Impact Features on Europa: Results of the Galileo Europa ...the distribution of Pwyll secondaries. MORPHOMETRY OF EUROPAN IMPACT CRATERS Although Europa’s surface is lightly cratered

Icarus151, 93–111 (2001)

doi:10.1006/icar.2000.6558, available online at http://www.idealibrary.com on

Impact Features on Europa: Results of the Galileo Europa Mission (GEM)

Jeffrey M. Moore

NASA Ames Research Center, MS 245-3, Moffett Field, California 94035E-mail: [email protected]

Erik Asphaug

Earth Sciences Department, University of California—Santa Cruz, Santa Cruz, California 95064

Michael J. S. Belton

Kitt Peak National Observatory, 950 North Cherry Avenue, Tucson, Arizona 85726

Beau Bierhaus

Southwest Research Institute, 1051 Walnut Street, Suite 426, Boulder, Colorado 80302

H. Herbert Breneman

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

Shawn M. Brooks and Clark R. Chapman

Southwest Research Institute, 1051 Walnut Street, Suite 426, Boulder, Colorado 80302

Frank C. Chuang

Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287

Geoffrey C. Collins

Department of Geological Sciences, Brown University, Providence, Rhode Island 02912

Bernd Giese

DLR, Institute of Planetary Exploration, Rudower Chaussee 5, 12489 Berlin, Germany

Ronald Greeley


James W. Head III


Steve Kadel


Kenneth P. Klaasen


James E. Klemaszewski


93

0019-1035/01 $35.00Copyright c© 2001 by Academic Press

All rights of reproduction in any form reserved.

94 MOORE ET AL.

Kari P. Magee


John Moreau


David Morrison

NASA Ames Research Center, MS 200-7, Moffett Field, California 94035

Gerhard Neukum

DLR, Institute of Planetary Exploration, Rudower Chaussee 5, 12489 Berlin, Germany

Robert T. Pappalardo


Cynthia B. Phillips

Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721

Paul M. Schenk

Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058

David A. Senske


Robert J. Sullivan

Cornell University, 308 Space Sciences Building, Ithaca, New York 14853

Elizabeth P. Turtle

Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721

and

Kevin K. Williams


Received May 12, 2000; revised October 6, 2000

During the Galileo Europa Mission (GEM), impact features onEuropa were observed with improved resolution and coverage wascompared with Voyager or the Galileo nominal mission. We sur-veyed all primary impact features >4 km in diameter seen on Eu-ropa (through orbit E19). The transition from simple to complexcrater morphology occurs at a diameter of about 5 km. We calcu-lated the transient crater dimensions and excavation depths of allcraters surveyed. The largest impact feature (Tyre) probably had atransient crater depth between 5 and 10 km and transported mate-rial to the surface from a depth of not greater than ∼4 km. Craters<30 km in diameter, such as Manannan and Pwyll, formed withintargets whose immediate subcrater materials exhibited nonfluid be-havior on time scales of the impact event, and that are capable,especially in the case of Pwyll, of supporting significant local to-

pographic loads such as a central peak. These craters are neverthe-less quite shallow, with very subdued floors, and we suspect thatManannan and Pwyll’s small depth-to-diameter ratios are due tothe isostatic adjustment of large-scale topography, facilitated bywarm, plastically deformable ice at depth. Morphological similar-ities between Callanish and Tyre strongly imply that conclusionsreached regarding Callanish in J. Moore et al. (1998, Icarus 135,127–145) also apply to Tyre, which was that Callanish is the conse-quence of impact into target materials that are mechanically veryweak at depth. New evidence that Callanish’s circumferential ringsformed before the proximal ejecta became immobile implies a low-viscosity substrate at the time of impact. We also report additionalevidence that a component of the proximal ejecta of Callanish wasemplaced as a fluid. Our observations of Pwyll secondaries supportthe conclusions stated in Alpert and Melosh (1999) that impacts

IMPACT FEATURES ON E

on icy bodies eject smaller fragments and that fragment size de-creases more gradually as velocity increases than observed for im-pacts on silicate bodies at equivalent ejection velocities. Examina-tion of Pwyll’s secondary craters reveals azimuthal variations, withejecta fragment sizes being larger near the center of a ray than offthe ray. Our initial analysis of the characteristic size distribution ofPwyll’s secondary craters shows that they form a differential slopeslightly shallower than−4. Similar steep slopes for small craters onGanymede imply that small craters there are mostly formed bysecondary impact, and the jovian system may thus be deficient insmall impacts relative to the environment of the terrestrial planets.c© 2001 Academic Press

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Key Words: Europa; cratering; satellites of Jupiter.

INTRODUCTION

The Galileo Orbiter, during theGalileo Europa Mission(GEM) phase of its mission, has observed a number ofpact features on Europa with improved resolution and coage compared withVoyagerand theGalileo nominal mission.Observations during theGalileonominal mission (Mooreet al.1998) revealed two basic types of impact features: (1) “clascomplex impact craters that grossly resemble well-preservenar craters of similar size but are more topographically subd(e.g., Pwyll); and (2) very flat circular features surroundedconcentric ring troughs that lack many typical geomorphic ftures of impact craters (e.g., raised rim, bowlshaped deprefloor, central peak), and which owe their identification as impfeatures largely to the field of secondary craters distributed rally around them (e.g., Callanish). Impact simulations sugthat features such as Callanish and Tyre would not be prodby impact into a solid ice target, but might be explained bypact into an ice layer of order of 10 km thick overlying a loviscosity material. Mooreet al.(1998) speculated that this lowviscosity material might be a global water layer.

This paper discusses analyses of europan impact featureserved in SSI (Solid-State Imager) data obtained during Gand focuses on: (1) a survey of all primary impact features son Europa>4 km; (2) the ejecta deposits of certain impact fetures, principally Cilix, Manannàn, and Tyre; (3) the “transitional” impact feature Tegid; (4) the implications of the toporaphy of Manannàn, Pwyll, Cilix, and Rhiannon; and (5) thdistribution of energy in impacts into icy targets as revealedthe distribution of Pwyll secondaries.

MORPHOMETRY OF EUROPAN IMPACT CRATERS

Although Europa’s surface is lightly cratered compared wmany other surfaces in the Solar System, the observed imcraters span a wide range of sizes and exhibit a variety of mphologies (Fig. 1). Crater morphology is influenced by theget’s material properties and near-surface structure. For ex
ple, (1) the minimum diameter at which complex craters fordepends upon the strength of the target material (e.g., Melo
UROPA: GEM RESULTS 95

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1989, pp. 144–150; Schenk, 1991); (2) Europa’s craterspear to be anomalously shallow compared to similarly scraters on other solid-surfaced bodies, which may be dupost-impact isostatic adjustment; and (3) the rings aroundtwo largest impact structures may have formed as a resuEuropa’s near-surface structure (e.g., Turtleet al. 1999). Ex-amination of europan craters reveals much about the natuEuropa’s near subsurface. To fully understand an impact eand the implications of the resulting crater morphology, inecessary to know the dimensions of the transient crater thexcavated upon impact. We have measured the diameterseuropan craters observed through the 19th orbit (E19) thalarger than 4 km (Table I). Along with final crater diameteTable I lists estimates of transient crater diameters and exction depths.

Crater diameters were measured using SSI images ographically reprojected around each crater’s center so that ving geometry would not affect the crater’s apparent shape. Ecrater diameter was determined from a circle fitted to sevpoints selected along the crater rim. The resulting diameterstheir RMS errors are listed in Table I. In some cases the Rerror is less than the resolution of the image because the imwas interpolated to a larger pixel scale. Two of Europa’s impstructures, Callanish and Tyre, have no obvious raised rimdifferent methods were used to estimate their diameters. Iedge of the concentric massifs corresponds to the final crim, Callanish’s diameter is 29 km and Tyre’s is 44 km. Aother method involves scaling from the edge of the continuejecta blanket using the relation derived by Joneset al. (1997)for palimpsests on Ganymede, and this predicts final rim dieters of 35 km for Callanish and 43 km for Tyre. Finally,upper limit on Callanish’s diameter can be determined fromlocations at which two preexisting ridges were truncated byimpact. Fitting a circle through these points results in a diamof 47.4± 1.0 km, yielding a result similar to that reportedMooreet al. (1998). Unfortunately, there are no obvious preisting features at Tyre upon which to base such an upper lFor these two craters we used the full ranges of final crateameter estimates in calculating the dimensions of the trancrater and excavation depths.

McKinnon and Schenk (1995) derived the following relatioship between final and transient crater diameters for Ganymwhich has similar gravity and crustal composition, basedvolume conservation during crater collapse,Df ∼ 1.176D1.108

tr ,whereDf is the diameter of the final crater andDtr is the diam-eter of the transient crater. They estimate that this relationis accurate to 15%. The ranges of transient crater diametersdicted by combining our fits’ RMS errors with the scaling retion’s uncertainty are listed in Table I. Craters in ice may undepost-impact isostatic adjustment that modify the dimensionthe final crater (e.g., Melosh 1989, pp. 154–161). However,calculated ranges are consistent with those predicted bas
msh,observations that indicate terrestrial transient crater diametersare 50 to 65% of their final crater diameters (e.g., Melosh 1989,

96
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IMPACT FEATURES ON EUROPA: GEM RESULTS 97

TABLE IEuropan Crater Diameters

Image Transient TransientOrbit Lat, long Diameter of circle RMS error resolution crater Crater depth Excavatio

Crater (or mission) Image no. (◦N, ◦W) fit to rim (km) of fit (km) (km/pixel) diameter (km) (km) depth (km)

Cormac E17 s0466676852.4 −37, 86 4.09 0.07 0.22 2.57–3.59 0.64–1.19 0.21–0.40Deirdre E17 s0466664352.1 −65, 208 4.25 0.03 0.22 2.69–3.69 0.67–1.22 0.22–0.41Niamh E14 s0440949842.1 21, 217 4.44 0.09 0.03 2.76–3.88 0.69–1.28 0.23–0Balor E17 s0466676913.5 −53, 94 4.47 0.05 0.22 2.81–3.88 0.70–1.28 0.23–0.43Angus E17 s0466676752.4 −13, 74 4.58 0.08 0.22 2.86–3.99 0.71–1.32 0.24–0.44Camulus E17 s0466676800.8 −27, 79 4.78 0.06 0.22 2.98–4.12 0.74–1.36 0.25–0.45Gwydion E17 s0466677052.7 −61, 79 4.90 0.07 0.23 3.04–4.23 0.76–1.40 0.25–0.47Dylan E17 s0466677052.5 −55, 82 5.06 0.14 0.23 3.10–4.40 0.77–1.45 0.26–0.48Aine E17 s0466665378.1 −43, 177 5.37 0.06 0.18 3.31–4.58 0.83–1.51 0.28–0.50Oisin E17 s0466664340.2 −52, 214 5.90 0.05 0.22 3.62–4.96 0.90–1.64 0.30–0.55Uaithne E17 s0466676880.2 −49, 88 6.42 0.12 0.25 3.87–5.41 0.97–1.80 0.32–0.60Diarmuid E17 s0466676914.2 −61, 97 7.85 0.21 0.22 4.60–6.53 1.15–2.16 0.38–0.72Brigid E15 s0449974265.2 11, 80 8.53 0.21 0.23 4.97–7.03 1.24–2.32 0.41–0Avagddu E14 s0440984865.7 1.2, 170 9.80 0.15 1.44 5.68–7.90 1.42–2.61 0.47–0Govannan E4 −38, 303 10.36 0.29 1.22 5.90–8.41 1.48–2.77 0.49–0.9Grainne E17 s0466676914.2 −60, 95 13.53 0.20 0.22 7.60–10.57 1.90–3.49 0.63–1.1Math E14 s0440955226.1 −26, 183 13.99 0.34 0.23 7.77–10.98 1.94–3.62 0.65–1.2Rhiannon E17 −81, 197 15.42 0.32 0.04 8.51–11.96 2.13–3.95 0.71–1.3Morvran Voyager 2 −6, 152 16.82 0.92 1.00 8.92–13.31 2.23–4.39 0.74–1.4Cilix E15 2.6, 182 18.36 0.70 0.03 9.80–14.21 2.45–4.69 0.82–1.5Amergin E17 s0466664600.1 −14, 230 18.56 0.35 0.21 10.08–14.10 2.52–4.65 0.84–1.5Maeve E15 s0449974426.3 58, 75 20.44 0.23 0.24 11.07–15.29 2.77–5.05 0.92–1Manannan E14 3, 240 21.77 0.83 0.22 11.43–16.57 2.86–5.47 0.95–1.8Pwyll E6 −25, 271 23.7 1.8 0.27 11.94–18.49 2.99–6.10 1.00–2.0Taliesin Voyager 2 −23, 137 27.7 1.7 0.75 13.88–21.01 3.47–6.93 1.16–2.3Tegid E14 0.5, 164 28.43 0.61 1.44 14.77–20.78 3.69–6.86 1.23–2.2Callanisha E4 −16, 334 29-47.3 1.0 0.12 15.34–32.94 3.69–10.87 1.23–3.6Tyrea E14 34, 146 43-44 — 0.17 22.34–30.23 5.59–9.98 1.86–3.3

Note.Craters observed through orbit E19 that have diameters larger than 4 km are listed in order of increasing size. Names are specified for thosewhich names have been assigned. Italics indicate proposed, but as yet unofficial, names. TheGalileo orbit (or other mission) during which the image we used tomeasure crater diameters was taken is specified, as well as the image number for those craters without official names. Diameters determined from a circe fit to thecrater rim are listed in the third column (See Footnotea) and the fits’ RMS errors are given in the fourth. (These errors can be less than the image resoldue to resampling of the image.) The sixth column lists the ranges of transient crater diameters calculated using Eq. 1 from (McKinnon and Schenk 1andincorporating both the RMS error of the fit diameters and the 15% uncertainty in the scaling relation (McKinnon and Schenk 1995). The seventh colutsranges of transient crater depths calculated from the ranges of transient crater diameters based on the observation that transient crater depths aretypically 1/4 to1/3 their diameters (e.g., Melosh 1989, p. 78). The final column lists ranges of excavation depths (i.e., the maximum depth from which material can be eained
into the ejecta) calculated using the experimental result that the excavation depth is roughly 1/3 the transient crater depth (e.g., Melosh 1989, p. 78).
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aRanges of final crater diameter estimates are given for Callanish and T

p. 138). Therefore, we assume that the effect of such isosadjustment on the scaling relation is negligible (but this doesimply that isostatic adjustment itself is negligible).

The transition between simple bowl-shaped europan craand more complex morphologies occurs at a diameter of appimately 5 km. Craters with diameters of 6 km already exhicentral peaks, but several 5-km-diameter craters do not (FigSimple craters probably formed from transient cavities that wtoo small to undergo crater collapse. It should be noted thatsimple craters McKinnon and Schenk’s scaling relation undestimates the transient crater diameters. Transient crater dewere calculated from transient crater diameters in Table I, ba
on observations that the depth of a transient crater is 1/4 to 1/3 itsdiameter (Melosh 1989, p. 78). The larger craters Cilix,“Maev
re, which do not have obvious rims.

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(proposed name), and Pwyll appear to exhibit central peakscentral peak complexes); therefore, if there is a liquid walayer on Europa, the overlying ice must be sufficiently thick ththe disrupted regions around∼10–18 km diameter,∼3–6 kmdeep transient craters do not penetrate it completely. As nin Moore et al. (1998), this suggests that the ice must beleast several kilometers thick. The calculated depths of transcavities for the largest diameter values for Callanish and Tare∼10–11 km, which perhaps may represent the depth tfluid-rich region if their morphologies are a manifestationsuch a region (Mooreet al.1998).

Ranges of excavation depths (the maximum depth from wh

e”material is ejected from the crater cavity) were also calculatedusing the experimental result that the excavation depth is∼1/3

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98 MOORE

FIG. 2. A ∼4.4-km-diameter crater provisionally named Niamh, locatat (21◦N, 217◦W). Craters of this size do not have central peaks, although othonly a couple of kilometers larger do, implying that∼5 km marks the simple-to-complex transition in europan crater morphology. To first order, Niamhindistinguishable from other craters of this size seen elsewhere on airlessatellites. (Portion of image PICNO 14E0027. The view is oblique. Illuminatis high and from the upper left. North is to the right.)

the transient crater depth (e.g., Melosh 1989, p. 78). In genthe excavation depths are quite small; using the largest posdiameter estimate for Callanish, the maximum depth from whejecta can originate is only 3.6 km. This implies that the souof the dark, red material observed around some europan cris within a few kilometers of the surface, which may have implications for the red material’s origin.

INDIVIDUAL IMPACT FEATURES

Cilix

Although the characterization of an impact crater is relativstraightforward, sometimes its recognition is not. The impcrater Cilix provides an excellent example of how image resotion, lighting conditions, and surface albedo influence interptation. Based on∼2 km/pixelVoyagerdata, the 15-km-diameterroughly circular feature located at 2◦S, 180◦W was interpretedto be an impact crater (Smithet al. 1979), and was given thename Cilix. In 1996, Cilix was imaged by theGalileo SSI at1.6 km/pixel under moderate-Sun illumination (incidence an∼53◦). Based on these data and a stereo complement imagquired byVoyagerat similar resolution, Cilix was interpreteas a positive-relief feature, perhaps a dome or flat-topped bstanding 1.0± 0.5 km above the plains, surrounded by a daannulus (Beltonet al. 1996). The positive relief of Cilix was
observed again when the feature was imaged just beyondterminator, with glancing sunlight illuminating an indistinct cir
ET AL.

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cular form barely extending above the local horizon. DuringGalileo Europa Mission an opportunity arose on the 15th or(E15) for Cilix to be imaged at higher resolution (at aboutand 165 m/pixel) under lower incidence angles (Fig. 3). Steand color (green, violet, and 1-µm filter) data obtained on thisorbit revealed that Cilix is, in fact, an 18-km-diameter impacrater with a central peak complex. The stereo coverage of Cwas used for generating a digital terrain model (DTM) of tregion (see Fig. 4) whose preliminary analysis was reporteGieseet al. (1999).

Cilix exhibits an elongate central peak complex surrounby a flat crater floor, terraced walls, a circular rim, and reddibrown continuous ejecta blanket. The central peak complecomposed of two prominent massifs (Fig. 3, black arrows). Tlarger of these massifs is oriented roughly NW–SE and is 12.5 km. The smaller 0.5 by 1 km, NE–SW-oriented massilocated immediately east of the larger massif. Both massifshibit ∼300 m relief. The central peak complex is located incenter of the crater floor. The crater floor exhibits only a ftens of meters of relief and is mottled by numerous subkilomter reddish-brown patches. Although these patches are evin nearly every quadrant of the crater floor, they are relativmore abundant in the southeast quadrant, and less abundthe northeast and northwest. The western crater walls haveto two terraces, while terraces are typically absent on the win the eastern half of the crater. Ejecta deposits just beyondeastern side of the crater rim appear to partly obscure aexisting ridge system, suggesting these deposits are thicsome places along Cilix’s rim, the combination of the ejectaposit and the old ridge system develops local relief of∼500 m(determined from stereo photogrammetry measurements ofimages), which probably represents the relief measured ein the mission that led to the butte misinterpretation (Belet al. 1996). Elsewhere the rim is typically∼300 m above thelocal surface. Cilix’s rim is seen to be circular and continous in both the E15 data and on-terminator data obtainedthe third orbit. The floor of Cilix is at the same general evation as the surrounding terrain beyond its continuous ejblanket. Cilix’s dark continuous ejecta blanket ranges in wifrom 8 to 20 km measured from the crater rim with diffusebounded ray-like extensions. Within the continuous ejecta this a deposit of very dark material in a topographic low formia partial moat around the southeast portion of crater lying∼3–5 km beyond the rim crest (Fig. 3, white arrows). The plainsyond the continuous ejecta exhibit numerous secondary imcraters.

Cilix exhibits the same suite of landforms seen for cratits size on other icy satellites (although Cilix is significanshallower). The “classical” crater landform suite of Cilix impliethat it formed entirely within a target that behaved brittlely ovthe crater-forming event. As can be seen from Table I, Cilix ha modeled transient crater depth between 2.4 and 4.7 km. Iftransient crater depth range is valid, and our interpretatio
the
-Cilix forming entirely within a brittle substrate is correct, thenwe infer that Europa’s crust is solid, and over short time scales

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FIG. 3. The 18-km-diameter dark-ray crater Cilix, located at 2◦S, 180◦W. The mosaic is four, 40 m/pixel images (PICNOs 15E0025, 29, 33, and 34) superpon a single 165 m/pixel image (PICNO 15E0020) all acquired during orbit E15. Black arrows point to the two central peak massifs. White arrows poinry
dark material in a topographic low forming a partial moat around the southeast portion of crater. Illumination is high and from the right. North is up. See Fig. 4 for
i

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brittle, at least to a depth comparable with the range transcrater depth estimates at this location.

Manannan

The∼22-km-diameter impact crater Manannàn (3◦N, 240◦W)was imaged during orbit E14 in color, in stereo, and at restions as high as 20 m/pixel, although at high (∼20◦) incidenceangles. These images complement others acquired during oG1 and E11. Manannàn (Fig. 5 and Fig. 6) is about the samsize as crater Pwyll (diameter∼24 km) described and discussin Moore et al. (1998), but Manannàn has fewer characteristics in common with classic (or lunar-like) crater morphologSeen under comparable illumination, Manannàn’s rays are lesprominent than those of Pwyll, suggesting that Manannàn isolder. Manannàn has no central peak, but several massifsdistributed across its floor. A DTM constructed from stereo cerage indicates the largest massif, located about half a cratdius east of the crater center, is about∼5 km across and∼200 mhigh. Smaller massifs and ridges are distributed across the c
floor, but the DTM indicates the floor itself is, on average, levLocal relief of the rim massifs above the crater floor interior
ent

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∼200 m. Like Cilix, Manannàn’s crater floor appears to lie at tsame general elevation as the surface exterior to the crater aproximal ejecta. There is a scarp-bounded∼5-km-diameter pit∼80 m deep located just east of the crater floor center (FigThis pit corresponds to the bright spot seen within the cratethe G1 image reported by Mooreet al. (1998). Since no ejectis observed associated with the pit, its origin as a smaller, mrecent impact crater seems unlikely. Centered within this pa well-defined asterisk-shaped dark feature of no perceptiblief, which may be an extrusion emerging from radial fissu(dark central unitin Fig. 6, seedc).

There is no well-developed nested terracing along therior of Manannan’s rim. Uplifted material forming the rim imassively exposed along the west wall of the crater, extenin places∼5 km into the crater interior (crater rim and inte-rior massif unit in Fig. 6, seecrm). In contrast, rim materiahas limited exposure on the east rim, appearing only in ainward-facing scarps. A pedestal-like break in slope occurthe continuous ejecta at∼4 to 7 km beyond the rim crest. ThDTM indicates that the pedestal ramparts rise a few tens of

el.isters higher than the rest of the pedestal (see point “P” on theManannan topographic profile in Fig. 4).

using

100 MOORE ET AL.

FIG. 4. Digital topography models (DTM) of europan craters Cilix, Manannàn, and Pwyll have been generated from stereo coverage of these featuresa parallax-measuring image autocorrelation algorithm. The vertical exaggeration is 4 : 1. Solid lines are DTM results; dashed lines are inferred topography over
areas of no data connecting areas for which there are data. The abbreviat r
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observations of individual craters and the Crater Topography section for d

We find two obvious and interesting deposits just inside aoutside Manannàn. The inner deposit, which is largely confinewithin the crater rim, forms a rolling surface that is textured wsmall blocks or hills<100 m across (crater floor unitin Fig. 6,seecf). At the margin of the deposit, near the crater rim, thilly texture gives way to a roughly radial texture and ultimatea lobate margin. This marginal presentation is best seen wthis deposit is in contact with the patch of dark ejecta (Fig. 7arrow). The contact here is broadly lobate. We interpret crfloor material to be mostly impact melt that collected within tinitial cavity of the crater and in some places lapping overeastern crater rim (as at point “D” on the Manannàn topographicprofile in Fig. 4, and Fig. 7A, arrow).

The second deposit is material seen just outside the craterWe are not (yet) referring to the dark patches but insteadthick, radially lineated deposit of continuous ejecta (continuousejecta unitin Fig. 6, seece). The margin of this deposit is lobat
in many places. Individual lobes are typically 1 to 1.5 km lonand up to∼500 m wide. Examples of these lobes can be seen
ions represent,P, pedestal;R, rim; CP, central peak;D, deposit contact. See text undetails.

nddth

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Fig. 7B (arrows). This deposit was described from E11 data “pedestal” deposit by Mooreet al. (1998), who proposed thapedestal topography around craters on Europa formed by mglacier-like deformation of warm continuous ejecta depositsmostly water ice. If this hypothesis is valid, then the lobes assated with this deposit may represent warm, plastically deformice coming from the lower portion of the deposit. Alternativethe lobes may be a different material emerging (draining?) fror under the main pedestal deposit.

The dark patches on the pedestal deposit appear as coaing radially arrayed streaks with diffuse boundaries in solocations (Fig. 5). Surface texture within the dark patchessimilar to adjacent brighter patches elsewhere within the ctinuous ejecta unit, so if the dark materials are deposits theytoo thin to be resolved, or at least sufficiently thin that thederlying surface shows through. The dark patches most likrepresent a veneer of primary, late-stage ejecta emplaced b
gin
tically on top of an earlier pedestal deposit, although an erosionalorigin or albedo patterning in a coherent ejecta unit cannot be

sedl


FIG. 5. The∼22-km-diameter ray crater Manannàn, located at 3◦N, 240◦W. The mosaic is three, 20 m/pixel images (PICNOs 14E0006, 7, and 8) superpoon a single 79 m/pixel image (PICNO 14E0027) all acquired during orbit E14. Illumination is moderately high and from the right. The insert at the lowereft is a
low (from the right) Sun, 218 m/pixel view of Manannàn acquired during orbit E11 (PICNOs 11E0012 and 14). See Fig. 6 for a geologic map of the area covered
. N

cla

htwaoawo

ee

ex-irls)

If thede-ar

s-ll.te-`n itwe

by the 20 m/pixel images and Fig. 4 for a topographic profile of this crater

ruled out. The darker material may represent either impacontaminated melt, or melt excavated from a deeper, darkbeneath Europa’s surface.

The radial orientation of individual linear streaks within tdark patches implies that the top of the underlying “pedesdeposit did not undergo significant nonradial distortion folloing deposition of the dark patches. This lack of differential (imuthal) distortion in the pedestal deposit is consistent with1998 hypothesis that the pedestal topography is due to rstrain due to the downslope (off-rim) slow, laminar “glacial” floof warm ice. If “pedestal” deposits were formed by the flowan ejected fluid-like slurry whose motion continued for sevminutes after emplacement (which is an alternative hypoth
for pedestal deposit formation), then we might expect that flof this slurry would continue beyond the time the dark patch
orth is up.

tor-yer

eal”-

z-urdial

fralsis

were ballistically emplaced and that this movement would bepressed by turbulence-induced distortions (e.g., curls and swof the dark patch streaks. This appears not to be the case.“pedestal” deposit underlying the streaks did deform, thisformation had to be very laminar and radial with no curvalinedisplacement.

Manannan has fewer characteristics in common with clasic (or lunar-like) crater morphology than does Cilix or PwyNevertheless, it exhibits an intact inward-facing rim, tall inrior massifs, and a∼100-m-thick pedestal deposit. Manannanmuch more resembles other “classic” europan craters thadoes the ringed impact features Callanish and Tyre. Thusinfer as we did for Cilix that Manannàn formed entirely within

owesa brittle substrate. Consequently, we suggest that Europa’s crustwas solid at this location at least to a depth comparable with the

102
MOORE ET AL.

sentation


FIG. 7. (A) View of Manannan crater floor material and its contact with the material beyond. The arrow points to an example of the broadly lobate pre
of this contact (portion of image PICNO 14E006, 20 m/pixel). (B) View of Manannàn continuous ejecta material and its contact with the material beyond. Arrows
1

-

, or

point to several tongue-like lobes of this material (portion of image PICNO

range of Manannàn transient crater depth estimates of 2.865.47 km.

Tegid

The crater Tegid (0.5◦S, 164◦W) was imaged under near-terminator conditions in a∼1.5 km/pixel global mosaic acquired

The Manannàn image base is shown in Fig. 5 and that of Tyre is shown in Figorthographic.

4E008, 20 m/pixel). North is up.

toin E14 (Fig. 8). An inward-facing enclosed scarp∼30 km in di-ameter is most likely the crater rim. A circular unit with somewhat less relief extends∼30 km from the crater center. Thecentral portion of the crater floor appears to be a raised disk

e text.

an broad flattened dome. Tegid may represent a class of craterthat is transitional between the “classic” (though very flattened)

FIG. 6. Geological maps of the Manannàn crater (top) and the Tyre impact feature (bottom) showing the distribution of the major units discussed in thThe Manannàn unit abbreviations representt, central pit scarps;dc, dark central unit; cf, crater floor unit; crm, crater rim and interior massif unit; ce, continuousejecta unit; s, secondary impact sculpture. The dashed trace indicates the rim crest. The Tyre unit abbreviations representbu, preimpact background units:sc,smooth central unit; ri , rough inner unit; ce, continuous ejecta unit; am, annular massif unit; s, secondary impact craters;au, post-impact non-Tyre-related units.

. 9. Note that Tyre image base in this figure is point perspective while that in Fig. 9 is

unrpret theough very

104 MOORE ET AL.

FIG. 8. The crater Tegid (0.5◦S, 164◦W) was imaged at low Sun in a∼1.5 km/pixel global mosaic acquired in E14 (PICNO 14E0055) and at high Sand high phase during E19 at∼900 m/pixel (PICNO 19E0001). Both images are reprojected around the center of Tegid and to the same scale. We inteinward-facing∼30 km-diameter scarp to represent the crater rim. Tegid may represent a class of crater that is transitional between the “classic” (th
flattened) complex impact craters just smaller than Tegid (e.g., Manannàn) and t oints
n

aew1

ea

rbr

igmiim

o

nd-

tedof

eyre

sim-oreired

calf theh-in a

rialThe

rget

dthe

t G7

ich

to dark crescent-shaped albedo mark on top of the topographic dome see

complex impact craters just smaller than Tegid (e.g., Pwyll)the “rimless” multiringed impact features Callanish and Tyr

A second image of Tegid was obtained on orbit E19 at lophase angle and∼900 m/pixel. Comparisons of E19 and Eimages of Tegid (both reprojected around the center of Tegidto the same scale in Fig. 8) show that the dark central cresvisible in the E19 image does not correspond to minor realong the margin of the central dome, but is actually an albfeature located on top of the central dome in the E14 im(Fig. 8, black arrow). Also interesting is that the pedestal ejedeposit visible in the E14 image corresponds to some ofalbedo features visible in the E19 image.

One of the main science objectives of the E19 Tegid obsetion was to determine the size distribution of secondaries athe impact feature as a measure of the size of its transient cwhich, in turn, would provide an estimate of the location ofrim. At ∼900 m/pixel no dark dot patterns were seen that mindicate secondaries (as were seen around Tyre under silighting at∼600 m/pixel). One possibility is that no secondar≥1 km were formed, which would imply that Tegid’s crater rdiameter is considerably smaller than the (∼45-km-diameter)equivalent crater rim of Tyre (consistent with our estimatea∼30-km diameter for Tegid). Tegid thus remains an unusimpact-generated landform whose gross features are now swhat characterized but whose close inspection awaits anoday.

Tyre and Callanish

◦ ◦
A 170 m/pxl mosaic of Tyre (centered at 34N, 146 W) wasacquired under near-terminator conditions during E14 (Fig
he “rimless” multiringed impact features Callanish and Tyre. Black arrow pin the E14 image.

nd.er4andcentliefdogectathe

va-outater,itshtilar

es

ofualme-ther

Fig. 6). A general geologic map focusing on the endogenic laforms within this region beyond Tyre was given in Kadelet al.(2000). Here we will limit ourselves to landforms associaexclusively with Tyre and provide a more detailed analysisthe impact feature. Tyre was seen less well byVoyagerand byGalileoon orbit G7 (Mooreet al.1998). With the E14 images wcan now recognize and characterize the geologic units of Tand compare them with those seen at Callanish, which isilar in both morphology and size, and was analyzed by Moet al.(1998) using 120 m/pixel, near-terminator images acquduring orbit E4.

Tyre, like Callanish, can be divided into several morphologiunits and other associated features (Fig. 6). At the center oTyre impact feature is a 15–20 km wide, flat, relatively higalbedo and smooth textured patch of material located withshallow scarp-enclosed depression (smooth central unitin Fig. 6,seesc), interpreted to be impact melt and/or possibly mateemplaced as a fluid from a source beneath the brittle crust.main,∼45-km-diameter inner deposit (rough inner unitin Fig. 6,seeri ) surrounds thesmooth central unit. Therough inner unitisinterpreted to be composed of impact melt and broken up tamaterial.

The material beyond and encircling therough inner unitischaracterized by a relatively smoother surface (continuous ejectaunit in Fig. 6, seece), in those places where it is not disrupteby subsequent tectonics. This unit largely corresponds withdarker, redder surface forming annular deposits imaged a(see Mooreet al.1998, for details).

The corresponding continuous ejecta unit at Callanish, wh

. 9,was imaged at∼5 times better resolution and at similar light-ing during orbit E26 as Tyre on orbit E14 (Fig. 10), displays a

uring


FIG. 9. The large impact feature Tyre, centered at 34◦N, 146◦W. The mosaic is four, 170 m/pixel images (PICNOs 14E0038, 39, 41, and 42) acquired d
orbit E14. Illumination is low and from the left. This is a feature-centered orthographic reprojection. North is up. See Fig. 6 for a geologic map of Tyre.
tse

xtw

ns

al

range of textures. At one textural endmember, the unit consispatches of very small, equidimensional hills that mantle preisting topography, and at the other endmember it forms smomaterial that appears to pond in low areas and embays preeing relief. As can be seen in Fig. 10, there are many places withis unit in which the texture is transitional between these t
endmembers. Mooreet al. (1998) speculated that the Callanishcontinuous ejecta unitwas emplaced in a fluidized state, perhap
ofx-

othist-

hino

as a slurry of liquid and solid material. The E26 observatioreinforce this interpretation.

Tyre’scontinuous ejecta unitextends as far as∼100 km fromthe impact feature’s center. Within thecontinuous ejecta unitarea number of elongate concentric massifs (annular massif unitinFig. 6, seeam) that are interpreted as uplifted target materi

saround which thecontinuous ejecta unitwas emplaced. Thesesame unit types were recognized at Callanish. As at Callanish,

aeeso

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ic in

er-t so-

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Forand

106 MOORE

FIG. 10. A 25 m/pixel view of Callanish continuous ejecta along the eside of the feature acquired during orbit E26. Callanish’s continuous ejare characterized by a range of textures. At one textural endmember, thconstitutes patches of very small, equidimensional hills that mantle preexitopography (such as around the location marked with an X), and at theendmember forms smooth material that appears to pond in low areas and epreexisting relief (such as the areas pointed out with arrows). Also noteCallanish’s circumferential troughs largely predate the emplacement, or atsolidification, of the continuous ejecta unit, as the ejecta deposit marked ahas flowed into the troughs above and below the “X”. The apparently very rformation of the troughs (i.e., after impact but before ejecta immobilizatistrongly supports the conclusion (Turtle 1998, Turtleet al. 1999) that a nearsurface layer of the target, at least at Callanish, behaved as a very low-viscmaterial. (Portion of image PICNO 26E0005. Centered at 16◦S, 333◦W. Illu-mination is moderately low and from the left. North is up.) Inset at top sho
location of E26 high-resolution view on E4 coverage of Callanish.
ET AL.

stctaunit

tingthermbaythateast“X”pidn)

osity

ws

the continuous ejecta unitappears to be confined to localslightly lower areas (as inferred from subtle shape from shing). This unit is broken by a number of concentric troughs anumerous fractures. These troughs are more abundant eathe crater center. The troughs are interpreted to be tectonorigin and may be graben.

E26 images of Callanish (Fig. 10) show that the circumfential troughs largely predate the emplacement, or at leaslidification, of thecontinuous ejecta unit, which is opposite theinferred sequence of events based on lower resolution E4ages reported in Mooreet al.(1998). The apparently very rapiformation of the troughs (i.e., after impact but before ejeimmobilization) strongly supports the conclusion (Turtle 199Turtle et al. 1999) that a near surface layer of the target,least at Callanish, behaved as a low-viscosity material, sucliquid water. A similar relationship between ring troughs andcontinuous ejecta unitcannot be verified at Tyre in the existinimaging.

Beyond Tyre’scontinuous ejecta unit, there are numerousmall pits, many with raised rims, some of which form pchains oriented radially to the center of Tyre. These pits probaare secondary impact craters. It is the presence of these sdaries that permits an unambiguous interpretation of Tyre aimpact feature. We estimate the equivalent crater diameterwhere the rim would be, if it were more prominent) of Tyre to∼45 km, based on the onset of ejecta sculpting, and the peery of therough inner unit. If Tyre’s equivalent crater diameteis∼45 km, then its transient crater depth is estimated to be 5to 9.98 km.

In Mooreet al. (1998) we concluded that Tyre and Callaniwere not the consequence of impact into a solid ice target,instead might be explained by impact into an ice layer of orde10 km thick overlying a low-viscosity material. This conclusiowas based on the liquid behavior at the time of emplacemof the proximal ejecta, the lack of any classic crater-rim facand our modeling of concentric ring formation. Our understaing of the data examined through E26 continues to support1998 conclusion. However, it must be noted that a low-viscosfluid-rich layer at∼10 km depth may be equally well explaineby a brine-rich zone associated with convecting ice (HeadPappalardo 1999, Collinset al.1999), as it could be by the toof a purely liquid layer (e.g., ocean).

Crater Topography

The craters on Europa>10 km in diameter are substantialshallower than similarly sized craters reported on Ganymand Callisto (Schenk 1991). This can be explained by isosadjustment involving near-surface materials too soft to supsignificant long-wavelength topography over time, or by flooing of crater cavities by fluid or melt. Either of these explanatiomight either posit or refute the existence of a very-near-sur(∼2 km) europan ocean such as proposed by Greenberget al.(1999), so further study of these larger craters is warranted.now we provide some first-order analysis of these structures
implications for the europan interior. Mooreet al.1998 proposed

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IMPACT FEATURES ON

that craters smaller than 30 km in diameter such as PwyllManannan formed within an initially brittle crust, then isostatcally adjusted to their present form. This still appears to bebest explanation.

Stereo coverage of Cilix, Pwyll, and Manannàn was acquiredduring GEM (Fig. 4). Additionally, in an attempt to assess teffects of a potentially colder and stiffer polar lithosphe(Ojakangas and Stevenson 1989), we imaged the crater Rhiato evaluate its depth/diameter ratio from shadow measuremRhiannon (81◦S, 197◦W) has a diameter of∼15 km, a rim-to-floor depth of∼400 m, and an average rim to ejecta blanheight of∼175 m (Fig. 11). The depth/diameter ratio of∼1 : 40makes Rhiannon proportionally much deeper than Manannàn orPwyll, whose floors are essentially level with the surfaceyond the continuous ejecta. However, the rim-to-floor reliefRhiannon is similar to that for 18-km-diameter Cilix (Fig. 4implying that the rheology of the lithosphere under Cilix aRhiannon was similarly stiff, even though one is located atequator and the other near a pole, and has remained at leastiff since these events. Recent work by Stevenson (2000)gests that the rheology of Europa’s ice shell has no latituddependence.

FIG. 11. Rhiannon (81◦S, 197◦W) has a diameter of∼15.4 km and a rim-to-floor depth of∼400 m. Rhiannon’s relief is similar to that of 18-km-diametCilix. This implies that the rheology of the lithosphere under Cilix and Rhiannare similar, even though one is located at the equator and the other near a
This is a crater-centered orthographic reprojection of 25 m/pixel image PICN17E0070. North is up.

nd-he

eenonnts.

et

e-or,dhest asug-al

rn

pole.O

Digital topography models (DTMs) of several europan crahave been generated from stereo coverage of these featuretypical vertical resolution of a few tens of meters (Fig. 4) (Gieet al. 1999, where details of the DTM generation are giveAn important result of this work is that the largest “classcraters, Pwyll and Manannàn, have floors that are at the samelevation as the plains beyond their rims. The greatest rassociated with these features is that of their central peakthe case of Pwyll, the central peak rises∼800 m above thefloor, some∼300 m above the average rim height (Fig. 4).contrast, central peaks rising above rims are rare for the Mand Mars (Schenk 1989). The presence of a significant cepeak implies load-bearing materials exist in the excavation zimmediately beneath the crater, rather than, for instance,uid water. Assuming that the ice beneath the crater maintits strength despite disruption during the impact, a lower lion the thickness of the ice needed to support the centralcan be estimated from flexure. Assuming that 100 m of vertdisplacement from flexure, if present, would be recognizain the DTM of Pwyll, the ice under the central peak wouhave to be at least 0.9 km thick (as derived using an expresfrom Turcotte and Schubert, 1982, Eq. 3–131, p. 126) to preflexure-induced vertical displacement of this amount. Since 12.0 km of ice is ejected during the impact the preimpact ice thness must have been at least 2.6–3.6 km, and it was probablythicker.

The overall shallowness of Pwyll and Manannàn may be ex-plained by plastic deformation of the crust due to warm ice unthese craters in order to restore the broader impact-genemass loads upon the local crust to a gravitational equipotial. Only the topography of very short (horizontal) wavelengfeatures, such as central peaks and crater rim-ridges, havrelaxed in the time since crater formation. Central peaks arthis scenario passively carried by the rising floor to their anolous elevation relative to the crater rim. This may explainrelatively unusual height of Pwyll’s central peak.

Pwyll Secondaries

No major events on Europa are known to postdate∼24-km-diameter impact crater Pwyll, although Bierhauset al.(2000) discuss possible modification in the Conamara Chregion after the emplacement of Pwyll secondaries. Either Pformed quite recently or smaller impacts are extraordinarilyfrequent. In either case, Pwyll formed on a rather “clean slawith little cratering history underlying the crater, its rays, aits secondaries. Bierhauset al.(1998) compared crater densitiinside and outside a Pwyll ray and determined that Pwyll secdaries compose at least 90% of the small craters along porof its ray traces. Pwyll secondaries are readily identified neacrater as well as across the trailing hemisphere of Europa aciated with extensive crater rays, providing an enormous bto impact modeling and crater statistics. For this reason, Euis a perfect “witness plate” for studying large-scale impact ocomes on icy satellites.

Europa’s crust may not be typical of planetary lithosphe
so some caution should be taken in using cratering models and

es

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ters

eter,ndary

108 MOORE

model constraints derived from studying europan impact ftures and applying the results to other bodies. Ejection ofondaries mainly involves the near-surface rheology at “eatime” in the impact event (e.g., Melosh 1989, p. 92), but folithospheric thickness perhaps only equal to several projediameters, this boundary effect may be important (Mooreet al.1998). And so, while we can study Europa secondaries to leabout the impact behavior of ice, we must keep in mind tthere may be differences between cratering on Europa, andinstance, cratering on Ganymede where the brittle crust mathicker.

Vickery (1986, 1987) developed a simple and robust tenique for transforming secondary crater locations and diameinto plots of ejection velocity versus ejecta diameter. We happlied this same technique to derive the size-velocity distrtion of Pwyll’s ejecta fragments from the diameter and distaof Pwyll secondary craters by measuring secondaries at atant site where there was very-high-resolution coveragePwyll ray and those in the Pwyll near field. Distance from tprimary crater, assuming a spherical Europa, gives us the fment ejection velocity, which equals the impact speed of aondary projectile on an airless world. Secondary crater diamcombined with this speed, assuming an ejection/impact ang45◦, together with scaling assumptions for how secondary crdiameters relate to impactor diameters, then provides anmate for ejecta fragment diameter. The advantage of Pwythis regard is that ejecta fragments launched at 1 km/s haveobserved (as secondary craters) 1000 km away in the Conaregion. In contrast, Vickery’s analysis of cratering on Mars,Moon, and Mercury could not be extended to ejection velocibeyond a few 100 m/s, since distant (faster) secondaries areamong the much more numerous background craters on tbodies.

Our analysis is presented in Fig. 12. Our findings are similaAlpert and Melosh (1999), except that our fragment diameare a few percent larger, likely due to systematic differenin measuring the original crater diameters. Our data showejecta fragments reached velocities over 1 km/s. Since Prays extend beyond the region we measured, ejection velocmust have been even higher in some cases. The distance traby ejecta from Pwyll to the secondaries we measured is overthe circumference of Europa; larger impacts such as thoseformed Callanish and Tyre must send ejecta over greater porof the surface, if the speed of launched ejecta scales with cdiameter.

We have also investigated (Asphauget al.1999) how the sizesand speeds of secondary craters around Pwyll and Tyre vaa function of azimuth, measured in a radial coordinate fracentered upon the source crater. For Pwyll, we find that sondary craters near the Conamara Chaos,∼1000 km from theprimary, range from 100 m (the smallest we measured in eframe) to 0.5 km diameter and were created by fragments∼10
to∼100 m in diameter, depending on the particular scaling retion used (cf. Vickery 1986, 1987). Figure 13 summarizes the
ET AL.

a-ec-rlyatile

arnat

, forbe

h-ersveu-cedis-f aeag-ec-ter,of

tersti-ineenarae

eslostese

torseshatylltieseled0%thatonsater

y ase

ec-

ch

la-

FIG. 12. A plot of Pwyll secondary-crater-producing fragment diameteagainst ejection velocity. The diameters of the fragments were derived from msurements of secondary craters and then scaled using the relationship repin Vickerey (1986). Secondaries were measured both at a site∼1000 km fromthe impact where there was very-high-resolution coverage of a ray and thothe Pwyll near field. See text for discussion.

results. By studying these azimuthal variations, we find taverage fragment sizes decrease away from the center oray. This implies that ejecta fragment masses may approacorder of magnitude larger along a ray center than elsewherea given ejection speed. We also observed azimuthal variain secondary craters adjacent to Tyre, which implies that retively large masses can be ejected at high speed as a resu

FIG. 13. Fragment diameter versus azimuth, for Pwyll. Secondary craassociated with a ray near the Conamara Chaos,∼1000 km from the primary,range from 100 m (the smallest we measured in each frame) to 0.5 km diammeasured in a radial coordinate frame centered upon the source crater. Seco

secrater diameters were then converted to fragment sizes (circles= gravity regime,porous target; triangles= strength regime). See text for discussion.

E

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IMPACT FEATURES ON

FIG. 14. Differential size-distribution for Pwyll secondary craters, mesured in the high-resolution (∼10 m/pix) sequence E12CHAOS01. The solline represents the−4.4 slope for lunar secondaries measured by Chapm(1968); note that the steep slope exhibited by the Pwyll secondary craterdistribution is similar to that of the lunar secondary craters. The error barsfor the square root ofn, wheren is the number of craters in each size bin. Stext for discussion.

asymmetries in the distribution of energy within the ejecta ctain. Such ejecta size asymmetries have provided insight towunderstanding the ejection of large, intact fragments fromsurfaces of planets, since large fragments launched alonghave a greater likelihood of escaping a planet.

We have also conducted a general statistical analysis ofondary craters on Europa. Again, this work is uniquely posson Europa because the secondary cratering flux is easilycerned from the primary flux. Detailed information about sondaries is required to assess two fundamental questions aicy satellite surfaces. First, determination of the characterisize distribution of secondary craters on icy targets providequantitative means of disentangling the secondary from themary cratering fluxes. The relative contribution of primaries asecondaries to Europa’s crater population continues to be ater of important dispute (e.g., Zahnleet al.1998, Neukumet al.1997), which has led to divergent age estimates for Europa’sface. Second, these data provide an opportunity to compardistribution of secondary craters around Europan impact featagainst secondary fields on terrestrial planets, and also ag
the outcome of numerical models (Mooreet al. 1998). Suchanalyses could have direct bearing, for instance, on the prod

-dansizearee

r-ardthe

ray

ec-bledis-c-bouttics apri-nd

at-

ur-theresinst

tion, abundance, and size distribution of purported circumjovdebris from very large impact events on the Galilean satellisuch as the Gilgamesh collision on Ganymede (ShoemakeWolfe 1982, Shoemaker 1996, Zahnleet al.1998).

Our preliminary analysis of the characteristic size distribtion of secondary craters shows that distant Pwyll secondameasured in the Conamara Chaos region possess a differslope slightly shallower than−4. This is similar to the−4.4slope measured by Chapman (1968) for a population of lucraters that he classified as likely secondaries (Fig. 14). Wenot take this comparison too far, however, because the Eudata are from only one secondary cluster at a single distafrom one crater, while the Chapman counts are from severagions, likely sampling numerous secondary clusters at varydistances from their primary crater. With this caveat in minour results nonetheless suggest that the steep slope in thesize distribution below 1 km on Europa reported in Chapmet al.(1998) is caused by distant secondary craters. Similar sslopes for small craters on the other icy Galilean satellites mimply that small craters on these objects are mostly formedsecondary impact. If so, the jovian system may be deficiensmall impacts relative to the environment of the terrestrial plets (Chapmanet al.1998).

CONCLUSIONS

1. A survey of all primary impact features>4 km in diam-eter seen on Europa (in images fromGalileo’s nominal andEuropa missions, through orbit E19) was performed. Weculated the transient crater dimensions and excavation deof these craters and note that even the largest impact fea(Tyre) probably had a transient crater depth between 5 and 1and transported material to the surface from a depth not grethan∼4 km. Craters the size of Pwyll and Manannàn (∼22 kmdiameter) had transient crater depths of between 3 and 6 kmtransported material to the surface from a depth not greater∼2 km. This implies that the source of the dark, red mateobserved around some europan craters is within a few kiloters of the surface. A simple-to-complex transition in europcrater morphology is observed to occur at diameters of∼5 km.Craters with diameters≥6 km exhibit central peaks, whereaseveral 5-km-diameter craters do not.

2. We are now convinced that Callanish and Tyre are bimpact features. This conclusion is based on the presencpits and radially arrayed chains of pits we interpret to be sondary craters surrounding both features. Tyre has now bobserved at comparable resolution and lighting to that ofCallanish nominal mission observations. Tyre’s strong resblance to Callanish implies that the conclusions reached reging Callanish in Mooreet al. (1998) also apply to Tyre, whichwas that Callanish is the consequence of impact into targetterials that are mechanically very weak at depth. A new ob

uc-vation that Callanish’s circumferential rings formed before theproximal ejecta became immobile further supports the inference

me

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110 MOORE

of a very-low-viscosity substrate at the time of formation. Walso observe new evidence for the fluid behavior of the proxiejecta material around Callanish at the time of emplacemHowever, it must be noted that a low-viscosity, fluid-rich layat depth may be equally well explained by a brine-rich zonesociated with convecting ice, as it could be the top of a puliquid layer (e.g., ocean).

3. Craters<30 km in diameter, such as Manannàn and Pwyll,formed within targets whose immediate subcrater materialshibited coherent behavior on time scales of the impact eventthat are capable, especially in the case of Pwyll, of supporsignificant local topographic loads such as a central peak. Tcraters are nevertheless quite shallow, with very subdued floand we suspect that Manannàn’s and Pwyll’s small depth-todiameter ratios are due to the isostatic adjustment of large-stopography facilitated by warm, plastically deformable icedepth.

4. Europa’s nearly pristine surface provides a unique “witnplate” recording the outcome of a recent large-scale crateevent (Pwyll) into an icy target. We previously found, on thesis of a numerical experiment (Mooreet al.1998), that the number and distribution of Pwyll secondaries are more consiswith impact into solid ice than impact into a thin shell overlyiliquid water. Our analysis supports the conclusions of AlpertMelosh (1999) that impacts on icy bodies eject smaller fragmthan impacts on silicate bodies at equivalent ejection velociand that fragment size decreases more gradually as velocicreases. Further analyses of Pwyll secondaries have reveaazimuthal variance, with ejecta fragment sizes being largerthe center of a ray than off the ray. Our preliminary analysisthe characteristic size distribution of Pwyll’s secondary crashows that it forms a differential slope slightly shallower th−4. Similar steep slopes for small craters on Ganymede imthat small craters there are mostly formed by secondary impand the jovian system may thus be deficient in small imprelative to the environment of the terrestrial planets.

ACKNOWLEDGMENTS

We thank Louise Prockter and Nadine Barlow for their careful reviews ofpaper. We also thank Moses Milazzo for processing some of the images uthis report. This investigation was funded by NASA’s Galileo Project.

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Impact Features on Europa: Results of the Galileo Europa ...the distribution of Pwyll secondaries. MORPHOMETRY OF EUROPAN IMPACT CRATERS Although Europa’s surface is lightly cratered