morphology and origin of palimpsests on ganymede based on

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Icarus 164 (2003) 197–212www.elsevier.com/locate/icaru

Morphology and origin of palimpsests on Ganymede based on Galiobservations

Kevin B. Jones,a,∗,1 James W. Head III,a Robert T. Pappalardo,a,2 and Jeffrey M. Mooreb

a Department of Geological Sciences, Box 1846, Brown University, Providence, RI 02912, USAb NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035-1000, USA

Received 8 September 2002; revised 2 April 2003

Abstract

Palimpsests are large, circular, low-relief impact scars on Ganymede and Callisto. These structures were poorly understoodVoyager-era analysis, but high-resolution Galileo images allow more detailed inspection. We analyze images of four Ganymedean ptargeted by Galileo: Memphis and Buto Faculae, Epigeus, and Zakar. Ganymedean craters and Europan ring structures are usehelp better understand palimpsests, based on morphologic similarities. From analysis of Galileo images, palimpsests consist of funits: central plains, an unoriented massif facies, a concentric massif facies, and outer deposits. Using as a tie point the locatistructures where secondary craters begin to appear, outer deposits of palimpsests are analogous to the outer ejecta facies oconcentric massif facies of palimpsests are analogous to the pedestal facies of craters; and the unoriented massif facies and centanalogous to crater interiors. These analogies are supported by the presence of buried preexisting structure beneath the outer twoof buried structure beneath the inner two units. Our observations indicate that palimpsest deposits represent fluidized impact ejethan cryovolcanic deposits or ancient crater interiors. 2003 Elsevier Inc. All rights reserved.

Keywords:Ganymede; Europa; Impact processes; Cratering

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1. Introduction

Palimpsests are large, bright, circular, low-relief impscars found on Ganymede and Callisto (Smith et al., 19Although it is accepted that they formed in response topact, the details of their formation were not well understobased on analysis of Voyager data. From Voyager obsetions (Fig. 1), consensus was not reached on what the bdeposit of a palimpsest represents, or where the origcrater rim is located within a palimpsest.

Several principal theories have been put forth concernboth the means of emplacement of the palimpsest deand the location of the original crater rim. These hypotses can be grouped into three theories on emplacemen

* Corresponding author.E-mail address:[email protected] (K.B. Jones).

1 Present address: Department of Geosciences, Gould-Simpson Bing, 1040 E. 4th St., University of Arizona, Tucson, AZ 85721-0077, US

2 Present address: Astrophysical and Planetary Sciences DepartmeLaboratory for Atmospheric and Space Physics, Campus Box 392, Unsity of Colorado, Boulder, CO 80309, USA.

0019-1035/03/$ – see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/S0019-1035(03)00128-3

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three theories on what is represented by visible palimpmorphology.

The three hypotheses on emplacement of bright matinvolve

(1) large-scale impact-triggered extrusions,(2) fluid-rich slushy ejecta, and(3) dry, solid ejecta.

(1) Thomas and Squyres (1990) postulated that somepacts in the early geologic history of Ganymede could hpenetrated its thin primordial lithosphere, allowing the extsion of warm, buoyant, asthenospheric material. Whenmaterial reached the surface, it would have spread radand formed circular extrusive deposits.

(2) Greeley et al. (1982) and Fink et al. (1984), baon experimental simulations, hypothesized that promptlapse of central peak craters in low-strength or fluidizedterials could produce palimpsest-like structures. SimilaCroft (1983) proposed that impacts by unusually largehigh-velocity objects could have resulted in primarily fluflow instead of typical granular flow during the modificati

http://www.elsevier.com/locate/icarus

198 K.B. Jones et al. / Icarus 164 (2003) 197–212

Fig. 1. Voyager images of palimpsests (A) Memphis Facula, (B) Epigeus, (C) Zakar, and (D) Buto Facula.

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stage of crater formation, based on theoretical modelingsuggested that this fluid flow could account for the oddpearance of these structures.

(3) The remaining hypotheses of Passey and Shoem(1982), Hartmann (1984), Lucchitta and Ferguson (19and Schenk (1996), discussed below, implicitly assuejecta with a significant dry, solid component.

Ideas for what the palimpsest margin represents coof

(1) the outer continuous ejecta facies,(2) the inner ejecta or pedestal facies, and(3) the transient cavity limit or original crater rim.

Although these ideas assume dry, solid ejecta, they calso apply to cratering processes involving more fluid-rejecta as proposed by Greeley et al. (1982), Croft (19and Fink et al. (1984).

(1) Passey and Shoemaker (1982) proposed thapalimpsest margin is analogous to the edge of a conous ejecta deposit like those commonly surrounding imcraters. This hypothesis was based on the presence oondary craters surrounding some palimpsests, beginjust beyond the palimpsest margin. Secondary cratersrounding ordinary craters on Ganymede begin to appeabeyond the continuous ejecta margin (Iaquinta-Ridolfi

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Schenk, 1995). This hypothesis places the original crrim well inside the palimpsest.

(2) Based on an extensive study of fresh complex craon Ganymede, Schenk (1996) concluded that outer maof palimpsests are analogous to the inner or “pedestalcies of the continuous ejecta blanket of craters on Ganym(e.g., Fig. 2) (see Horner and Greeley, 1982). Schemapping of craters on Ganymede revealed a well-defirelationship between pedestal facies diameter and crim diameter. He then tied this empirical relationshippalimpsests using the penepalimpsest Nidaba (19◦ N,124◦ W), in which he found the outer ejecta facies, pedefacies, and crater rim to be identifiable and to occur atexpected locations based on this relationship.

(3) Based on the remarkable circularity of palimpseand on comparisons with moat craters on Ganymede, Hmann (1984) and Lucchitta and Ferguson (1988) hyposized that palimpsest margins are located near and assowith former crater rim deposits of palimpsest-forming ipacts.

In Section 2, we summarize the characteristics ofGalileo targets analyzed here. In Section 3, we discussgeologic units seen in high- and regional-resolution imaof these targets. Sections 4 and 5 contain our comparof palimpsests to large craters on Ganymede and tostructures on Europa, respectively. Section 6 comparemorphometry of each of these types of impact structures

Morphology and origin of palimpsests on Ganymede 199

Fig. 2. Reprojected Galileo image of crater Achelous, showing locations of pedestal (inner ejecta) facies, outer (continuous) ejecta facies, and secondary craterfield. North is to the top in this and all subsequent images, except as indicated.

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2. Galileo targets

Galileo imaged four palimpsests on Ganymede: MempFacula during the first Jupiter orbit, designated G1, Epigduring orbit G2, Zakar during orbit G7, and Buto Facuduring orbit G8. Memphis Facula and Epigeus were imaat high resolution. Zakar and Buto Facula were imagea regional scale complementing and providing contextthe high-resolution images. Image characteristics are smarized in Table 1.

Memphis Facula (16◦ N, 133◦ W) is 355 km in diame-ter and is located in the dark terrain of Galileo Regio. Tpalimpsest was imaged as a strip of 8 summation-m(400× 400 pixels) images at about 60 m/pixel, providinga radial sample across most of the palimpsest (Fig. 3).very center of Memphis Facula, however, was not imagand the 28-km-diameter crater Hay-tau and its ejecta obsmuch of the original inner palimpsest surface within theaged area.

The 370-km-diameter palimpsest Epigeus (23◦ N,181◦ W), located at the convergence of Uruk and NippSulci, was imaged as a strip of 9 complete and 1 pasummation-mode frames at about 90 m/pixel (Fig. 4A). Thisimage strip provides a complete radial sample across

palimpsest, similar to that provided by the Memphis Facimages.

The Epigeus images suffered from inadvertent overcpression, reducing their effective resolution below the pcompression 90 m/pixel. Four of the frames contain loslessly compressed “truth windows” 96 pixels square, hever, which reveal the uncompromised surface texturedetailed morphology.

Zakar (30◦ N, 335◦ W), a palimpsest approximately 28km in diameter and located in a complex region of grooterrain, was imaged almost in its entirety as a singlem/pixel frame (Fig. 5A). Buto Facula (12◦ N, 203◦ W),located in Marius Regio, was imaged almost in its entiras a two-frame mosaic at 180 m/pixel (Fig. 6A). Althoughthe Zakar and Buto Facula images are lower resolution,provide greater areal coverage and therefore more extecontext than the Memphis Facula and Epigeus images, aolutions higher than obtained by Voyager.

Voyagers 1 and 2 previously imaged these four palimsests (Fig. 1). Memphis Facula was imaged at 0.7 km/pixel(Voyager image FDS 20638.29), Epigeus at 1.0 km/pixel(20637.05), Zakar at 1.1 km/pixel (16405.30), and ButoFacula at 1.3 km/pixel (20635.45).


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Fig. 3. Memphis Facula. Strip of reprojected Galileo images placed over reprojected Voyager image. Lettered boxes show locations of enlargemenFig. 9.Single white lines delineate the palimpsest margin and boundaries between labeled surficial facies. Double white lines delineate subconcentricughs withadjacent infacing scarps. Within Memphis Facula, the boundary between the concentric massif facies and the outer deposits is approximately coient witha prominent scarp and trough.

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Table 1Geometric information and resolution of selected Galileo images

Image number Incidence Emission Phase Resoluangle (◦) angle (◦) angle (◦) (m/pixel)

Memphis Facula:s0349760100 38.7 17.2 49.4 65.1s0349760122 39.2 17.4 49.7 63.4s0349760145 39.6 17.7 50.0 61.6s0349760168 40.0 18.0 50.1 59.8s0349760200 40.3 18.3 50.0 57.9s0349760222 40.6 18.7 49.9 56.1s0349760245 40.9 19.2 50.0 54.3s0349760268 41.0 19.8 49.5 52.3

Epigeus:s0359945481 31.8 34.9 14.7 89.4s0359945484 31.2 35.3 15.0 89.2s0359945488 30.4 35.8 15.4 88.9s0359945500 29.8 36.2 15.7 88.6s0359945504 29.0 36.7 16.3 88.4s0359945507 28.4 37.1 16.8 88.2s0359945511 27.7 37.6 17.5 87.9s0359945514 27.1 38.0 18.1 87.7s0359945518 26.3 38.6 18.9 87.5s0359945521 25.7 39.0 19.5 87.3

Zakar:s0389917900 48.72 65.65 37.21 440.8

Buto Facula:s0394532139 79.71 15.01 69.35 193.1s0394532152 82.85 17.65 68.95 195.0

Based on Voyager 2 images, Memphis Facula wasscribed by Passey and Shoemaker (1982) as consistia relatively smooth central region about 100 km acrosuniform bright annulus surrounding the central regioncontaining subconcentric ridges or hummocks, and an omottled unit. Secondary craters were considered presethe outermost unit and outside the palimpsest but wereeasily distinguishable from other, nonsecondary craters

The low resolutions and high sun angles of Voyagerages prevented observation of primary topography acthese features. The high-resolution Galileo images of Mphis Facula and Epigeus were also taken under condiof high solar illumination (Table 1). Without the presenof shadows or obvious shading, topography is difficult tofer directly from these images. Based on stereo imageother areas on Ganymede, surface brightness typicallyrelates with topography (Oberst et al., 1999). Steeper, tgraphically high areas, such as ridge crests and cratecrests, tend to have high surface brightnesses, while flalocally low areas, such as crater and trough floors, tenappear darker. Patterns of surface brightness can therbe used to qualitatively infer topography or slope. Morevorable sun angles in Galileo images of Buto FaculaZakar clearly reveal relief. This topography helps chaterize palimpsest surface texture, and allows us to brothe descriptions of palimpsest surface facies to include tgraphic descriptions.


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Fig. 4. Epigeus. (A) Reprojected Galileo image strip placed over reprojected Voyager image. Lettered boxes show locations of enlargements(B) Geologic sketch map of the area outlined in Fig. 4A. Heavy lines delineate the palimpsest margin and boundaries between surficial facies. Circleepresentcraters, and irregular outlines represent massifs. Near the palimpsest center, the network of subradial lines shows the locations and extent of fractures.

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The literal, nonplanetary definition of a palimpsest isparchment that has had its original writing scraped awperhaps incompletely, and overwritten. The lack of topogphy noted in Voyager images of these features supportedoriginal definition. Galileo images reveal that palimpsestsicy satellites are not simply “ghosts” showing through ovprinted topographic features, but basic geologic and tographic elements.

3. Palimpsest units

Galileo images of Memphis and Buto Faculae, Epigeand Zakar, along with their corresponding Voyager ctext images, allow mapping and description of four distipalimpsest surface units. These units are thecentral plains,the unoriented massif facies, the concentric massif facies,

and theouter deposits. Secondary craters are present beyothe outer deposits in each of these palimpsests.

3.1. Central plains

The innermost palimpsest unit, thecentral plains(Fig. 7A), consists of a bright, relatively smooth regipunctuated by low 500-m- to 3-km-diameter knobs conctrated toward the palimpsest center. The central plains fthe inner palimpsest floor, extending out to approxima15% of the palimpsest radius. In places, the plains emmassifs of the surrounding unoriented massif facies, win other areas the plains are scarp-bounded.

The highest-resolution (90 m/pixel) images of the centraplains were obtained in Epigeus. At the center of Epigethere is a 25-km-radius region of subradial dark-flootroughs or fractures (Fig. 8A). These troughs range from


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Fig. 5. Zakar. (A) Reprojected Galileo image placed over reprojected Voyager image. (B) Geologic sketch map of the area outlined in Fig. 5A. Subtriclines delineate the palimpsest margin and boundaries between labeled surficial facies. Small circles represent craters, and small dotted circlethin thepalimpsest represent buried secondary craters and crater chains. Dashed lines, particularly in the southwest portion of the palimpsest, representburied groovesHatchured lines represent infacing scarps.

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proximately 150 to 250 m across, are generally linear,merge in dendritic or anastomosing patterns. The troughhibit two orthogonal preferred orientations, trending norsouth and east–west. A bright, rough-textured, lobate deoccupies the central 15 to 20 km of this region, and locsurrounds troughs up to 25 km from the palimpsest ceThis bright region consists of massifs 200 to 300 m acseparated by 200 to 300 m. The fracturing present at theter of Epigeus is not noted at the centers of Buto FaculaZakar, possibly due to the lower resolution of images oflatter palimpsests.

In Voyager images, the central region of Zakar appeato be an uplifted dome (Squyres, 1981), but Galileo ima

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(including stereo combination with Voyager images) shthat this interpretation was an illusion of albedo, as the ctral plains are a low-lying unit.

We interpret the central plains to be formed from soified impact melt and chunks of solid ejecta. Radial frtures seen in the central plains of Epigeus may result fpost-impact viscous relaxation, resulting in compressand brittle deformation at the palimpsest center (Halal., 1981). This viscous relaxation would produce tensioforces outside the former crater rim, as expressed bycentric graben and scarps farther from the palimpsestter.


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Fig. 6. Buto Facula. (A) Reprojected Galileo images placed over reprojected Voyager image. Lettered boxes show locations of enlargement(B) Geologic sketch map of the area outlined in Fig. 6A. Subconcentric lines delineate the palimpsest margin and boundaries between labeled surfacies.Circles represent craters. Scalloped lines within the palimpsest represent limits of buried secondary crater chains, and dashed lines represent buried grooves.

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3.2. Unoriented massif facies

Surrounding the central plains is theunoriented mas-sif facies(Fig. 7B), a mottled brightness unit consistingbright, evenly distributed, unoriented curvilinear massifsmegahummocks 3 to 10 km in length and 1 to 5 km acrwhich in turn are textured by smaller hummocks 100 m1 km across. The massifs decrease in height within 120 km of the boundary with the central plains. In placthis unit appears to be dissected into plateaus by nardark-floored troughs 100 to 200 m across (Fig. 9A). Tboundary between the unoriented massif facies and therounding concentric massif facies is not sharp but canmapped where the arrangement of massifs changes fromoriented to concentric with respect to the palimpsest ceThe unoriented massif facies forms an annulus from appimately 15 to 40% of the palimpsest radius.

In the middle of the unoriented massif unit of Epigetwo distinct rows of massifs separated radially by 6 kmoriented circumferentially, in contrast to the unoriented msifs throughout the remainder of this unit (Fig. 4B). The

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two rows of massifs appear to be infacing scarps like thnoted in the concentric massif facies and outer deposiMemphis Facula and Zakar.

We interpret the unoriented massif facies as a jumbmass of ejecta located within the transient cavity.

3.3. Concentric massif facies

The concentric massif facies(Fig. 7C) is a region ofbright massifs arranged in arcs concentric to the palimpcenter. These massifs are 1 to 5 km across and 10 tothan 30 km in length, and are separated by darker, rollmassif-free areas 4 to 6 km across. In places, the maappear to be dissected into plateaus by narrow dark-flotroughs (Fig. 9B). The concentric massif facies occupieannulus from about 40 to 70% of the palimpsest radius.

Some massifs in the concentric massif facies are grouin arcuate segments approximately 1 km in width andvariable length that are not concentric to the palimpsbut enclose circular massif-free regions 3 to 7 km inameter. We interpret these circular arrangements of ma


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Fig. 7. Enlargements showing surface texture of palimpsest facies wButo Facula (see Fig. 5 for locations). (A) Central plains, (B) unorienmassif facies, (C) concentric massif facies, and (D) outer deposits.

and enclosed massif-free areas as secondary craters resfrom palimpsest formation buried beneath this unit. The sondary craters within the concentric massif facies are msubdued than those in the outer deposits, described bsuggesting burial here by a thicker overlying deposit.

200- to 600-m-width dark-floored troughs, possibly psent in all facies within Epigeus and Memphis Facu

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are most clearly presented in the concentric massif faarranged in patterns suggesting an origin related to ldrainage of a fluid ejecta component.

An infacing sinuous scarp with an adjacent dark-flootrough 1.2 to 1.8 km in width is located near the ouboundary of the concentric massif facies in Memphis Fa(Fig. 9C). Zakar contains several similar infacing subccentric scarps within this facies and at the proximal eof the outer deposits. These scarps may result from mocation stage or post-modification stage inward slumpinlarge sections of palimpsest.

The concentric massif facies within Zakar exhibits an opattern of albedo (Fig. 5A). The unit can be divided infour approximately equal annuli of alternating albedo,innermost and third zones being brighter than the secondoutermost zones. The higher albedo zones are mottledscale of 1 to 2 km, while the lower albedo zones are moton a scale of about 5 km. Reasons for this brightness vtion are not known but may be related to radial changeejecta composition or unresolved surface texture.

We interpret the concentric massif facies as a zonejecta-mantled preexisting crust that has been stressefractured by the palimpsest-producing impact. Post-impviscous relaxation formed concentric infacing scarps inmarily this facies.

3.4. Outer deposits

The outer deposits(Fig. 7D) consist of relatively gentle rolling topography, containing scattered 100- to 500moderate-brightness low hummocks, and mottled brighton a scale of about 5 km. This unit appears relatively smoat Galileo resolutions (as high as 60 m/pixel). The outerdeposits contain many fewer large bright massifs thanconcentric massif facies. Most outer deposit massifsarranged in arcs 0.5 to 1 km wide and 1 to 6 km loThese arcs combine to form crescents or circles 3 to 7

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Fig. 8. Enlargements and geologic sketch maps of selected areas of Epigeus (see Fig. 4 for locations). North is to the upper left. (A) Network of radracturesat the palimpsest center. Circles represent craters. (B) Secondary craters visible outside the palimpsest (to the right of the heavy line) are shos large,irregular circles both individually and in chains. Massifs (outlined) form circles and show the locations of buried secondary craters and chains.


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Fig. 9. Enlargements and geologic sketch maps of selected areas of Memphis Facula (see Fig. 3 for locations). (A) Unoriented massif facies. Darshowlocations of several narrow dark-floored troughs. (B) Concentric massif facies surface texture, partially obscured by the crater Chrysor. Dark lines again showlocations of several narrow dark-floored troughs. (C) A large dark-floored trough and infacing scarp. (D) The palimpsest margin, seen as a low outfag scarp.Curved lines within the palimpsest illustrate narrow dark-floored troughs. Sets of short parallel lines illustrate north- to north–northwest-trending streaks.(E) Possible secondary craters are outlined. North- to north–northwest-trending streaks are shown as short lines crossing crater rims and in surrounding terrain.

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in diameter, and these circles in turn are often groupesubradial chains (e.g., Fig. 8B). We interpret these featas buried chains of secondary craters. Many such busecondary crater chains can be seen within the outerposits.

The outer deposits, like the unoriented and concenmassif facies, appear to be dissected in places by na(about 200 m across), dark-floored, scarp-bounded tro(Fig. 9D), and cut by uncommon infacing scarps. Thtroughs and scarps may form during gravitational relaxaof the palimpsest as viscous flow is directed inward.

Palimpsest edges as revealed by Galileo are quite circoverall, but are locally lobate on a 10-km scale and apponly in places to be extremely low, outfacing scarps.

The eastern edge of Buto Facula overlaps an older c40 km in diameter (Fig. 6). The rim of this crater outsidepalimpsest is clearly exposed and visible, and the remder of the crater rim can be traced beneath the palimpalthough the expression of the rim within the palimpsesmuted (Moore et al., 1998). This strongly suggests thatouter deposits mantle preexisting topography, and thatmantling stops at the palimpsest margin.

Within Epigeus and Zakar, primarily toward the disedge of the outer deposits, hummocks are arranged inear rows with similar linearity, trend, and spacing to thoof grooves present just outside these palimpsests, suggethese hummocks are the topographic expression of bugrooves (e.g., Fig. 5B).

We interpret the outer deposits as a continuous ejectaposit mantling preexisting terrain and secondary craters

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3.5. Secondary craters

Exposed small craters surround each palimpsest imby Galileo (e.g., Fig. 8B). These craters range in diamter from approximately 2 to 7 km, occur both singly aarranged in subradial chains of several craters, are ofilar freshness, and are exposed only beyond the palimpmargin (i.e., the distal boundary of the outer deposits).craters decrease in areal density with increasing distfrom the palimpsest. These characteristics are consiwith an interpretation as secondary craters created bypalimpsest-forming impact.

Secondary craters are also noted within the outerposits and concentric massif facies, as discussed abThese craters are buried by ejecta, however, and are nclearly visible as the exposed secondary craters outsidpalimpsest.

Unlike a typical isotropic distribution of secondary crters, secondary craters associated with Zakar are disticoncentrated to the northeast and southwest of the palimBuried secondary craters and crater chains are infewithin the outer deposits and the outer half of the concenmassif facies (Fig. 5B). These crater chains are not puradial to the palimpsest; instead they exhibit a preferencenortheast-southwest trends. The nonisotropic distributiosecondary craters and crater chains suggests that the imthat produced Zakar may have been oblique, or that improcesses were controlled by oriented structural weaknein the lithosphere.


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3.6. Surrounding terrain

Beyond the outer deposit limits, preexisting terrain isvisibly mantled by palimpsest ejecta. The primary alteraof surrounding terrain as a result of palimpsest formaseems to be the creation of secondary craters and cchains. A less common alteration may be the formatiosubconcentric infacing scarps resulting from inward sluming of large sections of palimpsest and surrounding terra

In Epigeus and Zakar, both located within groovedrain, buried regional grooves can be traced into the odeposits as linear rows of hummocks before disappeaas discussed above. In the larger areal coverage of thkar image, however, the presence of the palimpsest caseen to have affected the trend of grooves located outhe palimpsest. Prominent grooves are present outsidekar to the northwest, southwest, and southeast (Fig.In several areas, but particularly in the southwest quadof Zakar, these grooves can be traced into the outerposits, where they appear subdued and buried (Fig.The trend of the grooves within and immediately adcent to the palimpsest appears to have been affectepalimpsest formation: grooves that are linear outsidepalimpsest become curvilinear and subconcentric nearwithin the palimpsest. Some grooves merge with subccentric infacing scarps in the concentric massif faciesouter deposits. The trends of grooves in and surroundingpalimpsest could be a result of inward slumping takingvantage of preexisting structural weaknesses along groor possibly of groove formation taking advantage of pexisting structural weaknesses along subconcentric sluassociated with the palimpsest. As no grooves truly cutpalimpsest without being deformed and deflected, it is mlikely that Zakar formed after groove formation in this arand that inward slumping utilized preexisting weaknesalong these grooves.

4. Comparisons to Ganymedean craters

An examination of typical Ganymedean craters helpclarify relationships between palimpsest facies and the ltions of the former rim and continuous ejecta limits.

Large craters on Ganymede commonly exhibit two ctinuous ejecta facies: an inner, elevated, scarp-bou“pedestal” of hummocky ejecta surrounding the rim, aa thinner, outer, radially textured ejecta blanket (SchenkRidolfi, 2002). In Voyager images, poor resolution obscuthe outer ejecta facies, and Horner and Greeley (1termed these craters “pedestal craters,” in reference toapparent similarity to Martian pedestal craters containonly a single layer or “pedestal” of continuous ejecta. Tpresence of the outer ejecta facies, confirmed by Galileoservations, shows that a more proper Martian analog isdouble-layered ejecta crater (Barlow et al., 2000).

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A high-resolution (180 m/pixel) Galileo image of theGanymedean crater Achelous (Fig. 2) provides the cleaexample of the textural differences between the pedestaouter ejecta facies.

The pedestal facies of Achelous is the topographicroughest unit surrounding the crater, rough-textured nearim and less so towards the distal edge of the unit. This dedge is lobate in plan view and slopes distinctly down toouter facies. The most prominent grooves from the surroing grooved terrain are visible through the pedestal faciewithin several km of the crater rim. Only the most promnent grooves are visible beneath the pedestal, howevereven they appear quite subdued. Although grooves areble through this unit, the mantling ejecta blanket is thickethe pedestal facies than in the outer facies. We interprepedestal deposit as analogous to the topographically rconcentric massif facies of palimpsests, in which preexistopography and secondary craters are visible buried bena thick blanket of ejecta.

The outer continuous ejecta of Achelous, particulanear its distal edge, appears radially textured, perhappressing the topography of underlying secondary crchains. Grooves can be distinctly seen through the outecies, although groove detail is obscured. The finest grovisible outside this ejecta blanket cannot be traced intoejecta, but larger, more prominent grooves can clearlyseen beneath the outer facies. The distal edge of thefacies is thin and feathered. The outer ejecta facies of Aelous appears smoother overall than its pedestal faciesmantles the underlying grooves much more thinly than dthe pedestal. This facies is similar to the outer depositpalimpsests, which appear relatively smooth and mantleondary craters, preexisting craters, and grooves more tthan does the concentric massif facies.

Beyond the outer ejecta facies, numerous exposedondary craters and radial secondary crater chains are aent. Most of these craters are 1 to 2 km in diameter. Basethe presence and distribution of exposed secondary crawhich begin just beyond the outer ejecta facies of Aclous and just beyond palimpsest margins, we infer thatedge of the continuous ejecta of a Ganymedean craterresponds to the margin of a palimpsest. Similarly, basethe above morphology, we infer the crater rim to correspapproximately to the boundary between the unorientedconcentric massif facies of a palimpsest.

Martian double-layered ejecta craters are hypothesizeform as a result of either impact into a volatile-rich tarmaterial (Carr et al., 1977) or atmospheric gases proing a fluidizing medium for some impact ejecta (Schuand Gault, 1979; Schultz, 1992). The similar double-layemorphology of Ganymedean craters and palimpsestsnot result from atmospheric effects, unless Ganymedean atmosphere in the past, or impacts created a tranlocal atmosphere. This suggests that the double-layeredphology of Ganymedean impact structures, and perhap


ogy,tiles

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the

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ies ispear

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analogous Martian double-layered ejecta crater morpholmay result from the presence of near-surface target vola

5. Comparisons to Europan ring structures

Galileo imaged two ring structures on Europa: Callanand Tyre. These structures have characteristics that clomatch those of palimpsests on Ganymede, suggesting ailar formation mechanism.

5.1. Callanish

The ring structure Callanish was imaged during orbitas a two-image mosaic at a resolution of 120 m/pixel, andalso with three partial frames during orbit E26 at a resoluof 25 m/pixel (Fig. 10A) (see Moore et al., 1998, 2001).these images, Callanish displays four facies that morphoically match the four palimpsest facies. As in palimpsesubconcentric troughs are noted in the concentric masscies, outer deposits, and beyond the edge of the outeposits, and small secondary craters begin just beyondouter deposit limits (Fig. 10B).

The central plains of Callanish (thebright central lobateunit of Moore et al., 1998) are relatively flat, bright plaifilling the center of the structure and covered with evenly dtributed low knobs from 200 to 500 m across. These plaare smooth at scales greater than about 500 m. The boun

.

-

-

y

between the central plains and the unoriented massif facnot everywhere distinct. In places, the central plains apto embay the massifs of the unoriented massif facies.

The unoriented massif facies (therough inner unitofMoore et al., 1998) consists of low hummocks or masabout 1 km across separated by approximately 1 kmpanses of a surface that appears rough at 120 m/pixel. Somehummocks are curvilinear and up to 3 km in length. Thummocks or massifs are not appreciably oriented, in ctrast to those of the concentric massif facies, and are mclosely spaced and topographically lower than those ofconcentric massif facies.

The concentric massif facies consists of massifs ab2 km across and between 5 and 15 km long arrangedcentrically to Callanish, separated radially by 2 to 3of a flat surface that appears smooth at scales greater500 m. These massifs contain the greatest topographic rpresent in Callanish. The massifs end abruptly at the boary with the outer deposits. Subconcentric graben and ining scarps are present in the outer portion of this unit andmore nearly concentric than the scarps noted in any ofpalimpsests on Ganymede. Throughgoing lineations trable from outside Callanish can be seen within this unitappear buried and degraded. This facies correspondstheannular massifsand portions of thesmoother outer flowunit mapped by Moore et al. (1998).

The outer deposits (portions of thesmoother outer flowunit of Moore et al., 1998) are smooth at 120 m/pixel and

Fig. 1cture. Man

Fig. 10. The multi-ring structure Callanish on Europa. (A) Mosaic of reprojected Galileo images. (B) Geologic sketch map of the area outlined in1A.Boundaries between facies are indicated by dashed lines. Walls of concentric graben are denoted by curved lines concentric to the ring struylineations (double straight lines) and secondary craters (small circles) are shown beyond the outer deposits. One large lineation crossing the ringstructure isburied and difficult to trace near and within the unoriented massif facies. This lineation is more clearly visible, but still buried, within the concentric massiffacies and outer deposits. The lineation is clearly exposed outside the ring structure. These observations are consistent with the former crater rimlocated at theboundary between the unoriented and concentric massif facies.


rroundi
Fig. 11. Galileo image of the multi-ring structure Tyre on Europa. Central region of uniform high albedo corresponds to the central plains. Sungconcentric features are subconcentric massifs, scarps, and graben.
hisuterfea

peaof

e bun-ateding

ss irateosi

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bero-in inentsits

ederiedcedre itn th

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600-helikeo thesifs,ding

outhin

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contain scattered low 200- to 300-m-diameter knobs. Tunit is the smoothest unit present in Callanish. The odeposits appear to cover and possibly embay preexistingtures at their outer edge. Many preexisting features apto be gradually “exhumed” within 5 km of the distal edgethe outer deposits. Throughgoing lineations are traceablpartially buried within this unit. The majority of subconcetric graben and troughs associated with Callanish are locin this unit. The margin of the outer deposits, and of the rstructure itself, is somewhat lobate.

Scattered exposed secondary craters 1.5 km and lediameter are located beyond the outer deposits. These chave the greatest areal density adjacent to the outer depand decrease in areal density away from the ring strucSecondary craters are not visible within Callanish itself.

Galileo images of Callanish allow another constraint toplaced on original crater rim diameter in addition to that pvided by palimpsest observations. The preexisting terrathe Callanish region, consisting of crisscrossing lineamand ridges, allows better interpretation of the ejecta limand the former crater rim location than terrain on Ganymdoes. One large lineation in particular can be seen buby ejecta in the outer ring structure units, and can be traup to the inner edge of the concentric massif facies whedisappears. It reappears at this same facies boundary o

-r

t

nrsts.

e

other side of Callanish, suggesting that the inner edge oconcentric massif facies may approximate the edge offormer crater rim.

5.2. Tyre

Tyre, another ring structure, was imaged in color atkm/pixel during orbit G7, and at 170 m/pixel under nearterminator conditions during orbit E14 (Fig. 11). From thigher resolution Tyre images, it is apparent that Tyre,Callanish, contains a central smooth region analogous tcentral plains, a region of concentrically arranged masand numerous subconcentric troughs or scarps surrounthe structure.

The central plains of Tyre (thesmooth central unitofKadel et al., 2000) are a slightly hummocky region ab20 km in diameter. This high-albedo region is located wita shallow, scarp-bounded depression.

The unoriented massif facies (corresponding to Kadeal. (2000)’srough inner unitand innermostannular massifunit) is about 50 km in diameter and surrounds the cenplains. The unoriented massif facies is comprised of eqand elongated massifs exhibiting no preferred orientatio

The concentric massif facies (all but the innermost ption of theannular massif unitof Kadel et al. (2000)) con


on-kmthan

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inges,

ran-tion

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outeruc-ies),assi

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Nowcom-rding

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sists of curvilinear chains of elongate massifs, oriented ccentric to the center of Tyre. This unit extends up to 100from the center of Tyre, and appears texturally smootherthe unoriented massif facies.

The outer deposits (thediscontinuous ejecta unitof Kadelet al., 2000) show ejecta-mantled secondary craters, ridand plains. Exposed small secondary craters and cchains oriented radially to the center of Tyre are visibleyond the edge of the outer deposits.

Like the terrain surrounding Callanish, the preexistterrain in the Tyre region consists of lineaments and ridgsome of which are obscured by Tyre and some of which tsect it (Fanale et al., 2000), suggesting lineament formaboth before and after the impact that produced Tyre.

6. Morphometry

In addition to the morphologic evidence noted abomorphometric similarities exist between palimpsest, craand ring structure facies. Schenk (1991, 1993) has also nmorphometric similarities between craters and palimpsand has tabulated central pit, crater rim, pedestal facand outer deposit dimensions of numerous large craterGanymede from Voyager images (Table 2). Figure 12 shthe radii of these palimpsest, crater, and ring structure faplotted against concentric massif facies (for palimpsestsring structures) or rim (for craters) radius. These morpmetric observations support the analogies drawn frommorphologic observations. Morphometrically, the outer apedestal ejecta facies of craters are analogous to theand concentric massif facies of palimpsests and ring sttures. The crater rim (the inner edge of the pedestal facthen, is analogous to the inner edge of the concentric mfacies in palimpsests and ring structures.

Schenk and Ridolfi (2002) provide an equation relatGanymedean continuous ejecta diameter to crater diamTheir equation also supports our analogies within the limof observational uncertainty.

7. Discussion

So far, we have described the morphology of palimpseand shown their morphologic and morphometric simila

,r

r

f

.

Fig. 12. Graph showing correspondence of facies between Ganymepalimpsests and craters and Europan ring structures. Solid data pointresent measurements on palimpsests and ring structures. Open datarepresent measurements on craters.

to Ganymedean craters and Europan ring structures.we discuss the implications that these observations andparisons have for the processes of and hypotheses regapalimpsest emplacement.

Based on an extensive morphologic and morphomestudy of craters on Ganymede, Iaquinta-Ridolfi and Sch(1995) found that for these craters, the edge of continuejecta can be approximated by the location where expsecondary craters start to appear. The presence of cleaposed secondary craters just outside the margins of allpalimpsests imaged by Galileo, but not inside, suggestspalimpsest margins correspond to continuous ejecta lim

Preexisting structures overlapped by palimpsest marprovide further evidence that these margins corresponcontinuous ejecta limits. Grooves exposed outside Epig(Fig. 4A) and Zakar (Fig. 5) can be traced through the mgins of these palimpsests into the outer deposits. Withinouter deposits, the grooves remain visible, but appear buby a blanket of ejecta. The large crater overlapped bymargin of Buto Facula (Fig. 6), which appears half expoand unmodified and half buried by ejecta, shows clearly

entral pitm)

Table 2Approximate facies radii for selected Ganymedean palimpsests and craters, and Europan ring structures

Palimpsest or ring Outer deposits Concentric Unoriented Central plains Crater name Outer Pedestal Crater rim Cstructure name (km) massif facies (km) massif facies (km) (km) facies (km) facies (km) (km) (k

Callanish 45 33 20 5 Achelous 49 35 20Tyre 70 46 25 9 Sebek 93 60 32 7Buto Facula 130 85 49 23 Isis 88 58 36 8Zakar 143 104 51 21 Melkart 141 92 54 17Memphis Facula 172 124 69 21 Osiris 150 92 54 17Epigeus 177 140 85 37 Enkidu 152 61 24


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the mantling ejecta deposit ends at the palimpsest margEpigeus (Fig. 8B), Zakar (Fig. 5), and Buto Facula (Fig.secondary crater chains that are exposed and clearly voutside the palimpsest boundary extend, mantled, benthe outer deposits and concentric massif facies, implyingboth of these units consist of ejecta blanketing the undeing topography.

Comparisons with craters on Ganymede allow us totimate where within a palimpsest the original crater rimlocated. The four surface units identified in Galileo imaof palimpsests are both morphologically and morphomrically similar to surface units identified in several craton Ganymede as imaged by Galileo and Voyager. Uthe location where secondary craters begin to appearside these craters and outside palimpsests as a tie poinouter deposits of palimpsests are found to be analogothe outer ejecta facies of craters; the concentric massif faof palimpsests are analogous to the inner ejecta, or pedfacies of craters; and the unoriented massif facies and ceplains are analogous to the crater interior (Fig. 12).

If the boundary between the unoriented massif faciesthe concentric massif facies does correspond to the onal crater rim, as is suggested by this scaling relationsno buried topographic features such as secondary crateregional grooves should be present within the unorienmassif facies or central plains of palimpsests. This is csistent with Galileo observations. Buried secondary crchains noted in Epigeus, Zakar, and Buto Facula are preonly in the outer deposits and the outer half of the concenmassif facies, and buried grooves and preexisting crateronly visible in the outer deposits. Additionally, the greaclarity with which these buried structures are seen neapalimpsest margin suggests that the mantling layer thinwards this margin, as expected if the outer deposits andcentric massif facies are palimpsest continuous ejecta.

The many preexisting lineaments and ridges surrounthe ring structure Callanish on Europa (Fig. 10) allowother constraint to be placed on both the continuous elimit and original crater rim location. These lineamentspear undisturbed outside the margin of the outer deposiCallanish, but disappear or are thickly mantled interiothis margin, suggesting that, like the outer deposits limiGanymedean palimpsests, the outer deposits limit of Caish corresponds to the edge of continuous ejecta. One prnent lineament traced into Callanish is visible up to the pwhere it crosses the boundary between the unorientedconcentric massif facies (Moore et al., 1998). This lineamis not seen within the unoriented massif facies, but reappat the boundary with the concentric massif facies on the oside of Callanish. The locations of disappearance and rpearance of this lineament support the idea that the bounbetween the unoriented and concentric massif facies appimates the original crater rim.

The infacing subconcentric scarps present in theGanymedean palimpsests analyzed here suggest that inslumping occurred following palimpsest formation. Ad

h

-e

l,l

r

t

e

-

-

d

-y-

d

tionally, the network of subradial fractures present at the cter of Epigeus (Fig. 8A) suggests that plastic flow and upof ice underlying the palimpsest center may have occurThis flow could have resulted from isostatic adjustmentlowing the creation of a cavity in the then-warm lithospheor from diapiric flow triggered by the thinner, weaker regof lithosphere in the region of greatest excavation depthward slumping of large sections of palimpsest could hproduced minor compression at the palimpsest center,the resulting volume increase at the center contributinthe uplift.

Both Memphis Facula and Epigeus were imaged at hresolution so that any flow structures or pools of impmelt would probably be detected if present, as predictethe palimpsest formation hypotheses of large-scale imptriggered extrusions (Thomas and Squyres, 1990) orplacement of fluid-rich ejecta (Greeley et al., 1982; Cr1983; Fink et al., 1984). The central plains of both Mephis Facula and Epigeus are only smooth in comparisoother sections of palimpsest; they still appear rough atresolution and may actually be quite rugged. If the cenplains formed by freezing of ponds of impact melt or fluejecta, they may have been modified and roughened stheir formation. This roughness may also result from chuof buoyant ice freezing in place in a pool of slushy ejecThe dark-floored troughs hundreds of meters across, prein both Memphis Facula and Epigeus, may be the resufluid ejecta locally draining around large coherent bloof solid ejecta into low-lying areas immediately followinthe palimpsest-forming impact. The relatively low albedothese areas may result from downslope movement ofmaterial as seen elsewhere on Ganymede.

There are three primary means of palimpsest materialplacement that can be evaluated based on these observa

(1) formation by large-scale post-impact extrusions of waplastically deforming ice (Thomas and Squyres, 199

(2) formation by liquid or slushy ejecta as proposedGreeley et al. (1982), Croft (1983), and Fink et(1984); and

(3) formation by dry, solid ejecta as in lunar cratering.

The lack of large-scale radial flow structures in palimpsis inconsistent with warm ice volcanic flows emanatfrom the palimpsest center. The lack of herringboneterns (Oberbeck and Morrison, 1973) and relative smoness of the outer palimpsest units seem inconsistent withejecta emplacement such as occurs on the Moon (Ober1975), although lunar herringbone patterns are typicnoted surrounding relatively fresh craters (Schultz, 19and could have been eroded from around palimpsests. Aatively smooth low-relief ejecta blanket with local clusteof massifs, similar to the surface textures in Ganymedpalimpsests, is expected from emplacement of fluidiejecta or an ejecta slurry with a considerable fractionimpact melt. Additionally, the observation of small da


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floored dendritic troughs, which could have formed aresult of fluid ejecta drainage, support a partially fluid ejeemplacement.

Similarities between Ganymedean craters and palisests, as well as the observation that, based on superpoand degradational state, palimpsests tend to be oldercraters of equivalent size, suggest that the lithospherGanymede has changed over time. Earlier in the geolhistory of Ganymede, its lithosphere may have been thinor had a higher temperature gradient, causing large impto produce high-fluid-content ejecta, forming palimpseMore recent impacts of similar size were into a thicklithosphere or one with a lower temperature gradient, foing more typical craters. This inference is supported bypresence of relatively young palimpsest-like impact scsuch as Callanish and Tyre, on the thin lithosphere ofropa. The presence of double-layered ejecta surrouncraters on Ganymede hints at the role of target volatileejecta emplacement (Carr et al., 1977).

8. Conclusions

Analysis of Galileo images of Memphis Facula, EpigeZakar, and Buto Facula provides evidence that contradseveral of the hypotheses proposed for palimpsest formaNo evidence is seen in any of these palimpsests at up tm/pixel resolution for large-scale radial flow structures twould be expected if the extrusion suggested by ThomasSquyres (1990) had taken place. The presence of troin Memphis Facula and Epigeus, possibly resulting frdrainage of a fluid ejecta component, does support thetially fluid ejecta hypotheses of Greeley et al. (1982), C(1983), and Fink et al. (1984). The presence of buried sttures such as grooves and secondary craters within thedeposits and concentric massif facies of Epigeus, ZakarButo Facula rules out the possibility that the palimpsest mgin corresponds to the former crater rim (Hartmann, 19Lucchitta and Ferguson, 1988), as these preexisting stures would have been located interior to the crater rimbeen destroyed during palimpsest formation. Similarly,buried secondary craters noted within the concentric mafacies in these palimpsests rule out the pedestal depospothesis of Schenk (1996), as that hypothesis places tburied secondaries interior to the former crater rim.

The continuous ejecta hypothesis of Passey and Smaker (1982) is supported by the presence of exposedondary craters beginning just beyond the palimpsest mabecause the extent of the continuous ejecta deposit asated with craters on Ganymede can be approximated blocation where secondary craters begin to appear (IaquRidolfi and Schenk, 1995). The presence of grooves, ocraters, and secondary crater chains that are clearly expoutside the palimpsest margin and visibly mantled with mterial within this margin also supports this view. Additioally, the morphologic and morphometric similarity of su

n

.

r

-e

--

i-

-

d

face units seen in the four palimpsests imaged by Galilethose of craters on Ganymede and ring structures on Eusupports the continuous ejecta hypothesis. Further, theof buried secondary craters or preexisting structure withe unoriented massif facies or the central plains in all fpalimpsests imaged by Galileo and within these same uin Callanish and Tyre is consistent with the palimpsest mgin being equivalent to the continuous ejecta deposit limithe boundary between the unoriented and concentric mfacies corresponds to the location of the original crater rThe possibility of fluid-rich ejecta is consistent with the cotinuous ejecta hypothesis of Passey and Shoemaker (1We conclude, then, that the palimpsest deposit is a fluid-continuous ejecta deposit resulting from an impact intoice-rich target.

Acknowledgments

We gratefully acknowledge the work of Galileo Projescientists and engineers in planning and acquiring theseand Herb Breneman, Scott Murchie, and Dave Sensketheir planning work. We thank Geoff Collins and LouiProckter for fruitful discussions and comments, and NadBarlow for a helpful and constructive review. This effort wfunded by the NASA Galileo Project as part of the SoState Imaging Team through a contract from JPL to JaW. Head III.

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