dark terrain on ganymede: geological mapping and ...dark terrain on ganymede: geological mapping and...

ICARUS 135, 317–344 (1998)ARTICLE NO. IS985981

Dark Terrain on Ganymede: Geological Mapping and Interpretation ofGalileo Regio at High Resolution

Louise M. Prockter, James W. Head, and Robert T. Pappalardo

Department of Geological Sciences, Box 1846, Brown University, Providence, Rhode Island 02912E-mail: [email protected]

David A. Senske

Sterling Software, Jet Propulsion Laboratory, Pasadena, California 91109

Gerhard Neukum, Roland Wagner, Ursula Wolf, Jurgen Oberst, and Bernd Giese

Institute of Planetary Exploration, DLR, Berlin D-12489, Germany

Jeffrey M. Moore

NASA Ames Research Center, MS 245-3, Moffett Field, California 94035

Clark R. Chapman

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

Paul Helfenstein

Department of Astronomy, Cornell University, Ithaca, New York 14853

Ronald Greeley

Geology Department, Arizona State University, Tempe, Arizona 85287-1404

H. Herbert Breneman

Jet Propulsion Laboratory, Pasadena, California 91109

and

Michael J. S. Belton

National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, Arizona 85719

Received November 3, 1997; revised April 6, 1998

found on furrow and crater floors. We also find high albedoDuring its first two encounters with Ganymede, the Galileo units which include crater rims, furrow rims, and isolated knobs

and massifs. Other features include an intermediate albedospacecraft obtained images of a 16,500 km2 portion of GalileoRegio, a large expanse of dark terrain, at high resolution (76–86 lobate feature interpreted to be a palimpsest and a hummocky

unit interpreted to be impact ejecta. Several processes are inter-m/pixel). Through mapping of the G1 and G2 target sites withinGalileo Regio, we are able to characterize geological units based preted to have occurred within Galileo Regio. These include

tectonic deformation, mass wasting, sublimation, resurfacingon their morphology and relative albedo. We find three gener-ally low albedo units: an intermediate albedo plains unit, a by impact ejecta, and possibly cryovolcanism and isostatic ad-

justment. We observe that the NW–SE trending furrowslower albedo plains unit, and the lowest albedo unit which is

3170019-1035/98 $25.00

Copyright 1998 by Academic PressAll rights of reproduction in any form reserved.

318 PROCKTER ET AL.

(Lakhmu Fossae) in Galileo Regio are degraded and are cross- affected by several different geological processes. Tectoniccut by the younger N–S trending furrows (Zu Fossae). We also structures are prevalent, with the majority of the brightfind several other tectonic features which may be minor faults terrain comprised of subparallel grooves at a variety ofor fractures related to one or other of these systems. Through scales and orientations (e.g., Pappalardo et al. 1998, Collinsmapping and crater size–frequency distributions, we are able et al. 1998). The dark terrain is crossed by furrows, vastto propose a stratigraphy for the Galileo Regio target site. The concentric systems of flat-floored troughs, most of whicholdest features in the area are high albedo knobs and massifs, are hypothesized to be impact-related (McKinnon andwhich are interpreted to be remnants of early impact-related

Melosh 1980, Schenk and McKinnon 1987, Murchie et al.features and furrow rims. These may have formed at approxi-1990). Galileo imaging goals (Carr et al. 1995) specificmately the same time as the intermediate and low albedo plainsto Ganymede’s dark terrain include characterization ofunits and the furrow systems. The lowest albedo unit of furrownonice components, the origin of spectral and albedo heter-floors probably subsequently evolved through sublimation andogeneities, the formation of furrows, the global structuremass wasting. Much of the northeast portion of the target areaand origin of the furrow systems, the mode of emplacementwas subsequently obscured by one of the youngest units, ejectaof dark terrain, the nature of the breakup of dark terrain,from an impact just to the north. We use our mapping of the

high-resolution images of Galileo Regio to evaluate three end- and mass wasting processes.member models for the formation of dark terrain: (1) the crust In this paper, we utilize geological mapping and crateris dark throughout, (2) material on the surface is the result of size–frequency distributions to address many of these pri-a low albedo cryovolcanic layer over a higher albedo crust, and mary goals. Stereo imaging allows us to understand the(3) dark material is distributed in small quantities throughout high-resolution topography of a portion of the target area.the crust, and geological processes have acted to concentrate We concentrate on the detailed morphology of the globallow albedo material on the surface. Although it is possible that furrow systems, the mode of emplacement of dark terrain,elements of more than one of these models are present within the stratigraphy of dark terrain, and surface processesthe dark terrain, we find that the third model, that of a thin which have shaped and modified dark terrain. We investi-veneer of low albedo material, best fits observations of Galileo

gate several surface processes including mass wasting, im-Regio. 1998 Academic Presspact gardening, contamination of the surface by infallingKey Words: Ganymede; geological processes; satellites, gen-meteoritic debris, sputtering, transfer or loss of volatileeral; satellites of Jupiter; surfaces, satellite.constituents by sublimation at low latitudes, isostatic relax-ation, and cryovolcanism.

I. INTRODUCTION

II. GALILEO REGIO HIGH RESOLUTIONThe first regional resolution images of Ganymede wereTARGET AREAobtained in 1979 by the Voyager spacecraft and showed

the surface to be divided approximately equally into dark,This study focuses on a target site within Galileo Regio,

heavily cratered terrain, and bright terrain with subparallelthe largest contiguous region of dark terrain on Ganymedegrooves (Smith et al. 1979a, 1979b, Shoemaker et al. 1982).(Fig. 1a). The G1 target was initially chosen to image anThe bright terrain is composed of relatively clean waterunusual feature which appeared to be an old, degradedice, while the dark terrain is found to contain a fractionimpact crater partially buried by resurfacing materialsof some low albedo component on the surface or admixed;(Murchie et al. 1989; Heliopolis Facula in Figs. 1b and 1c),estimates of the amount of this contaminant range fromand to simultaneously capture an intersection of furrowsless than 10% (Clark 1980) to as much as 45% (Spencerfrom the Lakhmu and Zu Fossae sets (Figs. 1a–1c). The1987b). This contaminant is sufficient to make the surfaceG1 observation comprises four images (Table I) coveringappear dark at visible wavelengths; global average singlea region approximately 110 km (north to south) by 150scattering albedos for dark terrain range from 0.42 , w ,km (east to west), at p77 m/pixel. The G1 images (Fig.0.72 (Helfenstein 1985).1b) were obtained in a mode which provided compressionThe first Galileo spacecraft images of Ganymede wereby means of least-significant-bit and line truncation; theobtained in June 1996 (Belton et al. 1997). The first (G1)G2 image is integer cosine transform (ICT) compressedand second (G2) orbits focused on Ganymede and pro-at a ratio 8.3 : 1. From the G1 images, the area was seenvided images of unparalleled resolution, some more thanto have extremely heterogeneous albedo; to aid in thean order of magnitude higher than those obtained by Voy-interpretation of the complex topography, an additionalager 17 years previously.image was taken of a portion of the same region (approxi-The primary Galileo imaging goals for Ganymede weremately one third of the G1 target site, Figs. 1c and 2) onto extend the coverage from that of Voyager and to obtainthe G2 orbit. The overlapping images were taken at solarhigher spatial resolution (Carr et al. 1995). Based on Voy-

ager imaging, the surface of Ganymede was thought to be illuminations which were almost identical, but at camera

GEOLOGICAL MAPPING OF GALILEO REGIO 319

FIG. 1. (a) Voyager image (FDS 20636.59) of Galileo Regio, the largest contiguous region of dark terrain on Ganymede. This image clearlyshows the NW–SE trending Lakhmu Fossae and N–S trending Zu Fossae furrow systems which cross the region. The box shows the location ofthe Galileo G1 high resolution images. (b) The Galileo G1 mosaic superimposed on Voyager data (processing by the Jet Propulsion Laboratory).The resolution of the Voyager image is p1 km/pixel, while the average resolution of the superimposed Galileo mosaic is 77 m/pixel. The G1 mosaicis reprojected to a simple cylindrical projection and measures 150 km (E–W) by 110 km (N–S). North is up; solar illumination is from WSW. SeeTable I for specific image data. (c) Locations of the major features and the location of the G2 image data within the G1 mosaic.

320 PROCKTER ET AL.

TABLE I

Incidence Emission Phase Slant range ResolutionOrbit S-clock Picture number angle (8) angle (8) angle (8) (km) (m/pixel)

G1 s0349759013 G1GSGREGIO01 29.5 47.5 20.9 7660 77.83G1 s0349759026 G1GSGREGIO01 31.0 49.5 21.2 7650 77.75G1 s0349759039 G1GSGREGIO01 30.2 48.8 21.2 7570 76.90G1 s0349759052 G1GSGREGIO01 28.7 46.6 20.7 7450 75.70G2 s0359944739 G2GSGLLREG02 24.8 40.7 54.4 8460 86.00

positions sufficiently separated to allow stereo viewing of junction between furrows from two distinct sets (LakhmuFossae and Zu Fossae; Fig. 1a), which can be resolved intothe scene. The G2 image resolution is 86 m/pixel, similar

to that of the G1 image. From these data, a digital terrain high albedo rim and low albedo floor units. In addition tothe targeted degraded impact feature (Heliopolis Facula),model (DTM) has been constructed (Fig. 3), as discussed

in detail by Giese et al. (1998). The DTM covers a region a 20-km crater (Ea) is visible in the central lower half ofthe region, and a 50-km crater (Khepri) is located slightlyof 63 3 102 km; it has a spatial resolution of 200 m/pixel

and relative height variations of up to 1.5 km. Due to to the north of the target area (Figs. 1b and 1c). We recog-nize three plains units of differing albedo, two of whichlimitations in the digital image correlation approach, spa-

tial resolution of the DTM is a factor of 3 poorer than are distributed relatively evenly throughout the region; thethird is found in furrow and crater floors. High albedothe original images, but nevertheless this model proves

invaluable in the interpretation of morphological features material is observed in the form of isolated knobs andmassifs. To the west of the imaged area is a plateau of highand surface processes within Ganymede’s dark terrain.

The G1 Galileo Regio images include morphological albedo material (Figs. 1b and 1c), apparently composed oflinear hummocks.features characteristic of the dark terrain, including the

FIG. 2. The Galileo G2 image of Galileo Regio covers an area 102 km (N–S) by 63 km (E–W). The viewing geometry of the Galileo G2 targetis sufficiently different from that of G1 to allow stereo viewing (Table I). Topographic data is thus obtained of several important morphologicalfeatures in the area, including Heliopolis Facula in the east of the image, Ea in the lower center, and the high albedo hummocky plateau to thewest. North is up; solar illumination is from WSW; image projection is orthographic.


FIG. 3. Perspective view of Galileo Regio constructed using G1 data and the Digital Terrain Model described in Giese et al. (1998). View isfrom the southwest. Image produced by DLR, Berlin.

III. GEOLOGICAL MAP AND DISCUSSION OF rain information) allow the photometric properties of thesurface to be studied and the spatial distribution of highMAPPING PROCEDUREand low albedo material to be related to its topography.

From the reprojected and mosaiced G1 and G2 images The plains units in the Galileo Regio site are relatively(Fig. 4a), we have constructed a geological map on the flat, which indicates that our estimates of relative albedobasis of relative albedo and stratigraphic relationships (Fig. are realistic and are not influenced unduly by topography.4b). Crater size-frequency statistics have also been deter- In areas where topography is significant, we take into con-mined, and these are discussed in Section VI. The mapping sideration the illumination geometry, as discussed in moreunits can be divided into three broad categories. These detail in the surface processes section that follows. Thus,include high albedo units, which include furrow rims, crater the terms ‘‘high,’’ ‘‘intermediate,’’ and ‘‘low’’ albedo arerims, and isolated knobs and massifs, low albedo units, all used in a relative sense.including an intermediate plains unit, a low albedo plains Next, we describe the distribution of each unit and illus-unit, and the lowest albedo unit found in the floors of trate the morphological characteristics of each.furrows and craters, and impact features, which includecraters, palimpsests, and impact ejecta. We discuss each of a. High Albedo Unitsthese units in detail below. Although they are structurally

High albedo units are generally topographically elevatedrelated units, we have treated the rims and floors of furrowsabove their surroundings and may be embayed by sur-as distinct mapping units, as the high resolution of therounding plains. These units are found in the form of iso-target area allows us to readily distinguish the differentlated knobs and massifs (Figs. 4a and 4b; m1), a high pla-units which comprise the furrows. Figure 4c is a structureteau to the west of the target area (Figs. 4a and 4b; m2),map of the area.and furrow and crater rims (Figs. 4a and 4b; m3). Each isWe use the term ‘‘albedo’’ in this study to indicate rela-discussed in detail below.tive albedo, as absolute albedos have not yet been deter-

mined for the Galileo Regio target area. DTM data along i. Isolated massifs. Distributed primarily along thenorthern part of the target area, these isolated blocky mas-with orthoimages (images rectified on the basis of the ter-

322 PROCKTER ET AL.


FIG. 4. (a) Mosaic of Galileo G1/G2 images in simple cylindrical projection, with a grid overlay for easy identification of features discussed inthe text. (b) Geological map of the area showing the major unit classifications discussed in the text. The units in this map are the same as thoseused for crater counting statistics (see Fig. 12). (c) Detailed structure of the map area. The two most prominent Lakhmu and Zu furrows are shownin light and dark tone and Heliopolis Facula and Ea are hatched for reference.

324 PROCKTER ET AL.

sifs and knobs (m1) generally stand higher (,1 km) than ing away from the Sun and can lead to confusion wheninterpreting the data; it is easy to ‘‘reverse’’ the topography.the surrounding plains (Figs. 4a, 4b, and 5a). They have a

relatively high albedo and are commonly angular in plan Figure 3 shows a perspective view of the area, which clari-fies the topography despite these interesting albedo effects.view, suggesting that their formation or modification may

have been partially controlled by tectonic structure. The These features are discussed in detail in Section V. Furrowrims rise up to 900 m above the surrounding plains, andmassifs are in places embayed by surrounding low albedo

plains. These massifs and knobs are found predominantly stereo imaging reveals that the highest elevation in thetarget area is found where two furrow rims cross (E6,in association with the furrows and rimless troughs in the

north and east of the area (e.g., as seen in C3 and D7 of F6 of Figs. 4a and 4b). The rims are usually linear andcontinuous (e.g., E5 in Figs. 4a and 4b), but may be discon-Figs. 4a and 4b), and are commonly associated with the

intermediate albedo plains unit (but this is not a clear tinuous and appear to be embayed by surrounding plainsin places. A narrow (pfew hundred meters), low albedoembayment relationship; one feature appears to grade into

the other in places). feature is observed near parts of the crests of some of thefurrow rims and just below the crests of the larger craterIt is likely that the massifs are the remnants of furrow

or crater rims which have been modified tectonically by rims, including Ea (Fig. 4b). Detailed furrow morphologyis discussed in Section IV.small-scale fractures, resulting in isolated portions of rim

crests and adjacent plains. Thus, the blocks tend to belarger than the width of individual furrow or crater rims b. Low and Intermediate Albedo Unitsin this area. Fewer massifs are observed in the southern

Several plains units are found within the Galileo Regiopart of the target area, where furrow rims are generallysite, some of which appear to be impact related. Thesemuch more coherent, supporting the idea that the massifsunits are characterized by their albedo and morphology.are rim remnants.Lower albedo units can be subdivided into two plains units,

ii. Linear massifs/hummocky plateau. To the west of pi and pd (Figs. 4a and 4b), and a furrow and crater floorthe target area is a high albedo, hummocky unit (m2), unit, pdf (Figs. 4a and 4b). Plains units are also foundwhich appears to form a high-standing plateau which rises within the 20-km crater Ea, (pcf; Figs. 4a and 4b) and to,1 km higher than troughs to its north and south (E–F, the immediate north of this crater (ps; Figs. 4a and 4b).1–2 of Figs. 4a and 4b; Fig. 5b). The unit is comprised of An intermediate plains unit (pl) is found within Heliopolisa series of linear hummocks approximately 2 km in width Facula in the east of the area (Figs. 4a and 4b), and aand several kilometers long, which are parallel or curve hummocky unit (ph) is distributed in the northeast of thearound each other. Individual hummocks are morphologi- target (e.g., Figs. 4a and 4b).cally similar to subdued furrow rims. Pockets of low albedo

i. Intermediate albedo plains. The intermediate plainsmaterial are observed between the hummocks. At Voyager(pi) unit is relatively smooth, with a somewhat rolling to-resolution, this feature appears to be the rim of a furrowpography, and appears to be more heavily cratered than(Fig. 1a), but at Galileo resolution, furrows tend to havethe low albedo plains unit (Figs. 4a and 4b; Fig. 5d). Thisnarrow, relatively well-defined rims. However, the plateauis one of the most spatially abundant units and is fairlyis associated with a smaller furrow rim that trends fromevenly distributed throughout the study area. The interme-the southwestern side of Ea to the southeastern portiondiate plains typically are slightly elevated with respect toof the plateau. The plateau has a similar albedo and eleva-their lower albedo counterparts, and relatively sharp con-tion to the furrow rim and may be a continuation of it. Atacts are in places visible between the two. There is nolow albedo feature, interpreted to be a furrow floor, cutsclear correlation between the intermediate plains unit andthrough the plateau.any of the furrow systems or other geologic units in the

iii. Furrow rims. High albedo furrow rims (m3) are area, and the unit is spatially interspersed with the lowerfound bounding many, although not all, of the furrows in albedo plains unit.the target area, particularly in the southeast of the region(F6 and G7 of Figs. 4a and 4b; Fig. 5c; models of formation ii. Low albedo plains. The low albedo plains (pd) are

similar in nature to the intermediate albedo plains, butfor furrows are discussed in detail in Section IV). The rimsare generally between 3 and 5 km in width and may be have a lower albedo and appear to be topographically

low (Figs. 4a and 4b; Fig. 5e). Relatively smooth, with arounded to triangular in shape, often with a well-pro-nounced ridge crest. Despite their generally high albedo, sometimes rolling topography, the plains often appear to

embay adjacent features, particularly topographically highfurrow rims commonly appear dark on their south-facingslopes and have low albedo streaks on their north-facing units such as furrow rims and massifs (e.g., C3 of Fig. 4b).

This unit is found throughout the target area and is notslopes (e.g., Fig. 8). This is contrary to what is expected,namely that sun-facing slopes are brighter than slopes fac- obviously associated with any other units, other than being


FIG. 5. Each suite of images shown is a representative selection of one of the geological units in the Galileo Regio target area. This figureillustrates the variations in character and relative albedo between the different units discussed in the text. All images are oriented with north upwardand solar illumination from the WSW. Each image section is approximately 5 km across.

326 PROCKTER ET AL.

spatially interspersed with the intermediate albedo to scalloped in outline and in places form distinct contactswith the surrounding low albedo plains. Stereo shows thatplains unit.this unit rises up to 500 m above the surrounding plains.iii. Furrow and crater floors. The furrow floors (pdf)The height of this deposit may result from a thick layer ofhave the lowest relative albedo observed within the targetcontinuous ejecta or possibly from cryovolcanism, perhapsarea and are clearly visible crosscutting the region (e.g. F7eroded into a scalloped margin by processes such as subli-of Figs. 4a and 4b). The material in furrow and crater floorsmation (see Section V.d). If the deposit is cryovolcanic, itsappears to be of the lowest albedo observed within thetopography could result from a very viscous cryolava, ortarget area (Figs. 4a and 4b; Fig. 5f). Crater floor materialfrom a series of stacked thinner flows. To the south andof similar appearance and occurrence is estimated to havesouthwest of the feature, furrow rims are visible roughlya mean albedo of 0.25 6 0.04 from Galileo imaging offollowing the curve of its margin. These rims probablyUruk Sulcus (Helfenstein et al. 1997). This is consistentformed later and may have been structurally controlled bywith an estimate of 0.22 6 0.05 for similar dark flooredHeliopolis. The lobate plains unit is interpreted to be acraters imaged by Voyager (Schenk and McKinnon 1991).palimpsest deposit.The furrow floors are the lowest topographic features

found and can be p200 m lower than the surrounding vii. Hummocky plains. These plains units (ph, Fig. 5j)plains. Sharp contacts are commonly visible between the are found in the northeast corner of the target area (A–C,furrow floors and the adjacent topographically higher fur- 6–8 of Figs. 4a and 4b). The unit has a heterogeneousrow rims. Stereo imaging shows that the floors range in albedo resulting in a mottled appearance and is comprisedmorphology from V-shaped to flat and hummocky (e.g., of hummocky material, with individual hummocks measur-Fig. 6), and may vary in width from a few km to 10 km ing p1 km. The unit obscures almost all other units, exceptwide (detailed furrow geometry is discussed in Section IV). for some portions of furrow rims (e.g., at B8 of Figs. 4aSmall craters are visible within the floor unit, but these and 4b), and two large (p10 km) craters, whose rims arerarely have wide or coherent high albedo rims (unlike still partially visible (A8 of Figs. 4a and 4b). This suggeststhose formed on other plains units), suggesting that the that the unit is relatively young and has a significant thick-furrow floors may be composed of relatively loose material. ness. Crater depth–diameter relationships for Ganymede

(Schenk 1993) suggest that the craters were approximatelyiv. Crater floor plains. The central portion of the crater600 m deep at the time of formation. Since parts of theEa (F 4–5 in Figs. 4a and 4b) is floored by relatively lowrims of these craters are completely obscured, it is likelyalbedo plains material (pcf, Fig. 5g). Stereo imaging showsthat the hummocky unit has embayed them to a depththat the floor of the crater is fairly smooth and has a convexof several hundred meters at this location. This unit isshape, its central region rising several hundred metersinterpreted to be the ejecta from Khepri just to the northabove the base of its inside wall (see Fig. 10 for a detailed(Figs. 1b and 1c). Further from Khepri, the unit appearsview). The unit forms distinct, lobate contacts with theto bank up against existing topography (e.g., B6 and C7crater wall in several places, most notably on the southeastof Figs. 4a and 4b) and may be only a few hundred metersrim (F5 of Fig. 4b). In the center of the crater floor unitthick. The unit forms a sharp, lobate contact with higherare dark linear features, 1–2 km in length, arranged radi-topographic units, implying that it may have been em-ally. These are interpreted to be fractures and may beplaced as a continuous ejecta deposit.related to the mode of emplacement of this unit.

v. Smooth plains. Immediately to the north of the cra-c. Structureter Ea, a smooth, mid-albedo unit with lobate or scalloped

margins (ps) is seen (E 4–5 on Figs. 4a and 4b; Fig. 5h). The interpreted structure of the Galileo Regio targetThe unit is almost as high topographically as the north rim site is shown in Fig. 4c. Most structural features in theof the crater and appears to embay part of the region region are clearly related to the furrow systems. Scarpsbetween two northwest-trending furrows. The proximity indicate furrow rims, while troughs have no obvious rims.of this plains unit to the crater Ea, along with its texture, The Lakhmu furrow, which trends from NW to SE acrossalbedo, and elevation, suggest that it may be part of the the central portion of the map, has several scarps associatedcontinuous ejecta from that impact event. with it, particularly at its southeastern end where it inter-

sects the prominent N–S trending Zu furrow. These scarpsvi. Lobate plains. Lobate plains (pl) comprise Helio-polis Facula in the eastern part of the target area (Figs. correspond to the rims of the furrows, range from p10 to

p50 km in length, and generally appear to face in toward1b and 1c; C–E 7–8 in Figs. 4a and 4b; Fig. 5i), which hasa mid to high albedo, and is relatively smooth, although it the furrow floor. At the northwestern end of the Lakhmu

furrow, scarps are not observed; here, narrow troughs arehas several craters upon it which are probably secondariesfrom the impact which formed the crater Khepri to the visible, ranging from p15 to p30 km in length. The struc-

tures of this furrow show an anastomosing and intersectingnorth. The subcircular margins of the deposit are lobate


pattern, characteristic of rifts on Earth (e.g., Harding and furrow set within Galileo Regio, the Lakhmu Fossae, con-sists of primarily NW–SE trending concentric (and someLowell 1979).

The N–S trending Zu furrow, which crosscuts the subradial) furrows arranged around what may be a largecentral palimpsest (Murchie et al. 1990). The Zu FossaeLakhmu furrow, is defined primarily by inward-facing

scarps, and is interpreted as a graben. The scarps (corre- contain primarily N–S trending furrows and are younger,transecting the Lakhmu furrows (Cassachia and Stromsponding to the furrow rims) are spaced approximately

10–15 km apart measured from the rim crests. At the 1984). Schenk and McKinnon (1987) suggest that Zu Fos-sae are related to the impact which formed the Lakhmunorthern end of this furrow, two scarps both face to the

west and no graben are present. To the northeast of the Fossae, although other workers (Cassachia and Strom1984; Murchie et al. 1990) propose the Zu furrows arearea, subdued furrow rims are present; these have been

partially obscured by a younger, hummocky unit. Small arranged subradially around a point about 1700 km eastof the palimpsest at the center of the Lakhmu Fossae.troughs are seen trending NW–SE through the outer parts

of Heliopolis Facula (E8 of Fig. 4b) and are probablyassociated with the Lakhmu furrow. It is not clear whether b. Models of Furrow Formationthey pre- or postdate Heliopolis.

Several workers have discussed the origin of the furrowIn the southwest part of the imaged area, additional

systems on Ganymede. McKinnon and Melosh (1980) in-scarps and troughs are observed trending primarily NW–

terpreted the furrows to be graben formed in an exten-SE. The majority of these are probably associated with a

sional and largely axially symmetric regime; they proposedLakhmu furrow that trends through the southwest corner

the source for stress to be the collapse of a large impactof the image mosaic. One scarp trends from the western

crater, analogous to their interpretation of the Valhallaside of Ea (F4 of Fig. 4b) through to the hummocky plateau

multiringed structure on Callisto. The type and extent ofto the west (F1 of Fig. 4b), where it becomes a narrow

the fault pattern would depend on the scale of the impacttrough. Just to the north of this feature is a parallel trough,

and the rheologic structure of the satellite. An impactwhich cuts through Ea (F 4–5 of Fig. 4b) and appears to

origin has been questioned on the basis of constant in-cut through the hummocky plateau (E1 of Fig. 4b). This

terfurrow spacing (Cassachia and Strom 1984) and devia-feature, like several others including a trough which runs

tions from concentricity of some Ganymede furrowsE–W across the center of the image (D 3–6 of Fig. 4b),

(Zuber and Parmentier 1984), although the latter havehas no obvious rim associated with it. This suggests that

been suggested to be due to inherent properties of impact-these small rimless troughs may be fractures rather than

induced multiringed structures on icy satellites (Schenkfurrows such as those discussed above, and they may be the

and McKinnon 1987).result of different formation processes than the furrows.

Murchie et al. (1990) proposed an origin for the ZuWe infer that the structural history of this region began

Fossae of isostatic uplift over a large-scale mantle thermalwith extension in the northeast–southwest direction to

anomaly centered p1700 km east of the center of theform the Lakhmu furrows followed by extension in the

Lakhmu Fossae. Schenk and McKinnon (1987) find thateast–west direction to form the Zu furrow, along with

this furrow system is related to the impact which formedother minor stresses, either coincident or later.

the Lakhmu system, however.McKinnon and Melosh (1980) proposed that the raisedIV. FURROW MORPHOLOGY

furrow rims probably formed during the relaxation of long-wavelength components of initial furrow relief. This woulda. Furrow Systemsresult in the graben floors rising, and bounding escarp-

Furrows on Ganymede are found only in the older, heav-ments would probably be rotated and carried upward. This

ily cratered dark terrain (Smith et al. 1979b). Observationstheory can now be tested from stereo imaging which reveals

from Voyager showed that furrows have wide (5–10 km),the detailed furrow morphology.

flat floors, are bounded by sharp, raised rims, and rangefrom 50 to several hundred kilometers long (Smith et al.

c. Galileo Furrow Imaging1979b, Shoemaker et al. 1982). Interfurrow spacing is fairlyuniform at p50 km, although the furrows are generally Using stereo-based topographic data (Giese et al. 1998)

we have constructed profiles across the two prominentspaced more closely near the center of a concentric furrowsystem (Passey and Shoemaker 1982). furrows observed in the Galileo Regio target area. Figures

6a and 6b show the locations of the profiles. Profiles 1–7At least three main systems of furrows have been identi-fied from Voyager data (Zuber and Parmentier 1984, (Fig. 6c) are taken across the NW–SE trending Lakhmu

furrow. This furrow exhibits much more subdued reliefSchenk and McKinnon 1987, Murchie et al. 1990). Lakhmuand Zu Fossae are located in the antijovian hemisphere, and lower slopes than the Zu furrow which crosscuts it.

Furrow rims rise from 100 to 600 m above the furrow floorcrossing both Marius Regio and Galileo Regio. The main

328 PROCKTER ET AL.

FIG. 6. Stereo-derived topography allows profiles to be measured across parts of the Galileo Regio target area. (a) Locations of 7 profilestaken at intervals along the NW–SE trending Lakhmu furrow. (b) Locations of 7 profiles taken at intervals along the N–S trending Zu furrow.Profiles across the (c) Lakhmu furrow and (d) Zu furrow. Filled arrows indicate the position of the furrow floor; open arrows indicate the approximateposition of the rim crests. The profiles are arranged such that the floor of the furrow in each is aligned with the others, in order to facilitatecomparison of rim and floor geometry. The horizontal distance scale of the profiles has been expanded for clarity, and their vertical exaggerationis 53. Profiles across the Lakhmu furrow show generally shallower slopes and a less pronounced floor than do those of the Zu furrow. Note thepoint where the Zu furrow crosses the Lakhmu furrow (profiles 4 and 11); the east rim on profile 4 and the west rim on profiles 11 are the highestpoints in the area.

and ,400 m above the surrounding terrain. Rims range V-shape to relatively flat and measures from p2 to p10km across. The generally subdued relief of this Lakhmuup to p5 km in width. The southwestern rim of this furrow

is relatively unbroken, but the northeastern rim is com- furrow suggests that it is degraded in comparison to theZu furrow.prised of discontinuous high albedo knobs and in some

places is absent altogether. Slopes range from 38–138 on Profiles 8–14 (Fig. 6d) were measured across the youngerN–S trending Zu furrow and show rims which rise fromthe western walls and 28–108 on the eastern walls, with

average slopes of 98 and 68 for the western and eastern p200 to 1200 m above the furrow floor and ,900 m abovethe surrounding terrain. Rims range from p3 to p5 km inwalls, respectively. The furrow floor varies from a shallow


width. The western wall is highest at the junction where to resolve (Pappalardo et al. 1997a). The Galileo Regiotarget site is several hundred kilometers from any groovethis furrow crosscuts the NW–SE trending furrow of the

Lakhmu Fossae system (profile 11). At its northern and lanes and appears to be a relatively ‘‘pristine’’ area ofdark terrain. Nevertheless, tectonic activity is probably thesouthern points within the imaged area, the floor of the

furrow is V-shaped, but at its center, where the two furrow major factor in the evolution of dark terrain and the shap-ing of the surface morphology.sets cross (profiles 11 and 12), the floor is relatively flat

and wide, measuring 5 km. Wall slopes range from 28–388b. Impact Crateringon the western walls and 68–308 on the eastern walls. Aver-

age slopes are 208 and 158 on western and eastern walls, re- Impact cratering is clearly a significant process withinspectively. Ganymede’s dark terrain. In the Galileo Regio target site,

Detailed observations of the furrow systems, along with we have mapped the distribution of craters down to thecareful designation of the different geological units present limit of resolution (Fig. 7). Just to the north of the Galileoin the Galileo Regio target area, enable us to determine Regio target area is a 50-km crater, Khepri, the southernthe range of surface processes which may potentially take rim of which is just imaged by Galileo (Figs. 1b and 1c).place in this type of terrain. These processes are discussed The material immediately around the Khepri crater has ain the next section. hummocky texture and mottled appearance (Figs. 4b and

5j), and we interpret this to be a continuous ejecta deposit.V. SURFACE PROCESSES INTERPRETATIONS This interpretation is supported by the crater distribution

map (Fig. 7), which shows that virtually all the larger cratersBased on Voyager data, several different processes have in this area have been obscured by this deposit. Further-

been predicted or inferred to occur on the surface of more, the deposit extends to approximately one craterGanymede. Observations of craters show that Ganymede radius from the crater rim, which is generally found to behas apparently suffered crater losses on the dark terrain the case for continuous ejecta of craters on all Solar Systemin the diameter range p10–50 km when compared with bodies regardless of size (e.g., Melosh 1989). The onlyCallisto (Woronow et al. 1982). From Voyager images, features which remain visible through the hummocky de-this has been interpreted to show that either widespread posit are two partial crater rims and parts of furrow rimsvolcanic resurfacing has taken place within Ganymede’s (e.g., A8 and B8 of Fig. 4b). The crater distribution alsodark terrain (Murchie et al. 1990), or that its crust became shows an increase in small craters interpreted as secondar-rigid and hence able to retain craters at a later time than ies beyond the limit of the continuous ejecta. These obser-Callisto’s (Woronow et al. 1982). Our observations of dark vations indicate that resurfacing through impacts is a veryterrain within Galileo Regio at high resolution indicate efficient process within Ganymede’s dark terrain. The mot-that several resurfacing processes apparently have taken tled appearance of the hummocky deposit suggests thatplace. Each of these mechanisms is discussed in the follow- some relatively inefficient mixing may have taken placeing section in approximate decreasing order of their rela- between the impactor and the target or that the target wastive importance. heterogeneous.

Few craters within the Galileo Regio target area showa. Tectonismany indication of an ejecta blanket. This may be a result of

One of the primary processes which has acted to shape their size; ejecta deposits are seen from the 50-km diameterthe dark terrain is tectonism (Fig. 4c). This is evident in Khepri and possibly to the north of the 20-km diameterthe vast furrow systems that crosscut almost all known Ea, rather than around smaller craters. It may be that thesedark terrain (e.g., Figs. 1a and 4c) and are seen at high smaller craters do not excavate higher albedo materialresolution in the Galileo Regio target site (Fig. 1b). The which would be visible against the dark terrain; this wouldfurrows have disrupted the dark terrain through extension, suggest that the region is dark near the surface or thatand their morphology has triggered processes of mass wast- significant impact volatilization occurs, leaving only a darking and isostatic relaxation. The isolated knobs and massifs lag (Helfenstein 1986). Another alternative is that thein the study area are most likely furrow and crater rims ejecta deposits excavate volatile-rich ice, which is easilywhich have been affected by tectonic activity. Tectonic eroded by processes such as sublimation, sputtering, and/disruption is suggested by the polygonal morphology of or micrometeorite bombardment.some of the massifs. In addition, very fine-scale fractures At large scales, dark terrain has greater variations inappear to cut through some crater rims (Head et al. 1997). elevation and steeper slopes than grooved terrain (Giese

In other regions of dark terrain, particularly those closer et al. 1998). However, unlike the grooved terrain, darkto grooved terrain, fractures are more prevalent than in terrain within our high resolution target area appears ‘‘soft-the Galileo Regio study area and features (such as furrows) ened’’ at small scales. This is particularly evident in the

rounded crests of furrow and crater rims and the overallmay become so tectonically disrupted that they are difficult

330 PROCKTER ET AL.

FIG. 7. The distribution of craters in Galileo Regio mapped to the limit of the resolution. This distribution demonstrates that there is a dearthof craters in the northeast of the region, corresponding to the area immediately south of the crater Khepri (see Fig. 1b). It is interpreted that theejecta blanket from Khepri has filled in the majority of the preexisting craters. There is a second area concentric to and outside of this region,which appears to have a slight increase in the number of craters. This is interpreted to be the result of secondary cratering from the Khepri event.

rolling, hummocky appearance of the plains. This softening significantly modify the pristine morphology of a planetaryeffect is probably attributable to micrometeorite bombard- surface. Evidence of mass wasting has been observed onment given the ancient age of this area. A reasonable other Galilean satellites, in particular Io and Callistocomparison can be made to morphologies within the lunar (Schaber 1982, Schenk and Bulmer 1997, Moore et al.highlands (e.g., Schultz 1976). 1997). Moore et al. (1997) suggest that degradational mass

wasting on Ganymede and Callisto is consistent with ‘‘dry’’c. Mass Wasting sliding or slumping, simple impact erosion, and the evolu-

tion of the regolith. Gravity and oversteepened slopes areMass movement phenomena may be used as indicatorsof the strength and stability of a planetary crust and can all that is necessary for this mass wasting to occur. Expected


site. The origin of these features is uncertain; they mayrepresent a low albedo subsurface layer exposed by thecratering process, but this does not explain its presencealong furrow rim crests. Alternatively, these features maybe cracks or crevices filled with loose dark material formedby the removal of material from under a competent surfacelayer causing a subsiding or overhang collapse (Mooreet al. 1997). If the dark, linear features observed at thefurrow and crater rim crests are indeed fractures resultingfrom the uplift of the rims, then these zones of weaknessmay act to focus subsequent tectonic deformation.

Mass wasting is definitely a process which has occurredon Ganymede, and may still be continuing at a slow rateto the present day. It has no doubt acted to degrade someof the higher standing topography (and may contribute tosublimation degradation, discussed below), but the factthat many high standing features still remain suggests that

FIG. 8. Low albedo streaks on north-facing slopes (a). The streaks mass wasting is not a dominant process, or that its effectsappear to originate at the crest of the slope and widen toward its base.

may be partially offset by processes such as isostatic ad-This is characteristic of mass wasted deposits. Lobate contacts at the footjustment.of the slopes (b) may indicate the extent of talus deposits. Linear, low

albedo features seen at or just below the crest of furrow rims (c) andcrater rims may be an exposed low albedo subsurface layer, or they may d. Sublimation Degradationbe cracks filled with loose dark material.

A plausible explanation for the concentration of lowalbedo material in topographically low regions and on thesun-facing slopes in the Galileo Regio target area is thelandforms resulting from mass wasting on Ganymede were

too small to be confidently recognized in Voyager images, process of sublimation and redeposition of volatiles. Thisprocess has been proposed for several Solar System bodiesbut Galileo imaging has provided strong evidence that

mass wasting has occurred in the dark terrain. (McCauley et al. 1979, Moore et al. 1996) including theGalilean satellites. Galileo images of Callisto have shownOn several of the north- and northeast-facing slopes in

Galileo Regio, low albedo streaks are observed (Fig. 8). a terrain which appears blanketed by a substantial regolithand the degradation process on this satellite may be aidedThe slopes upon which they occur are generally topograph-

ically high (a few hundred meters). Despite the fact that by sublimation degradation (Moore et al. 1998).Sublimation processes relevant to icy bodies have beenthere is some high DN saturation of nearby terrains, their

geological relationships and reproducibility in both the G1 modeled by several workers (Lebofsky 1975, Purves andPilcher 1980, Squyres 1980, Spencer 1987a, Colwell et al.and G2 images make us confident that the streaks are not

image artifacts. The streaks appear to commence and are 1990, Moore et al. 1996). Spencer (1987a) modeled thermalsegregation of water ice on the icy Galilean satellites andnarrow at the crest of the slope on which they occur and

widen as they progress to the base of the slope, resulting predicted that sublimation is the most important processfor the redistribution of ices at subkilometer spatial scales.in an inverted V-shape. This geometry is distinctive of mass

wasting deposits, and the dark streaks are interpreted as Within the Galileo Regio target area, slopes that facesouthward appear to have a lower albedo than their north-the result of material which has moved downslope. This

interpretation is reinforced by the presence of a low albedo facing counterparts (Fig. 9a), opposite of what is expectedgiven the solar illumination. Because of the latitude of the‘‘apron’’ at the foot of many of the slopes, interpreted to

be talus (Fig. 8). The streaks are not observed on the south- Galileo Regio target, net solar illumination will be greateston the south-facing slopes. This albedo dichotomy is inter-and southwest-facing slopes, but this may be due to the

fact that these sun-facing slopes have generally low albedo preted to represent a relatively thin veneer of the darkermaterial for two reasons, (1) the angle of slopes exhibiting(see the next section). Because of the low albedo of the

slopes, the contrast between the slope and any low albedo the low albedo deposit is not noticeably different fromthose without, suggesting that the deposit is not particularlystreaks present is probably too low to be discriminated at

this resolution. thick, and (2) crater morphology does not appear to beaffected by the presence of this low albedo material com-Low albedo linear features are visible along the crests

of some of the furrow rims and just below (i.e., inside) the pared to craters on bright slopes. If this were a deep layerof material compared to the crater diameter, crater mor-crests of crater rims (Fig. 8c) in the Galileo Regio target

332 PROCKTER ET AL.

FIG. 9. (a) The south-facing slopes of the furrow rims in the Galileo Regio target are predominantly of low albedo (a), while the crest andnorth facing slopes are of higher albedo (b). Dashed line shows inferred position of ridge crest. Solar illumination is from the WSW. (b) Profilesacross a 5-km crater in the study area show that the albedo within these features does not correspond to expected illumination geometry. Dashedlines correspond to DN values, while heavy lines correspond to topography (topographic profiles appear ‘‘stepped’’ because of the resolutionlimitations of the DTM). Profile A–A9 shows that the sun-facing slope has one of the lowest DN values for the entire crater bowl, rather than oneof the highest. The north-facing slope has a very high DN value, almost as high as the rim, indicating that it does not have the low albedo covering,or that it may have a covering of frost. Profile B–B9 is taken across the crater from east to west and shows low to intermediate DN values on bothslopes. Each slope would receive an approximately equal amount of solar illumination throughout the day, and both appear to have similarly lowto intermediate DNs. Profile C–C9 is taken in the direction of solar illumination at the time the image was obtained. The sun-facing slope showsthe lowest albedo seen within the crater floor, rather than the highest, as might be expected. The slope facing directly away from the current solarillumination still has relatively high DN, despite potential shadowing effects. (c) Thermal segregation models show that the low albedo slopes maybe the result of a sublimation lag. Volatiles are removed from warmer dirty ice more efficiently than from colder clean ice. The net result is a darklag deposit overlying the dirty ice, through which little further sublimation can occur, while volatiles may be redeposited onto cold clean ice as frost(after Spencer 1987a).


phology might be expected to show evidence of this. The mediate-albedo material forming a scarp-bounded plateaureaching several hundred meters above the dark terrain.dark material is interpreted as the covering material based

on evidence that mass wasting deposits are dark and that The scarps around Heliopolis are irregular to crenulate inplan view, possibly implying that the facula material ismaterial which collects in topographically low areas is of

very low albedo. Similarly, the geometry of the dark streaks undergoing erosion by scarp retreat, perhaps related tosublimation (Moore et al. 1998).interpreted as mass wasting deposits (see previous section)

is inconsistent with downslope wasting of a bright veneer. Sublimation is considered to be the primary process con-tributing to the albedo heterogeneity of the dark terrainUsing the DTM (Giese et al. 1998) and the G1 Galileo

Regio images, we have taken a series of topographic and at small scales. It may aid mass wasting and thus contributessignificantly to degradation of morphology at small scales.DN profiles across the four largest craters in the area and

across each of the furrow systems. Crater profiles weretaken in the N–S and E–W directions, in order to charac-

e. Isostatic Adjustmentterize variations between DN values and topography. Pro-files were also taken along the SE–NW direction of the Viscous relaxation has been suggested to be the primary

cause of the subdued relief of many impact structures oncraters, along the strike of solar illumination at the timethe images were obtained. Furrow profiles were taken per- icy satellites (e.g., Passey and Shoemaker 1982, Shoemaker

et al. 1982). New high resolution Galileo observations ofpendicular to the furrow, generally SW–NE. In general,slopes which face the Sun (i.e., south-facing) are expected Ganymede show that isostatic adjustment is not a primary

process in shaping the morphology of grooved terrainto have relatively high DN values due to solar illumination,while those facing away from the sun are expected to have (Pappalardo et al. 1998). Consideration of the viscoelastic

properties of ice suggest that most relaxation would occurrelatively low DN values, due to photometric shading. Re-sults from our profiles show that in general, the sun-facing soon after topography develops when stresses are highest,

and that thermal gradient has surprisingly little effect onslopes do not correlate with high DN values; on the con-trary, they tend to show low to intermediate DN values, final topography (Hillgren and Melosh 1989).

Many dark terrain craters have been observed to haveindicating low albedo slopes. Figure 9b shows an exampleof profiles across one of these craters. An interesting fea- very subdued topographic relief, often with flat or convex

floors, which may be evidence of viscous relaxation (Passeyture of some of the larger craters (including Ea) is a highalbedo region along the southeastern portion of the rim. and Shoemaker 1982). Alternatively, slightly convex crater

floors have been interpreted as deposits of viscous cryovol-This feature may result from a combination of a relativelyhigh albedo portion of the north-facing crater rim and canic material (Schenk and Moore 1995). High resolution

Galileo images show convex floors in some craters, suchscattering due to the solar illumination from the southwestat the time the images were obtained. as that in the crater Ea (F 4–5 of Fig. 4b), which may be

the result of isostatic rebound. Many of the furrow rimsThe dichotomy between north- and south-facing slopesis interpreted to be the result of sublimation and deposition in Ganymede’s dark terrain are elevated compared with

the average topography and are higher than might be ex-of volatiles on the surface, specifically H2O. South-facingslopes could be subjected to the sublimation of volatiles pected given their commonly degraded appearance (Fig.

6). The highest topography in the Galileo Regio targetfrom the regolith. As vapor is removed, the low albedocontaminant responsible for the albedo of the dark terrain area is found to be at the point where the northern and

western rims from the two major furrows intersect (E–Fis expected to be left behind, forming a silicate-rich subli-mation ‘‘lag’’ (Fig. 9c). This may explain why the south- 6 of Fig. 4b). We interpret these high standing furrow rims

as uplifted, probably as a result of isostatic adjustment.facing slopes tend to be darker. As this system evolves,the low albedo lag deposit is expected to build up until no Models utilising new Galileo imaging data may be able

to confirm isostatic rebound as a significant process infurther sublimation of volatiles can take place, and thesystem effectively ‘‘chokes’’ itself. Condensation of vapor modifying dark terrain topography.may occur on the colder, north-facing slopes, resulting inthese slopes having a relatively high albedo.

f. CryovolcanismFrost is a predominant feature in Ganymede’s northern

latitudes, as shown by Voyager and Galileo observations Several of the icy satellites show evidence of resurfacing,and cryovolcanism has been proposed as the likely mecha-(e.g., Smith et al. 1979b, Denk et al. 1997). Galileo images

at high northern latitudes (p608 N, p1708W) show that nism for this by several workers (Parmentier and Head1979, Wilson and Head 1983, Squyres and Croft 1986,frost appears to be preferentially deposited on colder

north-facing slopes and in places may be several meters Allison and Clifford 1987, Murchie and Head 1988, Un-derwood et al. 1997). Dark terrain on Ganymede has beenor more in depth (Pappalardo et al. 1997b). In Galileo

Regio, Heliopolis Facula appears to be composed of inter- interpreted by several workers to consist of an older, heav-

334 PROCKTER ET AL.

ily cratered surface buried by multiple blankets of cryovol-canic material (Croft and Strom 1985, Croft and Goudreau1987, Murchie et al. 1989, Croft et al. 1990). Evidence sup-porting this interpretation includes depleted densities ofsmall craters, embayment of large craters, and complexage relations of furrows and dark materials. Cassachia andStrom (1984) suggested that smooth areas concentrated inGalileo Regio may have been material extruded alongplanes of structural weakness related to the furrows. Mur-chie et al. (1990) proposed that cryovolcanism was directlyrelated to the formation of the furrow systems on Gany-mede, and Lucchitta et al. (1992) suggested that the lighteralbedo of furrow rims could be perhaps due to the additionof endogenic materials. More recently, Schenk and Moore(1995) have proposed that several different cryovolcanicstyles exist on the surface of Ganymede, including extru-sion of high viscosity materials, potentially explaining thepresence of domes within dark terrain crater floors. In-creased resolution from Galileo imaging allows us to testthese hypotheses, particularly with respect to the originof the smooth material found interrelated to the furrowsystems. Galileo resolution allows characterization of cra-ter and furrow walls down to a resolution of less than ahundred meters and identification of any features resem-bling vents. Stereo imaging allows us to gauge topographyand estimate the thicknesses of any potential flow-like fea-tures.

There are several features in the Galileo target areawhich could be interpreted as cryovolcanic. Some contactsbetween lower and higher albedo units are lobate in planview (Fig. 10). A particularly good example of this is foundimmediately to the west of Ea, where a low albedo tongue-like feature (F4 of Fig. 4b) is observed to have lobatecontacts to its north and south (Fig. 10d). The scarp aroundHeliopolis Facula (Fig. 10g) could possibly have formedthrough relatively viscous cryovolcanism, or by multiplelow-viscosity flows.

One of the most unique features in the Galileo Regiotarget area is the floor of the crater Ea (F 4–5, Fig. 4b).The DTM (Fig. 3) shows that the crater floor is severalhundred meters higher at its center than at the base of therim walls; in fact, the floor is almost as high as the craterrim in places. Parts of the crater floor are of differentalbedo than others and there are features present whichare reminiscent of terrestrial lava flows, including: lobate

FIG. 10. Some evidence of possibly cryovolcanic deposits is seen incontacts between the floor and the crater walls; small, linearthe Galileo Regio target area. On the northeastern rim of Ea, apparent

radial features at the center of the domed floor which might terraces appear to be filled with low albedo deposits (a). Distinct lobatebe interpreted as cooling cracks (Fig. 10c) and low albedo contacts are present in the southeast of the crater (b), and radial cracks

are present in the crater’s center (c). Just outside the crater, a darkmaterial apparently filling in the terraces on the northeast‘‘tongue’’ of low albedo material with a lobate southern margin (d) hasrim of the crater, which might be interpreted as flowsmorphology analogous to some terrestrial flows. On the west side of thecoming in from the plains (Fig. 10a). Our observationscrater floor, fingers of dark material are present (e). Heliopolis Facula

suggest that cryovolcanism is a viable candidate as the ( f) is a uniform albedo unit which rises several hundred meters aboveagent for modification of this crater, although the evidence the surrounding plains. This feature has a somewhat lobate margin (g),

which in places forms distinct contacts with the plains units (ph).for dark cryovolcanism is equivocal.


We do not see clear evidence of smooth (potentially terrain and on Callisto’s ancient surface (McCord et al.1997), consistent with this model. These hypotheses willcryovolcanic) dark materials associated with the furrows.

Although low albedo materials exist as plains units and at continue to be tested with Galileo.the bottom of craters and furrows, these materials are

h. Magnetospheric Interaction and Sputteringhummocky and homogeneous in albedo and are probablyderived from various mass wasting processes rather than There is some evidence that sputtering may be a signifi-volcanism. This interpretation is supported by the presence cant process in the dark terrain of Ganymede, based onof V-shaped, rather than flat, furrow floors (e.g., profiles calculations of sputtering rates and observations of dark1 and 3, Fig. 6c; profiles 9 and 14, Fig. 6d), and the lack ray craters (Conca 1981, Schenk and McKinnon 1991).of recognisable volcanic source vents. Thus, if cryovolcan- While the rate of sputter ablation would be reduced by aism has occurred within the Galileo Regio target site, it lag deposit of rocky debris (Shoemaker et al. 1982), thedoes not appear to be widespread, and no obvious source process of impact gardening would be expected to distrib-vents are observed. ute a sputter-produced deposit throughout the regolith,

somewhat negating the effects of the lag deposit. If sput-tering rates of ice are greater than the rate of thermalg. Addition of Meteoritic Materialsublimation, rehomogenization of the surface would pre-

Several workers have suggested that the dark terrain on vent the segregation of the dark material into bright frostGanymede is the result of the addition of a low albedo and dark lag deposits (Spencer 1987a).contaminant to clean ice at the surface. Squyres (1980) The extreme albedo heterogeneity and north/south-fac-suggested that a dark contaminant admixed into ice might ing slope dichotomy within the Galileo Regio target areabe expected in the near-surface as the result of micromete- implies that sputtering in this region must occur at a rateorite bombardment; moreover, Hartmann (1980) sug- significantly less than sublimation. Therefore, sputteringgested that volatiles would vaporize during impacts, leav- is not expected to be a major influence on the surfaceing the more refractory stony components behind. Dark morphology of this area of Ganymede. In other regions,ray craters on Ganymede have been proposed by Conca notably at Ganymede’s poles, sputtering is expected to(1981) to have an ejecta blanket which is a mixture of ice dominate over sublimation and may significantly affectand projectile contamination, and Schenk and McKinnon surface morphology and albedo (Johnson 1985, 1997, Spen-(1991) proposed that cumulative impacts of objects domi- cer 1987a, Hillier et al. 1996). It should be noted that thenated by D-type asteroids and comets may be largely re- recent discovery of Ganymede’s significant magnetic fieldsponsible for the albedos and colors of dark material on (Kivelson et al. 1996) will necessitate reworking of theGanymede and Callisto. magnetospheric sputtering models.

Voyager and Galileo imaging shows craters with verylow albedo deposits on both the dark and the grooved

VI. STRATIGRAPHY OF THE GALILEOterrain (Shoemaker et al. 1982; Helfenstein et al. 1997). ItREGIO TARGEThas been proposed that the dark floor deposits may repre-

sent impact melt from which volatile constituents were losta. Stratigraphic Relationships

due to vaporization (Helfenstein 1986; Helfenstein et al.1997), or that they are concentrated deposits of dark im- Our geological mapping (Fig. 4b) has revealed that Gali-

leo Regio has a complex history. We have determined apactor material (Schenk and McKinnon 1991). Recentmodelling based on Galileo imaging of kilometer-scale stratigraphy based on relative albedos and crosscutting

relationships (Fig. 11a), and the broad sequence of unitfresh-appearing impact craters with low albedo floor de-posits, has shown that the spectral normal albedo of dark emplacement is schematically illustrated in Fig. 11b. The

20-km crater Ea (F 4–5 of Fig. 4b) is one of the youngestterrain can be successfully modelled using a simple arealmixture of 55% dark floor material and 45% average bright features in the area. Ea also postdates the plains units and

the Lakhmu furrow close to it (Fig. 11b). The plains unitterrain material (Helfenstein et al. 1997). This suggests thatmeteoritic infall was important in Ganymede’s history and to the immediate north of the crater (ps), interpreted to be

an ejecta blanket, obscures part of a NW-trending Lakhmumay account for the characteristic low albedo of dark ter-rain. Since there was a much greater impact flux during furrow in the area, and part of the surrounding plains units,

and so postdates them.the time of formation of dark terrain, it is expected thatthere would be a much greater delivery and/or concentra- From the crater distribution in Galileo Regio (Fig. 7) it

is clear that there is an absence of large craters in the areation of dark material at that time, than during the periodof later grooved terrain formation. Galileo Near Infrared immediately adjacent to Khepri (A–B, 7–8 of Fig. 4b),

where they are obscured by the unit interpreted to beMapping Spectrometer (NIMS) data indicate that similarnon-ice contaminants are inferred within Ganymede dark Khepri ejecta (ph). This deposit almost obscures topo-

336 PROCKTER ET AL.

FIG. 11. (a) Stratigraphic column determined by means of crosscutting relationships among units. Unit designations are as described in Fig.4b; detailed relationships are discussed in text. (b) Schematic illustration of the inferred history of the Galileo Regio target site. (A) Oldest featuresinclude plains units, and Heliopolis Facula is in the east. (B) Lakhmu Fossae form, trending NW–SE. Plateau of linear massifs forms in the west.(C) N–S trending Zu Fossae form. (D) Crater Ea forms in southern part of area; Khepri impact crater forms just outside area to northeast, blanketingthe northeast portion of target site with ejecta.

graphically high Lakhmu furrow rims (m3), therefore Heliopolis Facula may be one of the oldest featuresKhepri postdates the formation of this furrow system. and certainly appears degraded compared to some otherEjecta from the impact also appears to embay the N–S palimpsests in the dark terrain. It is likely that Heliopolistrending Zu furrow, indicating that this furrow system was predates the adjacent furrow systems, as there is no observ-also formed before the Khepri impact event (Fig. 11b). able embayment of the furrows by material from HeliopolisThe deposit interpreted to be Khepri ejecta partially em- nor secondaries from the presumed impact.bays and therefore postdates Heliopolis Facula (pl). Sec- The oldest units in the Galileo Regio high resolutionondaries from the same impact appear to be present on target area are interpreted to be the high albedo massifsHeliopolis and much of the surrounding terrain, further and knobs (m1). These appear embayed in places by furrowindicating that Khepri postdates these features and is one floors (pdf) and by plains units (pd), and some of theof the youngest units in the area. massifs appear to be associated with the intermediate al-

Polygonal plains areas are commonly seen between fur- bedo plains units (pi). Stratigraphically, these plains appearrows and troughs, and it is likely that the high albedo to be cross cut by the furrow systems and probably predateisolated massifs (m1) were modified to their present form them, although they may have undergone subsequent mod-by later tectonism. Some tectonic activity appears to have ification.occurred late in the history of the target area; a smallrimless trough is seen trending through the northern part b. Crater Size–Frequency Distributionsof Ea (Fig. 4c), and continues through part of the high

We have carried out detailed studies of the crater size–standing plateau (m2).frequency distribution in this area for comparison to ourThe north–south trending Zu furrow crosscuts Lakhmustratigraphic interpretations from mapping and to derivefurrows (Fig. 11b) and is therefore younger. (For the pur-the relative ages of units with ambiguous contacts (Fig.poses of the stratigraphic column in Fig. 11a, we have12). Some caution should be attached to inferred relativetreated both furrow systems as one unit). This interpreta-ages, for several reasons. First, the measured areas aretion is supported by profiles across the two systems whichrelatively small and do not always yield good statistics. Inindicate that the Lakhmu furrow is subdued compared toaddition, in very low albedo areas such as furrow floors,the Zu furrow and is therefore probably older. The highit is difficult at this resolution to adequately resolve fea-standing hummocky plateau (m2) is of a similar albedotures such as crater rims. Similarly, it is difficult to discernand topographic elevation to one of the Lakhmu furrowfeatures within the very high albedo terrains, such as therims (m3) to the west of Ea (Fig. 4b) and appears to be amassifs. Finally, because of the many slopes within thecontinuation of this rim, implying that it may have formed

contemporaneously (Fig. 11b). target area, it is likely that craters may be removed by


mass wasting, so that crater statistics may be inaccurate in of the unit at the time of its emplacement, while theyounger age corresponds to a resurfacing age. This resur-these areas. Despite these uncertainties, the crater size–

frequency distributions (Fig. 12) broadly verify our strati- facing could be by means of a variety of mechanisms, butthe fact that the units which show resurfacing are almostgraphic findings. For example, the high albedo massifs (m1)

seem to be the oldest unit and the furrow floors (pdf) the exclusively topographically high massifs and topographi-cally low furrow floors, strongly suggests downslope move-youngest. The relative ages of some units are not separable

on the basis of crater frequencies alone, such as the inter- ment of material as the dominant resurfacing mechanism.mediate and low albedo plains units (pi and pd).

The use of a lunar-like chronology model (Neukum VII. DISCUSSION AND INTERPRETATION1997) (which assumes that the Ganymede crater Gilgameshis the last basin formed at the end of the heavy bombard- Galileo imaging has revealed that Ganymede’s dark ter-

rain is extremely heterogeneous, and hence it seems likelyment 3.8 billion years ago) gives a range of ages for ourunits from about 4.2 byr for the high albedo massifs (m1, that the ice and nonice components are distributed un-

evenly across the surface. The lowest albedo areas in ourm2), to 3.8 byr for the furrow floors (pdf). The intermediateand low albedo plains units (pi, pd) have intermediate ages target region are found at the bottom of furrows (Fig. 4b,

pdf) and within crater floors. High albedo material is foundof around 4 byr, and the hummocky plains unit (ph) hasan age of around 3.9 byr. These ages are consistent with at topographically high elevations and is in the form of

crater and furrow rims (Fig. 4b, m3), a high standing pla-an ‘‘average’’ Galileo Regio age obtained from these andother Galileo data, which is also consistent with that ob- teau (Fig. 4b, m2), and massifs (Fig. 4b, m1). Plains units

are intermediate or low in albedo (Fig. 4b, pi, pd) and aretained from Voyager crater counts (Neukum 1997). Thelunar-like chronology model is one of two widely divergent distributed somewhat randomly across areas of intermedi-

ate elevation.models however; recent dynamical modelling by Zahnleet al. (submitted) suggests a much younger age for Gil- Tectonic activity has been instrumental in shaping the

dark terrain, as is evident from the presence of the furrowgamesh of 0.7 byr, with uncertainties of a factor of 5. Thismodel implies that Ganymede’s dark terrain may be systems, smaller fractures, and the high albedo knobs and

massifs which are among the oldest units found withinyounger than previously supposed. However, the relativeages of our crater size–frequency distributions hold regard- Galileo Regio. On the basis of their relative age, their

topographic elevation, and their often polygonal plan view,less of absolute age.Figures 12a and b show the relative ages of the units these knobs and massifs are interpreted to be the remnants

of crater or furrow rims which have been tectonically dis-as determined by crater size–frequency distributions (andrelative age ranges for individual unit areas are shown membered and further degraded by mass wasting and subli-

mation erosion. Micrometeorite bombardment is likely toin Fig. 12b). The high albedo units present difficulty inobtaining reliable crater statistics, due to their hummocky contribute to an overall ‘‘softening’’ of the dark terrain

compared with the grooved terrain.nature, but data from the isolated massifs and knobs, m1,indicate that they may be the oldest features in the study The presence of low albedo south-facing slopes appears

to be the result of a thin veneer of dark material overlaidarea. The hummocky plateau unit, m2, is also inferred tobe very old. These data suggest that of the three plains on a brighter substrate. The dark material may be concen-

trated on the surface by various processes, particularlyunits, the intermediate and low albedo units pi and pd arethe oldest, while the furrow floor unit pdf is the youngest. that of sublimation, in which volatiles are preferentially

removed from sun-facing slopes, leaving a dark lag deposit.The intermediate albedo material unit, pi, has a craterdensity which would place it at approximately the same The heterogeneity in Galileo Regio cannot be explained

by conventional photometric behavior of uniform albedoage as Heliopolis Facula, pl. The floor of the crater Ea,pcf, and the smooth unit to its north, (interpreted to be material (Oberst et al., in preparation), and thus we favour

concentration of a lag deposit as a likely explanation for theits ejecta blanket) ps, appear to be toward the youngerend of the relative age scale. The hummocky plains unit, slope dichotomy, rather than highly unusual photometric

behavior at this stage. The depth of the lag deposit remainsph, is the most markedly different in age relative to itssurroundings. Crater counts have also been carried out on to be determined, but it is unlikely to be more than a few

meters (or tens of meters on the slopes). The relativethe floors of other large ($0.8 km) craters in the area;these are inferred to be relatively old. These inferred rela- brightness of the north-facing slopes may be due to the

deposition of frost by cold trapping.tive ages compare favorably with the stratigraphy proposedin Fig. 11a. Low albedo streaks on slopes are interpreted to result

from mass wasting. Due to the talus-like geometry of theEvidence for mass wasting comes from crater counts onindividual units which show subtle inflections indicative of dark streaks, we favor the interpretation that the streaks

are formed through the removal by mass wasting of a thintwo ages. It is likely that one of these is the original age

338 PROCKTER ET AL.

FIG. 12. (a) Cumulative crater counts for the units described in Section III, with error bars as shown. Each graph includes counts for multiplerelated units, such as furrow floors and rims, and plains units. (b) Schematic diagram illustrating the relative crater ages of each map unit based oncrater counting statistics. If a lunar-like chronology is adopted, the diamonds indicate the nominal relative age of the unit, while the lines indicatethe range derived from counts across different counting areas of the same map unit from which this age is derived. Units for which there is norange are those in which there was only one unit area to count (e.g., the floor of the crater Ea, pcf). The bar across the bottom of this figurerepresents ages ranging from 4.3 byr (oldest) to 3.7 byr (youngest). Dots indicate a possible resurfacing age; this is only significant on the furrowfloor unit (pdf), the massifs (m1), and the hummocky plateau unit (m2). LC represents the floors of the larger ($0.8 km) craters in the study area.If a different impact chronology is adopted, these ranges represent the approximate relative ages of each counted unit.

veneer of low albedo material from the surface, revealing tion (Murchie et al. 1990, Luchitta et al. 1992); instead, thelighter albedo of the furrow rims may be due to the tectonica high albedo subsurface layer below. The fact that the

streaks are themselves dark and overlie a brighter substrate uplift of a relatively high albedo substrate, perhaps subse-quently modified by erosional processes.suggests that the mass wasted material is of a different grain

size and hence exhibits different scattering properties, and/ Heliopolis Facula is interpreted to be a palimpsest be-cause it is smooth, has a uniform intermediate albedo, andor the material is of a different composition. The presence

of the lowest albedo material in furrow and crater floors rises several hundred meters above the surrounding plains.The smallest palimpsests discussed by Shoemaker et al.is consistent with the interpretation that dark material of

different composition has moved downslope and has col- (1982) are p50 km in diameter, and Heliopolis is approxi-mately this size; however, its lobate margins suggest thatlected in topographic lows. Crater counting statistics sug-

gest that resurfacing events have taken place on the slopes it may have been initially larger but has undergone erosionof its margins. It is also possible that this feature is cryovol-of topographically high units and within lower elevation

units, supporting this interpretation. canic.Possibly the best candidate for cryovolcanism is foundThere is no unequivocal evidence for cryovolcanism

within the dark terrain. We have yet to locate features within the floor of the crater Ea. Since the crater floorshows a low albedo deposit with lobate margins, featureswhich may be source vents. Lobate features are observed,

which may indicate contacts between ice-lava flows and which appear to be terraces covered with low albedo de-posits, and central radial cracks, we interpret this unit assurrounding topography; however, these features are gen-

erally found at the base of slopes, which also exhibit dark a possible cryovolcanic deposit. The floor may be domeddue to isostatic rebound; modeling is required to furtherstreaks, interpreted to be the result of mass wasting. Hence

the lobate contacts may indicate the limit of talus deposits. address this issue.Stereo observations yield profiles across the furrows inSome smaller craters appear to be embayed, but it is not

certain whether this is the result of cryovolcanism or of the target area which are consistent with graben morphol-ogy. The slopes of the furrows are significant (rangingthe deposition of unconsolidated, mass wasted material or

sublimation lag deposits. We see no evidence to support from 6 to 208, on average), but they are not as steep as ispredicted for normal faults. This may be due to modifica-predictions that cryovolcanism is related to furrow forma-


face and the size of the study region, it is not possible todetermine conclusively the origin of the furrows and wecontinue to investigate this issue. Reevaluation of the cen-ter of curvature of the Lakhmu Fossae, given improvedcoverage and coordinate control from Galileo observa-tions, will aid in the interpretation of the origin of the ZuFossae and other furrow systems.

Models of Dark Terrain Formation

Three models of dark terrain formation are proposedhere (Fig. 13), and we examine the relative merits of eachbased on the various observations of the Galileo Regiohigh resolution images and the surface processes dis-cussed above.

i. Dark crust model. The first model (Fig. 13a) is onein which Ganymede’s entire crust is dark, suggesting thatthe low albedo contaminant was present upon Ganymede’sFIG. 12—Continuedformation and that the crust is relatively homogeneousthroughout. This model is the simplest endmember whichcan explain the presence of the dark terrain on Ganymede.tion of the slopes by mass wasting or isostatic adjustment.Presumably this dark crust would overlie a ‘‘clean’’ iceSeveral of the stereo-derived profiles show a break in slopeasthenosphere in order to explain Ganymede’s palimpsestsnear the base of the wall which may indicate talus, andas crust-penetrating impacts. This model would explain thecould explain why the slopes of the younger Zu furrowpresence of low-albedo plains units on the dark terrain,are twice as steep as those of the older, more degradedand variations in albedo between these units could resultLakhmu furrow, if the slopes of the older furrow havefrom varying levels of sublimation and subsequent lagbeen shallowed by significant mass wasting. The shallowerbuildup resulting from small differences in slope. In thisslopes of the older set could also be explained if the systemmodel, only very large impacts would excavate relativelyformed at a time when Ganymede’s lithosphere was‘‘clean’’ material from the asthenosphere, such that onlywarmer and weaker and therefore may not have been ablethese impacts would have the bright central deposit charac-to support significant topography. Isostatic rebound mayteristic of the palimpsests. This is consistent with observa-be partially responsible for uplifting the rims of the furrowtions of palimpsests, which are generally at least 50 km insystems and may account for their significant relativediameter (Shoemaker et al. 1982). If Heliopolis Facula isheight despite their age and apparent modification by massa palimpsest, as we have interpreted it to be, then its rela-wasting processes.tively small size could provide some constraints on theThe morphology of the two furrow systems is remarkablymaximum depth of dark material in this model. Low albedosimilar and appears to differ only in the degree of modifi-streaks on slopes are interpreted as the result of a masscation as a function of age. This is consistent with a similarwasting of a sublimation lag.origin for both furrow sets. In both sets, the western wall

The major shortcoming of this model is that it is hardis slightly steeper than the eastern wall. If the Lakhmuto explain the presence of bright slopes, crater and furrowFossae result from an impact with its center p2000 kmrims, and massifs, unless they result from local frost deposi-from the study area, as proposed by Murchie et al. (1990),tion. If so, an initial albedo heterogeneity would have beenthen these scarps would be expected to be outward-facing,required to initiate thermal segregation and brighten highas is observed for the outer Valhalla scarps and has beentopography. For this model, if the high albedo materialmodeled based on asthenospheric flow (Melosh 1982). Theof grooved terrain was emplaced cryovolcanically (e.g.,impact structure which initiated the Lakhmu Fossae wouldAllison and Clifford 1987), it would require a source regionhave been almost as large and old as the Valhalla structurebelow Ganymede’s dark outer layer; alternatively, the ma-(Murchie et al. 1990, Passey and Shoemaker 1982), so suchterial forming the grooved terrain may have undergonecomparisons are reasonable. Thus, our observations aresome modification before emplacement at the surface (e.g.,consistent with the impact model, but it should be notedsettling out of the dark contaminant in a ‘‘cryomagma’’that our study area is extremely small compared with thechamber) resulting in its ‘‘cleaner’’ appearance.size of these furrow systems, and this slope variation could

simply result from lithospheric heterogeneities or other ii. Dark cryovolcanism model. In this model (Fig. 13b),the dark material on the surface of Ganymede’s dark ter-processes. Given the heterogeneity of the dark terrain sur-

340 PROCKTER ET AL.

rain is the result of low albedo cryovolcanic extrusionsemplaced over a relatively clean, high albedo crust (Croftand Strom 1985, Croft and Goudreau 1987, Murchie andHead 1988, Murchie et al. 1989, Croft et al. 1990). Thisscenario is suggested by the presence of apparently lobatefeatures within the Galileo Regio target site (e.g., Fig. 9).Low albedo streaks would form through mass wasting ofpreviously emplaced cryovolcanic materials, or sublima-tion lag deposits, and the central bright portion of palimp-sests would result from clean crustal material being upliftedat the time of impact or from clean ice cryovolcanismsubsequent to formation. Sublimation could act to removevolatiles, such as ammonia (e.g., Murchie and Head 1988)from cryovolcanic deposits to leave behind a darker lag,resulting in the heterogeneity of the plains. High albedotopographically elevated regions including massifs and fur-row and crater rims can be readily explained in this modelas areas which have not been obscured or modified bycryovolcanism. The high albedo hummocky plateau region(m2) in the west of the target area has what appears to below albedo ‘‘pockets’’ of material, which can be explainedwithin this model as resulting from cryovolcanic flows mov-ing downslope from higher source regions and ‘‘ponding’’in low areas.

An argument against this model is the fact that the DTMshows the G1 area is extremely hummocky. Flows areexpected to pond in low areas. Profiles across furrows (Fig.6) show that they have floors which are far from smooth,indicating that if cryovolcanism was present in this area,either it did not occur in the furrows, or the cryovolcanismwas highly viscous and stalled before flooding of the furrowfloors could occur, or the furrows were subsequently modi-fied by tectonism relatively soon after modification. If thefurrows were modified long after their formation, we wouldexpect to see evidence of this in crater statistics.

It is possible that cryovolcanism was responsible for theFIG. 13. Possible models of dark terrain crustal structure and forma-tion. (a) Dark crust model. This model assumes that the crust is dark formation of some of the dark terrain units in Galileothrough to the asthenosphere below. This model is consistent with the Regio (e.g., the intermediate and low albedo plains), butpresence of low albedo plains units and palimpsest formation (excavation the evidence is tenuous. Murchie and Head (1988) pro-of ‘‘cleaner’’ material at depth). However, it is inconsistent with the

posed that ammonia-rich volcanic mixtures bearing thepresence of high albedo slopes; some extreme variation in scatteringlow-pressure NH3 phase would ablate three times higherproperties or significant frost deposition would be required to explain

these. (b) Dark cryovolcanism model. This model assumes that the dark than pure H2O ice. Thus, if some units on Ganymede arematerial on Ganymede’s surface is the result of low albedo cryovolcanic formed from ammonia-rich magmas with a silicate fraction,material emplaced over a relatively clean higher albedo crust. This is these might be expected to undergo significant sublimationconsistent with the presence of dark plains units, high albedo palimpsests,

and form a darker silicate lag on their dark surfaces. Hence,and massifs and furrow rims. However, this is inconsistent with observa-this is a possible way to produce the low albedo materialtions of furrow floors, which are commonly V-shaped and hummocky,

and hence do not appear to be flooded with cryovolcanic materials. Also, found throughout the dark terrain.no certain source vents have been identified. (c) Thin, dark veneer model.

iii. Thin, dark veneer model. In this model (Fig. 13c),This model suggests that the low albedo material within Ganymede’sdark terrain consists of a thin lag deposit overlying cleaner ice with Ganymede’s lithosphere is largely comprised of clean ice,small amounts of darker material distributed throughout. This material but it has a small amount of a low albedo component mixedis proposed to have been added through asteroidal and cometary impacts

in heterogeneously. Surface processes such as sublimationand was later concentrated on the surface by processes such as sublimationand impact volatilization are proposed to concentrate theand mass wasting. This is consistent with all of our observations and is

our favored model for dark terrain crustal structure and origin. dark contaminant on the surface, resulting in a dark, thin


lag deposit. This model proposes that the low albedo con- the model in which low albedo material is concentratedonto the surface as a thin veneer. However, it is possibletaminant was emplaced by projectile infall as Ganymede

was accreting, and that it was not removed as Ganymede that elements of all three models are present. By examiningother Galileo observations of dark terrain, we can test andunderwent differentiation and/or the material was contrib-

uted after differentiation. Mixing models indicate that the refine these models further.dark contaminant could be D-type asteroidal material

VIII. OTHER GALILEO OBSERVATIONS(Helfenstein et al. 1997); cometary material is equallyAND FUTURE WORKlikely, especially in this region of the Solar System (Levison

and Duncan 1997). Hence, below the surface, Ganymede’sWe are continuing to test these three models of darkcrust would appear relatively bright, only slightly darkened

terrain evolution through comparison with other regionsby the heterogenous distribution of minor amounts of lowof dark terrain imaged by Galileo. Dark terrain undergoesalbedo material. This material may be less abundant ata marked change in character when it is adjacent to areasincreased depth, where higher temperatures are moreof grooved terrain; the morphology of furrows and craterslikely to have enabled it to settle toward the silicate mantleappears subdued and the dark terrain is frequently dis-during the planet’s differentiation, and where only veryrupted by fractures parallel to the trend of the grooveslarge impacts could resupply contaminant material. In this(Prockter et al. 1997). New Galileo data is being used tomodel, low albedo material on the surface of Ganymededocument the variation in morphology of the dark terrainis concentrated atop a heterogeneous crust in the form of aas grooved terrain is approached, and this is expected tolag deposit, by processes such as sublimation, mass wasting,provide insight as to how the grooved terrain forms. Weprojectile contamination, and/or impact volatilization.also plan analyses of furrows in other regions of GanymedeThis model suggests that the low albedo plains on the(such as terrain of transitional morphology and arounddark terrain are the result of a lag deposit, and conse-groove lanes), in order to further test models of their originquently there would be spatially variable concentrationsand evolution, and to test these models against multiringedof low albedo material on the surface. The plains units pisystems on Callisto.and pd that we have identified do not have such a low

Recent NIMS results (McCord et al. 1997) show thatalbedo as the floors of furrows and craters, because thecandidate materials for the dark contaminant include CO2 ,plains are relatively flat. In such regions, the sublimationSO2 , and organic materials such as tholins containing C–Hprocess would proceed to a certain point, then wouldand CIN. Future work will correlate NIMS results with‘‘choke’’ itself off, while dark material could be moreSSI observations in order to better understand how thereadily supplied to furrow floors from the slopes above.dark contaminant is associated with different surface fea-This model readily explains the presence of high albedotures within the dark terrain, in order to further constrainknobs, massifs, and furrow and crater rims, as these wouldthe distribution of the dark component throughout thebe essentially ‘‘bedrock’’ (or ‘‘bed-ice!’’). On slopes, pro-crust.cesses such as sublimation would act to leave a relatively

thin lag deposit, which is subsequently removed by massGanymede vs Callisto

wasting, revealing a fresh, higher albedo slope. The masswasted low albedo veneer would accumulate in the topo- Based on Voyager observations, it was expected that

Ganymede dark terrain might look similar at high resolu-graphic lows, causing them to darken with time. This modelis consistent with palimpsest formation, in which the cen- tion to Callisto’s dark surface (e.g., Carr et al. 1995); how-

ever, Galileo observations have shown this is not the case.tral high albedo deposit is formed when large impacts exca-vate through more contaminated crust to areas of cleaner, Galileo images show that the principal difference between

Ganymede and Callisto at small scales is that Callisto ap-relatively mobile material. Smaller impacts would be ex-pected to mix the darker lag deposit on the surface with parently has a relatively deep blanket of largely smooth,

mid-albedo material covering most of its surface (Benderhigher albedo, cleaner material from below. The ejectafrom Khepri, which has a mottled texture, may result from et al. 1997, Klemaszewski et al. 1998, Moore et al. 1998),

whereas Ganymede dark terrain is extremely heteroge-heterogeneous mixing of a low albedo surface layer witha cleaner substrate. This model is consistent with all of our neous in albedo and does not appear to have a deep blan-

keting layer. Furrow and crater rims on Callisto are narrowobservations of the Galileo Regio high resolution targetsite; thus, it is our favored hypothesis for the crustal struc- and highly degraded, while those on Ganymede are de-

graded to different degrees but are generally rounded andture of dark terrain.From the above discussion, it is clear that multiple pro- wide with respect to the dimensions of the furrow or crater.

Although there appears to be mass wasting on both moons,cesses have occurred on the surface of Ganymede to formthe dark terrain. The three models proposed are end-mem- that occurring on Ganymede does not seem to have created

a great amount of loose debris at the foot of slopes, whereasber models only, and at this stage of our analysis we favor

342 PROCKTER ET AL.

mass wasting on Callisto may have created the dark blan- the formation or evolution of this region of dark terrain.Galileo observations of the fine-scale morphology and to-keting material on that satellite. The floors of the Gan-

ymede furrows and craters are lower in albedo than the pography of furrows of the Lakhmu Fossae and Zu Fossaeare consistent with a normal faulting origin, perhaps im-surrounding plains, whereas these features on Callisto ap-

parently have the same grossly uniform albedo material pact-induced.Three models of dark terrain formation have been pro-surrounding and within them. This indicates that surface

processes occur somewhat differently on the two bodies; posed and evaluated. We find that the most plausible modelto explain our observations is one in which the dark terrainoverall, mass wasting appears to be far more important on

Callisto than on Ganymede, but local mass wasting and consists of a thin, low albedo veneer overlying a cleanersubstrate that contains small amounts of admixed darksublimation are probably important factors in concentrat-

ing a dark lag deposit in the troughs and craters on Gan- meteoritic material. The veneer is proposed to result fromconcentration of this dark material on the surface by subli-ymede.

The geometries of the furrows on the two moons are mation processes, aided by impact gardening, impact vola-tilization, and mass wasting.similar at high resolution, but there are variations in the

heights of the furrow rims and in their degradation state.This suggests differences in the state of the early litho- ACKNOWLEDGMENTSspheres of the two bodies, leading to variations in the rate

We thank Robert Strom and an anonymous reviewer for their thought-of isostatic adjustment subsequent to furrow formation orful and helpful comments on this manuscript. L. P. thanks The Openvariations in the way the furrows initially formed. We willUniversity, UK. This work was supported by the NASA Galileo Project.

continue to analyze and compare the morphologies anddegrees of modification of features on these two bodies to

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dark terrain on ganymede: geological mapping and ...dark terrain on ganymede: geological mapping and...

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