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Icarus 177 (2005) 69–88www.elsevier.com/locate/icaru

The Zamama–Thor region of Io: Insights from a synthesis of mappitopography, andGalileo spacecraft data

David A. Williamsa,∗, Laszlo P. Keszthelyib, Paul M. Schenkc, Moses P. Milazzod,Rosaly M.C. Lopese, Julie A. Rathbunf, Ronald Greeleya

a Department of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287-1404, USAb Astrogeology Team, U.S. Geological Survey, Flagstaff, AZ, USA

c Lunar and Planetary Institute, Houston, TX, USAd Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

e NASA Jet Propulsion Laboratory, Pasadena, CA, USAf Department of Physics, University of Redlands, Redlands, CA, USA

Received 14 June 2004; revised 10 March 2005

Available online 17 May 2005

Abstract

We have studied data from theGalileo spacecraft’s three remote sensing instruments (Solid-State Imager (SSI), Near-Infrared MSpectrometer (NIMS), and Photopolarimeter-Radiometer (PPR)) covering the Zamama–Thor region of Io’s antijovian hemisphereduced a geomorphological map of this region. This is the third of three regional maps we are producing from theGalileo spacecraft dataOur goal is to assess the variety of volcanic and tectonic materials and their interrelationships on Io using planetary mapping tesupplemented with all availableGalileo remote sensing data. Based on theGalileo data analysis and our mapping, we have determinedthe most recent geologic activity in the Zamama–Thor region has been dominated by two sites of large-scale volcanic surface chZamama Eruptive Center is a site of both explosive and effusive eruptions, which emanate from two relatively steep edifices (ZamA and B) that appear to be built by both silicate and sulfur volcanism. A∼100-km long flow field formed sometime after the 1979Voyagerflybys, which appears to be a site of promethean-style compound flows, flow-front SO2 plumes, and adjacent sulfur flows. Larger, possistealthy, plumes have on at least one occasion during theGalileo mission tapped a source that probably includes S and/or Cl to producepyroclastic deposit from the same vent from which silicate lavas were erupted. The Thor Eruptive Center, which may have been ato Voyager, became active again during theGalileo mission between May and August 2001. A pillanian-style eruption at Thor includetallest plume observed to date on Io (at least 500 km high) and new dark lava flows. The plume produced a central dark pyroclas(probably silicate-rich) and an outlying white diffuse ring that is SO2-rich. Mapping shows that several of the new dark lava flows arothe plume vent have reoccupied sites of earlier flows. Unlike most of the other pillanian eruptions observed during theGalileo mission, the2001 Thor eruption did not produce a large red ring deposit, indicating a relative lack of S and/or Cl gases interacting with the magmthat eruption. Between these two eruptive centers are two paterae, Thomagata and Reshef. Thomagata Patera is located on a largmesa and shows no signs of activity. In contrast, Reshef Patera is located on a large, irregular mesa that is apparently undergoingthrough erosion (perhaps from SO2-sapping or chemical decomposition of sulfur-rich material) from multiple secondary volcanic cen 2005 Elsevier Inc. All rights reserved.

Keywords: Satellites of Jupiter; Io; Geological processes; Volcanism; Surfaces (satellite)

* Corresponding author.E-mail address: david.williams@asu.edu(D.A. Williams).

0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2005.03.005

1. Introduction

On September 21, 2003, NASA’sGalileo mission toJupiter ended after 26 years of planning and operations, al-most 14 years in space, and 35 orbits of Jupiter completed.

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70 D.A. Williams et al. /

The data returned from the jovian system byGalileo overthe last∼8 years, supplemented by data from Earth-batelescopes, the Hubble Space Telescope (HST), and thecember 2000 Jupiter flyby of NASA’sCassini spacecraft,revolutionized our understanding of Jupiter and its inaction with its rings, moons, and magnetic field, andthe geological activity and evolution of the Galilean salites (for a review seeBagenal et al., 2004). These newinsights include the recognition of extremely diverse styof volcanism on Io, the innermost Galilean satellite (eGeissler, 2003; Kargel et al., 2003; McEwen et al., 20Lopes and Williams, 2005). Very high temperature, possibultramafic or superheated basaltic volcanism is interspewith very low temperature, likely sulfur and sulfur dioide (SO2) volcanism including both explosive and effusieruptive styles. One method to characterize Io’s volcanicversity is to utilize geologic mapping to determine the disbution of volcanic materials and to identify the stratigraprelationships. Previously, such mapping was used onVoy-ager data to understand the geology of the well-imaged sjovian hemisphere of Io(Moore, 1987; Greeley et al., 198Schaber et al., 1989; Whitford-Stark et al., 1991; Crownal., 1992).

We have produced a series of regional geologic mof selected areas of Io’s antijovian hemisphere, whichpoorly-imaged byVoyager but well-imaged byGalileo. Ourfirst map(Williams et al., 2002, 2003)focused on the ChaacCamaxtli region, which contains a diverse range of pate(volcano-tectonic depressions) and flucti (flow fields). Osecond map(Williams et al., 2004)focused on the CulannTohil region, which contains multicolored paterae andlarge mountain with interesting stratigraphic relationshHere we present our third regional map focusing onZamama–Thor region, which contains two active plume sand flow fields related to unusual shield-like rises and colike structures. Each of these regions was chosen for mping because of the volcanic diversity within and amongregions, and because of the availability of multiple setsGalileo remote sensing data from repeated flybys at a varof resolutions to characterize the volcanism in these regi

As was done for our previous maps, we utilize variodata sources to study our map region, including imaobtained by theGalileo Solid-State Imager (SSI), multspectral data from the Near Infrared Mapping Spectrom(NIMS) to provide temperature estimates and compositioinformation, and thermal data from the PhotopolarimetRadiometer (PPR) to monitor thermal emission from cregions. Where stereo imaging was available, SSI imawere also used to create Digital Elevation Models (DEMsinvestigate topographic relationships.Galileo images werecompared to low-resolutionVoyager images in search osurface changes. Our goals in this paper are to discusgeology of the Zamama–Thor region and (1) determine

variety of volcanic features, eruption styles, and composi-tions that are present, (2) discuss how this region has evolvedsince theVoyager flybys and during theGalileo mission, and

s 177 (2005) 69–88

-

e

(3) compare and contrast this region with the others we hmapped to search for similarities and differences in ertion styles and products that can be applied to investigaterange of volcanic activity in this hemisphere of Io.

2. Background

2.1. Mapping approach

The first mapping paper by Williams and colleagu(Williams et al., 2002, 2003)contained a review of previous geologic mapping of Io usingVoyager images and adiscussion of the techniques and approach used to permapping withGalileo data. Briefly, we follow establishetechniques for planetary mapping to determine the typevolcanic materials and structural features present andstratigraphic sequence in which they were emplaced (Shoemaker and Hackman, 1962; Wilhelms, 1990, 19Tanaka et al., 1994). Because all of our mapping is basedalbedo variations, morphology, and/or color or spectral vations inGalileo spacecraft data (i.e., we have no in situformation on rock ages or lithologies), strictly speakingplanetary maps are “geomorphologic” rather than “geolic”. Based on comparison with results from terrestrial fimapping, only under ideal circumstances is remote sensbased planetary mapping able to provide accurate thdimensional distributions of stratigraphic relationships amaterial units. Nevertheless,Wilhelms (1990)and Mooreand Wilhelms (2001)point out that planetary geologic maping remains a necessary and useful technique to aid ininterpretation of planetary surfaces until ground truth canobtained.

2.2. Galileo SSI color interpretation of Io

Besides Earth, Io is the most colorful object in the Slar System, in that its surface shows a wide range ofors from black to red, to orange, to yellow, to green,white. Comparison ofGalileo and Hubble Space Telescocolor data with laboratory studies has shown that the difent colors probably represent volcanic materials of difent chemical composition and/or emplacement styles (Geissler et al., 1999; Spencer et al., 2000a). Unlike otherplanetary bodies without such a broad spectrum of coon Io it is logical to attempt to use color to aid in the denition and characterization of material units, but with socaveats (see below). Several recent papers (e.g.,Simonelliet al., 1997; Geissler et al., 1999, 2000; Kargel et al., 19Lopes-Gautier et al., 2000; Spencer et al., 2000a; Doual., 2002, 2004; Schmitt and Rodriguez, 2003) discuss newinsights into, and complexities related to, Io’s color sptrum, and we summarize the essential points below.

First, it is important to note that, although theGalileo SSIproduces color images that are closer to “true” color (i.e.,color as would be seen by the unaided human eye) than any

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Mapping the Zam

previous spacecraft camera, red–green–blue compositeimages that are based onGalileo’s 756-nm, green and violefilters are still false color images, but which give an appromation of true color(Klaasen et al., 1984). The use of filterswith wavelengths beyond that of normal human visionoften help accentuate the colorful deposits on Io’s surfaAlso, the SSI’s filters are not capable of producing exa“true” color images. We use the color data to aid inrecognition of surface features that may not follow morplogical or structural/topographic boundaries typically visiin the greyscale imagery. Also typically, many structuralmorphological features are not visible in the color ima(Schenk et al., 2001). When the low spatial resolution, lowphase angle color images and the high spatial resoluhigh incidence angle greyscale data sets are merged,possible to visualize the relationships between colorfulterials with morphologic or topographic features all in oimage. This is a useful aid to mapping.

Second, surface colors on Io change their hues wchanges in lighting conditions. For example, increasphase angle causes polar deposits and some light-coplume deposits around active volcanic centers to brighwhereas materials in the equatorial band darken(Geissler etal., 2000). These color changes are attributed to the pence of thin, fine-grained SO2 frosts that are transpareunder low-phase illumination but that become visible unhigh-phase illumination(Geissler et al., 2000). These prop-erties can be a problem for color interpretation, becauseGalileo spacecraft’s various encounters with Io had widdifferent geometries, resulting in color imaging at a vaety of spatial resolutions and phase angles(Simonelli et al.,1997; Geissler et al., 1999, 2000). Thus, for consistency ingeologic mapping, it is important to use color images twere obtained at a uniform, preferably low, phase angle.all of the color interpretation used in our Io geologic mawe utilize theGalileo orbit C21 Io enhanced color mosaicthe antijovian hemisphere (July 1999), which was obtaiat a phase angle of 4◦.

Third, in terms of the actual colors observed inGalileoimages, Io can be subdivided into four main color un(Geissler et al., 1999): red materials, yellow materials, grawhite materials, and black materials. Red materialsfound either as regional red-orange units in polar regionsas local red patches and rings found on or around somtive vents (e.g., Pele). The red has been interpreted to cfrom short-chain sulfur molecules (S3, S4) that result, in thecase of the polar units, from breakdown of cyclooctal su(S8) by charged particle irradiation(Johnson, 1997), and inthe case of the red patches and rings by condensationS2-rich volcanic gases in the plumes of active vents(Spenceret al., 2000a). Alternatively, recent studies ofGalileo NIMSspectra of the red diffuse deposit south of Marduk combiwith laboratory analyses suggest that some red deposits

result from solid sulfuryl chloride (Cl2SO2) or sulfur dichlo-ride (Cl2S) that condensed on SO2 snow from Cl-bearinggases in active plumes(Schmitt and Rodriguez, 2003).

Thor region of Io 71

r

d

-

Yellow materials, which cover about 40% of Io’s surfa(Geissler et al., 1999), are found as either regional yellomaterials covering large expanses of equatorial plainsas local greenish-yellow patches observed near or in spaterae. Through a comparison ofGalileo SSI 6 filter (vi-olet, green, red, 756 nm, 889 nm, 968 nm; seeKlaasenet al. (1984)for filter bandpasses) color data to the labratory spectra of>650 rocks and minerals,Geissler et al.(1999)suggested that the yellow materials were most content with cyclo-octal sulfur (S8) with or without a coveringof SO2 frosts deposited by plumes. An alternative hypoesis presented byHapke (1989)suggested that the yellocolor on Io could be produced by polysulfur oxide aS2O without requiring large quantities of elemental sulfGeissler et al. (1999)also suggested that the rare greeniyellow patches could be composed either of some typsulfur compound contaminated by iron, or lava flows coposed of silicates rich in olivine or pyroxene with or witout sulfur-bearing contaminants. This interpretation of“green spots” is consistent with the locations of these uin or near paterae, suggesting intimate interaction betwsilicate lava and either sulfurous flows or plume depo(McEwen et al., 2000; Williams et al., 2000).

Gray-white materials, which cover about 27% of Io’s sface(Geissler et al., 1999), are found as extensive equatrial plains and as diffuse rings around active vents. Thmaterials have been extensively studied using the NIMSstrument, which had several detectors that are particusensitive to SO2 and whose data were analyzed to assgrain size and abundance (e.g.,Douté et al., 2001, 20022004). Carlson et al. (1997)interpreted the white materiato be coarse- to moderate-grained SO2 snow, which likely re-sult from plume fallout that has undergone recrystallizat(Douté et al., 2001, 2002). However, high spatial resolutioNIMS data showed that color alone is not in itself a goindicator of SO2 distribution or granularity, because SO2 isoften mixed with contaminants of other colors (e.g.,Lopes-Gautier et al., 2000; Douté et al., 2002, 2004).

Black materials, which cover about 1.4% of the surfa(Geissler et al., 1999), are mostly restricted to very dark ptera floors, lava flow fields, or dark diffuse materials nor surrounding active vents. Most of these materials colate with active or recently active hot spots(Lopes-Gautieret al., 2000, 1997; Lopes et al., 2001). Geissler et al. (1999found fromGalileo multicolor studies of the black materiathat their visible/near-IR spectra were most consistent wMg-rich ortho-pyroxene (enstatite or bronzitehypersthenas indicated by their strong 0.9 µm absorption. The darkterials are most likely silicate lava flows (within flucti), lavlakes (within paterae), or pyroclastic deposits (within diffudeposits near paterae), of mafic to ultramafic compositThe evidence for high-temperature ultramafic material

based in part on Geissler’s spectral analysis, and in part onthe high (>1200◦C) temperatures estimated for some activeeruptions (e.g.,McEwen et al., 1998b).

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72 D.A. Williams et al. /

Finally, the colors of some surficial deposits changethey age in Io’s heavy radiation environment. For exaple, irradiation of pale yellow S8 results in the development of the metastable short-chain molecules S3 and S4,which range from brownish-yellow to grayish-yellow to oange(Nash and Fanale, 1977; Nelson and Hapke, 19Steudel et al., 1986; Hapke and Graham, 1989; Nelsoal., 1990). Red materials tend to be very ephemeral,fade over a period of a few months at most vents(McEwenet al., 1998a; Geissler et al., 2004). Dark materials tendto brighten with time, as they develop coatings of sfurous materials from plume deposits(Nash et al., 1986).Bright materials tend to darken over time, as they inact with underlying or superposed materials (e.g.,Williamset al., 2002). Some materials were found to completechange color, such as the red unit on the floor of Pillantera that changed to green over several months(Phillips,2000; Keszthelyi et al., 2001). Phillips (2000)suggestedthat this color change was caused by a reaction of shchain red sulfur (S3, S4) molecules with warm, dark silicate lava. Recently,Kargel et al. (1999)reviewed the effectsof various processes and interactions on sulfurous maals, and they found that color changes to sulfurous maals occur on timescales of days to years. A more thoro

study of these type of surface changes may provide a use

Fig. 1. Galileo SSI image of Io’s antijovian hemisphere, showing prominenmapped: Chaac–Camaxtli(Williams et al., 2002, 2003), Culann–Tohil(Williams e

s 177 (2005) 69–88

3. Galileo data of the of the Zamama–Thor region

3.1. SSI imaging

Fig. 1 contains a map showing the location of tZamama–Thor region relative to the other regions mapusingGalileo data, andFig. 2 provides an overview of thZamama–Thor region.Table 1lists the approved and provisionally approved names of the volcanic features inZamama–Thor region, which include two new pateraeoccur on mesa-like rises, Thomagata and Reshef PatFigs. 2a and 2bare the highest resolution color imagesthe region from theVoyager andGalileo missions, respectively. Fig. 2cis theGalileo SSI regional mosaic that servas the base for our mapping (observation I32TERMINOctober 2001:360 m/pixel). SeeTable 2for a list ofGalileoorbit designations. I32TERMIN02 was a low-Sun obsvation that illuminated topographic features such as ridand scarps, making it ideal for mapping structural featuA quick study of these images indicates that unlikeChaac–Camaxtli and the Culann–Tohil regions, there hbeen two obvious large-scale changes in this region inpast 20 years. The first change is in the formation of themama Eruptive Center, which was not present in the 1

-Voyager images. Zamama was first detected by theGalileo

urce;

ful aid in determining relative ages at some localities onIo.

NIMS and SSI as a hot spot and a 60-km-high plume soduring orbit G1 (June 1996:Lopes-Gautier et al., 1997

t named features and regional mosaics. Those outlined in light gray have beent al., 2004), Zamama–Thor (Williams et al., this paper).

lution

Mapping the Zamama–Thor region of Io 73

Fig. 2. (a) Best combinedVoyager 1 and 2 color coverage of the Zamama–Thor region from the 1979 flybys, with a spatial resolution of∼5–20 km/pixel(Smith et al., 1979a, 1979b); (b) bestGalileo SSI color imaging of the Zamama–Thor region from the orbit C21 (July 1999) flyby, with a spatial resoof 1.4 km/pixel and obtained at a low-phase angle (4◦). Note the presence of the Zamama flow field, which was not present during theVoyager flybys;

(c) bestGalileo SSI mosaic of the Zamama–Thor region from the orbit I32 (October 2001) flyby, with a spatial resolution of 360 m/pixel and obtained at alow-Sun angle (incidence angle= 85◦) to enhance topography. Note the surface changes at the Thor Eruptive Center, due to the large eruption that produceda 500-km-high plume (seeFig. 5).

under

74 D.A. Williams et al. / Icarus 177 (2005) 69–88

Table 1Named volcanic features in the Zamama–Thor region (refer toFig. 2)

Name Lat (N) Lon (W) Size (km) Hot spot Notes Origin of name

Volund Eruptive Center 25.0 184.2 N.G. Y Detected byVoyager only Germanic supreme smith of the godsZamama Eruptive Center 18.0 174.0 N.G. Yb Detected by NIMS, SSI on several flybys Babylonian Sun, corn, and war godThomagata Pateraa 24.9 165.6 50 N Depression atop shield-like structure Chibcha fire spirit and storm godReshef Pateraa 27.1 157.9 55 N Depression atop shield-like structure Phoenician god of lightning, Sun, and thSurya Patera 22.5 151.0 130 Yb Detected by NIMS in I27 Hindu Sun godArinna Fluctus 31.4 149.3 120 N Hittite Sun goddessThor Eruptive Centera 40.7 133.8 N.G. Yb Source of I31 500-km-high plume Norse god of thunder

7

8

9

3

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wn

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e to

tralnu-e-ith

;-

cho to00-

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aste-g-

ing;nd

N.G., not given.a Name provisionally approved by the IAU.b FromLopes et al. (2001).

Table 2Galileo mission ancountersa

Mission Flyby/orbit Date

NOM J0b December 7, 1995NOM G1 June 29, 1996NOM G2 September 6, 1996NOM C3 November 6, 1996NOM E4 December 18, 1996NOM E5b January 20, 1997NOM E6 February 20, 1997NOM G7 April 3, 1997NOM G8 May 7, 1997NOM C9 June 27, 1997NOM C10 September 18, 199NOM E11 November 7, 1997GEM E12 December 16, 1997GEM E13b February 10, 1998GEM E14 March 29, 1998GEM E15 May 31, 1998GEM E16b July 21, 1998GEM E17 September 26, 199GEM E18b November 22, 1998GEM E19 February 1, 1999GEM C20 May 5, 1999GEM C21 June 30, 1999GEM C22 August 14, 1999GEM C23b September 16, 199GEM I24 October 11, 1999GEM I25 November 26, 1999GEM E26 January 4, 2000GMM I27 February 22, 2000GMM G28 May 20, 2000GMM G29 December 28, 2000GMM C30 May 25, 2001GMM I31 August 6, 2001GMM I32 October 16, 2001GMM I33b January 17, 2002GMM A34b November 5, 2002GMM J35b,c September 21, 200

a Orbit letter designates primary remote sensing target: J, Jupiter;E, Europa; G, Ganymede; C, Callisto; A, Amalthea; NOM,Galileo Nom-inal Mission; GEM,Galileo Europa Mission; GMM,Galileo MillenniumMission.

b No SSI data collected on these orbits.c Galileo impact into Jupiter.

McEwen et al., 1997; Davies et al., 1997). The Zamamaflow field was first seen by the SSI south of Volund duringorbit G2 (September 1996:McEwen et al., 1998a). The Za-

mama plume (Fig. 3) was observed byGalileo SSI in globalcolor, eclipse, and plume monitoring images taken duorbits G1, G8, E11, and E14 but was not observed inages taken during orbits G7 or C21, indicating that unthe Prometheus plume, the Zamama plume was intertent. Surface color changes around the Zamama plumeinclude bright red diffuse deposits that faded to red-broover several years(McEwen et al., 1998a), and diffuse whitedeposits near the contact between the eastern dark flowand the bright plains. These white deposits are thoughbe similar to the white, SO2-rich flow-front plume depositsseen at Prometheus (cf.,Milazzo et al., 2001). The Zamamaflow field was observed at high resolution (35–40 m/pixelwith 400 m/pixel context) during orbit I24 (Fig. 4), and, al-though these images were garbled by radiation damagthe SSI(Keszthelyi et al., 2001), they showed that the E–Wtrending Zamama flow field is fed by a well-defined cenvent that is the source of the lava. The flow field has crelated flow margins and narrow linear flows typical of tubfed compound pahoehoe-like flow fields associated wother Promethean-style eruptions(McEwen et al., 2000Keszthelyi et al., 2001). Sinuous bright flows are also visible to the south, suggestive of sulfur lavas (cf.,Williams etal., 2001, 2002).

The second large-scale surface change, visible inFigs. 2band 2c, is in the area of the Thor Eruptive Center, whiwas the source of the largest volcanic plume seen on Idate. The Thor Eruptive Center was the source of a 5km-high plume, detected during the orbit I31 flyby (Agust, 2001) as part of a pillanian-style(Turtle et al., 2004)eruption (Fig. 5). The plume produced a central zonedark diffuse material and a ring of white diffuse matersurrounding the vent.Douté et al. (2004)report (based onstudies of NIMS data) that the white diffuse ring is at lepartly composed of solid SO2, and has enriched the prexisting regional SO2 deposits in this area. It is interestinto note that this pillanian-style eruption with a tall, fewhundred-km-high plume did not produce a large red rdeposit as typically occurs at Pele(Keszthelyi et al., 2001Radebaugh et al., 2004), and has occurred at Tvashtar a

Surt (Turtle et al., 2004; F. Marchis, pers. comm., 2004) atsimilarly northern latitudes. The absence of a red diffuse ringmay be indicative of a lack of juvenile or meteoric sulfur

ptured the

Mapping the Zamama–Thor region of Io 75

Fig. 3.Galileo SSI observations of the Zamama plume: (a and b) Enhanced global color observations during the orbit E11 flyby (November 1997) cabright, umbrella-shaped, 100-km-high Zamama plume in eruption; the region appears blue-white in color images, and is thought to be SO2 gas in the plume;

(c) at left, the Zamama plume is visible as a whitish blur over the Zamama flow field in this image from the orbit E14 flyby (March 1998), whereas at bottom

ly 19 14 flybysmeth

the

a-hot,

ama24us-

l-odelim-nt att

right the plume is not visible in this image from the orbit C21 flyby (Jubut not during the G7 or C21 flybys, demonstrating that (unlike the Pro

and/or chlorine interaction with the Thor magma duringI31 eruption.

3.2. NIMS imaging

Galileo’s NIMS and SSI instruments detected the Zmama (initially referred to as South Volund) plume andspot as early as the orbit G1 flyby(Lopes-Gautier et al.1997; McEwen et al., 1997; Davies et al., 1997), and NIMS

detected additional hotspot activity at Zamama during theorbit G2, C3, E4, C10, possibly E11, E15 and C21 flybys(Lopes-Gautier et al., 1999). Lopes-Gautier et al. (1999)thus

99). The Zamama plume was observed during the G1, G8, E11, and Eeus plume) the Zamama plume is intermittently active.

classified Zamama as a persistent hot spot. The Zamhotspot is also visible in NIMS observations from orbits Iand I32 (Fig. 6), and it was detected in December 2001ing the Keck telescope adaptive optics system(Marchis etal., 2005). Davies et al. (1997)assessed the style of vocanism at Zamama using a two temperature–area mfit to combined, near-simultaneous G1 NIMS and SSIages, and they found that Zamama had a cool compone∼450 K (∼180◦C) covering∼50 km2, and hot componen

◦ 2

at∼1100 K (∼830 C) covering∼0.1 km . They interpretedthese data to represent the cooled crust of a lava flow fieldwith smaller hot cracks or breakouts, similar to terrestrial

1 imageAMA02,ear)

pahoehrupts

76 D.A. Williams et al. / Icarus 177 (2005) 69–88

Fig. 4. Galileo SSI high-resolution observations of the Zamama flow field obtained during the orbit I24 flyby (October 1999). (a) The global C2shows the location of the Zamama flow field, which is seen more clearly in the regional view at upper right. (b) This view (observation I24ZAM400 m/pixel), although degraded by radiation damage to the SSI (see discussion inKeszthelyi et al., 2001), clearly shows that the east–west trending, linZamama flow field emanates from a central vent source(Keszthelyi et al., 2001), later found to be associated with a shield-like structure(Schenk et al., 2004rather than a fissure as was first proposed(McEwen et al., 1998a). (c) A 10-frame, high-resolution image strip (observation I24ZAMAMA01, 35–40 m/pixel)straddles the southern margin of the flow field, and reveals the crenulated flow margins and narrow, linear flows indicative of tube-fed, compoundoeflow fields such as those seen on Kilauea in Hawaii(Bruno et al., 1992; Keszthelyi et al., 2001). These and other observations suggested that Zamama e

like Prometheus and Amirani, producing promethean-style lava flow fields(Keszthelyi et al., 2001). Note also the sinuous, relatively higher albedo flows at

images-resolutio

lower left, most likely indicative of sulfur lavas (cf.,Williams et al., 2001, 2002).

Fig. 5. Galileo SSI images from the orbit I31 flyby (August 2001) that revealed the pillanian-style Thor eruption. At left, stretched violet-filterrevealed the Thor plume reached at least 500 km above Io’s surface, making it the largest plume eruption seen on Io to date. At right, this lown

(19.6 km/pixel) global color observation shows that the Thor eruption produced a central dark diffuse deposit encircled by a white diffuse ring deposit.NIMS detected SO2 in the I31 plume(Lopes et al., 2004)and SO2 is typically found in or near white materials on Io’s surface(Carlson et al., 1997;Geissler et al., 2000; Douté et al., 2001, 2002).

t approx-by NIMSed).atures are att 28uring orbitrted

tialNIMS

are de-

Mapping the Zamama–Thor region of Io 77

Fig. 6.Galileo NIMS observations of Zamama manually registered onto SSI image C21COLOR01. (a) NIMS observation of Zamama during I24 aimately 9.5 km/NIMS pixel. The white outline represents the area observed by NIMS (including the rectangular area over the Zamama flow) andwhile “riding along” with SSI (i.e., NIMS collected data while the scan platform slew from one SSI target to the next, and thus fewer DN were collectFourareas show thermal emission at NIMS wavelengths. The main Zamama flow shows thermal emission along all the observed length and temperleast 650 K. A small area in Volund (24◦ N, 176◦ W) shows thermal emission. A more extended area to the west of Volund shows thermal emission a◦ N,190◦ W and 27◦ N, 189◦ W. These are probably part of the same volcanic system and appear to correlate with a similar hot spot observed by SSI dG1 at approximately 28.1◦ N, 192◦ W (McEwen et al., 1998a). A fourth area, at 32◦ N, 199◦ W, had not previously been detected, and thus is here repoas a new hot spot. (b) NIMS observation of Zamama during I24 at 1 km/NIMS pixel (observation 24INZAMAMA01, inset). This is one of the highest sparesolution observations obtained by NIMS during theGalileo mission. The white outline represents the area observed by NIMS (bottom left) and bywhile “riding along” with SSI (southwest to northeast strip, fewer DN collected by NIMS). Areas showing thermal emission at NIMS wavelengths

picted in red. Temperatures are at least 400 K. A lower limit is given because NIMS obtained this observation in reflected sunlight (seeLopes et al., 2004, for

m-t of

li-I24

spot

ataingnthes

n

usebal

1),il-nsitypot23,tive

description of technique).

basaltic flow fields (rather than sulfur flows as the hot coponent temperature was above the sulfur boiling poin∼720 K (∼445◦C): seeTheilig, 1982). The Davies et al.(1997)study of Zamama was one of the first usingGalileodata of Io to confirm earlier hypotheses (e.g.,Carr, 1986;Johnson et al., 1988) that some of Io’s volcanoes erupt sicate rather than sulfurous materials. The analysis of theNIMS Zamama observations included in this paper (Fig. 6)has led to the discovery of a previously undetected hotat 32◦ N, 199◦ W, southeast of Lei–Kung Fluctus.

The Thor hot spot was first detected by NIMS in I31 das an extremely bright region consisting of two overlapphot spots(Lopes et al., 2004). The main hot spot locatiofits well with the estimates from SSI of the source ofplume(Turtle et al., 2004). The main hot spot (Thor) wa

so bright that it saturated NIMS at most wavelengths. Us-ing the unsaturated pixels surrounding the saturated regionNIMS obtained a lower limit on the power output (at 4.7 µm)

from the I31 Thor event and this value (1.4× 1011 W µm−1:Lopes et al., 2004) is about half that of the C9 Pillan eruptio(∼3 × 1011 W µm−1: Davies et al., 2001). NIMS also de-tected SO2 in the I31 plume(Douté et al., 2004). SO2 fromthe I31 plume is thought to be the source of the white diffring seen around the Thor Eruptive Center in the I31 gloSSI color images (Fig. 5). Although it is not known when theThor eruption started (prior to the I31 flyby in August 200Lopes et al. (2004)concluded that NIMS captured this planian event during the early stages, based on the inteof the power output and on the lack of detection of a hot sat this location in observations obtained in C30 (May2001, 3 months earlier). NIMS observed the Thor ErupCenter again during I32, at both regional (83–93 km/NIMSpixel) and local scales (13–16 km/NIMS pixel). Two hot

,spots were clear in the highest spatial resolution observa-tion (Lopes et al., 2004): Thor (formerly I31A, temperature>800 K), coinciding with lava flows shown in an SSI image

Icaru

ig-est1Bs atf the

een

e-ma

forr-

es-n inthantely-

78 D.A. Williams et al. /

(Turtle et al., 2004), and a second, fainter hot spot (desnated I31B) that coincided with a small patera to the wof Thor. Although the relationship between Thor and I3is unclear, the occurrence of activity at the two locationapparently the same time suggests that they are part osame system(Lopes et al., 2004).

3.3. PPR imaging

Galileo’s Photopolarimeter–Radiometer (PPR) has b

used to map thermal emission from Io’s hot spots using

Fig. 7. Galileo PPR temperatures overlain on simultaneously collected SSasterisks (*) indicate the measured temperature: (a) the I24 Zamama obserZamama–Thor region (PPR resolution 42 km).

s 177 (2005) 69–88

Rathbun et al., 2004). PPR obtained observations simultanously with SSI during both the I24 high-resolution Zamaobservation and the I32 regional observation we usedmapping (Fig. 7). Initial analysis of the I24 Zamama obsevation was presented by(Spencer et al., 2000b)(see theirFig. 3). A reanalysis of these PPR data (with a 2.2-km rolution) is superimposed on the SSI mosaic and showFig. 7a. These data show that the dark flows are warmerthe surrounding terrain, with a temperature of approxima180 K (Spencer et al., 2000b). The dark flows do not com

urews

both daytime and nighttime observations and at both global(low) and local (high) resolutions(Spencer et al., 2000b;

pletely fill the PPR field of view, so the actual temperatof the flows is higher. Further, neither the bright lava flo

(a)

(b)

I images. The ovals indicate the field of view of the PPR instrument and thevation (PPR resolution 2.2 km), and (b) the I32 Terminator observation covering the

ama–

em-

are

imethe

uratee inureZa-

ica-e-

e-01,omvedage

Mapping the Zam

nor the white deposits have any obvious effect on the tperature profile where they are covered by the PPR.

The PPR data taken in I32 (with a 42-km resolution)superimposed on the SSI mosaic and shown inFig. 7b. Un-fortunately, the PPR data were taken during the daytwith an open filter, so reflected sunlight contaminatesdata. The result is that the temperatures are not acc(measured values are higher than actual) and cannot bterpreted quantitatively, only qualitatively. The temperatincreases sharply in the area of the active hot spots,

mama and Thor, while the background temperature varies

Fig. 8.Galileo SSI context mosaic (a) and coregistered DEM from stereo (b)of I32 images is 360 m/pixel. Image is∼1580 km across. Three mountainsregion. The central block of white pixels corresponds to the mesa-like unit (

Thor region of Io 79

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3.4. Topographic data derived from SSI imaging

Galileo SSI images, along withVoyager images, havebeen used to study topography on Io through appltion of 3D stereo photogrammetry and 2D photoclinomtry to produce Digital Elevation Models (DEMs: for dtails seeSchenk and Bulmer, 1998; Schenk et al., 202004). Topographic data for our map area are derived frtwo techniques. Semicontrolled topography was derifrom analysis of C21–I24 regional stereo image cover

at ∼1.4 km/pixel spatial resolution. The derived DEM

from west to east. No other temperature anomalies are ob-served in this area.

(method described inSchenk and Bulmer, 1998, andSchenket al., 2004) has a vertical precision of∼500 m. A second

mosaic of Zamama–Thor region, produced from I32 and I24 images. Resolution(M) are visible in this mosaic (a), but fall outside the Zamama–Thor mappingme) surrounding Reshef Patera (RP), as is visible inFig. 2c.

Icaru

iontical

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of aphicari-contry,ck o

ears inifica-ion,ral

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80 D.A. Williams et al. /

stereo DEM of this region was obtained using low-resolutE14 and C10 images. This DEM has comparable verresolution but at much lower spatial resolution (∼10 km).Both stereo DEMs are somewhat compromised by radiadamage to the images, which gives the DEMs a specappearance. Nonetheless, areas of high contrast are weresented in the DEM.

Additional high-resolution DEM data of the ZamamEruptive Center were derived from 2D photoclinometanalysis of the I32 mosaic (note that only the Zamamacan be so mapped due to the fact that sun angles deceastward across the I32 mosaic: seeSchenk et al., 2004,for a description). These data have a vertical precisionfew tens of meters, but are subject to increasing topograuncertainties over longer distances due to photometric vations. Unfortunately, the stereo data cannot be used totrol the longer wavelength uncertainties in photoclinomedue to the poor stereo data near Zamama related to lalocal scene contrast in the original images.

Although the data are relatively noisy, there do not appto be any major regional topographic trends or featureour DEMs of the Zamama–Thor region unrelated to specgeologic features (Fig. 8), namely, shield volcanoes or meslike plateaus. There are at least 3 mountains in the regalthough none lie within the I32 mapping mosaic. Sevevolcanic features are associated with broad topographicof 2–3 km relative height and slopes of no more than adegrees. These include the large fan-shaped set of darkflows of the Volund Eruptive Center due north of Zamamand the western small dark flow field of Arinna FluctuThese are probably analogous to the very low shield vonoes of the type described bySchenk et al. (2004). No majorrelief is apparent at the Thor Eruptive Center or the easdark flow fields of Arinna Fluctus (Fig. 8), but the DEM dataare relatively sparse and noisy in this region. This is duthe facts that the stereo images of Thor were obtained bethe fresh dark flows apparent in the I32 images formedthe region otherwise has low contrast.

Schenk et al. (2004)examined specific constructs in thZamama area using I24 and I32 imaging data. Theyscribed the two conical mountain-like structures westsouthwest of the Zamama flow field as steep-sided shvolcanoes with small summit pits and flattened crests, whwe hereafter refer to as Zamama Tholus A and ZamTholus B (Fig. 2c). Zamama Tholus A (18◦ N, 175◦ W)is ∼40 km wide and∼1.5 km high with average slopes o∼4◦ (Schenk et al., 2004), in which the central depressiocorresponds geographically to the central vent observeI24ZAMAMA02 with ∼15 radial flows (typically 20–50 kmlong and<5 km wide:Schenk et al., 2004) extending downthe slopes of the edifice. The central region within∼5–15 km of the summit has steeper slopes of 8–9◦ (Schenket al., 2004). Zamama Tholus B is also∼40 km wide and

∼1–1.5 km high(Schenk et al., 2004), but is the source ofonly one visible∼10-km-long, east-trending, narrow flowon its eastern flank.Schenk et al. (2004)suggested from their

s 177 (2005) 69–88

-

e

-

f

a

DEM analysis that Zamama Tholus A and B might be ctered on low (<500 m) broad rises with associated scathat are visible in the SSI images around their perimetA third feature, a small conical mound with a central pithereafter refer to as Zamama Tholus C, was identifiedSchenk et al. (2004)as being∼15 km across and∼250 mhigh, with no associated volcanic deposits.

North and east of the Zamama Eruptive Center arepaterae on mesa-like rises, Thomagata Patera and RPatera (Fig. 2). The mesa-like rise containing Reshefclearly visible in the center of the DEM (Fig. 8b), althoughThomagata Patera is not. Previous studies described Thgata Patera as a roughly kidney-shaped central depre56× 26 km in size and∼1.2–1.6 km deep, that is just weof center on top of a raised ovoid mesa>100 km acrosswith a western scarp (based on shadow measurementsis ∼200 m high(Turtle et al., 2004). Because it was thoughto lie on a structurally distinct rise,Jaeger et al. (2003)clas-sified Thomagata Patera as a mountain of “ambiguous mphology.”Schenk et al. (2001)did not include it as a mountain due to its low relief and because the bounding scairregular (scalloped) rather than linear, indicative of degration seen at scarps elsewhere on Io (e.g., Tvashtar Meeast and southeast scarps).

4. Material units and map production

From a thorough analysis of the variousGalileo dataof the Zamama–Thor region and a comparison topreviously-mapped Chaac–Camaxtli and Culann–Tohilgions, we have identified 11 material units in this region. Fdescriptions and interpretations of each are given inTable 3and color type localities are given inFig. 9. In all cases relative albedos were taken from the I32 mosaic and colors fthe C21 mosaic. The albedo, color and morphological difences were all combined to define and characterize maunits. In summary, at the scale ofGalileo SSI regional-resolution images (i.e., hundreds of meters per pixel), wedivide Io into essentially five types of materials: plains (ylow and white), patera floors (bright and dark), flows (brigdark, and undifferentiated), mountains (lineated and motlarge mountain blocks and smaller tholus structures),diffuse deposits (white, yellow, red, green, and black).all of these subunits are found in the Zamama–Thor reg(seeTable 3).

Bright plains materials cover more than 70% of Io’s sface (Geissler et al., 1999), and consist of yellow, graywhite, and (in polar regions) red to red-brown color unBright plains are thought to consist of the silicate upper cof Io, mantled by dark silicate and bright sulfurous expsive and effusive deposits from Io’s many active volcan(McEwen et al., 2000; Williams et al., 2002). In our map

area we have attempted for the first time to subdivide thebright plains materials into subunits based on color, shape,and textures. White bright plains (pbw) may be dominated

Mapping the Zamama–Thor region of Io 81

Fig. 9. Type examples of geologic units in the mapped area. Yellow bright plains material, pby; white bright plains material, pbw; dark patera floor material, pfd;ateriaor de

tm-

esteratotheili-

vely

dur-stillingma-2;nar-te-

ten-als

tede

zedarko-theeldainre-ce.ghtowstlsorticu-

bright patera floor material, pfb; mountain material, m; bright tholus mmaterial, dw; dark diffuse material, dd; red diffuse material, dr. SeeTable 3f

by coarse- to moderate-sized grains of SO2 snow(Carlson etal., 1997; Douté et al., 2001, 2002), whereas yellow brighplains (pby) may be dominated by other sulfur-bearing copounds (e.g., S8, SnO, and S2O) or SO2 mixed with thesecompounds(Hapke, 1989; Geissler et al., 1999).

Patera floor materials are found in (and sometimaround) these volcano-tectonic depressions. The pafloors span the full range of colors, from bright whiteyellow-orange to dark black. These colors suggest thatcompositions of patera floor materials include mixes of scates and various sulfurous compounds, including relatipure sulfur dioxide in some cases(Williams et al., 2002,2004). The processes that create this variety of materialsing paterae formation are necessarily complex and arenot well understood. Hypotheses include caldera-formcollapses, tectonic basins filled with lava, and the exhution of sills (Radebaugh et al., 2001; Williams et al., 200Keszthelyi et al., 2004). The spatial and spectral resolutioof Galileo SSI images is generally sufficient to enable chacterization of only two broad subunits of patera floor ma

rials (Table 3), bright (sulfur-dominated) and dark (silicate-dominated). Although there is evidence of lava flows in partsof some patera floors, there is sufficient ambiguity regarding

l, tb; undifferentiated flow material, fu; dark flow material, fd; white diffusescription. Illumination is from the southeast (lower right).

emplacement mechanisms in available imagery (e.g., potial role of lava lakes) to justify treating patera floor materias a separate material unit from flow materials(Williams etal., 2004).

Flow materials are typified by their generally elongamorphology (lengths� widths) and sharp contacts with thother units(Williams et al., 2002, 2004). Like the paterafloor materials, flow materials are generally characteriusing color and albedo as bright (sulfur-dominated) or d(silicate-dominated), although in the I32TERMIN02 msaic there are no unambiguous bright flows. Only inI24 highest-resolution mosaic of the Zamama flow fi(Fig. 4) are bright flows discernable southwest of the mflow field. Flow materials are interpreted to be thesult of one or more outpourings of lava onto the surfaAlbedo variations in the dark flows are generally thouto be indicative of age on the surface: the freshest flare the darkest (e.g.,McEwen et al., 1998a; Williams eal., 2002). Correlation between NIMS and SSI data ashows that the darkest materials appear to be active, pa

larly striking examples are seen in Emakong and RadegastPaterae(Lopes et al., 2004). Older flows have intermedi-ate albedos and colors and ill-defined contacts with other

its

or,es

ablerm

to

ern

he

82 D.A. Williams et al. / Icarus 177 (2005) 69–88

Table 3Descriptions and interpretations of geomorphologic material units found in the Zamama–Thor region of Io, based onGalileo data analysis

Unit R–G–B Color Description Interpretation

pby,yellowbrightplainsmaterial

243-157-205(pink)

Layered, textured surface, in various shades of yellow.Panchromatic albedo variation is considerable, but generallyintermediate between dark and bright patera floor materials.Plains near volcanic centers are mantled by various types ofdiffuse materials or contain superposed lava flows. Plainscontain scarps, grooves, pits, mesas, graben-like depressions,and/or channel-like features in some regions, in which scarpheights range typically from 50–100 m. At higher resolution,textured plains surface appears hummocky (i.e., periodic,km-scale mounds) and quite variable, from ill-defined on thestratigraphically lowest layers to prominently ridged on thestratigraphically highest layers (which tend to occur awayfrom active volcanic centers). No high-resolution images ofthe plains were obtained in this region, but near Chaac Pateraand Ot Mons individual hummocks were seen to be bright,irregular mounds in a matrix of darker, smoother material.

Silicate upper crust of Io mantled by (dominantly)sulfur-rich materials in brighter areas, with buried orpartly-buried silicate flows in darker areas. Surface deposformed by a combination of volcanic plume fallout (i.e.,diffuse deposits near specific vents such as Zamama, Thand Surya) and frost deposition (i.e., frozen sulfurous gasfrom indeterminate sources that condense out of Io’s variatmosphere, covering wide areas). Processes that may fothe hummocky texture include: (a) SO2 sublimation,(b) dunes deposited by pyroclastic activity(McEwen et al.,2000), (c) tectonic modification of crustal materials,(d) gravitational slumping (e.g.,Moore et al., 2001), (e) tidalworking of light surface materials (e.g.,Bart et al., 2004).Orientations of the hummocks have been shown to beconsistent with current tidal flexing(Bart et al., 2004).

pbw,white brightplainsmaterial

255-255-255(white, dashedborder)

Layered, textured surface, white to white–gray in color.Panchromatic albedo variation is considerable, but generallyintermediate between dark and bright patera floor materials.Hummocky texture and mantling may occur as describedabove.

Silicate upper crust of Io mantled by (dominantly) coarse-intermediate-grained sulfur dioxide (SO2) snow (seeDoutéet al., 2001) and perhaps other sulfur-bearing compoundsthat have been recrystallized from plume fallout.

pfd,dark paterafloormaterial

249-87-16(dk. orange)

Dark gray to black surface with some variation in albedo andtexture. Has distinct contact with surrounding terrain, and isfound in topographic depressions that occur within paterae. Insome regions at higher resolution (50 m/pixel) this materialappears smooth and dark, and often correlates with NIMS andPPR hot spots; highest-resolution images of this materialoccur in the floor of Chaac Patera(Williams et al., 2002)andshowed that this material can contain an interwoven mixtureof relatively bright and dark features, irregular hummocks,and pits. Usually little sulfur dioxide is present, based onprevious studies using NIMS data(Lopes et al., 2001).

Silicate lava flows that may or may not be coated bysulfurous materials (as indicated by brighter colors) orintermingled with various warmer silicate and sulfurousflows; black surfaces are likely warm, recently-emplaced,coalesced silicate lava flows, or crusted lava lakes(Lopes etal., 2001; Davies et al., 2001; Radebaugh et al., 2004).

pfb,brightpatera floormaterial

150-147-19(lt. orange)

Bright pinkish-white to red-orange unit with smooth surface athigh and medium resolution. Has distinct contact withsurrounding terrain, usually found within paterae but can alsooccur on or beyond their rims.Galileo NIMS data indicate anenhanced signature of sulfur dioxide in the white topinkish-white material on several paterae floors in otherregions.

White, yellow and orange surfaces containing significantamounts of impure SO2, requiring cold temperatures.Surfaces may be formed as coatings on cold silicate lavaflows in inactive paterae (e.g., Thomagata Patera), asprimary flows or ‘ponds’ of sulfur or SO2 (e.g., SW TohilPatera), or as melting or sublimating plains material beingremobilized by heat from intrusions (e.g., paterae in theChaac–Camaxtli region:Keszthelyi et al., 2004).

m,mountainmaterial

164-111-78(lt. brown)

Yellow to yellow-white unit that resembles bright plains, butthat occurs on topographically-elevated structures (e.g.,mesas, based on shadows). Unit is cut by a few grooves, butlacks the well-developed lineations found in lineatedmountain material on Tohil Mons in the Culann–Tohil region(Williams et al., 2002, 2003). Although there are severaldomes of bright material off of the northern scarp of the unit,there is no widespread mottled texture like that found inmottled mountain material on Tohil Mons (at availableresolutions). The bounding scarp is well-defined around mostof the unit (harder to detect on the SE side), although ithighest on the western face.

An uplifted crustal block (mountain) that is undergoingerosion on all sides. Although no faults are visible, theshadows visible in low-Sun images suggest that the westside is higher than the eastern side, supporting thehypothesis that it is a faulted crustal block.

tb,brighttholusmaterial

248-0-63(red)

Yellow-brown unit that resembles bright plains but that makesup elliptical raised edifices that are separated from thesurrounding plains by a distinct, steep scarp. Some edificesmay contain a circular central pit. Irregular radial grooves

Steep shield volcanoes (Zamama Tholi A, B, C) that are tsource for both dark (silicate) and bright sulfurous lavaflows. Thomagata Patera appears to be a shallow shieldvolcano.

occur along the edges of some edifices. The edifices may bepartially covered by various diffuse deposits.

(continued on next page)

to

sn

d/or

es.7

n

;

Mapping the Zamama–Thor region of Io 83

Table 3 (continued)

Unit R–G–B Color Description Interpretation

fd,dark flowmaterial

20-28-137(violet,Subunit 2)181-60-155(purpleSubunit 1)

At medium resolution, appears as smooth dark (black) lobateflows with lengths much greater than their widths. Contactswith surrounding terrain are sharp, and flows often extendtoward and into apparent topographic lows (as indicated bymapping of scarps). Variation in albedos and cross-cuttingrelations can be used to define age relationships in some casesto separate younger (fd2) from older (fd1) flows. High-tempe-rature hot spots usually correlate with dark flows as has beennoted in NIMS and PPR observations(Lopes et al., 2001;Williams et al., 2002)both inside and outside paterae. Forexample, the Zamama hot spot in NIMS data appears tocorrelate with the prominent∼100-km long dark flow field.

Lava flows of warm silicate materials from mafic orultramafic silicate eruptions (seeMcEwen et al., 1998b;Williams et al., 2000). Range of albedos in dark flows duevariation in lava composition, coating of flow surfaces bysulfurous pyroclastic materials or condensates, or othereffects of aging in the ionian environment.

fu,undifferen-tiated flowmaterial

100-197-219(lt. blue)

Terrain consisting of bright and dark flows with a range ofalbedos. Contacts are not distinct, making it difficult todistinguish individual flow units.

Lava flows of indeterminate type, mostly likely from variouVoyager-era or earlier silicate effusive eruptions, older thamaterials defined as fd. Range of albedos due to coating offlow surfaces by pyroclastic materials or condensates, anthe effects of aging in the ionian environment.

dd,dark diffusematerial

Two-sizedspotting

Dark brown to black unit that appears to thinly mantleunderlying materials, occurring mostly around fresh darkflows in the Thor Eruptive Center.

Explosively-emplaced pyroclastic deposits rich in mafic toultramafic silicates, and possibly black sulfur in some casSpectral analysis of the dark diffuse deposit from the 199Pillan eruption is consistent with Mg-rich silicates(Geissleret al., 1999).

dw,whitediffusematerial

NW-SEdiagonalhatching

White unit that appears to thinly mantle underlying materials,occurring mostly as radial streaks around parts of the Zamamaflow field, patches in the Thor eruptive center, and as astreamer adjacent to white plains material east of Zamama.This unit may also occur as a mantle that slightly obscures thetexture in other parts of the bright plains; however, if present itis so thin that we cannot confidently map its boundaries andthus have not included it on the map.

Explosively-emplaced pyroclastic deposits, probablydominated by sulfur dioxide. Volatiles may come fromsurficial deposits or near-surface “aquifers” of SO2 that areremobilized by nearby volcanic heat sources (as has beeinferred to produce the radial white streaks around thePrometheus flow field:Kieffer et al., 2000; Milazzo et al.,2001), or from condensation of SO2 gas from various plumeeruptions.

dr,red diffusematerial

NE-SWdiagonalhatching

Dark red to red-brown unit that thinly mantles underlyingmaterials, occurring primarily as an asymmetrical depositwest of the Zamama Eruptive Center and with decreasingoptical depth with increasing distance from the center.

Explosively emplaced pyroclastic deposits rich inmetastable, short-chain sulfur polymers (S3 and S4: Spenceret al., 2000a) and/or sulfur-bearing chlorides(Schmitt andRodriguez, 2003). The sulfur allotropes or chlorides couldact as colorizing contaminants within bright, transparentmaterial such as SO2 (Geissler et al., 1999). Source appearsto be primary magmatic S2 or Cl2 gas(Spencer et al., 2000aKeszthelyi et al., 2001; Schmitt and Rodriguez, 2003)coming from the Zamama vent.

repre units in

eri-

dosis-rps,un-ight1;

ac-ain-tec-s),

cts,ponwetivenon-

. Itma-

eric

ngag-ol-dis-

SeeFig. 9 for type localities in color. R–G–B color values in Column 2Fig. 10. Modified fromWilliams et al. (2002, 2004).

units, which are defined as undifferentiated flow matals.

Mountain materials generally have colors and albesimilar to bright plains materials, and are often only vible in low-Sun images where shadows highlight scaridges, grooves, and mountain peaks. About 150 motains have been identified on Io between 1–18 km in he(see Schenk and Bulmer, 1998; McKinnon et al., 200Schenk et al., 2001; Turtle et al., 2001; Jaeger et al., 2003). Inour previous mapping of the Culann–Tohil region we charterized three types of mountain materials: lineated (conting well defined ridges and grooves, interpreted to betonically modified, uplifted, autochthonous crustal block

mottled (containing lobes and dome-like hills, interpretedto be indicative of displacement of crustal materials bymass wasting processes), and Tholus (nontectonically de

sent R–G–B values used in Adobe Illustrator to produce colors of map

rived domical edifices, interpreted to be volcanic construprimarily shield or composite volcanoes depending umorphology and slopes of the edifices). In this regionhave mapped several tholi related to the Zamama ErupCenter and Thomagata Patera, as well as an apparentlyvolcanic, low rising mountain north of Reshef Pateracontains aspects of both lineated and mottled mountainterials, but is dominated by neither, justifying a more genclassification, mountain material, labeled unit “m.”

Diffuse deposits appear to thinly mantle underlyitopography in a manner characteristic of fine-grained frmental material, and typically occur on or near active vcanic centers. On Io, mappable diffuse units occur in five

-

tinct colors: yellow, white, black, red, and green(Williamset al., 2004). As discussed in Section2.2, these colors areinterpreted to be diagnostic of the dominant chemical con-

Icaru

inlfurnlynted

apsen

fea-ionscludsheefin

eshet the

ma-

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84 D.A. Williams et al. /

stituent: sulfur, sulfur dioxide, silicate, either short-chasulfur and/or sulfur chlorides, and products of silicate–sualteration, respectively. In the Zamama–Thor region, owhite, dark, and red diffuse deposits are clearly represein theGalileo data.

Because theGalileo SSI image we are using as a basemwas taken under low-Sun conditions, shadows are prethat can be used to identify a wide range of structuraltures, including scarps, grooves, ridges, and depressThe features that can be recognized on our basemap invarious caldera-like depressions at Thomagata and RePaterae (and between Reshef and Thor), scarps that dovoid hills at Zamama(Schenk et al., 2004)and ThomagataPatera, as well as an irregular mesa that surrounds RPatera, and a variety of ridges and grooves throughoumap area.

5. Mapping results

We have produced a geomorphologic map of the Za

ma–Thor region, based on theGalileo SSI mosaic I32TER-

Fig. 10. Geomorphologic map of the Zamama–Thor region, including corr

s 177 (2005) 69–88

t

.efe

f

is difficult to assess relative ages for some units, as wetime marker horizons (i.e., marker beds). Thus, all surfathat are mapped as the same geologic unit did not neceily form at the same time (as indicated by our stratigrapcorrelation). In each of the following sections we discspecific aspects of the geology of the Zamama–Thor reas inferred from our map and supporting data.

5.1. Zamama

Our mapping of the Zamama red diffuse deposit aprominent dark flow field indicates that they both emanfrom Zamama Tholus A, a central vent volcano. Howevthe white diffuse plume seen in some SSI observat(Fig. 3c) is centered on the dark flow field, indicating thit has a separate source. A similar situation, with a redfuse deposit at the primary vent and a bright plume centon a large lava flow field is seen at Prometheus(Keszthelyi etal., 2001). What is different about Zamama is the preseof small bright lava flows (Fig. 4c), presumably composelargely of sulfur, also emanating from or near the cen

vent. It may be that it is the sulfur-rich materials that are

port

MIN02 (Fig. 10). We have also derived a general stratigra-phy of the map units. However, it is important to note that it

the dominant shield-building element here.Clow and Carr(1980)suggested sulfur would have the strength to sup

elation of map units. Basemap is SSI observation I32TERMIN02, 360 m/pixel.

ama–

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Mapping the Zam

mountains at least�1 km high, which is close to the measured heights of the Zamama Tholi. The silicate lavasIo are thought to be ultramafic with very low viscositi(Williams et al., 2000), so they are poor candidates for costructing volcanic edifices. Nevertheless, Zamama is clea complex volcanic system, with at least two active cenconstructs associated with effusive and explosive volcaninvolving silicate and sulfurous materials. Dark flows aassociated with both Zamama Tholi A and B (bright floare also apparently associated with Tholus A), and plucontaining different volatiles are associated with ZamaTholus A, the central part of the flow field, and with the flofronts.

5.2. Thomagata and Reshef Paterae

Although no hot spots or volcanic surface changes hbeen detected at either Thomagata or Reshef Pateraemapping has provided insight into the nature of these sttures. Our analysis of the scarps at Thomagata suggestthey fit the Type A classification ofMoore et al. (2001)(scal-loped margins linked into adjoining alcoves with no terring and little or no clearly-visible debris downslope). Thtype of scarp is interpreted byMoore et al. (2001)to havebeen caused by SO2 sapping, plastic deformation of SO2,sulfur “ice” disaggregation by chemical decompositionS2O and polysulfur oxides(Hapke, 1989), or some type ofsolid-state creep. The shape of the mesa-like rise suggeus that it be classified as a large tholus, i.e., dormant volcmountain.

Turtle et al. (2004)noted that Reshef Patera consistsan irregular central depression∼60× 35 km in size, with amargin that is taller on the south than on the north (∼1.6 kmvs ∼1.3 km). Our mapping shows that Reshef is locateda roughly triangular-shaped mesa (also visible inGalileoC3 images). This irregular mesa appears to be undergdegradation driven by multiple magma sources, as exemfied by two dark depressions north of Reshef Patera (Fig. 2c).One of the depressions is forked, whose floor is covereddark material (unit fd2, seeFig. 10), in which one fork trendsE–W and the other trends NW–SE bending to N–S at its eAt the point where the two forks meet, the depression ein a south-trending groove. North of this forked depressis a portion of the mesa with a wider shadow on its wesface (Fig. 2c) than other parts of the mesa (shadow measments imply a height of∼350 m for this section, relativto 110–190 m to the sections just south, consistent withalbeit noisy results of the stereo analysis). It appears tseparating from the rest of the mesa, and because it apto stand higher than the rest of the mesa, we have mappas mountain material, although it could simply be a thicremnant section of the mesa. Multiple level or stepped mare observed elsewhere on Io.

There is at least one additional small, dark patera withinthis mesa. Our stereo topography indicates that this smalldark spot just 50 km north of Reshef, which appears to have

Thor region of Io 85

r

t

o

st

a low inward-facing scarp, is at the top and center of a brrise roughly 3-km-high lying astride the northeastern flaof the Reshef mesa complex (Fig. 8b). Several small darkflow-like features radiate from this patera, suggesting this also a low shield volcano. This shield volcano appearpost-date the mesa (and partly obscures its northeast flbut may predate or be contemporaneous with Reshef Pas dark flows from the two patera do not overlap.

When we discuss the degradation of Reshef’s mesaplateau, we mean melting or sublimation of S- and possSO2-rich crustal materials from the heat of the underlysilicate magmas associated with the dark features mentiabove. The deeper, SW end of Reshef Patera, the forkepression to the NW, and a smaller, dark patera to the nall contain dark materials indicative of recent silicate efsions, the heat from which would erode into the surroundsulfurous-rich plateau. Also, the triangular mesa mappemountain material north of the forked depression is cut bdark, NW–SE-trending groove which might be a fissuredark lavas that would continue the degradation of this mAlthough the limited resolution of existingGalileo data pro-hibit further analysis of the erosive mechanisms that maacting on these features, the proposed mechanisms fordegradation mentioned earlier (SO2 sapping, plastic deformation of SO2 or sulfur “ice,” disaggregation by chemicdecomposition of S2O and polysulfur oxides, or solid-stacreep:Moore et al., 2001) are all plausible.

5.3. Thor

Our mapping results support the hypothesis ofTurtle etal. (2004)that the newly-emplaced Thor dark flows from tI31 event (Figs. 2c, 10) correlate with the locations of olbright flows in the C21 color mosaic (Fig. 2b), suggestingthat the Thor Eruptive Center was active prior to theVoyagerflybys (i.e., no changes at this location were observed pto the I31 eruption). Thor is either a source of renewedcate volcanism that has covered former dark flows that wlong ago buried by bright pyroclastic materials, or Thora site where volcanic activity has changed from sulfurmagma producing bright flows prior to the C21 flyby (Ju1999) to silicate magma producing dark flows and darkroclastics during and/or after the I31 eruption. This laalternative would be consistent with the new hypothesisKeszthelyi et al. (2004), in which new paterae can form bexhumation of sulfurous materials mobilized by heat fran underlying silicate sill. Although the Thor vent appeto be located on a local rise, it is unclear if a larger pera is in the process of forming. There are, however, sevsmall paterae near the main Thor vent, including one wa recently (i.e., post C21) darkened floor. The NIMS dcovering Thor from orbit I31 show two hot spots (Thor aI31B) distributed across these smaller dark flows and pat

(Lopes et al., 2004, Fig. 5), suggesting that their magmaticplumbing systems are interlinked. We have mapped dark dif-fuse material around Thor from the I32 mosaic (Fig. 2c),

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which is also visible in the I31 global color image (Fig. 5),indicative of an explosive silicate component to the I31 evat Thor.

6. Discussion

As we mentioned earlier, both explosive and effuseruptions were observed and their corresponding depwere mapped at both the Zamama and Thor Eruptive Cters, and both of these centers produced large-scale suchanges either since theVoyager flybys (1979) or during theGalileo mission (1996–2001). Although there are no lavisible bright flow fields in this region, the high-resolutiomosaic of the Zamama flow field (Fig. 4) shows sinuousbright flows to the south, indicative of small-scale, perhsecondary, sulfur flows(Greeley et al., 1984; Williams et al2001). Thus, it is likely that both sulfurous and silicate lacompositions have been erupted in the Zamama–Thogion, consistent with our mapping of other regions of Iantijovian hemisphere(Williams et al., 2002, 2004). It is in-teresting to note that the Zamama magma source has atonce interacted with a S- and/or Cl-rich volatile sourceproduce the red diffuse deposit), whereas the Thor masource that produced the large, SO2-rich plume eruption didnot (i.e., no red ring deposit was produced as is typicamost but not all pillanian eruptions). This occurrence sports our contention(Williams et al., 2002, 2004)that thereis a complex interaction between silicate materials and vous volatiles in the crust and/or on the surface of Io, incling SO2 (Smythe et al., 1979), sulfur(Spencer et al., 2000a,and chlorine(Schmitt and Rodriguez, 2003). Even at a givengeographic location on Io, a volcanic vent may eruptcombination of explosive and effusive styles and silicatesulfurous magmas, with entrained sulfur, SO2, chlorine, orperhaps other volatiles, such that one eruption can be todifferent from the next.

7. Implications

A comparison of our Zamama–Thor map with thoof the Chaac–Camaxtli(Williams et al., 2002, 2003)andCulann–Tohil(Williams et al., 2004)regions demonstratehow 5 primary types of geologic material units (plains, pera floors, flows, mountains, and diffuse deposits) aretaposed on Io to create wildly different geologic regioThese differences are due to the relationship betweencanic and tectonic activity that occurs at and below Isurface, which can vary within a few hundred kilomet(i.e., the distance separating these mapped regions).haps the greatest source of variability in our maps isto the presence (or lack thereof) of an active volcanic c

ter or centers that resurface themselves in geologically shortime intervals. For example, both Zamama and Thor havestrongly affected their surroundings in this region, whereas

s 177 (2005) 69–88

e

st

-

in the Chaac–Camaxtli region there is not an equally dinant volcano that causes frequent resurfacing. AsGeissleret al. (1999, 2004)noted, the bulk of the surface changesIo due to volcanism are centered around a few very acvents, in which large areas of the surface (83%) have hadiscernable changes during theGalileo mission. The impli-cation of this fact is that volcanism on Io is dominated by dcrete plumbing systems that penetrate the crust at thesvery active vents (e.g., Pele, Tvashtar, Prometheus), sof which may be active for relatively long timespans. Aother implication relates to global mapping of Io, suggesa mapping approach centered on formations, i.e., sets olated geologic units focused on a specific source of act(patera, fluctus, mountain, etc.).

It seems clear that to truly understand the interrelavolcano-tectonic activity on Io from a global perspective wrequire global geologic mapping that integrates allGalileoandVoyager data (the latter of which has higher spatial relution coverage of the subjovian hemisphere). Such a glmap will serve as a framework for the continuing analyof Galileo data, will be a useful tool for correlating contining ground-based telescopic observations of Io with knosites of active volcanism, and will be a key element in plning any future spacecraft observations of Io from upcommissions. At present, only theNew Horizons/Pluto Jupiterflyby (which includes Io observations) is tentatively schuled (February 2007 assuming a February 2006 launch).next mission to the jovian system (the Jupiter Icy Moonsbiter, JIMO) has been deferred until late in the next deca

8. Conclusions

Using data from theGalileo spacecraft throughout it∼8 year tour of the jovian system, we have produced aomorphological map of the Zamama–Thor region of Iantijovian hemisphere. This map is the third of threegional maps produced to assess the variation in volcand tectonic surface features and the interrelationshipscan be identified using planetary mapping techniques.like the Chaac–Camaxtli and Culann–Tohil regions thatpreviously mapped, the Zamama–Thor region has been dinated by two sites of large-scale surface changes. Bothplosive and effusive eruptions occur at the Zamama ErupCenter (which formed sometime after theVoyager flybys),which are centered on a relatively steep edifice (ZamTholus A). The dominant effusive style at Zamama appeto be promethean, which is characterized by relatively(<200 km high) SO2 plumes and slowly-emplaced, inslated compound flow fields, and flow-front SO2 plumes.There is also evidence for sulfur flows from Zamama. Tred pyroclastic deposit at Zamama indicates that S and/ohave been incorporated occasionally into the plume ema

ting from the lava source vent.

The largest plume observed on Io to date (at least 500 kmhigh) erupted from the Thor Eruptive Center during the

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August 2001 flyby (I31). This large pillanian-style plumeruption produced a central dark (presumably silicate-rpyroclastic deposit and an outlying white diffuse ring (psumably SO2-rich), but no large red ring deposit, suggesta lack of S and/or Cl volatile interaction with the magmThe Thor vent area may have been active prior to Voyaand mapping shows that several of the dark lava flowsformed in 2001 around the vent region have reoccupiedof earlier bright flows, indicating a long interval betweeruptions or a change in magma composition from sulfusilicate.

The central part of the mapped region is dominatedtwo paterae, Thomagata and Reshef. Thomagata Patervolcanic crater that is located on a large shield-like mesashows no signs of activity. Reshef Patera is also a volccrater, but is located on a large, irregular mesa that is ungoing degradation through erosion from multiple secondvolcanic centers.

Acknowledgments

The authors would like to thank the many members ofGalileo spacecraft and instrument teams for many yearhard work that kept this mission running for 26 years, aensured the return of 8 years of great data from the josystem. This paper benefited from helpful reviews by SRowland and Elizabeth Turtle (twice). This work was suported by a NASA Planetary Geology and Geophysics gto R.G.

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