**GEOLOGIC MAP OF THE**

**GALILEO REGIO QUADRANGLE (Jg-3)**

**OF GANYMEDE**

By

James R. Underwood, Jr., Ruggero Casacchia,

Alex Woronow, and Michael J. Teeling

1997

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--2534, 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**

LIGHT MATERIALS

ls **Smooth material**---Forms plains of moderate to high albedo; smooth
except where secondary craters and crater chains are superposed.
Characteristically associated with grooved material that it may overlie
and grade into. Distinct boundaries with dark material. Appears to fill
throughgoing groove pairs that penetrate dark material, and some
exposures are bounded or crosscut by grooves. Representative area: lat
28° N., long 177°. Interpretation: Ice and less abundant silicate
material that fill topographic lows; probably extruded along extensional
fractures. May be younger or older than some of the grooved materials
with which it is associated

lg **Grooved material**---Has high albedo. Forms relatively level
surface interrupted by parallel to subparallel, curvilinear grooves,
tens to hundreds of kilometers long; locally, grooves short and poorly
ordered. Bounded in places by long, deep, linear, through-going grooves;
distinct boundaries with dark materials. Young palimpsest materials and
bright and slightly degraded crater materials superposed. Representative
area: lat 53° N., long 176°. Interpretation: Ice, containing few rock
fragments, that flooded areas where dark materials were displaced
laterally or downward, or both. Groove formation resulted from pervasive
development of extension fractures in the form of joints, faults, or
grabens

DARK MATERIALS

\[Constitute Galileo Regio, a subcircular area approximately 3,000 km
across that is separated by lanes of light materials from other large
areas of dark materials\]

ds **Smooth material**---Has low albedo; smoother than other dark
materials, with which it has indistinct boundaries. Very few small
craters less than 10 km diameter superposed. Closely associated with
furrows, especially in south half of quadrangle. Representative area:
lat 25° N., long 156°. *Interpretation:* Facies of dark material,
probably result of extrusion of ice and abundant rock fragments along
extensional crustal fractures associated with furrows

df **Furrow material**---Forms arcuate northwest-trending troughs in
undivided dark material flanked by rims that are estimated to be
approximately 100 m high. Individual furrows 6 to 20 km wide, 50 km to
several hundred kilometers long; material may compose much wider
subparallel sets. Characteristically has scalloped outer margins and
somewhat higher albedo than surrounding material. Spacing between
arcuate furrows ranges from 15 km to 100 km and averages 50 km. Craters
and palimpsests are superposed. Representative area: lat 48.5° N., long
165°. *Interpretation:* May be icy extrusive material that reached the
surface through fractures associated with furrows; alternatively, may be
upturned graben rims

d **Undivided material**---Has low albedo and irregular surface.
Elements of roughness, largely degraded or partly buried by regolith, or
both, are parallel short ridges and troughs and randomly distributed
circular ridges. All craters, palimpsests, and furrows are

superposed. Crater density relatively high but lower than in lunar
highlands and on Callisto. Representative area: lat 30° N., long 167°.
Correlative with dark furrowed material of Philus Sulcus quadrangle
(Murchie and Head, 1989) and Uruk Sulcus quadrangle (Guest and others,
1988). *Interpretation:* Forms oldest surface recognizable on Ganymede;
consists of ice-rock mixture that may overlie still older surface whose
irregularities may be faintly visible

CRATER MATERIALS

\[All craters interpreted to be of impact origin. Only craters larger
than about 20 km in rim diameter are mapped\]

c3 **Material of bright craters**---Forms very bright ejecta deposits;
bright rays common. Associated craters have relatively sharp rims.
Ejecta deposits may extend outward as far as two crater radii but do not
totally obliterate topography of underlying surface. Representative
area: lat 21.5° N., long 169°. *Interpretation:* Ejecta, rays, and other
crater materials associated with youngest craters. Postdates all other
material mapped

c2 **Material of slightly degraded craters**---Has albedo similar to or
somewhat higher than that of surrounding terrain. Narrow rims subdued;
some bright. Low-albedo material on some crater floors. Ejecta deposits,
where present, may extend outward one to two crater radii; a few show
shallow, outward-facing scarps similar to those of pedestal craters on
Mars (Horner and Greeley, 1982). Representative area: lat 35.3° N., long
147°. *Interpretation:* Material of intermediate age. Craters degraded
by some combination of impact, mass wasting, and ablation

c1 **Material of highly degraded craters**---Has albedo similar to that
of surrounding material. Highly subdued rims and degraded floors;
original impact morphology in places barely discernible. No ejecta
blankets visible. Representative area: lat 54.5° N., long 135.3°.
*Interpretation:* Oldest material. Degraded by some combination of
impact, mass wasting, and ablation

cm **Material of moat craters**---Forms large pit craters of
intermediate albedo. From center outward, consists of a central smooth,
flat or domed area surrounded by circular depression and inward-facing
scarp, crater rim or platform, and outward-facing scarp. Diameters range
from 30 to 70 km. Representative area: lat 29.5° N., long 165.5°.
*Interpretation:* Impact crater, but origin of distinctive
characteristics uncertain. Age comparable to that of ancient and old
palimpsest materials

cs **Material of secondary craters and chains**---Forms chains and
clusters of generally bowl-shaped craters less than 10 km diameter; most
chains several tens of kilometers long. Albedo ranges from low to high.
Representative area: lat 33° N., long 170°. *Interpretation:* Formed by
impact of debris from large craters

PALIMPSEST MATERIALS

p3 **Young material**---Forms multiring circular structures with
irregular but low relief and albedo comparable to light material.
Characterized by central smooth area surrounded by one or more
concentric ridges and pitted material. Both bright and slightly degraded
impact craters

superposed. Ejecta forms uneven surfaces mapped as pitted terrain.
Superposed both on light grooved and light smooth materials, and on dark
materials. Diameter of outermost ring of structure in the southwest
corner of quadrangle ranges from 250 to 300 km. Representative area: lat
24° N., long 179°. *Interpretation:* Large impact structures whose low
relief is result of viscous relaxation or impact into low-strength
materials, or both. Diameter of central smooth material may represent
size of original excavation cavity (Casacchia and Strom, 1984; Passey
and Shoemaker, 1982)

p2 **Old material**---Forms intermediate-albedo, subcircular features
that have diffuse outer limits and superposed crater density similar to
that of dark materials. Internal concentric ridges less discernible than
in young material, but central smooth area still recognizable. No
extensive ejecta deposits or secondary craters visible. Representative
area: lat 27° N., long 148°. *Interpretation:* Same as for young
palimpsest material

p1 **Ancient material**---Forms subcircular highly degraded structures
slightly higher in albedo and similar in roughness to surrounding dark
materials. Central smooth material common. Diffuse boundaries; no
internal structures. Superposed on dark material. Representative area:
lat 34° N., long 144.5°. *Interpretation:* Oldest palimpsests, probably
formed soon after furrow systems

ps **Smooth material**---High-albedo circular plains, generally 70 to 90
km across, in center of young, old, and ancient palimpsests.
Representative area: lat 27° N., long 148°. *Interpretation:* Origin
uncertain; may represent area of original cavity of excavation later
flooded or eliminated by viscous relaxation or doming

pp **Pitted material**---Albedo similar to that of target material.
Surface rough or irregularly sculptured; high density of craters less
than 7 km diameter; several discontinuous crater chains. Representative
area: lat 25° N., long 177.5°. *Interpretation:* Discontinuous ejecta
facies of young palimpsest material

> **Contact**---Dashed where approximately located; dotted where buried
>
> **Ridge**
>
> **Graben or grabenlike structure**---Dashed where subdued
>
> **Scarp**---Line at top; hachures point downslope
>
> **Linear depression**---Dashed where subdued; mostly in dark materials
>
> **Deep, linear depression**---Located in or at boundary of light
> materials
>
> **Furrow in dark materials**---Dashed were subdued; schematic
>
> **Abundant, closely spaced furrows in dark materials**
>
> **Sharp groove trend**---Schematic
>
> **Subdued groove trend**---Schematic
>
> **Shallow depression**
>
> **Crater rim crest**
>
> **Highly subdued or buried crater rim crest**
>
> **Vague outline of highly degraded circular structure**---Dotted where
> buried; possibly impact related
>
> **Moat crater**---Central dome surrounded by circular depression; dome
> symbol schematic
>
> **Central peak of crater**
>
> **Central pit of crater**---Symbol outlines depression; some lie at
> summit of subdued domes
>
> C**entral dome of crater**---Symbol schematic for small craters;
> locally transitional between central peaks and pits
>
> **Palimpsest ring**
>
> **Light ejecta**
>
> **Dark ejecta and dark crater floor**
>
> **Field of secondary craters**

**INTRODUCTION**

Ganymede, the largest of the 16 moons of Jupiter and the largest known
satellite in the Solar System, is 5,262 km in diameter (Burns, 1986) and
thus is larger than Mercury (diameter 4,878 km) and the Earth\'s Moon
(diameter 3,476 km) (Hartmann, 1985, p. 108). Ganymede is the fourth
largest body with a solid surface in the Solar System (after Earth,
Venus, and Mars). In 1610, Galileo Galilei discovered four large Jovian
moons, known as the Galilean satellites.Today, Ganymede is known to be
the seventh moon, and the third Galilean satellite, outward from
Jupiter. Ganymede is in synchronous rotation with Jupiter, and, with an
orbital distance from Jupiter of 1.070 ¥ 106 km (Morrison, 1982), it is
well within Jupiter\'s magnetosphere (Dessler, 1983).

Ganymede has a mass of 1.48 +/- 1023 g and a mean density of 1.94 g/cm3
(Burns, 1986). The relatively low mean density, combined with the
detection of water absorption in its reflectance spectra (Pilcher and
others, 1972), indicates that water ice is a major constituent of
Ganymede; absorption spectra together with relatively high albedo
indicate that water ice constitutes most of the surface material and may
contain small percentages of oxidized iron-bearing minerals (Clark and
others, 1986). Theoretical studies by Consolmagno and Lewis (1976) and
Cassen and others (1982) support the hypothesis that Ganymede is a
differentiated body with a rocky core and a water-ice mantle and crust.

The first spacecraft flybys of Jupiter and its satellites were by
Pioneers 10 and 11 in 1973 and 1974, respectively, but only
low-resolution, full-disc images were obtained (Morrison and Samz,
1980). Voyagers 1 and 2 made their closest approach to Ganymede in 1979
at distances of 115,000 km and 62,000 km, respectively (Morrison and
Samz, 1980); they acquired high-resolution images of Galileo Regio on
March 5 (Voyager 1) and July 9 (Voyager 2). The resolution of the best
images of the Galileo Regio quadrangle ranges from 0.5 to 2.0 km/pixel.
(See resolution diagram, this sheet.) Sun angles were 35° to 89° above
the horizon (Smith and others, 1979a,b). The images of the northern part
of the mapped area include the limb; these images are therefore highly
foreshortened and do not permit detailed mapping. Many features shown in
the poorly imaged eastern part of the quadrangle were identified on
full-disc, far-encounter images (for example, Voyager 2 image 0528J2-2).

The first geologic map of Ganymede and thus of the Galileo Regio
quadrangle was the preliminary, small-scale (1:42,500,000) map (Ferguson
and others, 1982) of that part of Ganymede imaged by Voyager spacecraft;
this map accompanied the description of the geology by Shoemaker and
others (1982). A geologic map of Galileo Regio also was prepared by
Casacchia and Strom (1984, scale 1:16,800,000), and a geologic map of
the Galileo Regio quadrangle was made by Teeling (1986, scale
1:5,000,000).

Ganymede apparently is without high mountains and deep canyons. Study of
the bright limb of Ganymede revealed that in most areas it has
surprisingly little topographic relief, only about 1 km (Smith and
others, 1979a). The apparent flatness possibly is the result of
relaxation of the icy crust (Johnson and McGetchin, 1973; Parmentier and
Head, 1981).

The quadrangle includes most of Galileo Regio, the largest region of
contiguous dark materials on Ganymede. With the exception of the dark
polar caps on Io, Galileo Regio is the only surface feature on the
Galilean satellites easily identifiable from Earth (Smith and others,
1979b). That part of the regio in this quadrangle is a subcircular area
of dark cratered material about 3,000 km across. It is in Ganymede\'s
anti-Jovian hemisphere, whose central longitude is

approximately 180° (Smith and others, 1979a,b). The mappable area of the
quadrangle is limited in the east by the terminator and in the north by
the limb.

The geologic mapping of the quadrangle followed the principles set forth
by Wilhelms (1972). The map units were identified on the basis of
albedo, morphology, crater density, and geometric pattern.

Two types of units dominate the surface of the quadrangle: dark,
furrowed and cratered materials and light, grooved materials. The dark
materials, which are much more abundant, have a low albedo, a rough
texture, and a relatively high density of impact craters and are
apparently the oldest surface units on the satellite. The less densely
cratered and therefore younger light materials occupy about half of the
imaged area of Ganymede and testify to intense resurfacing; in the map
area, they are restricted to the western margin. Also in this quadrangle
are crater materials and crater palimpsests (Smith and others, 1979b, p.
943, 945), which are high-albedo, subcircular structures of low
topographic relief.

Shoemaker and others (1982) concluded that Ganymede has a "regolith of
pulverized and mixed surficial materials" formed by (1) impact gardening
by impact of meteoroids and secondary ejecta, (2) addition of meteoritic
debris to surficial material, (3) sublimation of volatiles at low
latitudes and possible reaccumulation at mid- and high latitudes, and
(4) ion sputtering produced by the interaction with the Jovian
magnetosphere. The low thermal inertia of the surface of Gany- mede
demonstrates that the surficial material must be composed of fine,
loosely packed, clastic particles (Shoemaker and others, 1982). Applying
a lunar regolith model, they estimated the thickness of the regolith on
Ganymede to be a few tens of meters in the anti-Jovian hemisphere in
which Galileo Regio lies. However, Veverka and others (1986), using
Monte Carlo simulations of regolith development, obtained a mean
thickness of 320 m and a median thickness of 160 m for dark material and
a mean thickness of 170 m and a median thickness of 45 m for light
material.

Full-disc images of Ganymede obtained by Voyager 2 revealed the presence
of north and south polar caps (Smith and others, 1979b). Poleward of 48°
latitude, a continuous cover of relatively high albedo material is
visible (Voyager image 0532J2-002); the material is interpreted to be a
thin polar cap of water ice (Squyres, 1981a). Because albedo differences
in the underlying material remain clearly evident in the area covered by
the polar frost, the frost is thought to be very thin or, alternatively,
to be pervasive in, and thus to have brightened, the underlying
surficial materials (Helfenstein, 1986). Clark (1981) showed that as
little as 0.1 mm of frost could produce the visible albedo of
Ganymede\'s polar caps.

Several mechanisms have been suggested to explain the presence and
distribution of polar frost on Ganymede (Purves and Pilcher, 1980;
Sieveka and Johnson, 1982; Shaya and Pilcher, 1984), but each mechanism
has problems (Johnson, 1985). Johnson proposed an alternative: because
of Ganymede's position well within the magnetic field of Jupiter,
energetic ions may penetrate millimeters or less beneath the surface,
producing a thin, light-scattering layer of ice poleward of 48°
latitude, whereas south of that latitude, thermal reprocessing prevents
the formation of such a layer (Johnson, 1985).

**STRATIGRAPHY AND TECTONICS**

DARK MATERIALS

Dark material is believed to be composed of water ice contaminated with
dark silicate material (Pollack and others, 1978). Lucchitta and others
(1992) suggest the darkening of the surface possibly is caused by (1)
ion sputtering (Conca, 1981; Clark and others, 1986) or ablation of ice
(Purves and Pilcher, 1980; Squyres,

1980b) that creates a surface concentration of silicate-rock particles,
or by (2) contamination of the crust by dark projectile material
(Pollack and others, 1978; Hartmann, 1980; Conca, 1981). The dark
material may be polygenetic.

The area of dark material has a distinctive orthogonal grain or fabric
created by conspicuous, gently arcuate, northwest-trending furrows and
less conspicuous but more pervasive northeast-trending furrows. Still a
third set, consisting of less abundant furrows spaced hundreds of
kilometers apart, is oblique to the others. Inasmuch as furrows are not
superposed on craters, the furrows must have formed when the crust was
brittle enough to fracture but too weak to preserve sizeable craters
(Casacchia and Strom, 1984). Because few furrows are modified by impact,
the rate of furrow formation must have been greater than the impact rate
(Casacchia and Strom, 1984).

The surface of the interfurrow areas is irregular and rough on a scale
of hundreds of meters. In places, arcuate ridge segments, possibly the
remnants of small craters, suggest that an older surface either was
degraded by viscous relaxation or was partly covered by a material of
endogenic origin. The lack of large crater-rim segments suggests that
the process was one of viscous relaxation (Lucchitta and others, 1992).
Also visible are the rims of abundant craters less than 10 km in
diameter, presumably of impact origin; their crests characteristically
have somewhat higher albedo than the surrounding dark material
(Casacchia and Strom, 1984), suggesting either that they are of
different material or that they have been modified in some way.

In addition to its low albedo, the dark material in Galileo Regio is
distinguished by its relatively high density of craters ranging in size
from the limit of resolution to about 50 km in diameter; only two
craters have diameters as great as 100 km. Typical densities of craters
greater than 10 km in diameter are 200 to 300 per 106 km2; local
densities may be as high as 400 (Shoemaker and others, 1982). Superposed
craters cover the entire range of crater ages.

Undivided material (unit d), which occupies most of the quadrangle, has
a normal albedo of 0.35 (Squyres and Veverka, 1981) and is in
conspicuous contrast to the higher albedo light materials that occur
along the western margin of the quadrangle. Smooth dark material (unit
ds) is scattered in low-lying areas marginal to topographic highs, such
as the rims of furrows and craters; it is more abundant in the southern
part of Galileo Regio where it appears to embay materials of widely
varied age, including dark material, furrow material, and old and young
crater materials (Casacchia and Strom, 1984). In many places the smooth
material is associated with arcuate furrows whose fractures may have
provided it with pathways to the surface. Casacchia and Strom (1984)
suggested that the increase in quantity of smooth dark material
southward indicates the presence there of a thinner crust, where access
to the surface would have been easier.

Furrow material (unit df) is interpreted to be a mixture of ice and rock
that has extruded from the fractures that bound the northwest-trending
arcuate furrows. These features are the dominant structural features in
Galileo Regio and have been designated collectively as Lakhmu Fossae.
The furrow material is so interpreted because the furrow-flanking ridges
have a slightly higher albedo and because their outer margins are
irregular. The arcuate furrows are irregularly spaced at intervals
ranging from 15 to 100 km; average spacing is about 50 km. Their widths
range from 6 to 20 km; their lengths from about 50 km to hundreds of
kilometers (Casacchia and Strom, 1984); the length of the longest
furrow, trending southeast from lat 50° N., long 175°, is 615 km. Their
depth was estimated to be as great as 500 m, and they are bordered by
ridges that rise to an estimated height of 100 m above the interfurrow
terrain (Shoemaker and others, 1982). In at least one locality the
distance across a furrow set from the outer limit of the flanking ridge
on each

side is 40 km (lat 32° N., long 165°). The furrows are linear, slightly
sinuous, and mostly parallel to subparallel; they also display a variety
of abrupt offsets and kinks. The furrows are interpreted to be grabens
(McKinnon and Melosh, 1980; Casacchia and Strom, 1984). Alternatively,
furrow rims may not be composed of extruded material but may be graben
rims that were elevated as the graben floors rose in response to viscous
relaxation of the faulted crust (Shoemaker and others, 1982).

Two other systems of linear fractures characterize the quadrangle and
the region as a whole. The first system is Zu Fossae, which has
fractures that trend northeast and thus are orthogonal to the arcuate
furrows. These fractures are more densely spaced and more subdued. They
are older than the arcuate furrows inasmuch as they are cut, but not
displaced, by them.

The youngest of the three systems contains widely spaced (500 to 600 km
apart) and less abundant furrows that cut both the arcuate and the
orthogonal sets of fractures. These furrows are straight and relatively
narrow; they trend north or north-northwest, oblique to the fractures of
the other sets. Although their geometry suggests that they are not ring
segments of a multiringed impact structure, they could be fractures
radial to such a structure (Casacchia and Strom, 1984).

The origin of the furrows is uncertain. The crudely concentric pattern
of the most conspicuous (the arcuate) set suggested to some that the
furrows were created by a major impact event (McKinnon and Melosh, 1980;
Shoemaker and others, 1982; Schenk and McKinnon, 1987). Murchie and Head
(1989) interpreted the arcuate furrow set to be derived from an impact
and superposed on fractures produced by tidal despinning, but also to
have been modified by volcanism and extensional tectonism. Casacchia and
Strom (1984), however, argued that the "\...age relationships,
morphology, and geometry of the furrow systems do not favor an origin by
impact or tidal stressing." They pointed to several problems with these
interpretations: (1) absence of the fracture system predicted to
accompany tidal despinning (Melosh, 1980); (2) lack of predicted
increased spacing of fractures outward from the center of a multiring
impact structure, according to the model of McKinnon and Melosh (1980);
(3) dissimilar ages of the three fracture sets, which suggest that they
were not produced simultaneously by the impact of a large meteor; (4)
dissimilar morphologies of the rimmed furrows in Galileo Regio and the
ridges of the Valhalla impact structure on a similar icy body, Callisto;
and (5) the strong implication of furrow morphology that the furrows
were produced by crustal tension rather than compression.

Further, the increasing quantity of dark smooth material southward in
Galileo Regio and the apparent association of this supposed endogenic
material with fractures presumed to have formed the furrows led
Casacchia and Strom (1984) to suggest that an upwelling mantle plume of
water uplifted a crust of uneven thickness, resulting in arcuate and
concentric extensional fractures. They interpreted the other sets of
furrows to be radial to subradial fractures also associated with the
uplift. Schenk and McKinnon (1987) contended, however, that a
hemisphere-wide upwelling plume is unlikely to have resulted in
sufficient extension to have created the arcuate furrows; they and
Murchie and Head (1989) favored an impact origin. The evidence is
considered to be equivocal without additional, higher resolution data.

A large (300 km diameter) circular structure near the center of the
quadrangle at lat 41° N., long 143° displays concentric ridges, furrows,
grabens, and albedo markings reminiscent of the surface expression of a
large impact structure. The origin of this feature is uncertain, but it
probably is volcanic or impact related. If volcanic, this structure may
record a hot spot where underlying materials have welled upward;
however, the ripplelike surface morphology of the structure suggests
that it was produced by the ancient impact of a large body (Underwood

and others, 1986). The structure has neither the relatively high albedo
nor the secondary impact craters commonly associated with large young
palimpsests, nor does it resemble the two 250-km-diameter domes
elsewhere on Ganymede described by Squyres (1980a). On the basis of this
structure in Galileo Regio, a morphologically similar one in eastern
Marius Regio (Jg--4), and part of a much larger one in northern Perrine
Regio (Jg--2), Schenk and McKinnon (1987) suggested that, together with
Lakhmu Fossae, these structures fit the large-crater part of Ganymede\'s
crater-size-frequency curve and are therefore indicative of the impact
origin of these structures.

LIGHT MATERIALS

Light materials, both smooth (unit ls) and grooved (unit lg), located
along the western margin, occupy only about 5 percent of the quadrangle.
Their relatively high albedo, 0.44 (Squyres and Veverka, 1981), may
reflect the presence, at least at the surface, of a larger proportion of
water ice than that found in the dark material. The boundaries with the
dark units are distinct, in most places marked by linear depressions or
boundary grooves interpreted as grabens. Although a few extensions of
light materials into Galileo Regio can be identified (for example, at
lat 35° N., long 179° and at lat 56° N., long 173°), no broad or
consistent pattern of embayment of one material by the other is
observed. Because crater densities in the light material are lower than
those in the dark, the light material is known to be younger.

The smooth material, which is restricted in areal extent, has an albedo
similar to that of the grooved material and is interspersed with it. The
surface of the smooth material is almost featureless except for small
superposed craters and rectilinear depressions (not mapped) interpreted
as areas of subsidence.

The formation of the smooth material may be analogous to the flooding by
basaltic lava of terrestrial rift zones (Squyres, 1981a, b). Because of
extensional stress on Ganymede, subsidence of light grooved material
permitted the extrusion of fluid onto the surface. The emplacement of
smooth materials within Galileo Regio quadrangle must have been confined
within the rifted zones because smooth materials have notably sharp
boundaries.

In the Uruk Sulcus quadrangle to the south, a few craters more than 12
km in diameter are surrounded by dark halos; the low-albedo material of
the halos may be derived from dark cratered materials buried 1.0 to 1.6
km beneath the surface (Schenk and McKinnon, 1985). This interpretation
favors an origin by shallow burial of dark materials by light materials
(Parmentier and others, 1982) rather than an origin from upwelling of
liquid water following either the spreading apart of dark materials or
their deep subsidence into the interior (Shoemaker and others, 1982). In
the Galileo Regio quadrangle, a dark-halo crater in light materials
occurs in Ur Sulcus at lat 52° N., long 178.5°; the crater is 60 km in
diameter, the halo is 160 km in diameter.

Elsewhere on Ganymede, most of the light material is characterized by
pervasive subparallel to parallel grooves that may be as deep as 700 m
but average 300 to 400 m, are spaced at intervals of 3 to 10 km (5 to 6
km is typical), are locally hundreds of kilometers long, and have
U-shaped troughs and somewhat flattened crests (Squyres, 1981a). In
neither of the small areas of light grooved material in the map area are
grooves as pervasive or as well ordered as elsewhere, which may reflect
the dominant influence of the nearby boundary of Galileo Regio. The
boundary has superposed on it craters as old as c2 age that show only
slight to no disruption, and it truncates ancient palimpsest material at
lat 44° N., long 179°.

Grooves have been interpreted to result from extension (Smith and
others, 1979a, b; Lucchitta, 1980; Squyres, 1980c, 1981a, b, 1982;
Golombek and Allison, 1981; Parmentier and others, 1982; Bianchi and
others, 1986; Murchie and others, 1986), which probably occurred during
a period of crustal expansion resulting from planetary differentiation
(Squyres, 1980c), phase changes in an ice mantle (Shoemaker and others,
1982), or global thermal effects (Zuber and Parmentier, 1984). Lucchitta
(1980) and Shoemaker and others (1982) suggested that mantle convection
may have driven the development of grooved terrain, although Squyres and
Croft (1986) demonstrated that convection-derived stress would be minor
compared with that produced by global expansion. It is possible,
however, that global expansion was minimal and that light grooved
materials resulted from a process unrelated to expansion, for example
\"violent hydrothermal activity\" generated at the core-mantle interface
(McKinnon, 1981).

In general, most grooves occur in the light grooved material, as do
elongate zones of light smooth materials. Also a few areas of dark
grooved materials occur in dark material. Thus, the term "grooved
material" does not necessarily specify the same material as "light
material," although most light materials are at least somewhat grooved.

Extending more than 1,000 km along the northwest margin of Galileo
Regio, that part of Ur Sulcus in the Galileo Regio quadrangle ranges in
width from 25 to 180 km. Projecting for 360 km southward into Ur Sulcus
from the northwest corner of the quadrangle, an irregular, elongate area
of dark material is enclosed in light material. Much of the southernmost
part of this area has an albedo that approaches that of the light
material, which suggests that part of the material may be transitional
between dark and light materials.

The boundary between Galileo Regio and Nippur Sulcus is marked locally
by a conspicuous boundary groove, and Ur Sulcus also has a conspicuous,
through-going, curvilinear groove about 600 km long. This long
through-going groove is crosscut but not displaced by a similar
curvilinear groove that extends southeastward and appears to extend into
the dark terrain at lat 55° N., long 173°. There, light material may be
just beginning to form at the expense of the dark material. At lat 54°
N., long 175°, an elongate irregular area of grooved intermediate-albedo
material may represent a transitional stage from dark to light material.
Similar processes may have taken place along the southwest margin of
Galileo Regio, where linear boundary depressions or grooves extend into
the dark material in two localities: (1) at lat 28° N., long 176°, the
penetration is about 200 km; (2) at lat 35° N., long 180°, the
penetration is about 60 km. At lat 25° N., long 177°, a crudely
rectangular area of intermediate-albedo material may be in transition
from dark to light material, perhaps by fracturing and subsequent ice
volcanism.

In the northwestern part of the quadrangle (for example, lat 55° N.,
long 173°), the boundary between the light and dark materials is
orthogonal to the arcuate furrow system. At the southwest margin of
Galileo Regio and outside the map area (lat 17° N., long 164°), the
light material-dark material boundary is orthogonal to the radial furrow
system. Thus, the west and south boundaries of Galileo Regio are each
parallel to subparallel to one of the three furrow-system trends,
suggesting that fractures associated with the furrows may have
controlled the breakup of the dark terrain (Casacchia and Strom, 1984).

The best images of the boundary between light and dark materials in the
map area do not provide definitive evidence of a difference in
elevation, although the impression elsewhere is that the light material
is slightly lower. Shoemaker and others (1982) observed that the dark
material is higher than the adjacent light material at many places, and
Lucchitta and others (1992) stated that in Memphis

Facula quadrangle to the southeast, the dark material of Galileo Regio
locally is higher than the light materials of Uruk Sulcus along their
east-west boundary.

PALIMPSEST MATERIALS

Ganymede's palimpsests, first clearly seen on Voyager 2 images of
Galileo Regio, are bright, circular to subcircular features whose
diameters in the map area range from 100 to 400 km and whose albedo is
similar to that of the light materials. In the following discussion, the
diameter of palimpsests is considered that of the total circular or
subcircular relatively high albedo zone.

Palimpsests apparently were the earliest impact features formed after
the development of furrows, when the crust became rigid enough to retain
impact scars (Casacchia and Strom, 1984). The subdued topography of
palimpsests is interpreted to be a result of viscous relaxation of the
crust or of the low strength of the early crust during impact (Smith and
others, 1979b; Parmentier and Head, 1981; Passey and Shoemaker, 1982;
Croft, 1983), or both. However, Croft (1983) argued that palimpsests are
not due to viscous relaxation; rather, they result directly from higher
than normal impact velocities.

Palimpsests, which show several degrees of degradation, consist of three
basic types of materials: ancient (unit p1), old (unit p2), and young
(unit p3). Ancient palimpsest material has a surface similar in
roughness and crater density to the dark material and predates the light
material. Most ancient palimpsests lack discernible internal structures,
and many of these palimpsests contain areas of smooth palimpsest
material (unit ps). Old palimpsest material has fewer superposed impact
craters, and many of these structures contain central areas of smooth
palimpsest material, some of which are surrounded by circular
depressions and ridges. Young palimpsest material forms multiringed
structures similar to impact basins observed on the surface of the
terrestrial planets, although more flattened.

Of the 15 palimpsests in or partly in the quadrangle, 13 occur in the
west half. The two largest and youngest (p3) palimpsests are 400 km in
diameter, near the southwest corner of the quadrangle, and 280 km in
diameter, near the southeast corner. These young palimpsests are
surrounded by ejecta deposits and fields of secondary craters that have
degraded significantly the material on which they formed, creating an
irregularly sculptured surface designated pitted palimpsest material
(unit pp). Of the remaining 13 palimpsests, 6 are old (p2) and average
about 150 km in diameter, and 7 are ancient (p1) and average 110 km in
diameter. This correlation of larger size of palimpsest with younger age
does not exist everywhere on Ganymede; in the Uruk Sulcus quadrangle,
for example, some of the largest palimpsests are the oldest (Guest and
others, 1988).

On the outer part of some palimpsests, as at lat 35° N., long 152°,
faint expressions (not mapped) of the underlying furrows are visible;
therefore, the high-albedo material there cannot be thicker than the
depth of the furrows, a few hundred meters or less (Shoemaker and
others, 1982). That relation may also suggest that in such places furrow
formation continued after the palimpsests were formed, but in any event
the observation shows that the cavity of excavation of the palimpsest
did not extend to the outer margin of the high-albedo zone (Casacchia
and Strom, 1984).

The outer edges of the bright palimpsests are too sharp and even to be
ejecta margins; they may be zones of seismic disruption, local heating,
or both (Strom and others, 1981). The cavities of excavation probably
are represented by (1) the bright areas (unit ps) at the center of most
palimpsests (Shoemaker and others, 1982) or (2) the subdued inner rings
or partial rings (Casacchia and Strom, 1984)

observed in all of the palimpsests in the map area with the exception of
four ancient ones.

Ancient palimpsests and highly degraded craters are known to be the
oldest crater forms on Galileo Regio because they are overlapped by
degraded craters. However, palimpsests and all craters greater than 10
km in diameter postdate the furrow systems (Casacchia and Strom, 1984).

CRATER MATERIALS

Impact craters in the quadrangle have a wide range of morphologies,
ranging from fresh craters with bright rays, sharp and well-defined rim
crests, and bright ejecta deposits to barely discernible features with
highly degraded and flattened rims. The morphologic characteristics of
impact craters also are related to size: the smallest, less than 20 km
in diameter, have central peaks; intermediate sizes, 20 to 40 km in
diameter, have central pits; and craters greater than 40 km in diameter
commonly show terraces and domical floors. The interpretation and
comparison of crater degradation states is complicated by (1) lack of
data on the response of crustal ice-rock mixtures to hypervelocity
impact and to subsequent relaxation by viscous flow, and (2) changing
physical properties of the crust as Ganymede cooled.

Bright crater material (unit c3) is the youngest unit of the region;
ejecta of these craters overlap all other mapped units. The largest c3
crater in the quadrangle, 75 km in diameter, is at lat 21.5° N., long
169°. There, bright rays extend outward as far as 150 km from its
center.

Material of slightly degraded craters (unit c2) has most of the
attributes of bright crater material but has lower albedo and no rayed
craters. Inasmuch as the unit is superposed on most other units of the
map area and is the most abundant crater material, the impacts that
produced it must have occurred throughout most of that part of the
history of Ganymede recorded by the present surface.

Material of highly degraded craters (unit c1) forms crater rims that
characteristically are highly subdued, and the craters generally have no
peaks or pits. The stratigraphic position of highly degraded craters
shows that they formed relatively early; locally, they seem to overlap
furrows, grooves, and crater palimpsests but also are, in places,
disrupted and cut by furrows in the dark terrain.

Moat craters (unit cm) have an anomalously large central pit, a domed,
smoother central area, and a depression or moat between the crater rim
crest and the arched or domed crater floor. The floor of moat craters
first flattened, then bowed upward as the crater rims and rim flanks
subsided by viscous relaxation (Passey and Shoemaker, 1982).

Several chains of secondary craters (unit cs) also have been mapped. The
crater Halieus (lat 35.5° N., long 168.5°) has several associated
well-developed secondary crater chains, one more than 100 km long.

A crater at lat 28° N., long 133° has an outward-facing scarp at the
apparent distal margin of the ejecta blanket and thus resembles
ejecta-flow craters on Mars. Image resolution is not adequate, however,
to identify radial-flow structures nor the thin, digitate flow lobes
typical of the outer margin of the ejecta blankets of similar craters on
Mars (Horner and Greeley, 1982).

Distinct, conspicuous, dark crater rays radiate outward for 50 km from a
20-km circular source at lat 59° N., long 150°. The resolution is poor
at that latitude, but the rays appear to be centered at a depression
about 100 km in diameter. Conca (1981) has suggested that the dark
crater rays there and elsewhere on Ganymede are the result of ablation
of projectile-contaminated ejecta, resulting in a surface-lag deposit of
dark material. Ablation could result from sputtering of water by charged

particles (Conca, 1981), still another of the possible effects of
Jupiter\'s powerful magnetic field.

Many craters of varied sizes have straight rim segments or are polygonal
in plan view. One or more of the straight segments is parallel to one or
more of the furrow systems of the dark material, which may reflect
control by the regional fracture pattern, either at the time of
formation of the crater by impact or subsequently during isostatic
adjustment and viscous relaxation of crater topography. Gravity-induced
mass movements also may have occurred. Examples of such craters are at
lat 23° N., long 125° (35 km diameter), lat 23° N., long 133° (50 km
diameter), lat 30° N., long 154.5° (40 km diameter), lat 32° N., long
122° (50 km diameter), and the large craters Kulia, at lat 34.5° N.,
long 115° (95 km diameter), and Halieus, at lat 35.5° N., long 168.5°
(90 km diameter).

Many dome craters are present in the Galileo Regio quadrangle and have
crater diameters of 30 to 90 km and dome diameters of 10 to 50 km.
Bianchi and Pozio (1985) and Moore and Malin (1988) studied the dome
craters in the Galileo Regio quadrangle and elsewhere on Ganymede with
inconclusive results. Dome craters may be the result of diapirism
triggered by crater formation or by viscous relaxation following crater
formation (Moore and Malin, 1988); their varied morphologies probably
reflect the changing physical characteristics of the evolving crust of
Ganymede (Bianchi and Pozio, 1985).

**GEOLOGIC HISTORY**

Ganymede\'s surface apparently does not provide direct evidence of the
earliest phase of its geologic history, that is, heavy bombardment and
accretion of particles of water ice and silicate-rich rock, nor of the
period of early planetary differentiation into a mantle, core, and weak
lithosphere. The first recorded event was the formation of dark
materials approximately 4.0 to 3.8 billion years ago (Shoemaker and
others, 1982). Viscous relaxation of the thin, weak lithosphere resulted
in the disappearance of most evidence of the accretion process
(Shoemaker and Wolfe, 1982) and "extensive resurfacing or viscous
relaxation may have obliterated a Callisto-like crater population"
(Murchie and Head, 1989).

The ice- and silicate-rich dark material may be the original global
crust of Ganymede formed just after accretion, or it may be material
extruded onto an older surface and altered by contamination with
meteoritic material or by ion sputtering, or both, creating an
increasingly dark lag deposit of silicate- and meteorite-rich material.
As Ganymede cooled and the lithosphere strengthened, craters less than
10 km in diameter began to be preserved. The lithosphere eventually
stiffened enough to permit widespread furrow formation, as the result of
(1) an external event such as the impact of a large meteor (Smith and
others, 1979; McKinnon and Melosh, 1980; Shoemaker and others, 1982;
Murchie and Head, 1989) or (2) internal processes such as the upwelling
of a large mantle plume of water ice (Casacchia and Strom, 1984).

The oldest set of furrows in the quadrangle is orthogonal to the arcuate
set of intermediate age; the youngest set consists of those oblique to
the other sets.The three sets must have formed relatively quickly
inasmuch as they are older than craters greater than 10 km diameter. The
earlier and larger impact events are represented by palimpsests; the
later and smaller impact events, by craters of varied size and degree of
degradation. In one locality just south of the map area, an arcuate
furrow seems to have undergone reactivation, as it shows faint evidence
of transecting the large palimpsest Memphis Facula. Dark smooth
material, associated with the furrows, was emplaced from the time the
furrows were formed through the period of formation of most craters.

Following the evolution of the furrow system and the emplacement of much
of the dark smooth material, the thickening crust was broken apart and
possibly sheared laterally as crustal expansion continued. Convection
cells may have operated within the mantle to carry the water ice of the
light material to the surface to fill the incipient voids developing
between blocks of dark material or to overspread their surfaces as they
foundered. The light grooved material formed by extension, but the
details of the process remain obscure. In some areas, the light material
has smooth surfaces that are younger than the grooved material. Young
palimpsests also formed after the emplacement of light materials.

The bright-ray craters, the youngest features in the quadrangle, have
not been significantly degraded. Processes that may be currently active
on Ganymede are occasional meteorite impacts, mass wasting, and ablation
resulting from either thermal migration of water, ion sputtering, or
both.

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