Icarus 164 (2003) 197–212
www.elsevier.com/locate/icarus
Morphology and origin of palimpsests on Ganymede based on Galileo
observations
Kevin B. Jones,a,∗,1 James W. Head III,a Robert T. Pappalardo,a,2 and Jeffrey M. Moore b
a Department of Geological Sciences, Box 1846, Brown University, Providence, RI 02912, USA
b NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035-1000, USA
Received 8 September 2002; revised 2 April 2003
Abstract
Palimpsests are large, circular, low-relief impact scars on Ganymede and Callisto. These structures were poorly understood based on
Voyager-era analysis, but high-resolution Galileo images allow more detailed inspection. We analyze images of four Ganymedean palimpsests
targeted by Galileo: Memphis and Buto Faculae, Epigeus, and Zakar. Ganymedean craters and Europan ring structures are used as tools to
help better understand palimpsests, based on morphologic similarities. From analysis of Galileo images, palimpsests consist of four surface
units: central plains, an unoriented massif facies, a concentric massif facies, and outer deposits. Using as a tie point the location in these
structures where secondary craters begin to appear, outer deposits of palimpsests are analogous to the outer ejecta facies of craters; the
concentric massif facies of palimpsests are analogous to the pedestal facies of craters; and the unoriented massif facies and central plains are
analogous to crater interiors. These analogies are supported by the presence of buried preexisting structure beneath the outer two and absence
of buried structure beneath the inner two units. Our observations indicate that palimpsest deposits represent fluidized impact ejecta, rather
than cryovolcanic deposits or ancient crater interiors.
 2003 Elsevier Inc. All rights reserved.
Keywords: Ganymede; Europa; Impact processes; Cratering
1. Introduction
Palimpsests are large, bright, circular, low-relief impact
scars found on Ganymede and Callisto (Smith et al., 1979).
Although it is accepted that they formed in response to impact, the details of their formation were not well understood
based on analysis of Voyager data. From Voyager observations (Fig. 1), consensus was not reached on what the bright
deposit of a palimpsest represents, or where the original
crater rim is located within a palimpsest.
Several principal theories have been put forth concerning
both the means of emplacement of the palimpsest deposit
and the location of the original crater rim. These hypotheses can be grouped into three theories on emplacement and
* Corresponding author.
E-mail address: [email protected] (K.B. Jones).
1 Present address: Department of Geosciences, Gould-Simpson Build-
ing, 1040 E. 4th St., University of Arizona, Tucson, AZ 85721-0077, USA.
2 Present address: Astrophysical and Planetary Sciences Department and
Laboratory for Atmospheric and Space Physics, Campus Box 392, University of Colorado, Boulder, CO 80309, USA.
0019-1035/03/$ – see front matter  2003 Elsevier Inc. All rights reserved.
doi:10.1016/S0019-1035(03)00128-3
three theories on what is represented by visible palimpsest
morphology.
The three hypotheses on emplacement of bright material
involve
(1) large-scale impact-triggered extrusions,
(2) fluid-rich slushy ejecta, and
(3) dry, solid ejecta.
(1) Thomas and Squyres (1990) postulated that some impacts in the early geologic history of Ganymede could have
penetrated its thin primordial lithosphere, allowing the extrusion of warm, buoyant, asthenospheric material. When this
material reached the surface, it would have spread radially
and formed circular extrusive deposits.
(2) Greeley et al. (1982) and Fink et al. (1984), based
on experimental simulations, hypothesized that prompt collapse of central peak craters in low-strength or fluidized materials could produce palimpsest-like structures. Similarly,
Croft (1983) proposed that impacts by unusually large or
high-velocity objects could have resulted in primarily fluid
flow instead of typical granular flow during the modification
198
K.B. Jones et al. / Icarus 164 (2003) 197–212
Fig. 1. Voyager images of palimpsests (A) Memphis Facula, (B) Epigeus, (C) Zakar, and (D) Buto Facula.
stage of crater formation, based on theoretical modeling. He
suggested that this fluid flow could account for the odd appearance of these structures.
(3) The remaining hypotheses of Passey and Shoemaker
(1982), Hartmann (1984), Lucchitta and Ferguson (1988),
and Schenk (1996), discussed below, implicitly assume
ejecta with a significant dry, solid component.
Ideas for what the palimpsest margin represents consist
of
(1) the outer continuous ejecta facies,
(2) the inner ejecta or pedestal facies, and
(3) the transient cavity limit or original crater rim.
Although these ideas assume dry, solid ejecta, they could
also apply to cratering processes involving more fluid-rich
ejecta as proposed by Greeley et al. (1982), Croft (1983),
and Fink et al. (1984).
(1) Passey and Shoemaker (1982) proposed that the
palimpsest margin is analogous to the edge of a continuous ejecta deposit like those commonly surrounding impact
craters. This hypothesis was based on the presence of secondary craters surrounding some palimpsests, beginning
just beyond the palimpsest margin. Secondary craters surrounding ordinary craters on Ganymede begin to appear just
beyond the continuous ejecta margin (Iaquinta-Ridolfi and
Schenk, 1995). This hypothesis places the original crater
rim well inside the palimpsest.
(2) Based on an extensive study of fresh complex craters
on Ganymede, Schenk (1996) concluded that outer margins
of palimpsests are analogous to the inner or “pedestal” facies of the continuous ejecta blanket of craters on Ganymede
(e.g., Fig. 2) (see Horner and Greeley, 1982). Schenk’s
mapping of craters on Ganymede revealed a well-defined
relationship between pedestal facies diameter and crater
rim diameter. He then tied this empirical relationship to
palimpsests using the penepalimpsest Nidaba (19◦ N,
124◦ W), in which he found the outer ejecta facies, pedestal
facies, and crater rim to be identifiable and to occur at the
expected locations based on this relationship.
(3) Based on the remarkable circularity of palimpsests
and on comparisons with moat craters on Ganymede, Hartmann (1984) and Lucchitta and Ferguson (1988) hypothesized that palimpsest margins are located near and associated
with former crater rim deposits of palimpsest-forming impacts.
In Section 2, we summarize the characteristics of the
Galileo targets analyzed here. In Section 3, we discuss the
geologic units seen in high- and regional-resolution images
of these targets. Sections 4 and 5 contain our comparisons
of palimpsests to large craters on Ganymede and to ring
structures on Europa, respectively. Section 6 compares the
morphometry of each of these types of impact structures.
Morphology and origin of palimpsests on Ganymede
199
Fig. 2. Reprojected Galileo image of crater Achelous, showing locations of pedestal (inner ejecta) facies, outer (continuous) ejecta facies, and secondary crater
field. North is to the top in this and all subsequent images, except as indicated.
2. Galileo targets
Galileo imaged four palimpsests on Ganymede: Memphis
Facula during the first Jupiter orbit, designated G1, Epigeus
during orbit G2, Zakar during orbit G7, and Buto Facula
during orbit G8. Memphis Facula and Epigeus were imaged
at high resolution. Zakar and Buto Facula were imaged at
a regional scale complementing and providing context for
the high-resolution images. Image characteristics are summarized in Table 1.
Memphis Facula (16◦ N, 133◦ W) is 355 km in diameter and is located in the dark terrain of Galileo Regio. This
palimpsest was imaged as a strip of 8 summation-mode
(400 × 400 pixels) images at about 60 m/pixel, providing
a radial sample across most of the palimpsest (Fig. 3). The
very center of Memphis Facula, however, was not imaged,
and the 28-km-diameter crater Hay-tau and its ejecta obscure
much of the original inner palimpsest surface within the imaged area.
The 370-km-diameter palimpsest Epigeus (23◦ N,
181◦ W), located at the convergence of Uruk and Nippur
Sulci, was imaged as a strip of 9 complete and 1 partial
summation-mode frames at about 90 m/pixel (Fig. 4A). This
image strip provides a complete radial sample across the
palimpsest, similar to that provided by the Memphis Facula
images.
The Epigeus images suffered from inadvertent overcompression, reducing their effective resolution below the precompression 90 m/pixel. Four of the frames contain losslessly compressed “truth windows” 96 pixels square, however, which reveal the uncompromised surface texture and
detailed morphology.
Zakar (30◦ N, 335◦ W), a palimpsest approximately 280
km in diameter and located in a complex region of grooved
terrain, was imaged almost in its entirety as a single 440
m/pixel frame (Fig. 5A). Buto Facula (12◦ N, 203◦ W),
located in Marius Regio, was imaged almost in its entirety
as a two-frame mosaic at 180 m/pixel (Fig. 6A). Although
the Zakar and Buto Facula images are lower resolution, they
provide greater areal coverage and therefore more extensive
context than the Memphis Facula and Epigeus images, at resolutions higher than obtained by Voyager.
Voyagers 1 and 2 previously imaged these four palimpsests (Fig. 1). Memphis Facula was imaged at 0.7 km/pixel
(Voyager image FDS 20638.29), Epigeus at 1.0 km/pixel
(20637.05), Zakar at 1.1 km/pixel (16405.30), and Buto
Facula at 1.3 km/pixel (20635.45).
200
K.B. Jones et al. / Icarus 164 (2003) 197–212
Fig. 3. Memphis Facula. Strip of reprojected Galileo images placed over reprojected Voyager image. Lettered boxes show locations of enlargements in Fig. 9.
Single white lines delineate the palimpsest margin and boundaries between labeled surficial facies. Double white lines delineate subconcentric troughs with
adjacent infacing scarps. Within Memphis Facula, the boundary between the concentric massif facies and the outer deposits is approximately coincident with
a prominent scarp and trough.
Table 1
Geometric information and resolution of selected Galileo images
Image number
Incidence
angle (◦ )
Emission
angle (◦ )
Phase
angle (◦ )
Resolution
(m/pixel)
Memphis Facula:
s0349760100
s0349760122
s0349760145
s0349760168
s0349760200
s0349760222
s0349760245
s0349760268
38.7
39.2
39.6
40.0
40.3
40.6
40.9
41.0
17.2
17.4
17.7
18.0
18.3
18.7
19.2
19.8
49.4
49.7
50.0
50.1
50.0
49.9
50.0
49.5
65.1
63.4
61.6
59.8
57.9
56.1
54.3
52.3
Epigeus:
s0359945481
s0359945484
s0359945488
s0359945500
s0359945504
s0359945507
s0359945511
s0359945514
s0359945518
s0359945521
31.8
31.2
30.4
29.8
29.0
28.4
27.7
27.1
26.3
25.7
34.9
35.3
35.8
36.2
36.7
37.1
37.6
38.0
38.6
39.0
14.7
15.0
15.4
15.7
16.3
16.8
17.5
18.1
18.9
19.5
89.4
89.2
88.9
88.6
88.4
88.2
87.9
87.7
87.5
87.3
Zakar:
s0389917900
48.72
65.65
37.21
440.8
Buto Facula:
s0394532139
s0394532152
79.71
82.85
15.01
17.65
69.35
68.95
193.1
195.0
Based on Voyager 2 images, Memphis Facula was described by Passey and Shoemaker (1982) as consisting of
a relatively smooth central region about 100 km across, a
uniform bright annulus surrounding the central region and
containing subconcentric ridges or hummocks, and an outer
mottled unit. Secondary craters were considered present in
the outermost unit and outside the palimpsest but were not
easily distinguishable from other, nonsecondary craters.
The low resolutions and high sun angles of Voyager images prevented observation of primary topography across
these features. The high-resolution Galileo images of Memphis Facula and Epigeus were also taken under conditions
of high solar illumination (Table 1). Without the presence
of shadows or obvious shading, topography is difficult to infer directly from these images. Based on stereo images of
other areas on Ganymede, surface brightness typically correlates with topography (Oberst et al., 1999). Steeper, topographically high areas, such as ridge crests and crater rim
crests, tend to have high surface brightnesses, while flatter,
locally low areas, such as crater and trough floors, tend to
appear darker. Patterns of surface brightness can therefore
be used to qualitatively infer topography or slope. More favorable sun angles in Galileo images of Buto Facula and
Zakar clearly reveal relief. This topography helps characterize palimpsest surface texture, and allows us to broaden
the descriptions of palimpsest surface facies to include topographic descriptions.
Morphology and origin of palimpsests on Ganymede
201
Fig. 4. Epigeus. (A) Reprojected Galileo image strip placed over reprojected Voyager image. Lettered boxes show locations of enlargements in Fig. 8.
(B) Geologic sketch map of the area outlined in Fig. 4A. Heavy lines delineate the palimpsest margin and boundaries between surficial facies. Circles represent
craters, and irregular outlines represent massifs. Near the palimpsest center, the network of subradial lines shows the locations and extent of fractures.
The literal, nonplanetary definition of a palimpsest is a
parchment that has had its original writing scraped away,
perhaps incompletely, and overwritten. The lack of topography noted in Voyager images of these features supported this
original definition. Galileo images reveal that palimpsests on
icy satellites are not simply “ghosts” showing through overprinted topographic features, but basic geologic and topographic elements.
3. Palimpsest units
Galileo images of Memphis and Buto Faculae, Epigeus,
and Zakar, along with their corresponding Voyager context images, allow mapping and description of four distinct
palimpsest surface units. These units are the central plains,
the unoriented massif facies, the concentric massif facies,
and the outer deposits. Secondary craters are present beyond
the outer deposits in each of these palimpsests.
3.1. Central plains
The innermost palimpsest unit, the central plains
(Fig. 7A), consists of a bright, relatively smooth region
punctuated by low 500-m- to 3-km-diameter knobs concentrated toward the palimpsest center. The central plains form
the inner palimpsest floor, extending out to approximately
15% of the palimpsest radius. In places, the plains embay
massifs of the surrounding unoriented massif facies, while
in other areas the plains are scarp-bounded.
The highest-resolution (90 m/pixel) images of the central
plains were obtained in Epigeus. At the center of Epigeus,
there is a 25-km-radius region of subradial dark-floored
troughs or fractures (Fig. 8A). These troughs range from ap-
202
K.B. Jones et al. / Icarus 164 (2003) 197–212
Fig. 5. Zakar. (A) Reprojected Galileo image placed over reprojected Voyager image. (B) Geologic sketch map of the area outlined in Fig. 5A. Subconcentric
lines delineate the palimpsest margin and boundaries between labeled surficial facies. Small circles represent craters, and small dotted circles within the
palimpsest represent buried secondary craters and crater chains. Dashed lines, particularly in the southwest portion of the palimpsest, represent buried grooves.
Hatchured lines represent infacing scarps.
proximately 150 to 250 m across, are generally linear, and
merge in dendritic or anastomosing patterns. The troughs exhibit two orthogonal preferred orientations, trending north–
south and east–west. A bright, rough-textured, lobate deposit
occupies the central 15 to 20 km of this region, and locally
surrounds troughs up to 25 km from the palimpsest center.
This bright region consists of massifs 200 to 300 m across
separated by 200 to 300 m. The fracturing present at the center of Epigeus is not noted at the centers of Buto Facula and
Zakar, possibly due to the lower resolution of images of the
latter palimpsests.
In Voyager images, the central region of Zakar appeared
to be an uplifted dome (Squyres, 1981), but Galileo images
(including stereo combination with Voyager images) show
that this interpretation was an illusion of albedo, as the central plains are a low-lying unit.
We interpret the central plains to be formed from solidified impact melt and chunks of solid ejecta. Radial fractures seen in the central plains of Epigeus may result from
post-impact viscous relaxation, resulting in compression
and brittle deformation at the palimpsest center (Hall et
al., 1981). This viscous relaxation would produce tensional
forces outside the former crater rim, as expressed by concentric graben and scarps farther from the palimpsest center.
Morphology and origin of palimpsests on Ganymede
203
Fig. 6. Buto Facula. (A) Reprojected Galileo images placed over reprojected Voyager image. Lettered boxes show locations of enlargements in Fig. 7.
(B) Geologic sketch map of the area outlined in Fig. 6A. Subconcentric lines delineate the palimpsest margin and boundaries between labeled surficial facies.
Circles represent craters. Scalloped lines within the palimpsest represent limits of buried secondary crater chains, and dashed lines represent buried grooves.
3.2. Unoriented massif facies
Surrounding the central plains is the unoriented massif facies (Fig. 7B), a mottled brightness unit consisting of
bright, evenly distributed, unoriented curvilinear massifs or
megahummocks 3 to 10 km in length and 1 to 5 km across,
which in turn are textured by smaller hummocks 100 m to
1 km across. The massifs decrease in height within 10 to
20 km of the boundary with the central plains. In places,
this unit appears to be dissected into plateaus by narrow,
dark-floored troughs 100 to 200 m across (Fig. 9A). The
boundary between the unoriented massif facies and the surrounding concentric massif facies is not sharp but can be
mapped where the arrangement of massifs changes from unoriented to concentric with respect to the palimpsest center.
The unoriented massif facies forms an annulus from approximately 15 to 40% of the palimpsest radius.
In the middle of the unoriented massif unit of Epigeus,
two distinct rows of massifs separated radially by 6 km are
oriented circumferentially, in contrast to the unoriented massifs throughout the remainder of this unit (Fig. 4B). These
two rows of massifs appear to be infacing scarps like those
noted in the concentric massif facies and outer deposits of
Memphis Facula and Zakar.
We interpret the unoriented massif facies as a jumbled
mass of ejecta located within the transient cavity.
3.3. Concentric massif facies
The concentric massif facies (Fig. 7C) is a region of
bright massifs arranged in arcs concentric to the palimpsest
center. These massifs are 1 to 5 km across and 10 to more
than 30 km in length, and are separated by darker, rolling,
massif-free areas 4 to 6 km across. In places, the massifs
appear to be dissected into plateaus by narrow dark-floored
troughs (Fig. 9B). The concentric massif facies occupies an
annulus from about 40 to 70% of the palimpsest radius.
Some massifs in the concentric massif facies are grouped
in arcuate segments approximately 1 km in width and of
variable length that are not concentric to the palimpsest,
but enclose circular massif-free regions 3 to 7 km in diameter. We interpret these circular arrangements of massifs
204
K.B. Jones et al. / Icarus 164 (2003) 197–212
are most clearly presented in the concentric massif facies,
arranged in patterns suggesting an origin related to local
drainage of a fluid ejecta component.
An infacing sinuous scarp with an adjacent dark-floored
trough 1.2 to 1.8 km in width is located near the outer
boundary of the concentric massif facies in Memphis Facula
(Fig. 9C). Zakar contains several similar infacing subconcentric scarps within this facies and at the proximal edge
of the outer deposits. These scarps may result from modification stage or post-modification stage inward slumping of
large sections of palimpsest.
The concentric massif facies within Zakar exhibits an odd
pattern of albedo (Fig. 5A). The unit can be divided into
four approximately equal annuli of alternating albedo, the
innermost and third zones being brighter than the second and
outermost zones. The higher albedo zones are mottled on a
scale of 1 to 2 km, while the lower albedo zones are mottled
on a scale of about 5 km. Reasons for this brightness variation are not known but may be related to radial changes in
ejecta composition or unresolved surface texture.
We interpret the concentric massif facies as a zone of
ejecta-mantled preexisting crust that has been stressed and
fractured by the palimpsest-producing impact. Post-impact
viscous relaxation formed concentric infacing scarps in primarily this facies.
Fig. 7. Enlargements showing surface texture of palimpsest facies within
Buto Facula (see Fig. 5 for locations). (A) Central plains, (B) unoriented
massif facies, (C) concentric massif facies, and (D) outer deposits.
and enclosed massif-free areas as secondary craters resulting
from palimpsest formation buried beneath this unit. The secondary craters within the concentric massif facies are more
subdued than those in the outer deposits, described below,
suggesting burial here by a thicker overlying deposit.
200- to 600-m-width dark-floored troughs, possibly present in all facies within Epigeus and Memphis Facula,
3.4. Outer deposits
The outer deposits (Fig. 7D) consist of relatively gentle rolling topography, containing scattered 100- to 500-m
moderate-brightness low hummocks, and mottled brightness
on a scale of about 5 km. This unit appears relatively smooth
at Galileo resolutions (as high as 60 m/pixel). The outer
deposits contain many fewer large bright massifs than the
concentric massif facies. Most outer deposit massifs are
arranged in arcs 0.5 to 1 km wide and 1 to 6 km long.
These arcs combine to form crescents or circles 3 to 7 km
Fig. 8. Enlargements and geologic sketch maps of selected areas of Epigeus (see Fig. 4 for locations). North is to the upper left. (A) Network of radial fractures
at the palimpsest center. Circles represent craters. (B) Secondary craters visible outside the palimpsest (to the right of the heavy line) are shown as large,
irregular circles both individually and in chains. Massifs (outlined) form circles and show the locations of buried secondary craters and chains.
Morphology and origin of palimpsests on Ganymede
205
Fig. 9. Enlargements and geologic sketch maps of selected areas of Memphis Facula (see Fig. 3 for locations). (A) Unoriented massif facies. Dark lines show
locations of several narrow dark-floored troughs. (B) Concentric massif facies surface texture, partially obscured by the crater Chrysor. Dark lines again show
locations of several narrow dark-floored troughs. (C) A large dark-floored trough and infacing scarp. (D) The palimpsest margin, seen as a low outfacing scarp.
Curved lines within the palimpsest illustrate narrow dark-floored troughs. Sets of short parallel lines illustrate north- to north–northwest-trending streaks.
(E) Possible secondary craters are outlined. North- to north–northwest-trending streaks are shown as short lines crossing crater rims and in surrounding terrain.
in diameter, and these circles in turn are often grouped in
subradial chains (e.g., Fig. 8B). We interpret these features
as buried chains of secondary craters. Many such buried
secondary crater chains can be seen within the outer deposits.
The outer deposits, like the unoriented and concentric
massif facies, appear to be dissected in places by narrow
(about 200 m across), dark-floored, scarp-bounded troughs
(Fig. 9D), and cut by uncommon infacing scarps. These
troughs and scarps may form during gravitational relaxation
of the palimpsest as viscous flow is directed inward.
Palimpsest edges as revealed by Galileo are quite circular
overall, but are locally lobate on a 10-km scale and appear
only in places to be extremely low, outfacing scarps.
The eastern edge of Buto Facula overlaps an older crater
40 km in diameter (Fig. 6). The rim of this crater outside the
palimpsest is clearly exposed and visible, and the remainder of the crater rim can be traced beneath the palimpsest,
although the expression of the rim within the palimpsest is
muted (Moore et al., 1998). This strongly suggests that the
outer deposits mantle preexisting topography, and that this
mantling stops at the palimpsest margin.
Within Epigeus and Zakar, primarily toward the distal
edge of the outer deposits, hummocks are arranged in linear rows with similar linearity, trend, and spacing to those
of grooves present just outside these palimpsests, suggesting
these hummocks are the topographic expression of buried
grooves (e.g., Fig. 5B).
We interpret the outer deposits as a continuous ejecta deposit mantling preexisting terrain and secondary craters.
3.5. Secondary craters
Exposed small craters surround each palimpsest imaged
by Galileo (e.g., Fig. 8B). These craters range in diameter from approximately 2 to 7 km, occur both singly and
arranged in subradial chains of several craters, are of similar freshness, and are exposed only beyond the palimpsest
margin (i.e., the distal boundary of the outer deposits). The
craters decrease in areal density with increasing distance
from the palimpsest. These characteristics are consistent
with an interpretation as secondary craters created by the
palimpsest-forming impact.
Secondary craters are also noted within the outer deposits and concentric massif facies, as discussed above.
These craters are buried by ejecta, however, and are not as
clearly visible as the exposed secondary craters outside the
palimpsest.
Unlike a typical isotropic distribution of secondary craters, secondary craters associated with Zakar are distinctly
concentrated to the northeast and southwest of the palimpsest.
Buried secondary craters and crater chains are inferred
within the outer deposits and the outer half of the concentric
massif facies (Fig. 5B). These crater chains are not purely
radial to the palimpsest; instead they exhibit a preference for
northeast-southwest trends. The nonisotropic distribution of
secondary craters and crater chains suggests that the impact
that produced Zakar may have been oblique, or that impact
processes were controlled by oriented structural weaknesses
in the lithosphere.
206
K.B. Jones et al. / Icarus 164 (2003) 197–212
3.6. Surrounding terrain
Beyond the outer deposit limits, preexisting terrain is not
visibly mantled by palimpsest ejecta. The primary alteration
of surrounding terrain as a result of palimpsest formation
seems to be the creation of secondary craters and crater
chains. A less common alteration may be the formation of
subconcentric infacing scarps resulting from inward slumping of large sections of palimpsest and surrounding terrain.
In Epigeus and Zakar, both located within grooved terrain, buried regional grooves can be traced into the outer
deposits as linear rows of hummocks before disappearing
as discussed above. In the larger areal coverage of the Zakar image, however, the presence of the palimpsest can be
seen to have affected the trend of grooves located outside
the palimpsest. Prominent grooves are present outside Zakar to the northwest, southwest, and southeast (Fig. 5A).
In several areas, but particularly in the southwest quadrant
of Zakar, these grooves can be traced into the outer deposits, where they appear subdued and buried (Fig. 5B).
The trend of the grooves within and immediately adjacent to the palimpsest appears to have been affected by
palimpsest formation: grooves that are linear outside the
palimpsest become curvilinear and subconcentric near and
within the palimpsest. Some grooves merge with subconcentric infacing scarps in the concentric massif facies and
outer deposits. The trends of grooves in and surrounding this
palimpsest could be a result of inward slumping taking advantage of preexisting structural weaknesses along grooves,
or possibly of groove formation taking advantage of preexisting structural weaknesses along subconcentric slumps
associated with the palimpsest. As no grooves truly cut the
palimpsest without being deformed and deflected, it is more
likely that Zakar formed after groove formation in this area,
and that inward slumping utilized preexisting weaknesses
along these grooves.
4. Comparisons to Ganymedean craters
An examination of typical Ganymedean craters helps to
clarify relationships between palimpsest facies and the locations of the former rim and continuous ejecta limits.
Large craters on Ganymede commonly exhibit two continuous ejecta facies: an inner, elevated, scarp-bounded
“pedestal” of hummocky ejecta surrounding the rim, and
a thinner, outer, radially textured ejecta blanket (Schenk and
Ridolfi, 2002). In Voyager images, poor resolution obscured
the outer ejecta facies, and Horner and Greeley (1982)
termed these craters “pedestal craters,” in reference to their
apparent similarity to Martian pedestal craters containing
only a single layer or “pedestal” of continuous ejecta. The
presence of the outer ejecta facies, confirmed by Galileo observations, shows that a more proper Martian analog is the
double-layered ejecta crater (Barlow et al., 2000).
A high-resolution (180 m/pixel) Galileo image of the
Ganymedean crater Achelous (Fig. 2) provides the clearest
example of the textural differences between the pedestal and
outer ejecta facies.
The pedestal facies of Achelous is the topographically
roughest unit surrounding the crater, rough-textured near the
rim and less so towards the distal edge of the unit. This distal
edge is lobate in plan view and slopes distinctly down to the
outer facies. The most prominent grooves from the surrounding grooved terrain are visible through the pedestal facies to
within several km of the crater rim. Only the most prominent grooves are visible beneath the pedestal, however, and
even they appear quite subdued. Although grooves are visible through this unit, the mantling ejecta blanket is thicker in
the pedestal facies than in the outer facies. We interpret the
pedestal deposit as analogous to the topographically rough
concentric massif facies of palimpsests, in which preexisting
topography and secondary craters are visible buried beneath
a thick blanket of ejecta.
The outer continuous ejecta of Achelous, particularly
near its distal edge, appears radially textured, perhaps expressing the topography of underlying secondary crater
chains. Grooves can be distinctly seen through the outer facies, although groove detail is obscured. The finest grooves
visible outside this ejecta blanket cannot be traced into the
ejecta, but larger, more prominent grooves can clearly be
seen beneath the outer facies. The distal edge of the outer
facies is thin and feathered. The outer ejecta facies of Achelous appears smoother overall than its pedestal facies, and
mantles the underlying grooves much more thinly than does
the pedestal. This facies is similar to the outer deposits of
palimpsests, which appear relatively smooth and mantle secondary craters, preexisting craters, and grooves more thinly
than does the concentric massif facies.
Beyond the outer ejecta facies, numerous exposed secondary craters and radial secondary crater chains are apparent. Most of these craters are 1 to 2 km in diameter. Based on
the presence and distribution of exposed secondary craters,
which begin just beyond the outer ejecta facies of Achelous and just beyond palimpsest margins, we infer that the
edge of the continuous ejecta of a Ganymedean crater corresponds to the margin of a palimpsest. Similarly, based on
the above morphology, we infer the crater rim to correspond
approximately to the boundary between the unoriented and
concentric massif facies of a palimpsest.
Martian double-layered ejecta craters are hypothesized to
form as a result of either impact into a volatile-rich target
material (Carr et al., 1977) or atmospheric gases providing a fluidizing medium for some impact ejecta (Schultz
and Gault, 1979; Schultz, 1992). The similar double-layered
morphology of Ganymedean craters and palimpsests cannot result from atmospheric effects, unless Ganymede had
an atmosphere in the past, or impacts created a transient
local atmosphere. This suggests that the double-layered morphology of Ganymedean impact structures, and perhaps the
Morphology and origin of palimpsests on Ganymede
analogous Martian double-layered ejecta crater morphology,
may result from the presence of near-surface target volatiles.
5. Comparisons to Europan ring structures
Galileo imaged two ring structures on Europa: Callanish
and Tyre. These structures have characteristics that closely
match those of palimpsests on Ganymede, suggesting a similar formation mechanism.
5.1. Callanish
The ring structure Callanish was imaged during orbit E4
as a two-image mosaic at a resolution of 120 m/pixel, and
also with three partial frames during orbit E26 at a resolution
of 25 m/pixel (Fig. 10A) (see Moore et al., 1998, 2001). In
these images, Callanish displays four facies that morphologically match the four palimpsest facies. As in palimpsests,
subconcentric troughs are noted in the concentric massif facies, outer deposits, and beyond the edge of the outer deposits, and small secondary craters begin just beyond the
outer deposit limits (Fig. 10B).
The central plains of Callanish (the bright central lobate
unit of Moore et al., 1998) are relatively flat, bright plains
filling the center of the structure and covered with evenly distributed low knobs from 200 to 500 m across. These plains
are smooth at scales greater than about 500 m. The boundary
207
between the central plains and the unoriented massif facies is
not everywhere distinct. In places, the central plains appear
to embay the massifs of the unoriented massif facies.
The unoriented massif facies (the rough inner unit of
Moore et al., 1998) consists of low hummocks or massifs
about 1 km across separated by approximately 1 km expanses of a surface that appears rough at 120 m/pixel. Some
hummocks are curvilinear and up to 3 km in length. The
hummocks or massifs are not appreciably oriented, in contrast to those of the concentric massif facies, and are more
closely spaced and topographically lower than those of the
concentric massif facies.
The concentric massif facies consists of massifs about
2 km across and between 5 and 15 km long arranged concentrically to Callanish, separated radially by 2 to 3 km
of a flat surface that appears smooth at scales greater than
500 m. These massifs contain the greatest topographic relief
present in Callanish. The massifs end abruptly at the boundary with the outer deposits. Subconcentric graben and infacing scarps are present in the outer portion of this unit and are
more nearly concentric than the scarps noted in any of the
palimpsests on Ganymede. Throughgoing lineations traceable from outside Callanish can be seen within this unit but
appear buried and degraded. This facies corresponds with
the annular massifs and portions of the smoother outer flow
unit mapped by Moore et al. (1998).
The outer deposits (portions of the smoother outer flow
unit of Moore et al., 1998) are smooth at 120 m/pixel and
Fig. 10. The multi-ring structure Callanish on Europa. (A) Mosaic of reprojected Galileo images. (B) Geologic sketch map of the area outlined in Fig. 11A.
Boundaries between facies are indicated by dashed lines. Walls of concentric graben are denoted by curved lines concentric to the ring structure. Many
lineations (double straight lines) and secondary craters (small circles) are shown beyond the outer deposits. One large lineation crossing the ring structure is
buried and difficult to trace near and within the unoriented massif facies. This lineation is more clearly visible, but still buried, within the concentric massif
facies and outer deposits. The lineation is clearly exposed outside the ring structure. These observations are consistent with the former crater rim located at the
boundary between the unoriented and concentric massif facies.
208
K.B. Jones et al. / Icarus 164 (2003) 197–212
Fig. 11. Galileo image of the multi-ring structure Tyre on Europa. Central region of uniform high albedo corresponds to the central plains. Surrounding
concentric features are subconcentric massifs, scarps, and graben.
contain scattered low 200- to 300-m-diameter knobs. This
unit is the smoothest unit present in Callanish. The outer
deposits appear to cover and possibly embay preexisting features at their outer edge. Many preexisting features appear
to be gradually “exhumed” within 5 km of the distal edge of
the outer deposits. Throughgoing lineations are traceable but
partially buried within this unit. The majority of subconcentric graben and troughs associated with Callanish are located
in this unit. The margin of the outer deposits, and of the ring
structure itself, is somewhat lobate.
Scattered exposed secondary craters 1.5 km and less in
diameter are located beyond the outer deposits. These craters
have the greatest areal density adjacent to the outer deposits
and decrease in areal density away from the ring structure.
Secondary craters are not visible within Callanish itself.
Galileo images of Callanish allow another constraint to be
placed on original crater rim diameter in addition to that provided by palimpsest observations. The preexisting terrain in
the Callanish region, consisting of crisscrossing lineaments
and ridges, allows better interpretation of the ejecta limits
and the former crater rim location than terrain on Ganymede
does. One large lineation in particular can be seen buried
by ejecta in the outer ring structure units, and can be traced
up to the inner edge of the concentric massif facies where it
disappears. It reappears at this same facies boundary on the
other side of Callanish, suggesting that the inner edge of the
concentric massif facies may approximate the edge of the
former crater rim.
5.2. Tyre
Tyre, another ring structure, was imaged in color at 600
km/pixel during orbit G7, and at 170 m/pixel under nearterminator conditions during orbit E14 (Fig. 11). From the
higher resolution Tyre images, it is apparent that Tyre, like
Callanish, contains a central smooth region analogous to the
central plains, a region of concentrically arranged massifs,
and numerous subconcentric troughs or scarps surrounding
the structure.
The central plains of Tyre (the smooth central unit of
Kadel et al., 2000) are a slightly hummocky region about
20 km in diameter. This high-albedo region is located within
a shallow, scarp-bounded depression.
The unoriented massif facies (corresponding to Kadel et
al. (2000)’s rough inner unit and innermost annular massif
unit) is about 50 km in diameter and surrounds the central
plains. The unoriented massif facies is comprised of equant
and elongated massifs exhibiting no preferred orientation.
The concentric massif facies (all but the innermost portion of the annular massif unit of Kadel et al. (2000)) con-
Morphology and origin of palimpsests on Ganymede
209
sists of curvilinear chains of elongate massifs, oriented concentric to the center of Tyre. This unit extends up to 100 km
from the center of Tyre, and appears texturally smoother than
the unoriented massif facies.
The outer deposits (the discontinuous ejecta unit of Kadel
et al., 2000) show ejecta-mantled secondary craters, ridges,
and plains. Exposed small secondary craters and crater
chains oriented radially to the center of Tyre are visible beyond the edge of the outer deposits.
Like the terrain surrounding Callanish, the preexisting
terrain in the Tyre region consists of lineaments and ridges,
some of which are obscured by Tyre and some of which transect it (Fanale et al., 2000), suggesting lineament formation
both before and after the impact that produced Tyre.
6. Morphometry
In addition to the morphologic evidence noted above,
morphometric similarities exist between palimpsest, crater,
and ring structure facies. Schenk (1991, 1993) has also noted
morphometric similarities between craters and palimpsests
and has tabulated central pit, crater rim, pedestal facies,
and outer deposit dimensions of numerous large craters on
Ganymede from Voyager images (Table 2). Figure 12 shows
the radii of these palimpsest, crater, and ring structure facies
plotted against concentric massif facies (for palimpsests and
ring structures) or rim (for craters) radius. These morphometric observations support the analogies drawn from our
morphologic observations. Morphometrically, the outer and
pedestal ejecta facies of craters are analogous to the outer
and concentric massif facies of palimpsests and ring structures. The crater rim (the inner edge of the pedestal facies),
then, is analogous to the inner edge of the concentric massif
facies in palimpsests and ring structures.
Schenk and Ridolfi (2002) provide an equation relating
Ganymedean continuous ejecta diameter to crater diameter.
Their equation also supports our analogies within the limits
of observational uncertainty.
7. Discussion
So far, we have described the morphology of palimpsests,
and shown their morphologic and morphometric similarity
Fig. 12. Graph showing correspondence of facies between Ganymedean
palimpsests and craters and Europan ring structures. Solid data points represent measurements on palimpsests and ring structures. Open data points
represent measurements on craters.
to Ganymedean craters and Europan ring structures. Now
we discuss the implications that these observations and comparisons have for the processes of and hypotheses regarding
palimpsest emplacement.
Based on an extensive morphologic and morphometric
study of craters on Ganymede, Iaquinta-Ridolfi and Schenk
(1995) found that for these craters, the edge of continuous
ejecta can be approximated by the location where exposed
secondary craters start to appear. The presence of clear, exposed secondary craters just outside the margins of all four
palimpsests imaged by Galileo, but not inside, suggests that
palimpsest margins correspond to continuous ejecta limits.
Preexisting structures overlapped by palimpsest margins
provide further evidence that these margins correspond to
continuous ejecta limits. Grooves exposed outside Epigeus
(Fig. 4A) and Zakar (Fig. 5) can be traced through the margins of these palimpsests into the outer deposits. Within the
outer deposits, the grooves remain visible, but appear buried
by a blanket of ejecta. The large crater overlapped by the
margin of Buto Facula (Fig. 6), which appears half exposed
and unmodified and half buried by ejecta, shows clearly that
Table 2
Approximate facies radii for selected Ganymedean palimpsests and craters, and Europan ring structures
Palimpsest or ring Outer deposits
Concentric
Unoriented
Central plains Crater name
Outer
Pedestal Crater rim Central pit
structure name
(km)
massif facies (km) massif facies (km)
(km)
facies (km) facies (km)
(km)
(km)
Callanish
Tyre
Buto Facula
Zakar
Memphis Facula
Epigeus
45
70
130
143
172
177
33
46
85
104
124
140
20
25
49
51
69
85
5
9
23
21
21
37
Achelous
Sebek
Isis
Melkart
Osiris
Enkidu
49
93
88
141
150
152
35
60
58
92
92
20
32
36
54
54
61
7
8
17
17
24
210
K.B. Jones et al. / Icarus 164 (2003) 197–212
the mantling ejecta deposit ends at the palimpsest margin. In
Epigeus (Fig. 8B), Zakar (Fig. 5), and Buto Facula (Fig. 6),
secondary crater chains that are exposed and clearly visible
outside the palimpsest boundary extend, mantled, beneath
the outer deposits and concentric massif facies, implying that
both of these units consist of ejecta blanketing the underlying topography.
Comparisons with craters on Ganymede allow us to estimate where within a palimpsest the original crater rim is
located. The four surface units identified in Galileo images
of palimpsests are both morphologically and morphometrically similar to surface units identified in several craters
on Ganymede as imaged by Galileo and Voyager. Using
the location where secondary craters begin to appear outside these craters and outside palimpsests as a tie point, the
outer deposits of palimpsests are found to be analogous to
the outer ejecta facies of craters; the concentric massif facies
of palimpsests are analogous to the inner ejecta, or pedestal,
facies of craters; and the unoriented massif facies and central
plains are analogous to the crater interior (Fig. 12).
If the boundary between the unoriented massif facies and
the concentric massif facies does correspond to the original crater rim, as is suggested by this scaling relationship,
no buried topographic features such as secondary craters or
regional grooves should be present within the unoriented
massif facies or central plains of palimpsests. This is consistent with Galileo observations. Buried secondary crater
chains noted in Epigeus, Zakar, and Buto Facula are present
only in the outer deposits and the outer half of the concentric
massif facies, and buried grooves and preexisting craters are
only visible in the outer deposits. Additionally, the greater
clarity with which these buried structures are seen near the
palimpsest margin suggests that the mantling layer thins towards this margin, as expected if the outer deposits and concentric massif facies are palimpsest continuous ejecta.
The many preexisting lineaments and ridges surrounding
the ring structure Callanish on Europa (Fig. 10) allow another constraint to be placed on both the continuous ejecta
limit and original crater rim location. These lineaments appear undisturbed outside the margin of the outer deposits of
Callanish, but disappear or are thickly mantled interior to
this margin, suggesting that, like the outer deposits limit of
Ganymedean palimpsests, the outer deposits limit of Callanish corresponds to the edge of continuous ejecta. One prominent lineament traced into Callanish is visible up to the point
where it crosses the boundary between the unoriented and
concentric massif facies (Moore et al., 1998). This lineament
is not seen within the unoriented massif facies, but reappears
at the boundary with the concentric massif facies on the other
side of Callanish. The locations of disappearance and reappearance of this lineament support the idea that the boundary
between the unoriented and concentric massif facies approximates the original crater rim.
The infacing subconcentric scarps present in the four
Ganymedean palimpsests analyzed here suggest that inward
slumping occurred following palimpsest formation. Addi-
tionally, the network of subradial fractures present at the center of Epigeus (Fig. 8A) suggests that plastic flow and uplift
of ice underlying the palimpsest center may have occurred.
This flow could have resulted from isostatic adjustment following the creation of a cavity in the then-warm lithosphere,
or from diapiric flow triggered by the thinner, weaker region
of lithosphere in the region of greatest excavation depth. Inward slumping of large sections of palimpsest could have
produced minor compression at the palimpsest center, with
the resulting volume increase at the center contributing to
the uplift.
Both Memphis Facula and Epigeus were imaged at high
resolution so that any flow structures or pools of impact
melt would probably be detected if present, as predicted by
the palimpsest formation hypotheses of large-scale impacttriggered extrusions (Thomas and Squyres, 1990) or emplacement of fluid-rich ejecta (Greeley et al., 1982; Croft,
1983; Fink et al., 1984). The central plains of both Memphis Facula and Epigeus are only smooth in comparison to
other sections of palimpsest; they still appear rough at high
resolution and may actually be quite rugged. If the central
plains formed by freezing of ponds of impact melt or fluid
ejecta, they may have been modified and roughened since
their formation. This roughness may also result from chunks
of buoyant ice freezing in place in a pool of slushy ejecta.
The dark-floored troughs hundreds of meters across, present
in both Memphis Facula and Epigeus, may be the result of
fluid ejecta locally draining around large coherent blocks
of solid ejecta into low-lying areas immediately following
the palimpsest-forming impact. The relatively low albedo of
these areas may result from downslope movement of dark
material as seen elsewhere on Ganymede.
There are three primary means of palimpsest material emplacement that can be evaluated based on these observations:
(1) formation by large-scale post-impact extrusions of warm,
plastically deforming ice (Thomas and Squyres, 1990);
(2) formation by liquid or slushy ejecta as proposed by
Greeley et al. (1982), Croft (1983), and Fink et al.
(1984); and
(3) formation by dry, solid ejecta as in lunar cratering.
The lack of large-scale radial flow structures in palimpsests
is inconsistent with warm ice volcanic flows emanating
from the palimpsest center. The lack of herringbone patterns (Oberbeck and Morrison, 1973) and relative smoothness of the outer palimpsest units seem inconsistent with dry
ejecta emplacement such as occurs on the Moon (Oberbeck,
1975), although lunar herringbone patterns are typically
noted surrounding relatively fresh craters (Schultz, 1972)
and could have been eroded from around palimpsests. A relatively smooth low-relief ejecta blanket with local clusters
of massifs, similar to the surface textures in Ganymedean
palimpsests, is expected from emplacement of fluidized
ejecta or an ejecta slurry with a considerable fraction of
impact melt. Additionally, the observation of small dark-
Morphology and origin of palimpsests on Ganymede
floored dendritic troughs, which could have formed as a
result of fluid ejecta drainage, support a partially fluid ejecta
emplacement.
Similarities between Ganymedean craters and palimpsests, as well as the observation that, based on superposition
and degradational state, palimpsests tend to be older than
craters of equivalent size, suggest that the lithosphere of
Ganymede has changed over time. Earlier in the geologic
history of Ganymede, its lithosphere may have been thinner
or had a higher temperature gradient, causing large impacts
to produce high-fluid-content ejecta, forming palimpsests.
More recent impacts of similar size were into a thicker
lithosphere or one with a lower temperature gradient, forming more typical craters. This inference is supported by the
presence of relatively young palimpsest-like impact scars,
such as Callanish and Tyre, on the thin lithosphere of Europa. The presence of double-layered ejecta surrounding
craters on Ganymede hints at the role of target volatiles in
ejecta emplacement (Carr et al., 1977).
8. Conclusions
Analysis of Galileo images of Memphis Facula, Epigeus,
Zakar, and Buto Facula provides evidence that contradicts
several of the hypotheses proposed for palimpsest formation.
No evidence is seen in any of these palimpsests at up to 60
m/pixel resolution for large-scale radial flow structures that
would be expected if the extrusion suggested by Thomas and
Squyres (1990) had taken place. The presence of troughs
in Memphis Facula and Epigeus, possibly resulting from
drainage of a fluid ejecta component, does support the partially fluid ejecta hypotheses of Greeley et al. (1982), Croft
(1983), and Fink et al. (1984). The presence of buried structures such as grooves and secondary craters within the outer
deposits and concentric massif facies of Epigeus, Zakar, and
Buto Facula rules out the possibility that the palimpsest margin corresponds to the former crater rim (Hartmann, 1984;
Lucchitta and Ferguson, 1988), as these preexisting structures would have been located interior to the crater rim and
been destroyed during palimpsest formation. Similarly, the
buried secondary craters noted within the concentric massif
facies in these palimpsests rule out the pedestal deposit hypothesis of Schenk (1996), as that hypothesis places these
buried secondaries interior to the former crater rim.
The continuous ejecta hypothesis of Passey and Shoemaker (1982) is supported by the presence of exposed secondary craters beginning just beyond the palimpsest margin,
because the extent of the continuous ejecta deposit associated with craters on Ganymede can be approximated by the
location where secondary craters begin to appear (IaquintaRidolfi and Schenk, 1995). The presence of grooves, older
craters, and secondary crater chains that are clearly exposed
outside the palimpsest margin and visibly mantled with material within this margin also supports this view. Additionally, the morphologic and morphometric similarity of sur-
211
face units seen in the four palimpsests imaged by Galileo to
those of craters on Ganymede and ring structures on Europa,
supports the continuous ejecta hypothesis. Further, the lack
of buried secondary craters or preexisting structure within
the unoriented massif facies or the central plains in all four
palimpsests imaged by Galileo and within these same units
in Callanish and Tyre is consistent with the palimpsest margin being equivalent to the continuous ejecta deposit limit, as
the boundary between the unoriented and concentric massif
facies corresponds to the location of the original crater rim.
The possibility of fluid-rich ejecta is consistent with the continuous ejecta hypothesis of Passey and Shoemaker (1982).
We conclude, then, that the palimpsest deposit is a fluid-rich
continuous ejecta deposit resulting from an impact into an
ice-rich target.
Acknowledgments
We gratefully acknowledge the work of Galileo Project
scientists and engineers in planning and acquiring these data,
and Herb Breneman, Scott Murchie, and Dave Senske for
their planning work. We thank Geoff Collins and Louise
Prockter for fruitful discussions and comments, and Nadine
Barlow for a helpful and constructive review. This effort was
funded by the NASA Galileo Project as part of the Solid
State Imaging Team through a contract from JPL to James
W. Head III.
References
Barlow, N.G., Boyce, J.M., Costard, F.M., Craddock, R.A., Garvin, J.B.,
Sakimoto, S.E.H., Kuzmin, R.O., Roddy, D.J., Soderblom, L.A., 2000.
Standardizing the nomenclature of martian impact crater ejecta morphologies. J. Geophys. Res. 105, 26,733–26,738.
Carr, M.H., Crumpler, L.S., Cutts, J.A., Greeley, R., Guest, J.E., Masursky,
H., 1977. Martian impact craters and emplacement of ejecta by surface
flow. J. Geophys. Res. 82, 4055–4065.
Croft, S.K., 1983. A proposed origin for palimpsests and anomalous pit
craters on Ganymede and Callisto. J. Geophys. Res. 88 (suppl.), B71–
B89.
Fanale, F.P., Granahan, J.C., Greeley, R., Pappalardo, R., Head III, J.,
Shirley, J., Carlson, R., Hendrix, A., Moore, J., McCord, T.B., Belton, M., 2000. The Galileo NIMS and SSI Instrument Teams, Tyre and
Pwyll: Galileo orbital remote sensing of mineralogy versus morphology
at two selected sites on Europa. J. Geophys. Res. 105, 22,647–22,655.
Fink, J.H., Gault, D.E., Greeley, R., 1984. The effect of viscosity on impact cratering and possible application to the icy satellites of Saturn and
Jupiter. J. Geophys. Res. 89, 417–423.
Greeley, R., Fink, J.H., Gault, D.E., Guest, J.E., 1982. Experimental simulation of impact cratering on icy satellites. In: Morrison, D. (Ed.),
Satellites of Jupiter. Univ. of Arizona Press, Tucson, AZ, pp. 340–378.
Hall, J.L., Solomon, S.C., Head, J.W., 1981. Lunar floor-fractured craters:
evidence for viscous relaxation of crater topography. J. Geophys.
Res. 86, 9537–9552.
Hartmann, W.K., 1984. Does crater “saturation equilibrium” occur in the
Solar System? Icarus 60, 56–74.
Horner, V.M., Greeley, R., 1982. Pedestal craters on Ganymede. Icarus 51,
549–562.
212
K.B. Jones et al. / Icarus 164 (2003) 197–212
Iaquinta-Ridolfi, F., Schenk, P., 1995. Ejecta deposits on the icy satellites:
deeper insights into the cratering process. Lunar Planet. Sci. 426, 651–
652. Abstract.
Kadel, S.D., Chuang, F.C., Greeley, R., 2000. Geological history of the Tyre
region of Europa: a regional perspective on Europan surface features
and ice thickness. J. Geophys. Res. 105, 22,657–22,669.
Lucchitta, B.K., Ferguson, H.M., 1988. Ganymede: “moat” craters compared with palimpsests and basins. Lunar Planet. Sci. 19, 701–702.
Abstract.
Moore, J.M., Asphaug, E., Belton, M.J.S., Bierhaus, B., Breneman, H.H.,
Brooks, S.M., Chapman, C.R., Chuang, F.C., Collins, G.C., Giese, B.,
Greeley, R., Head III, J.W., Kadel, S., Klaasen, K.P., Klemaszewski,
J.E., Magee, K.P., Moreau, J., Morrison, D., Neukum, G., Pappalardo,
R.T., Phillips, C.B., Schenk, P.M., Senske, D.A., Sullivan, R.J., Turtle,
E.P., Williams, K.K., 2001. Impact features on Europa: results of the
Galileo Europa Mission (GEM). Icarus 151, 93–111.
Moore, J.M., Asphaug, E., Sullivan, R.J., Klemaszewski, J.E., Bender, K.C.,
Greeley, R., Geissler, P.E., McEwen, A.S., Turtle, E.P., Phillips, C.B.,
Tufts, B.R., Head III, J.W., Pappalardo, R.T., Jones, K.B., Chapman,
C.R., Belton, M.J.S., Kirk, R.L., Morrison, D., 1998. Large impact features on Europa: results of the Galileo nominal mission. Icarus 135,
127–145.
Oberbeck, V.R., 1975. The role of ballistic erosion and sedimentation in
lunar stratigraphy. Rev. Geophys. Space Phys. 18, 337–362.
Oberbeck, V.R., Morrison, R.H., 1973. On the formation of the lunar
herringbone pattern. In: Fourth Lunar Sci. Conf. Proceedings, Vol. 1,
pp. 107–123.
Oberst, J., Schreiner, B., Giese, B., Neukum, G., Head, J., Pappalardo, R.,
Helfenstein, P., 1999. The distribution of bright and dark material on
Ganymede in relationship to surface elevation and slopes. Icarus 140,
283–293.
Passey, Q.R., Shoemaker, E.M., 1982. Craters and basins on Ganymede and
Callisto: morphological indicators of crustal evolution. In: Morrison, D.
(Ed.), Satellites of Jupiter. Univ. of Arizona Press, Tucson, AZ, pp. 379–
434.
Schenk, P.M., 1991. Ganymede and Callisto: complex craters and planetary
crusts. J. Geophys. Res. 96, 15,635–15,664.
Schenk, P.M., 1993. Central pit and dome craters: exposing the interiors of
Ganymede and Callisto. J. Geophys. Res. 98, 7475–7498.
Schenk, P.M., 1996. Origins of palimpsests and impact basins on
Ganymede. Lunar Planet. Sci. 27, 1137–1138. Abstract.
Schenk, P.M., Ridolfi, F.J., 2002. Morphology and scaling of ejecta deposits
on icy satellites. Geophys. Res. Lett. 29. 10.1029/2001GL013512.
Schultz, P.H., 1972. Moon Morphology: interpretations Based on Lunar Orbiter Photography. Univ. of Texas Press, Austin, TX, p. 218.
Schultz, P.H., 1992. Atmospheric effects on ejecta emplacement. J. Geophys. Res. 97, 11,623–11,662.
Schultz, P.H., Gault, D.E., 1979. Atmospheric effects on martian ejecta emplacement. J. Geophys. Res. 84, 7669–7687.
Smith, B.A., Soderblom, L.A., Beebe, R., Boyce, J., Briggs, G., Carr, M.,
Collins, S.A., Cook II, A.F., Danielson, G.E., Davies, M.E., Hunt, G.E.,
Ingersoll, A., Johnson, T.V., Masursky, H., McCauley, J., Morrison, D.,
Owen, T., Sagan, C., Shoemaker, E.M., Strom, R., Suomi, V.E., Veverka, J., 1979. The Galilean satellites and Jupiter: Voyager 2 imaging
science results. Science 206, 927–950.
Squyres, S.W., 1981. The topography of Ganymede’s grooved terrain.
Icarus 46, 156–168.
Thomas, P.J., Squyres, S.W., 1990. Formation of crater palimpsests on
Ganymede. J. Geophys. Res. 95, 19,161–19,174.