubtedlyThere is a low-albedo linear feature (possible fissure-fedconsists of plume fallout. Pillan and Babbar may form aflows) about 150 km long, surrounded by diffuse red andnew class of plume eruption on Io, perhaps analogous togray deposits, none of which were apparent in Voyagerlunar dark mantle eruptions (Wilhelms 1987). There areimages. There also appear to be some relatively brightnew dark flow-like features up to 75 km long near thenew flows extending WNW along the strike of the fissure.center of the dark fallout, northeast of Pillan Patera, whichHowever, the bright flow-like features may be more promi-appears to be the more precise vent region. A C10 eclipsenent in the Galileo images simply because the Volundimage resolves two hot spots, each about 75 km apart,plume has shut off and its deposits have faded.perhaps the vent and toe of the new lava flow. The intensity

Volund (Plate 3e) was a Voyager-era plume and hot in C10 was greatly reduced compared to C9 (Table VIII),spot just northwest of Zamama, erupting from the southern and the temperature was much lower (McEwen et al. 1998),end of a dark linear marking. There has been no sign of although plume activity continued.plume activity here during Galileo’s tour, but NIMS has

Pele provides the biggest plume and one of the mostdetected a hot spot at this location, distinct from Zamama.intense high-temperature hot spots on Io (see Fig. 3, Fig.It is conceivable that Volund and Zamama share the same6b, Plate 2, and Plate 3h). SSI has seen the hot spot inmagmatic system at depth.every appropriate image (orbits G1, C3, E6, C9, and C10).

Marduk. The G1, C9, and C10 eclipse images (Plate The SSI observations (Table VIII) all reached saturation;2) have revealed a high-temperature hot spot at the posi- only Pillan in C9 was more intense. In addition, NIMS

detected a high temperature (828 6 50 K with a single-tion of the Voyager-era plume and hot spot Marduk (Platetemperature fit) at Pele in orbit G2 (Lopes-Gautier et al.3d). The shape of the G1 spot is consistent with the pres-1997b). Pele was the highest-temperature hot spot seen byence of a double hot spot. The G2–Voyager comparisonVoyager (.650 K; Pearl and Sinton 1982). Ground-basedshows extensive surface changes (Fig. 9d), including darkobservers have sometimes detected a 3–5 em intensityflows and a bright red deposit (first detected by HST). Asimilar to Voyager, sometimes fainter (Spencer andplume was seen on the bright limb in orbit G8 (Fig. 3),Schneider 1996). The intensity level seems remarkably con-and plume-like features were seen in the E6, C9, and C10stant during the Galileo prime mission. Voyager also re-eclipse images. Galileo could acquire high-resolution ob-vealed a large area of low-temperature thermal enhance-servations (including stereo) over Marduk during the I24ment near Pele; this bimodal temperature distribution isflyby in 1999.difficult to model with lava flows (Howell 1997). The con-

Pillan (Plate 3h) has provided many surprises and is stancy of the high-temperature emission might be bestperhaps the best-observed eruption of Galileo’s nominal explained with an active lava lake, and the low-tempera-tour. It is also the best example of a very high-temperature ture component could be due to pyroclastics (i.e., the rederuption, with a magma temperature exceeding 1700 K and gray diffuse materials distributed radially near the(McEwen et al. 1998). Pillan Patera was a simple caldera vent; Plate 3h).with no albedo markings in 1979, although there was mar- Pele’s plume was detected by HST in July 1995 and Julyginal evidence for a darkening of Pillan between Voyagers 1996 (Spencer et al. 1997c) and by SSI in E4—December1 and 2 (McEwen 1988). The caldera appeared unchanged 1996 (Fig. 3). The SSI observation indicated a height ofsince Voyager 1 in G1 images. The SSI eclipse image did 460 km for Pele’s plume, much higher than the 300-kmnot reveal a hot spot at Pillan in G1, although a bright spot plume seen by Voyager 1 (and also much fainter). Pelewas seen at Reiden Patera 88 to the east. NIMS reported a may also be active (but even fainter) in July 1997 imageshot spot in orbits G2–E4 at Pillan, but not Reiden (Lopes- from HST (Spencer et al. 1997d). SSI has attempted toGautier et al. 1997a). In G2 images the entire caldera floor image Pele’s plume in orbits G1, G2 (against Jupiter’swas very dark, but then it brightened again in C3 and atmosphere), and with extended exposures in E6 and C9,subsequent orbits. The caldera is within the fallout zone but without success. However, the HST observationsof Pele, so surface changes may be quickly buried and (plume detected only at UV wavelengths beyond SSI’shidden. Without the G2 and C10 observations we might range of sensitivity) suggest that Pele could have beenhave concluded that Pillan was the only hot spot on Io not active during these encounters but invisible to SSI. Theassociated with low-albedo material. A plume was seen E4 image just barely detected Pele because of the combina-over this location by HST on July 5, 1997 (Spencer et al. tion of long exposure time and high phase angle. Surface1997d), a week after the C9 plume image of SSI (Fig. 3). changes at Pillan Patera (described above) also suggestIn C10 we saw a new 400-km diameter deposit over the that Pele has been active, and reddish rings west of the

dark Pillan fallout deposits seen in C10 (Plate 3h) suggestPillan region, similar in appearance to Babbar Patera to


that Pele’s plume was active simultaneously with Pillan’s Loki (Plate 3a) has been by far the most energetic hotspot on Io (in terms of total heat flow) over the past 20plume. Johnson et al. (1995) suggested that Pele may beyears (Veeder et al. 1994, Spencer and Schneider, 1996).a pure-gas ‘‘stealth’’ plume, but the plume observationsThere were two bright plumes northeast of Loki Pateracan also be interpreted as due to very fine particles (Spen-(at each end of the dark linear marking or ‘‘fissure’’) incer et al. 1997c). The prediction that stealth plumes might1979. Loki was quiescent in 1996 (Spencer et al. 1997b),be seen in eclipse, while supported for some plumes, hasso the absence of plumes during the early Galileo orbitsnot been verified for Pele. Note that this is probably notseemed reasonable. A new episode of thermal brighteningan emission angle effect, because the last C10 eclipse imageat Loki began shortly after the E6 encounter (Stansberrycovered Pele at a high emission angle and because we doet al. 1997b, Goguen et al. 1997), but plumes did not reap-see diffuse glows even at low emission angles elsewherepear. No large-scale topographic relief has been resolved aton Io. These observations suggest that Pele is indeed aLoki (Fig. 2), but the fact that the outline of the horseshoe-very ‘‘stealthy’’ plume, but does not have the exact charac-shaped dark region is unchanged since 1979 suggests thatteristics predicted by Johnson et al. (1995).it is topographically confined (i.e., in a caldera).Ultraviolet imaging and spectroscopy by HST has re-

The surface differences at Loki between Voyager 1 andvealed the presence of gaseous SO2 and S2 (with about aVoyager 2 are more striking than the differences between10 : 1 ratio) over Pele (Sartoretti et al. 1996, M. A. McGrath,Voyager and Galileo (Fig. 8b). Since 1979 the fissure NEpersonal communication, 1997). Kieffer (1982) determinedof Loki has extended slightly to the SE, perhaps due tothat a large plume like Pele could be driven by either SO2 ,new flows, and a dark spot to the west of the caldera hassulfur, or both. McEwen and Soderblom (1983) favoredbecome larger and more diffuse, possibly from a pyroclasticsulfur as the driving volatile in order to explain Pele’seruption. No differences are obvious in SSI images beforereddish suface deposits. NIMS has confirmed that Pele’sand after the post-E6 thermal brightening. Given that Lokifallout deposits are deficient in SO2 frost (Carlson et al.is the largest thermal emission source on Io, it is surprising1997). Hence, it now appears that Pele is driven primarilythat we do not see more changes here. However, this lackby SO2 , but also contains significant sulfur, and that theof visible change does not preclude vigorous resurfacingSO2 does not condense onto the red fallout zone and mustof the dark horseshoe. Dark calderas on Io seem to remainbe laterally transported to condense at more distal loca-dark only if they are active hot spots, so continual resurfac-tions. This could explain the transient brightenings south-ing by dark lava is a reasonable hypothesis. About 25% ofeast of Pele seen during Voyager 1 (McEwen 1988). TheIo’s global heat flow emanates from Loki, so we expectrays extending up to 800 km beyond the red ring (McEwenp25% of the resurfacing has occurred here as well. Theet al. 1989) are further evidence for extensive lateral trans-dark regions of Loki Patera cover an area of p50,000 km2,port of plume materials. SO2 should condense in the redand silicate flows or overturning lava lakes over only aring unless the temperature of the surface is elevated. Thesmall fraction of this area could account for the heat flow.bolometric albedo of the red material is probably quite

Two basic models have been described for Loki’s ther-high and should result in a surface no warmer than aboutmal emission: silicate flows (Carr 1986, Blaney et al. 1995,130 K and much colder at night (if in equilibrium withDavies 1996, Howell 1997) and sulfur lakes (Lunine andinsolation), so SO2 should condense here. Perhaps theStevenson 1985). The Loki hot spot may have a lowerplume contains large particles that cannot equilibrate withtemperature distribution than most active hot spots on Iothe gas and remain warm on the surface. However, it may(Stansberry et al. 1997b), but the peak temperatures arebe difficult to explain how Pele could contain both verynevertheless too high for sulfur. Although both silicatesmall particles (to make the plume visible at high phaselava and sulfur lakes may be present (Lunine 1989), theangles) and large particles that warm the surface, and yetlack of widespread surface changes does not seem consis-remain transparent at visible wavelengths and low to mod-tent with the evaporation and recondensation of largeerate phase angles.amounts of sulfur, as required by the sulfur lakes model.There are relatively few obvious Voyager-to-Galileo sur-

face changes at Pele (Fig. 6b), though it is interesting to Ra. Dramatic changes have occurred around the shieldsee that the Voyager 1 notch in the southern portion of volcano Ra Patera (3258 W, 88 S), including new flow-likethe deposit has not returned. This is consistent with the and diffuse deposits (Fig. 7). A brightening at Ra Pateraidea that a localized obstruction at or near the vent was was first seen from HST images and occurred betweenblown clear between the Voyager 1 and Voyager 2 flybys. March 1994 and July 1995 (Spencer et al. 1997a). TheHowever, the northern elongation of the plume deposits presence of new deposits with flow-like morphologies andhas remained, perhaps reflecting a more permanent asym- the absence of any intense hot spot at this location ismetry in the vent area. The dark diffuse deposits close to consistent with the eruption of sulfur flows, as previously

proposed for Ra Patera by Pieri et al. (1984). SSI discov-Pele’s vent also appear largely unchanged.

214 MCEWEN ET AL.

ered an active plume p75 km high at Ra Patera during lived or intermittent activity, reddish deposits, associatedG1 (Fig. 3 and Plate 2), but the plume has not been visible with high-temperature hot spots, and concentrated fromduring subsequent orbits, several with Ra very near the longitude 240–3608) and Prometheus-type (small, bright,bright limb. The visible plume probably disappeared be- long-lived, bright white deposits, and concentrated in anfore orbit G2. However, a faint diffuse glow in eclipse equatorial band). They also suggested that the Loki plumeshas persisted over this region in subsequent orbits. Bright were an intermediate type. They interpreted the Pele-typeyellow diffuse deposits just west and south of Ra’s vent plumes as due to sulfur gas vaporized by molten silicateswere anomalously bright on Io’s nightside in a RED G1 and the Prometheus-type plumes as due to SO2 vaporizedimage (but not VLT, GRN, or 756 images). This region by sulfur magma (temperature p400 K). Johnson et al.was seen glowing in the dark again in a G8 CLR image. (1995) proposed a potential third class: stealth plumesThe origin of this brightening, which is p100 times more (large, high-temperature, pure SO2 gas plumes) or thatintense than the diffuse glows seen in eclipse, remains a Pele-type plumes are actually stealth plumes. The thermo-mystery, but might be due to unusually forward-scattering dynamics in all three cases were described by Kieffermaterials illuminated by Jupitershine at a high phase (1982). Galileo observations have partially confirmed someangle. aspects of these models, but not others.

Acala is the apparent source of a plume seen only in Prometheus-type plumes. The most common type ofeclipse during G7, G8, and C9 (Plate 2). The plume has plume is 50–150 km high in reflected light, long-livednot been seen in sunlight in spite of limb views in G1 and (months to years), and is associated with high-temperatureC10, but eclipse images in these same orbits also do not (.1000 K) volcanism. All but one occurs within 6308 ofclearly show a plume over Acala, so it is possible that a the equator. Examples (Table VI) include Kanehekili,visible plume was present during the G7–C9 encounters. Amirani, Maui, Prometheus, Zamama, Volund, and Mar-However, C10 images do not show any apparent new duk. They are identical to the Prometheus-type plumesplume deposits surrounding Acala. Perhaps this is a stealth described by McEwen and Soderblom (1983), except thatplume (Johnson et al. 1995). SSI detected a bright hot spot we now know that they are usually associated with hotat Acala in C9 and faint spots in E6 and C10, but NIMS spots that include high-temperature components, probablyhas not detected a hot spot here due to low spatial resolu- exceeding 1000 K. Prometheus itself remains the best-ob-tion over this longitude. Acala was also the location of a served example, so it seems appropriate to call them ‘‘Pro-Voyager-era hot spot (McEwen et al. 1992a, 1992b). Acala metheus-type’’ plumes. We have also seen evidence fromFluctus is a complex field of flows and pyroclastic deposits eclipse images that the plume gases sometimes extend toand exhibited several changes (mostly new bright patches) much larger volumes than the visible plume (e.g., Prome-between Voyager and Galileo (Fig. 8a). theus in G8, Zamama in G1, Marduk in E4). The high

temperatures and plume components that are not easilySubjovian field of vents. There is a remarkable field ofseen via reflected light are characteristics of stealth plumes.about 26 apparent vents around the subjovian point (6158Hence, Prometheus-type plumes share characteristics oflatitude, 3308-308 longitude). These apparent vents showthe Prometheus-type plumes of McEwen and Soderblomup as faint bright spots seen in eclipse (Plate 2). It is unclear(1983) and stealth plumes of Johnson et al. (1995). SO2whether these are high-temperature hot spots, excitedremains the leading candidate for driving the plumes basedgases concentrated near vents, or some other phenomenon.on the ubiquity of SO2 on and around Io, but the SO2 isThere is also a fainter diffuse glow over this region, andprobably volatilized by contact (or close approach) of sili-over a region of similar dimensions at the antijovian point.cate magma rather than sulfur magma.Individual bright spots have not been resolved at the anti-

There is evidence for lateral migration of Prometheus-jovian region. The subjovian region contains a concentra-type plumes in several locations. Four of these plumestion of shield volcanos (Schaber 1982), but the locationshave been active during both Voyager and Galileo; twoof bright spots do not seem to have any obvious correspon-(Prometheus and Amirani) are clearly erupting from dif-dence to the shields. The subjovian point also corresponds

to ths subsolar point just prior to eclipse, so perhaps the ferent but nearby locations and two (Masubi and Marduk)bright spots are related to the solar radiation in some are uncertain. Zamama could be another example if dueunknown manner. to migration of Volund. There are high-temperature hot

spots and dark flows, not confined to a caldera floor, neareach of the Prometheus-type plumes and the intermediate-Types of Volcanic Eruptionssized plumes at Loki, Pillan, and Acala (see Plate 3 for

Classes of Volcanic Plumes several examples). These relations suggest that the near-surface lateral migration of silicate magma is needed forMcEwen and Soderblom (1983) described two classes

of volcanic plumes on Io: Pele-type (large, faint, short- plume activity, by volatilizing near-surface SO2 .


Pele-type plumes. Pele itself remains the only directly plumes have been seen to be larger in eclipse airglowthan in reflected light (although where a plume ends andobserved example of a ‘‘Pele-type’’ plume. As previously

discussed, we now know that the hot spot is long-lived, atmosphere or ionosphere begins is poorly defined), lend-ing further support to some sort of stealthy plume phenom-including the high-temperature component, and the plume

is frequently active but very difficult to observe at visible ena. Several plume-like features seen in eclipse seem tolack a contemporaneous visible counterpart: Acala (dis-wavelengths. It is probably more continuously active than

was envisioned by McEwen and Soderblom (1983) and cussed above), Ra (perhaps seen in E4 after the visibleplume vanished), a south polar feature seen in severalmay have been active but too faint to observe during the

Voyager 2 flyby. Why is Pele so different from Prometheus- eclipse images, and a north polar feature seen in C10eclipses. It is unclear whether the polar features are associ-type plumes? The key differences are that Pele’s plume is

much larger and fainter and deposits red materials rather ated with volcanism. To summarize, the stealth plume con-cept seems to have considerable merit but the details arethan bright white material (although there are smaller red

patches associated with many Prometheus-type plumes). currently confusing.Both plume types are associated with high-temperature Unique individual plumes. Several plumes appear tohot spots, are likely driven by SO2 , and may be continu- be unique individuals, especially Pele (discussed above)ously active for months to years. Perhaps Pele is driven by and Ra. Ra is like a Prometheus-type plume but no hotmagmatic gas rather than upper crustal volatiles? Further spot has been observed at this location and the surfacework is needed to explore these questions. deposits have unusual characteristics. The intermediate-

Have there been other ‘‘Pele-type’’ plumes? Large red- sized plumes (Loki, Acala, Pillan) are also unique com-dish plume deposits similar to the deposits of Pele have pared to other plumes and each other. It is interesting thatbeen seen around Surt, Aten, Babbar, and Euboea Fluctus. the unique plumes Pele, Loki, Acala, Ra, and Pillan allSome of these deposits are not as intensely red as those occur from longitude 240–3608, and that these are the onlyof Pele in the Galileo era, but the red materials fade over plumes observed in this longitude range. This longitudetime. We have no information other than the surface de- range is the ‘‘active sector’’ described by McEwen andposits, for which Pele remains the best analog among the Soderblom (1983).observed plumes. Prometheus is also unique in that it is unusually bright

and long-lived. Indeed, each plume and eruptive center isIntermediate-size plumes. The Loki plumes were inter-unique in some detail, but the Prometheus-type plumemediate in size and other characteristics compared to Pelegeneralization nevertheless seems valid. Perhaps the mostand Prometheus-type plumes (McEwen and Soderblomuseful groupings at present are just Prometheus-type1983). The Acala plume may be another intermediate ex-plumes and unique individuals.ample, although if has not yet been observed in reflected

light. Pillan is also larger than most Prometheus-typeEffusive Eruptionsplumes (120 km measured by Galileo in the VLT bandpass;

150 km measured by HST in the uv—Spencer et al. 1997d). There are many intense hot spots that are not associatedOrbit C10 provided information on the surface deposits with plumes or diffuse surface deposits. All of these hotof Acala and Pillan: nothing at Acala and 400-km dark spots correspond to dark flow-like features or calderas withgray deposits around Pillan (similar to Babbar Patera). dark floors, so they probably represent effusive eruptions

in flows and perhaps lava lakes. SSI can provide littleStealth plumes. Johnson et al. (1995) proposed the exis-detailed information about these eruptions until high-reso-tence of stealth plumes: large, pure SO2 gas plumes drivenlution images are acquired.by contact with high-temperature silicate magma. They

predicted that stealth plumes would be invisible via re-Global Distribution of Volcanismflected light but might be seen in eclipse airglow. In particu-

lar, they noted that Pele (as seen by Voyager) was very Active and inactive volcanic centers appear to be almostfaint and nearly a stealth plume. Galileo has shown Pele uniformly distributed over Io (Carr et al. 1998, R. Lopes-to be even fainter in visible light and HST has confirmed Gautier et al., manuscript in preparation). However, sev-the presence of SO2 gas in Pele, so it is certainly a stealthy eral types of volcanic activity are nonuniform. The activeplume. The large size of Pele suggests high exit velocities, plumes clearly prefer the equatorial region. The field ofwhich in turn is expected from the high-entropy character- bright vents occurs over the subjovian region, and a regionistics of stealth plumes (Kieffer 1982). It seems unlikely of diffuse glow seen in eclipse at low resolution suggeststhat Pele does not contain particulates, however, because that a similar field may exist over the antijovian region.a high-velocity jet is likely to entrain particulates and be- The sub- and antijovian regions may experience the great-cause something is needed to account for the SO2-poor est tensile stress at the surface, if similar to models for

Europa (Helfenstein and Parmentier 1983). The largeplume deposits. Meanwhile, several Prometheus-type

216 MCEWEN ET AL.

mountains on Io are uniformly distributed according to is it that a body which is being heated throughout, albeitperhaps nonuniformly, focuses so much of its heat intoCarr et al. (1988), but the limb profiles of Thomas et al.

(1988) suggest that the antijovian region is unusually one place while at the same time distributing much of itsheat broadly over its surface? The record of inactive hotsmooth. (The subjovian region is covered by lower resolu-

tion limb profiles, so the topography in this region remains spots preserved on Io’s surface suggests that these centersof heat loss occurred at different locations at differentunclear.) The region of Io from longitude 240–3608 has

distinctive color and albedo patterns and the plumes in times in the past with no particular place favored over anyother. Of course, we cannot know if any of these formerthis region display a greater range of characteristics than

plumes at other longitudes. The distribution of volcanism hot spots was comparable in size to Loki.Tidal heating in the deep mantle would result in largersuggests that the heat flow is globally uniform except for

(a) an enhancement at Loki and (b) a reduced heat flow scale convection perhaps with a relatively small numberof mantle plumes spaced widely apart and carrying mostfrom Bosphorus Regio (Lopes-Gautier et al. 1997a, Smythe

et al. 1997). Heat flow in the polar regions is poorly con- of the deeply generated heat. In this scenario it would beexpected that major volcanic constructs similar to Tharsisstrained. It is unclear how best to explain these patterns,

but the tidal heating mechanisms and resulting heat flow on Mars, Hawaii on Earth, and Atla and Beta Regioneson Venus would be found on Io. Perhaps the properties ofpatterns may provide part of the answer.Io’s surface materials and the surface thermal environmentpreclude the building of high volcanoes, but instead resultsTidal Heating Mechanismsin especially large calderas such as Loki. In any case, thelarge concentration of heat at Loki could plausibly origi-Io is losing heat at the phenomenal rate of about 1014

W. In contrast, the rate of heat loss from the much larger nate from tidal heating in Io’s deep mantle.Clearly, the way in which Io is heated and the mannerEarth is only about 4 3 1013 W. Io’s heat loss occurs

primarily from its active and recently active hot spots. The in which the heat makes its way to the surface are poorlyunderstood. The key to improving this state of affairs isestimate of Io’s heat flow is based mainly on ground-based

observations of infrared emission over the past two decades further observation of Io’s hot spots, topography, and com-position combined with new modeling efforts.(Veeder et al. 1994). It is likely that the ground-based

estimate is a lower bound on Io’s actual rate of heat lossFUTURE GALILEO OBSERVATIONSsince the measurements do not sense the conductive heat

flow. The distribution of hot spots over Io’s surface can beSSI will observe Io during all three phases of the Galileoexpected to reveal something about the nature of heating

Europa Mission (GEM): the Europa phase (orbits E12–within the interior.E19), the perijove reduction phase (orbits C20–C23), andBecause of the intense tidal heating and the apparentlythe Io phase (orbits I24–I25, close flybys of Io). There ishigh temperatures in Io’s interior, convection should bea constraint to minimize the encounter periods duringthe main internal mode of heat transfer. If tidal heating isGEM, so there will be fewer opportunities for distantpredominantly in an asthenosphere on the order of 100plume monitoring and eclipse observations. However,km thick (Segatz et al. 1988, Ross et al. 1990), convectionGEM does provide a number of unique opportunities, mostshould occur globally in the asthenosphere with centers ofimportantly via improved spatial resolution. Observationsupwelling and downwelling separated by a few hundredfor E12–C23 have already been planned, whereas detailedkilometers. In this case it would be anticipated that activeplanning for I24–I25 will occur in 1998–1999. Below ishot spots would pepper the surface at several hundreda summary.kilometer intervals (and with an equatorial concentration

of hot spots or more energetic hot spots). However, the 1. Eclipse observations have been planned for orbitsE12, E14, E15 (2 eclipses), and C22. The E12 image wasdistribution of hot spots might also be controlled by the

lithospheric structure. Currently active centers are typi- lost due to a pointing error. Other eclipse images willinclude the CLR and 1MC filters for measuring hot-spotcally separated by 500–1000 km, but prominent volcanic

centers that were probably recently active (i.e., in the last temperatures. NIMS will provide much additional moni-toring of hot spots during GEM. The E15 eclipse sequencescentury) have spacing more like 200–300 km. Accordingly,

it is plausible that the majority of Io’s hot spots are associ- were recently expanded to include the VLT, GRN, andRED filters, to attempt improved data on the atmosphericated with asthenospheric convection.

The major current exception to this uniform pattern is and plume airglow.2. Six-color coverage at 3.0 km/pixel will be acquiredLoki Patera, which alone accounts for about a quarter of

Io’s heat loss (Veeder et al. 1994). The heat being radiated in orbit E14. These images of Io’s antijovian hemispherewill provide the highest resolution six-color coverage in allfrom Loki is an order of magnitude larger than the heat

loss from all the Earth’s presently active hot spots. How of Galileo’s mission, especially important to search for


absorption bands typical of iron-rich silicates, which may For example, we could cover part of Marduk with a verticalprecision of p5 m, perhaps sufficient to model lava flowbe best exposed over small areas. This set of images will

also provide our best coverage before C21 of regions that rheologies. In I25 we pass under the south pole and canimage several targets, and we expect to attempt p26 m/will be visible and illuminated during the close flybys of I24

and I25, and may influence our selection of high-resolution pixel images of the Prometheus eruption column on thebright limb. NIMS and PPR (Photopolarimeter–targets (C21 may occur too late).

3. I24 context. A single frame will be acquired in E14 Radiometer) will acquire coordinated observations of alltargets.at 2.6 km/pixel and with a subsolar position identical to

that during the I24 closest approach, to put the highest- 10. High-resolution images (100–1000 m/pixel) will beacquired, about 70 summation-mode frames in I24 and 50resolution I24 images into regional context.

4. Low-phase (48) mapping will be acquired in E15, E18, in I25. There are many potential targets over the regionfrom longitude 708 to 2308. We expect to include coverageand C21 to provide global coverage in four colors. The

low-phase mapping enables (1) change detection via com- in 3 colors over a few especially interesting or colorfulareas such as Prometheus and Culann. We can also observeparison with low-phase images from Voyager, HST, Gali-

leo nominal mission, and future datasets and (2) uniform regions near the terminator at longitudes 80–1008 for opti-mal topographic shading. There are opportunities to imagemapping of phase-function behaviors via ratios with high-

phase mapping. The E15 color set was recently expanded the plumes of Pele and Pillan (if active) at about 1 km/pixel on the bright limb.to six colors to search for silicate absorption bands at loca-

tions including Pillan, Pele, Babbar, and Isum.5. High-phase mapping frames will be acquired in E15

ACKNOWLEDGMENTSat 758 phase angle. When combined with nominal-missiondata, this will complete global coverage at 70–908 phase Many thanks to the Galileo project and SSI team for acquiring thein 3 colors. images and to the Io Working Group for stimulating discussions. Detailed

reviews from John Spencer (Lowell Observatory), Diana Blaney (JPL),6. Moderate-resolution color/albedo mapping. The C21and Rosaly Lopes-Gautier (JPL) are greatly appreciated. We thank Jennyimages at 48 phase (mentioned above) will be acquired atBlue (USGS) and the IAU nomenclature committee for approving new1.26 km/pixel in the GRN filter and 2.52 km/pixel in thenames for features on Io. The images were processed and analyzed at

other three colors, thus providing the highest resolution the Planetary Image Research Lab (PIRL) using the ISIS software devel-Galileo color of Io to be acquired over an entire hemi- oped at USGS, Flagstaff, Arizona; we are grateful to the Flagstaff group

for their support. Special thanks go to Joe Plassmann and Linda Hickcoxsphere. This mosaic will also provide the highest resolution(LPL) for the operation of PIRL. This research supported by the Gali-coverage ever achieved from longitude 468 to 2108, thusleo project.completing near-global coverage by Voyager plus Galileo

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Active Volcanism on Io as Seen by Galileo SSI