**GEOLOGIC MAP OF THE**

**HATHOR REGION (Jg-15)**

**OF GANYMEDE**

By René A. De Hon, Andrew C. Leith,

and William B. McKinnon

1994

Prepared for the National Aeronautics and Space Administration

by

U.S. Department of the Interior, U.S. Geological Survey

> (Published in hardcopy as USGS Miscellaneous Investigations Series Map
> I--2388, as part of the Atlas of Jovian Satellites, 1:5,000,000
> Geologic Series. Hardcopy is available for sale from U.S. Geological
> Survey, Information Services, Box 25286, Federal Center, Denver, CO
> 80225)

Please direct questions or comments about the digital version to:

> Richard Kozak
>
> U.S. Geological Survey
>
> 2255 N. Gemini Drive
>
> Flagstaff, AZ 86001
>
> e-mail: rkozak@flagmail.wr.usgs.gov

**DESCRIPTION OF MAP UNITS**

MATERIALS OF HIGH TO INTERMEDIATE ALBEDO

**s Smooth material**---Mostly in smooth, bright, straight to arcuate
lanes. Few grooves or superposed craters. Albedo may be mottled. Typical
exposure at lat 75° S., long 200° (frame 0530J2). *Interpretation:*
Young plains-forming material emplaced as a fluid (water, slush, or
solid ice warm enough to flow)

g **Grooved material**---Forms grooved sets and straight to arcuate
grooved lanes. Groove lengths 10 to 500 km, spacing between grooves 4 to
12 km. Grooves commonly form or parallel contacts. Cut by abundant
parallel or subparallel grabenlike troughs with flat or U-shaped
profiles. High albedo where albedo distinctions can be made. Typical
exposure at lat 67° S., Iong 160° (frame 0533J2). *Interpretation:*
Similar to smooth material but cut by sub-parallel faults

r **Reticulate material**---Cut by abundant transecting grooves forming
small knobs. Moderate to high albedo; moderately to heavily cratered.
Typical exposure at lat 75° S., long 165° (frame 0658J2).
*Interpretation:* Old grooved material refractured by multiple episodes
of groove formation

pm **Mottled plains material**---Forms relatively smooth to rolling
mottled plains having low to moderate crater density. Moderate albedo.
Relative age uncertain. Typical exposure at lat 66° S., long 250° (frame
0662J2). *Interpretation:* Old cratered material. Apparent low crater
density may be attributed, in part, to viewing conditions or topographic
relaxation

l **Light materials, undivided**---Moderate- to high-albedo unit in area
of low-resolution images. *Interpretation:* Materials of diverse origins
and compositions, including grooved and smooth materials, plains-forming
materials, and crater rays

DARK MATERIALS

dg **Grooved material**---In arcuate to irregularly shaped, grooved
areas. Low albedo. Most exposuses associated with or cut by light
grooved material. Typical exposure at lat 71° S., long 206° (frame
0530J2). *Interpretation:* Cratered material tectonically disrupted by
same forces that created light grooved material elsewhere

dc **Cratered material**---Forms dark terrain containing many small
craters; grooves sparse or absent. Commonly occurs as polygons bounded
by light grooved material. Typical exposure at lat 73° S., long 200°
(frame 0530J2). *Interpretation:* Oldest crustal material exposed in map
area, probably water ice contaminated with dark, rocky meteoritic
debris. May form substantial impact-generated regolith

d **Dark materials, undivided**---Low-albedo unit in area of
low-resolution images. Relief and age uncertain, but probably older than
light materials

CRATER, BASIN, AND PALIMPSEST MATERIALS

\[Craters assigned to three morphologic classes that reflect their
degradational state and relative age. All crater, basin, and palimpsest
materials thought to have formed by impact. Only craters greater than 30
km in diameter mapped except secondary craters, pedestal craters, and
craters superposed on contacts between units. Basin materials mapped
separately because of their significance as a time-stratigraphic datum\]

Crater Materials

c3 **Material of bright craters**---Forms bright, sharp-rimmed craters
that have widespread ejecta blankets bright ray patterns, and abundant
secondaries. Superposed on all other materials

c2 **Material of partly degraded craters**---Craters have sharp to
subdued rim crests and ejecta blankets that extend less than one crater
diameter from rim crest. Mostly older than smooth material in bright
lanes, but generally younger than grooved materials

cs2 **Material of secondary craters**---Forms shallow, circular to
elongate craters that commonly overlap or are aligned in chains or
clusters

c1 **Material of highly degraded craters**---Forms craters with
incomplete rim crests, poorly defined ejecta blankets, and no secondary
craters

c **Crater materials, undivided**---Mapped where low-resolution images
prevent positive age identifictation. Even though age uncertain,
moderate to high albedo of most craters suggests young age

Gilgamesh Basin Materials

rg **Rugged material**---Hummocky or blocky material surrounding
Gilgamesh inner plain. Inner facies of ejecta from Gilgamesh basin

sg **Smooth and lineated material**---Mottled, dark material that has a
lineation radial to Gilgamesh or parallel to groove trends in adjacent
units. Outer facies of ejecta from Gilgamesh basin

csg **Material of secondary craters**---Forms mostly irregular craters
in overlapping chains surrounding Gilgamesh basin

Palimpsest Materials

ps **Smooth palimpsest material**---Makes up relatively featureless
plains that have circular outlines. Low albedo relative to surrounding
terrains. Typical exposure at lat 72° S., long 280° (frame 0664J2).
*Interpretation:* Extruded ice, ejecta fallback, or impact melt

p3 **Material of palimpsests having internal structures**--- Forms
circular structures having low, hummocky relief surrounding central
plain. Palimpsest materials (units p1 and p2) older than mapped
palimpsests not recognized in Hathor region. *Interpretation:* Impact
basins that have undergone some combination of rapid (post-excavation)
collapse, slow topographic relaxation, and volcanic burial

**Contact**---Dashed where approximately located; dotted where buried.
Includes domain boundaries within grooved and smooth materials

**Fault or narrow graben**

**Scarp**---Line at top of cliff; hachures point downslope

**Throughgoing, conspicuous groove**---Steep infacing scarps and flat
floor. Interpreted as graben

**Trend of sharp groove set**---Schematic

**Trend of subdued groove set**---Schematic

**Lineament**---Indistinct narrow linear depression, ridge, or break in
slope

**Crater rim crest**---Dashed where indistinct

**Crater rim crest**---Highly subdued or buried

**Pedestal crater**---Outer edge of ejecta blanket is low scarp

**Peak on crater floor**---Rugged individual or compound peaks near
center of craters

**Peak ring on crater floor**---Rugged ring of peaks near center of
crater

**Pit on crater floor**---Within craters larger than about 20 km in
diameter

**Palimpsest ring**

**Bright ray material**---Radial streaks or bright halos from bright,
fresh crater. Superposed on all other materials. Visible only at high
sun angle

**Field of secondary craters from crater Ptah**

**Field of secontary craters from Gilgamesh basin**

**INTRODUCTION**

Ganymede, the largest of Jupiter\'s sate llites and one and a half times
the size of Earth's Moon, shows evidence of surface processes that are
strikingly different from those of terrestrial bodies. In contrast to
the rocky crusts of the inner planets, the outer crust of Ganymede is
composed of a water-ice and rock mixture with ice predominating. The
surface has had a longer and more complex tectonic history than that of
the Moon or Mercury and a different style of volcanism than that of the
Earth, Venus, or Mars.

The global geology of Ganymede was mapped by Shoemaker and others
(1982). Ganymede's surface is chiefly composed of crater materials,
light materials, and dark materials (Smith and others, 1979a, b;
Lucchitta, 1980; Shoemaker and others, 1982). (Light and dark are
relative terms; even the dark material on Ganymede is brighter than the
lunar highlands.)

The most pervasive unit in the Hathor region is light grooved material.
The region also contains materials of the craters Isis and Ptah and of
two palimpsests, as well as ejecta from the Gilgamesh basin (in Jg--12,
centered about 150 km north of the Hathor map border).

Information for the geologic mapping of Ganymede comes from images
obtained by the Voyager 1 and 2 spacecraft during their flybys through
the Jovian system in 1979. Approximately 40 images of the Jg--15 region
were acquired, although half of the region was either not imaged at all
or imaged only at very poor resolution. Resolution and look angle are
varied even over the better covered half. Resolution ranges from poor
for obliquely viewed scenes to 550 m per pixel for the highest
resolution frames. This resolution is the best obtained for Ganymede by
Voyager (see resolution diagram). Illumination ranges from extremely low
sun angles along the 300° and 120° meridians, marking the approximate
location of the terminator during the Voyager 2 flyby, to about 65°
elsewhere. The low sun angles result in the loss of topographic detail
in shadows and the absence of albedo information; hence, correlation of
units between areas of high- and low-sun-angle illumination is
difficult.

Topography and morphology are difficult to discern in the poorly imaged
portion of the map region. Therefore, materials here are mapped as
albedo units and crater materials only. The albedo units may include any
or all of the units in the better imaged portion of the region or even
entirely different materials.

**STRATIGRAPHY**

Albedo, topography, and morphology are the characteristics used for
distinguishing the mapped units. Superposition and transection relations
in the map region indicate that the younger units tend to be brighter
than the older units. Also, smooth units generally appear to be younger
than rough units. The units (other than crater and basin materials) are
divided into two major albedo groups: high to intermediate and dark. The
groups are further subdivided on the basis of topography and morphology;
grooved and smooth materials of varied ages are present over more than
two-thirds of the better imaged half of the region.

The oldest material in Jg--15 is dark cratered material (unit dc), much
of which occurs in relatively small polygons outlined by grooved lanes.
Contacts with younger units are commonly abrupt structural boundaries
except where overlain by young crater materials. Although old, the
cratered material lacks remnants of large craters but retains abundant
small, degraded craters. Presumably, large craters and basins have been
destroyed by processes such as viscous relaxation, mass wasting, and
resurfacing, or possibly they did not form in the first place.

Some darkening of material units with time can be attributed to a
combination of magnetospheric sputtering, water-ice sublimation, and
accumulation of dark, carbonaceous, meteoritic debris on a surface that
is mostly water ice (Schenk and McKinnon, 1991). Albedo variations among
different units may also be due to variations in the original
ice-to-rock ratio of the units (Johnson and others, 1983; McKinnon and
Parmentier, 1986). The stratigraphy beneath dark material is not known
in this map region, but bright material excavated elsewhere on Ganymede
by large impacts indicates that at depths greater than 5 km the dark
material is underlain by ice less contaminated by silicates (Shoemaker
and others,1982, as discussed in McKinnon and Parmentier, 1986).

The mottled plains material (unit pm) is poorly resolved. This unit
forms a rolling plain and appears to be cut by light grooved material;
hence, it is older than this material. Despite its apparent age, the
mottled material does not appear to have a high density of superposed
craters. However, its exposures, as mapped, were viewed only obliquely
and at low resolution, so the mottled material may have a more complex
morphology and a higher crater density than are visible in the available
images.

Three types of grooved material occur in the map region: dark grooved
material (unit dg), reticulate material (unit r), and light grooved
material (unit g). The dark grooved material is preserved between some
younger grooved lanes and reticulate material. It is morphologically
similar to, and associated with or cut by, the light grooved material.
The knobby topography of the reticulate material is caused by a pattern
of grooves crosscutting at high angles. The light grooved material is
mapped where albedo distinctions can be made and forms curvilinear lanes
and sets cut by abundant parallel to subparallel troughs. The grooves
are spaced about 4 to 12 km apart (Grimm and Squyres, 1985), and within
any particular set the groove spacing is very consistent. Grooves are as
long as several hundred kilometers.Typical groove profiles have gentle
U-shapes or flat floors; crest-to-floor depths are 300 to 400 m
(Squyres, 1981). Some groove sets die out along strike, while others
terminate abruptly against older sets or converge in a fanlike pattern.
South of about lat 75° S., the light grooved unit is cut by many
juxtaposed, densely grooved sets; farther north the spacing is less
dense and grooves occur singly or in small groups.

Although mapped as a single stratigraphic unit, grooved material south
of about lat 75° S. is primarily composed of contiguous lanes of locally
varied ages. Within this area is a trend of increasing groove density
with increasing age. The least grooved lane coincides approximately with
the 285° meridian and is mapped as young smooth material (unit s) on the
basis of physiography and relative age.

In bright grooved terrains elsewhere on Ganymede, dark-halo craters are
evidence for the excavation of dark materials from beneath bright
materials. Estimates of the thickness of bright materials in Uruk Sulcus
(about lat 35° N. --10° S., long 140°--180°) range between 1.0 and 1.6
km (Schenk and McKinnon, 1985). Dark halo craters may not be seen in the
map region because of masking by polar frost or the low sun angle of the
images, but the thickness of bright materials in this region is assumed
to be similiar to that at Uruk Sulcus.

Smooth material forms bright lanes. This material commonly has a dark
mottling, suggestive of a thin layer of bright material overlying dark
material or of locally varied silicate content. The rare grooves occur
singly; most are at the lane margins. Apparently this unit has been
emplaced either by the filling of structural troughs (in areas where it
has sharp boundaries), or by the flooding of low-lying areas (where it
has a more mottled appearance and irregular or diffuse boundaries).

The smooth and grooved light materials are close to pure ice in
composition (Clark and others, 1986; McKinnon and Parmentier, 1986), but
their physical state during emplacement as water, slush, or ice has not
yet been determined. Exposures of the smooth unit and partly flooded
craters in the map region are compatible with liquid water-or slush,
whereas a possible flow front at lat 78.5° S., long 165.0°

(frame 658J2-001) is more indicative of the solid-state flow of ice. A
graben about 370 km long at lat 78° S., long 140° has been resurfaced
with smooth material. Ice, slush, or water appears to have spilled over
from this graben and partly buried surrounding grooved materials,
especially near one graben terminus. This flooding episode is known to
postdate the formation of the Gilgamesh basin, because the smooth
material buries secondary craters from Gilgamesh. Apart from some impact
materials, the smooth material appears to be the youngest unit in the
map region.

CRATERS, BASINS, AND PALIMPSESTS

Craters and basins are conspicuous features on Ganymede as on the Moon,
Mercury, and Mars. However, unlike the case on the terrestrial planets,
topographic relaxation may be a major factor in altering large-crater
and basin morphology on Ganymede. Crater and basin topographic
expression ranges from well-defined crater forms to circular,
high-albedo patches probably representing very relaxed or buried craters
(Smith and others, 1979b; McKinnon and Parmentier, 1986; Lucchitta and
Ferguson, 1988; Thomas and Squyres, 1990), or possibly craters with
little initial relief. These circular albedo patches are called
palimpsests (Passey and Shoemaker, 1982). Palimpsest-like features that
retain some topographic expression of rim and other structural units
(such as central pits) are termed penepalimpsests by Passey and
Shoemaker (1982), but their material is here classified as palimpsest
material (unit p3). The albedo distinction favors the identification of
palimpsests in regions of dark terrain, and high resolution favors the
identification of penepalimpsests or basinlike palimpsests. Ancient
palimpsests (units p1 and p2) are not recognized in this map region,
which may be due to the lack of widespread dark units.

Although some medium-sized (20- to 50-km diameter) old craters can be
seen in the Hathor region, no very large, old craters are preserved,
probably because of the dominance of grooved and smooth materials. A
relative lack of very large craters on older dark units elsewhere on
Ganymede is attributed to a combination of two factors: (1) topographic
relaxation, which may occur either soon after impact or over time as
viscous creep; and (2) a dearth of large impactors in the Jovian
projectile population compared with those responsible for the late heavy
bombardment on the terrestrial planets (Strom and others,1981; Woronow
and Strom, 1982; Chapman and McKinnon, 1986).

Highly degraded craters having incomplete rims (unit c1) are scarce.
Craters that have partly degraded rims (unit c2) are found both on
materials that are older than Gilgamesh ejecta and superposed on
Gilgamesh materials. Crater rim materials superposed on grooved
materials are locally cut by grooves, although the interiors of the
craters are mostly undisturbed. This observation suggests that some of
the partly degraded craters were formed after groove formation and that
the ejecta and rim deposits are following the underlying topography.
Bright craters having sharp rims (unit C3) and bright ejecta are
superposed on all other materials. The crater Ptah, 26 km in diameter,
appears to be the youngest crater in the region; its bright ejecta are
superposed on deposits of the 77-km diameter crater Isis.

Over the whole of Ganymede, including the Hathor region, central peaks
dominate in craters less than about 20 km in diameter (Greeley and
others, 1982; Passey and Shoemaker, 1982). In craters larger than 20 km
in diameter, such as Isis, the dominant interior structure is a central
pit (Passey and Shoemaker,1982). At still larger diameters, craters may
contain inner rings. An example is Anubis, a double-ringed crater within
a region of complex grooved terrain. Its outer rim is 100 km in
diameter; its prominent inner ring is about 40 km in diameter and
slightly off center. Although possibly a peak-ring basin as seen on the
Moon, Mercury, and Mars, Anubis may be chance superposition of two
impact craters or a large central-

pit crater. lt is highly shadowed in the available images, making a more
definite interpretation difficult. Secondary craters from the Gilgamesh
basin are superposed on the ejecta blanket of Anubis; thus it is older
than Gilgamesh.

About 5 percent of fresh craters on Ganymede have ejecta that terminate
in scarps (Horner and Greeley, 1982). These pedestal craters are more
abundant in the higher resolution images of this map region than
elsewhere on Ganymede, and they appear to be most common on the smooth
materials. We interpret their apparent abundance here to result from a
combination of their youth, the image resolution, and the material on
which they are superposed: pedestal formation may be ubiquitous on
Ganymede, but the pedestal scarps degrade with age, they cannot be
discriminated if the resolution is too low, and they are recognizable
only against smooth background material. The pedestal craters identified
in this map region appear to postdate the formation of the grooved units
and are thus relatively young.

A mountainous annulus of the Gilgamesh basin extends into the map region
from about long 145° to the terminator at long 115°, even though the
basin's 150-km-diameter central plain is centered at lat 62° S., long
124°, outside the map region. Material of the Gilgamesh basin is
subdivided into two facies: an inner rugged material (unit rg) and an
outer blanket of smooth and lineated material (unit sg). Although the
radial lineaments are more common in the outer unit, they do cut both
basin facies. The northern part of the contact of the two facies
coincides with a topographic offset, basin side down, that probably
consists of a low, irregular set of scarps and slopes that are probably
related to basin ring formation. The nearby major scarp in the inner
rugged material is an extension of the main ring of Gilgamesh, which may
be structurally equivalent to the Cordilleran rim of the Orientale basin
on the Moon (McKinnon and Melosh, 1980; Shoemaker and others, 1982).
Secondary craters from Gilgamesh are superposed on the outer edge of the
lineated materials and extend outward onto adjacent grooved materials.

Hathor and an unnamed basin (lat 72° S., long 281°) are twin palimpsests
having an ill-defined, smooth central depression sur rounded by an
irregular annulus of subdued, short, semiconcentric ridges; the width of
the annulus is approximately equal to the diameter of the central
depression. The ring symbol is placed on the first hummocky ring outside
the smooth inner plain. This ring is the most obvious topographic
feature of each palimpsest, but it probably does not correspond to the
rim crest of craters and well-defined basins. Hathor-related material
appears to be superposed on older groove sets, but younger groove sets
at lat 73° S., long 286° transect the palimpsest material. Because of
its better preserved topography and the superposition of its
secondaries, Hathor appears the younger of the two palimpsests. Both
Hathor and its companion basin are much more subdued than the Gilgamesh
basin, but they exhibit similar features such as a central plain and an
extensive annulus of hummocky material. Smooth plains material (unit
ps), darker than its surroundings, occurs on the floors of Hathor and
the unnamed palimpsest and is assumed to be ejecta fallback, impact
melt, or both.

**STRUCTURE**

The dominant structures of the region are subparallel groove sets and
long, bright lanes (sulci) that are smooth to moderately grooved. The
lanes sharply truncate topographic features in the older dark terrain,
which suggests that bright material is in many areas tectonically or
structurally confined by the dark material (Lucchitta, 1980; Parmentier
and others, 1982; Shoemaker and others, 1982). The absence of
demonstrated large lateral strike-slip displacements (Lucchitta, 1980;
Parmentier and others, 1982) and the concentricity of the ancient furrow
system expressed on sections of Galileo and Marius Regiones (Schenk and
McKinnon, 1987) are evidence that the dark material has not been widely
separated. Although dark-halo craters are not identified in Ganymede\'s
polar regions, elsewhere on

Ganymede they indicate that dark material has been excavated from
shallow depths beneath the bright material (Schenk and McKinnon, 1985),
thus providing further evidence that the dark material has not been
widely spread apart. The existence of dark material at shallow depths
beneath the bright lanes is not consistent with deep subsidence or
stoping into the interior of blocks of dark material. It is consistent
with the formation of bright material by rifting and filling where, in
relatively narrow linear zones, relatively clean ice was emplaced over
down-dropped blocks of dark cratered material.

The grooves are generally considered to be the result of extensional
tectonics (McKinnon and Parmentier, 1986; Squyres and Croft, 1986).
Single grooves are probably tension cracks or narrow grabens (Shoemaker
and others, 1982; Squyres, 1982); pairs of grooves are interpreted as
viscously relaxed grabens (Parmentier and others, 1982). Groove sets
could be sets of tension cracks or sets of horsts and grabens. Grooves
may be the surface expression of fractures in the underlying dark
material, or they may develop solely at shallow depth due to continuing
extension of the bright material (Murchie and others, 1986). They may
have formed as a result of necking instabilities in a brittle layer
overlying a more ductile substrate, as the strength of ice is strongly
temperature dependent (Fink and Fletcher, 1981; Herrick and Stevenson,
1990). The different orientations of the groove sets are probably
related to sequential episodes of bright-material emplacement (Golombek
and Allison, 1981), possibly reflecting different stress systems.

The stress sources responsible for grooved material were probably some
combination of global expansion due to continued internal
differentiation (Squyres, 1980) and superposed regional and local stress
fields (McKinnon, 1981; Parmentier and others, 1982; Zuber and
Parmentier, 1984; Grimm and Squyres, 1985). A plausible regional stress
source is convective upwelling; a plausible local source is the cooling
of emplaced bright material. The response of Ganymede's lithosphere to
these stresses was not spatially uniform. Within the map region a
dominant extensional stress direction is oriented along the 190°
meridian. Younger bright lanes near the crater Isis have a variety of
orientations, suggesting a more isotropic extensional stress field.

**GEOLOGIC HISTORY**

No record of the early high impactor flux (Shoemaker and Wolfe, 1982) is
preserved in the Hathor region. Early craters and basins were lost by
extensive resurfacing and topographic relaxation. Only small polygons of
densely cratered material remain, and these areas are characterized by
small craters that probably do not reflect the initial period of
cratering. The surface appears to have been been extensively modified by
endogenous processes. Dominant were extensional tectonics and
accompanying water-ice volcanism. The old, dark materials were
extensively fractured by normal faulting, and as blocks of the dark
material were down-dropped as grabens, they were nearly completely
covered by brighter, cleaner ice from beneath. The mottled plains unit
was probably emplaced at this time. Grooves in the bright materials then
apparently formed over a long period, as superposed crater densities on
the grooved materials differ considerably. Craters were not generally
preserved until after the major period of groove formation. The
palimpsests formed during this period, but the Gilgamesh multiring basin
formed after it. Late-stage eruptions of smooth material resurfaced and
partly flooded other units, but disruption of smooth material by groove
formation was minor. Last emplaced were young, bright, sharp-rimmed
craters.

**REFERENCES CITED**

Chapman, C.R., and McKinnon, W.B., 1986, Cratering of planetary
satellites, *in* Burns, J.A., and Matthews, M.S., eds., Satellites:
Tucson, University of Arizona Press, p. 492--580.

Clark, R.N., Fanale, F.P., and Gaffey, M.J., 1986, Surface composition
of natural satellites, *in* Burns, J.A., and Matthews, M.S., eds.,
Satellites: Tucson, University of Arizona Press, p. 437--491.

Fink, J.H., and Fletcher, R.C., 1981, A mechanical analysis of
extensional instability on Ganymede, *in* Reports of Planetary Geology
Program---1981: National Aeronautics and Space Administration Technical
Memorandum 84211, p. 51--53.

Golombek, M.P., and Allison, M.L., 1981, Sequential development of
grooved terrain and polygons on Ganymede: Geophysical Research Letters,
v. 8, p. 1139--1142.

Greeley, Ronald, Fink, J.H., Gault, D.E., and Guest, J.E., 1982,
Experimental simulation of impact cratering on icy satellites, *in*
Morrison, David, ed., Satellites of Jupiter: Tucson, University of
Arizona Press, p. 340--378.

Grimm, R.E., and Squyres, S.W., 1985, Spectral analysis of groove
spacing on Ganymede: Journal of Geophysical Research, v. 90,
p. 2013--2021.

Herrick, D.L., and Stevenson, D.J., 1990, Extensional and compressional
instabilities *in* icy satellite lithospheres: Icarus, v. 85, p.
191--204.

Horner, V.M., and Greeley, Ronald, 1982, Pedestal craters on Ganymede:
Icarus, v. 51, p. 549--562.

Johnson, T.V., Soderblom, L.A., Mosher, J.A., Danielson, G.E., Cook,
A.F., and Kupferman, Peter, 1983, Global multispectral mosaics of the
icy Galilean satellites: Journal of Geophysical Research, v. 88, p.
5789--5805.

Lucchitta, B.K., 1980, Grooved terrain on Ganymede: Icarus, v. 44,
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Lucchitta, B.K., and Ferguson, H.M., 1988, Ganymede: Moat craters
compared with palimpsests and basins, *in* Abstracts of papers submitted
to the Nineteenth Lunar and Planetary Science Conference, Houston, March
14--18, 1988: Houston, Lunar and Planetary Institute, part 2, p.
701--702.

McKinnon, W.B., 1981, Tectonic deformation of Galileo Regio and limits
to the planetary expansion of Ganymede, *in* Lunar and Planetary Science
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McKinnon, W.B., and Melosh, H.J., 1980, Evolution of planetary
lithospheres: Evidence from multiringed structures on Ganymede and
Callisto: Icarus, v. 44, p. 454--471.

McKinnon, W.B., and Parmentier, E.M., 1986, Ganymede and Callisto, *in*
Burns, J.A., and Matthews, M.S., eds., Satellites: Tucson, University of
Arizona Press, p. 718--763.

Murchie, S.L., Head, J.W., Helfenstein, Paul, and Plescia, J.B., 1986,
Terrain types and local-scale stratigraphy of grooved terrain on
Ganymede, *in* Lunar and Planetary Science Conference, 17th, Houston,
March 17--20, 1986, Proceedings, part 1: Journal of Geophysical
Research, v. 91, no. B13, p. E222--E238.

Parmentier, E.M., Squyres, S.W., Head, J.W., and Allison, M.L., 1982,
The tectonics of Ganymede: Nature, v. 295, p. 290--293.

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