**GEOLOGIC MAP OF THE**

**TIAMAT SULCUS**

**QUADRANGLE (Jg-9)**

**OF GANYMEDE**

By

S.K. Croft, Ruggero Casacchia,

and R.G. Strom

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--2328, 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

**g** **Grooved material**---High albedo (normal albedo ≈0.45, Johnson
and others, 1983); surface extensively cut by linear and curvilinear
grooves ranging in morphology from grabenlike grooves 3 to 5 km wide and
as much as several hundred kilometers long to barely discernible
lineations a few tens of kilometers long. Contact with underlying dark
material probably uneven. *Interpretation:* Ice with very few dark
impurities, emplaced as liquid flows over tectonically depressed dark
terrain. Grooved by extensional tectonic or volcanic processes or both

**s** **Smooth material**---High albedo (≈0.45); relatively unbroken,
flat. Few or no grooves and very low crater density. Gradational to
grooved material. Emplaced locally over pre-existing grooved material.
*Interpretation:* Same as grooved unit but not deformed by
groove-formation processes

**l** **Light material, undivided**---Material of high albedo (≈0.45),
either obscured by overlying ejecta or observed at resolutions too low
to assign confidently to smooth or grooved units. *Interpretation:*
Similar to other light materials

DARK MATERIALS

**dcr** **Dark cratered material**---Low albedo (≈0.35); superposed by
rare, very dark streaks. Moderately high crater density and irregular,
rough surface. Transected by long, west-northwest-trending troughs
having large (50 to 150 km) but irregular spacing and by subdued,
meandering troughs, parts of which may be degraded furrows. Some
flat-floored grabens bordering or within unit have somewhat darker
floors. *Interpretation:* Ice containing moderate amount of rock or
other impurities. Emplaced as "muddy," low-viscosity cryovolcanic flows.
Surface roughness caused by pervasive tectonic or cryovolcanic
structures and impact craters near limit of resolution. Troughs formed
in two stages: earlier north-northeast-trending segments caused by
stresses forming concentric furrow system, and later
west-northwest-trending segments due to stresses forming radial trough
system

**dcrm** **Moderately bright cratered material**---Fairly uniform albedo
(≈0.40) with a few darker patches. Moderately high crater density;
rough, irregular surface. Surface features similar to those on dark
cratered material. *Interpretation:* Same as dark cratered material but
contains somewhat fewer rocks or other impurities. Possibly emplaced
later than dark cratered material

**dcrd** **Very dark cratered material**---Very low albedo (≈0.30);
surface features similar to cratered material. Includes some deposits
mapped as dark lineated material by Murchie and Head (1989) in area of
overlap with Philus Sulcus quadrangle; within our map area, these
deposits lack the conspicuous linear depressions of their unit.
*Interpretation:* Same as dark cratered material but contains more rocks
or other impurities

**df** **Furrowed material**---Low albedo (≈0.35); high crater density.
Cut by arcuate furrows; rough texture between furrows. *Interpretation:*
Same as dark cratered material

**dl** **Lineated material**---Very low albedo (≈0.30, comparable with
that of very dark cratered material); extensively cut by sets of
groovelike linear depressions spaced 3 to 7 km apart. Edges of
individual depressions

somewhat more irregular than groove edges on light materials, but both
types of features have similar planforms and dimensions. Crater density
appears only slightly greater than on light materials. *Interpretation:*
Ice having relatively high impurity (rock) content; original cratered
surface greatly disrupted by tectonic activity associated with formation
of groovelike depressions

**d** **Dark material, undivided**---Material of low to moderate albedo
observed at resolutions too low to map as cratered or lineated material.
*Interpretation:* Similar to other dark materials

CRATER AND PALIMPSEST MATERIALS

**cs** **Smooth material at center of Ombos Facula**---Circular,
high-albedo patch in central depression of facula. *Interpretation:*
Refrozen impact melt consisting of nearly pure water ice; facula
interpreted as impact basin

**c3** **Rayed crater material**---Forms craters having conspicuous
bright or dark rays or very bright ejecta. Central peaks or pits in
larger craters. Superposed on all other map units. *Interpretation:*
Ejecta and other materials associated with freshest impact craters

**c2** **Partly degraded crater material**---Forms rim and interior of
crisp- appearing craters, but craters lack rays and material has
generally lower albedos than those of **c3** craters. Internal
structures similar to **c3** craters. *Interpretation:* Well-preserved
impact craters but older than rayed crater material

**c1** **Degraded crater material**---Forms rim and floor of craters
having moderately to highly degraded rim crests. Albedo similar to that
of surrounding terrain. *Interpretation:* Degraded impact craters

**p2** **Palimpsest material**---Material of subcircular patches of
fairly high albedo centered on subconcentric system of topographically
subdued ridges; southern palimpsest contains poorly defined central
smooth plain. No secondary craters or chains around palimpsests in this
quadrangle. Superposed by **c3** and **c2** craters. Other palimpsest
materials mapped in Uruk Sulcus quadrangle (units **p1** and **p3**,
Guest and others, 1988) not seen in map area. *Interpretation:* Possible
impact scars formed under unusual conditions: high-velocity impact or
impact into unusually warm substrate; collapse more complete than for
multiring basins; possible cryovolcanic origin or modification

**ps** **Palimpsest smooth material**---High-albedo material forming
smooth plains in central part of southern palimpsest. *Interpretation:*
Uncertain; may be refrozen impact melt or extruded cryovolcanic material

**Contact**---Dashed where approximately located, dotted where buried.
Includes boundaries between domains in light materials

**Conspicuous quasi-linear trough in dark materials**

**Subdued, meandering trough in dark materials**

**Groovelike depression in dark materials**

**Sharp groove trend**---Schematic

**Subdued groove trend**---Schematic

**Conspicuous throughgoing groove or groove bounding domain**

**Furrow**---Only representative number mapped

**Conspicuous light-floored groove**---Interpreted as graben

**Conspicuous dark-floored groove**---Interpreted as graben

**String faculae**---Chain of bright spots

**Crater rim crest**

**Highly subdued or buried crater rim crest**

**Inward-facing scarp on crater floor**

**Pit on crater floor**---Circle outlines depression

**Dome on crater floor**

**Basin ring**

**Palimpsest ring**

**Light ejecta**

**Dark ejecta**

**Dark mantling material or ejecta**

**INTRODUCTION**

Ganymede is the largest known satellite in the Solar System and the
third Galilean satellite outward from Jupiter. Its density (1.93 g/cm3)
and surface spectral characteristics indicate that its bulk composition
is abouthalf water ice and half rocky material, presumably of
carbonaceous chondriticcomposition (Morrison, 1982). Thus Ganymede is
the largest Solar Systemobject classified as an icy satellite.

The Tiamat Sulcus quadrangle lies in the equatorial region of Ganymede's
anti-Jovian hemisphere. The best images of the west half of the
quadrangle are low-resolution frames (≈30 km per pixel) obtained by
Voyager 1 in March 1979, whereas substantially higher resolution images
(≈1.6 km per pixel) were obtained of the east third by Voyager 2 in July
1979. (See resolution diagram.)

Our geologic interpretation is based on albedo, crater density,
morphology, texture, and structural patterns, following the planetary
geologic mapping conventions of Wilhelms (1972). However, interpretation
was difficult in this quadrangle for several reasons. First, the Sun was
within ≈20° of the zenith, virtually eliminating shadows, for the higher
resolution images. Second, viewing angles range from moderately to
extremely oblique in these images. Third, the higher resolution images
show extensive areas of the quadrangle blanketed by ejecta from the ray
craters Tammuz and Dendera Facula within the map area and from Punt
Facula in the Apsu Sulci quadrangle (Jg-13) to the south. Finally, the
use of terrestrial analogs in interpretation is much less reliable for
an icy satellite like Ganymede than for the terrestrial planets, because
water ice dominates the surface units. Ice has different material
properties than rock, and the effect of those differences on geologic
processes is poorly understood.

**PHYSIOGRAPHIC SETTING**

The surface of Ganymede is commonly divided into two distinct albedo
terrains: light and dark (Smith and others, 1979a,b). The dark terrain
is divided by the light terrain into a few large, subcircular regions
such as Galileo Regio centered in the Galileo Regio quadrangle (Jg-3)
and Nicholson Regio centered in the Namtar quadrangle (Jg-14); several
medium-size, more elongate or linear regions, such as Bernard Regio in
the Dardanus Sulcus (Jg- 6) and Misharu (Jg-10) quadrangles and eastern
Marius Regio in the Uruk Sulcus quadrangle (Jg-8); and complex clusters
of medium to small polygons (as seen in this quadrangle and in the
Dardanus Sulcus (Jg-6) and Apsu Sulci (Jg-13) quadrangles). The division
of the dark terrain into circular, linear, and small polygonal blocks
may be caused by the lithosphere underlying these regions being thinner
than the lithosphere beneath the large, circular regions (Croft and
Goudreau, 1987). Crater densities are generally lower on the light
terrain than on the dark, consistent with direct stratigraphic
indications that the light terrain is younger. The difference in albedo
between the two terrains is apparently due to differing fractions of
rocky material mixed with the ice. Estimates of the absolute volume
fractions of rock in terrains of different albedo are poorly
constrained: the albedo differences can be accounted for by only a few
percent rock (Clark, 1982) or as much as several tens of percent
(Helfenstein, 1986), depending on the relative sizes of ice and rock
grains in the surface layers.

The quadrangle covers the north two-thirds of a large, previously
unrecognized, circular and dark region similar in diameter (\~3,600 km)
and appearance to the Galileo and Nicholson Regiones. The regio
comprises the central, southern, and western parts of a large,
disjointed group of blocks of dark terrain collectively denoted as
Marius Regio (Masursky and others, 1986). To emphasize it as a distinct
geologic feature, this circular regio will be informally referred to
here as Southwest Marius Regio. It is recognizable on the global color
photomosaic of Ganymede in Johnson and others (1983). The regio is
bounded by Tiamat Sulcus on the east (in the Uruk Sulcus (Jg-8)
quadrangle) and northeast and by large, unnamed areas of light terrain
to the north (partly included in the

map area), south (in the Apsu Sulci (Jg-13) and Namtar (Jg-14)
quadrangles), and west (partly in the map area and extending to the
Harpagia Sulci in the Misharu (Jg-10) quadrangle). The approximate west
boundary of the regio is shown on the map by a screened line. Southwest
Marius Regio is partly broken up by lanes of light material. The broad
lanes of Kishar and Hursag Sulci penetrate to the regio's center, and
several smaller strips of light terrain radiate from the center.
Crosscutting by narrow lanes of light material has reduced the southeast
part of the regio to a maze of small polygons. The extent of light
terrain within the regio is comparable to that found in Nicholson Regio,
but both Nicholson and Southwest Marius Regiones are less intact than
Galileo Regio.

**STRATIGRAPHY**

DARK MATERIALS

On the basis of superposition relations and crater densities, the areas
of dark cratered terrain are interpreted to be the oldest surfaces on
Ganymede. However, the highest crater densities on the dark terrain are
only about one-third the density of a similar population of craters on
neighboring Callisto, even though both satellites should have had
virtually identical cratering histories. The apparent loss of craters on
Ganymede has been variously attributed to viscous relaxation (for
example, Passey and Shoemaker, 1982; Shoemaker and others, 1982) or
volcanic flooding (for example, Woronow and others, 1982; Croft, 1983b,
1985; Murchie and others, 1988). Croft (1981, 1988) showed that viscous
relaxation has not affected craters less than 50-60 km in diameter on
Ganymede. Similarly, finite-element modeling by Hillgren and Melosh
(1989) showed that topography on Ganymede is elastically supported and
that relaxation has little effect on small-wavelength features, or even
on moderate-size craters. Certainly such features would not be wholly
erased by viscous relaxation. Thus the low crater density on even the
oldest dark materials indicates that all of Ganymede has been completely
resurfaced by volcanic flooding. Several lines of evidence indicate that
dark materials were emplaced in a series of resurfacing events over a
prolonged period and that they were deformed by more than one tectonic
event: (1) crater densities differ by factors of 2 or more (Shoemaker
and others, 1982); (2) albedo differs regionally in the dark materials
by 15 to 20 percent; (3) surface texture differs regionally from smooth
to rough to ridged to grooved; (4) tectonic structures change in
morphology and regional strike from area to area within the dark
materials.

Many of these variations occur within the map area. The dark cratered
material (unit **dcr**) in the east-central part of the quadrangle has
an albedo near 0.35 (estimated from Squyres and Veverka, 1981, and
Johnson and others, 1983) and a locally rough texture due to randomly
oriented, irregular hillocks typically 5 to 10 km in size. The
moderately bright cratered material (unit **dcrm**) in the extreme east
has the same texture as the dark cratered material, but it is about 10
percent higher in albedo. (The extension of this unit eastward into the
Uruk Sulcus quadrangle was interpreted by Guest and others (1988) as
palimpsest material. The unit is interpreted differently here because of
its similarity to the other cratered material units and its lack of
characteristic palimpsest structural features and surface texture.) The
very dark cratered material (unit **dcrd**) also has texture similar to
that of the dark cratered material but is about 10 percent lower in
albedo. The boundaries between these three units are gradational at high
resolution but are recognizable in the low-resolution photomosaic of
Johnson and others (1983).

All three cratered units are cut by conspicuous quasi-linear troughs and
more subdued meandering troughs. Both types of troughs have edges with
topographic irregularities of the same dimensions as the hillocks and
depressions in the surrounding terrain, and they have large but
irregular spacings of 50 to 150 km. The quasi-linear troughs trend
roughly west-northwest, as do some segments of the subdued meandering
troughs. The quasi- linear troughs are part of a large tectonic system
of troughs oriented radially from a center near lat 22° S., long 135°
(Shoemaker and others, 1982; Casacchia and Strom, 1984;

Murchie and Head, 1987) in the Memphis Facula quadrangle (Jg-7).
Elements of this radial trough system can be traced over most of the
dark terrains adjoining the map area to the east and northeast.

The furrowed material (unit **df**) in the extreme northeast corner of
the quadrangle has (in this area) about the same albedo as dark cratered
material, but it is somewhat smoother in texture. This unit is cut by
grabenlike arcuate furrows with raised and scalloped edges that tend to
have slightly higher albedo than surrounding materials. These furrows
are part of a second large tectonic system whose elements are oriented
concentrically around a center near lat 10° S., long 175° in the Uruk
Sulcus quadrangle (Jg-8). This concentric furrow system forms a
conspicuous network that covers roughly the same area as the radial
trough system, and it has been suggested to be related to a giant impact
(Smith and others, 1979a; Schenk and McKinnon, 1987; Murchie and Head,
1987). In contrast to the quasi-linear troughs, the concentric furrows
in the furrowed material trend north-northeast within the map area and
have regular spacing of about 20 km. Except for the few furrows in the
small patch of furrowed material, the conspicuous network of concentric
furrows virtually ends just outside the map area to the east and
northeast. The only features representing a possible further extension
of the concentric trend into the map area are the
north-northeast-trending segments of some subdued meandering troughs. A
few of these segments resemble degraded furrows.

The dark lineated material (unit **dl**), which dominates the
southeastern part of the map area, is approximately as dark as the very
dark cratered unit, but it has a substantially lower crater density,
approaching that of the light material. This unit is characterized by a
roughly east-west-trending system of closely spaced (3 to 7 km),
groovelike, linear depressions substantially different in morphology
from the furrows and troughs in the dark material to the north. In many
places, the groovelike depressions have the same trends as grooves in
the adjoining light materials, and locally, particularly along the south
boundary of Kishar Sulcus, the depressions may be traced continuously
from the dark into the light units. The only difference between grooves
in the light materials and the depressions in the dark lineated
materials is the somewhat less regular edges of the depressions within
the latter, perhaps reflecting the rougher surface of the dark unit
relative to the light ones.

Dark materials thus comprise a complex set of different facies. The
explicit division by albedo of the otherwise similar cratered materials
(units **dcr**, **dcrm**, and **dcrd**) is made to emphasize this
complexity. The various albedo units are interpreted to be, if not
single large-scale flood deposits, multiple flow deposits emplaced
during a single cryovolcanic episode analogous to terrestrial
flood-basalt eruptions. Age relations among the three cratered material
units within the map area are difficult to establish. Significant
crater-density differences do occur on units distinct in albedo and
texture in eastern Marius and Galileo Regiones (Shoemaker and others,
1982; Croft and Goudreau, 1987), indicating that cryovolcanic
resurfacing in the dark terrain continued over an extended period, but
crater statistics within the Tiamat Sulcus quadrangle are poor because
of the high sun angle. Further, not all differences in crater density
are necessarily due to flooding. For example, tectonic features in the
dark lineated material are sufficiently closely spaced to disrupt and
obscure even small craters, and thus the apparently low crater density
of the lineated terrain is probably due more to tectonic disruption than
cryovolcanic flooding.

Stratigraphic relations are similarly problematic. If the furrowlike
segments of the meandering troughs are partly flooded elements of the
concentric furrow system, then the moderately bright cratered material
is younger than the furrowed material. Alternatively, the meanders may
reflect local tectonism unrelated to the concentric system. This
alternative implies breaks in the concentric furrow system similar to
breaks in the pattern of concentric features around the Valhalla
structure on Callisto (Smith and others, 1979b), which may be indicative
of local variations in lithospheric structure. In this case, the

stratigraphic relation between the furrowed and the moderately bright
cratered materials cannot be unambiguously established. Similarly, the
patch of dark cratered material near lat 5° N., long 217° has no
furrowlike elements within the map area, but conspicuous concentric
furrows do occur on the patch's extension into the Uruk Sulcus
quadrangle. If the furrows originally covered the entire patch in both
quadrangles, then the part of the patch within the map area was
resurfaced later than its extension in the Uruk Sulcus quadrangle still
exhibiting the furrows. Alternatively, the furrows may not have formed
on that part of the patch within the map area.

The albedo differences of the dark units are inferred to result from
differences in an entrained rocky component or from the presence of
minor carbon-bearing coloration compounds. Age-related darkening
mechanisms, such as charged particle irradiation, meteoritic
contamination, or ice ablation, may account for some of the albedo
differences but not all, because some of the darkest units (for example,
the smooth, dark units in Galileo Regio) are among the youngest
(Casacchia and Strom, 1984; Guest and others, 1988). The albedo
differences indicate horizontally distinct surface units in the dark
materials. If the variations in dark and light ejecta in the map area
(see below) are due to excavation of subsurface layers of different
compositions, then the dark terrain also has vertically distinct layers
on the order of hundreds of meters thick (Croft and Strom, 1985). The
minimum cumulative thickness of the dark-terrain units must be
sufficient to bury all previous crater topography, on the order of a few
kilometers. The maximum thickness is poorly constrained.

LIGHT MATERIALS

The light materials in the east half of the map area form several bands
100 to 200 km wide that cross the dark terrain in a dominantly
west-northwest direction, though other minor trends are present. Tiamat
Sulcus itself merges to the north into an enormous area of light terrain
comparable in size with the large, circular, dark regiones. Most of the
light terrain is grooved material (unit **g**); the grooves are
curvilinear and have smooth edges and a range of sizes. Smaller grooves
tend to occur in parallel groups or sets bounded by somewhat larger
grooves, whereas conspicuous grooves occur singly or in pairs. The most
conspicuous grooves are grabenlike, having a definite break in slope
between the sides and floor, as opposed to the more sinusoidal cross
section of most grooves (Squyres, 1981), though this distinction may be
a resolution effect. The conspicuous grooves cut across all other
grooves in the light materials and at places cross into dark materials.
But even there, most conspicuous grooves retain their high-albedo
floors. In the map area, however, the floors of several conspicuous
grooves change from light to dark as they bound or cross into the dark
terrain. In a few places (for example, at lat 7° S., long 218°), the
dark floor materials are even darker than the surrounding dark terrain.
Conspicuous dark-floored grooves also occur sporadically in the light
terrain in the northeast corner of the map area (lat 16° N., long 217°)
and on the south edge of Busiris Facula.

Smooth material (unit **s**) occurs as patches of various sizes within
the grooved material. The albedos of the two units are
indistinguishable, and subdued grooves occur in many places in the
smooth material. Thus the two units appear gradational, and distinction
between them is somewhat arbitrary. Smooth material bordering the
conspicuous grooves is the youngest of the light materials (Murchie and
others, 1986).

The "string faculae," or chains of bright spots, are curious albedo
features found around the junction of Kishar and Hursag Sulci. These
spots are as bright as the lightest crater rays, and they might be
interpreted as such. However, no crater is associated with them, and
some of the strings correlate with local structural trends, which
perhaps indicates an internal origin.

Two relations observed within the map area indicate that groove
formation is a separate process from light-terrain emplacement: (1)
smooth light material occurs without grooves, indicating that grooves
and light material are not everywhere associated; and (2)

the continuation of grooves on light units into linear depressions on
dark units in Kishar Sulcus indicates that groove-forming processes can
affect dark terrain as well as light. Grooves are commonly interpreted
to be extensional tectonic features (see Shoemaker and others, 1982,
Squyres and Croft, 1986, and references therein), an interpretation that
is supported by the transition of grooves into linear depressions in
southern Kishar. However, the presence of patches of the younger smooth
unit along the lengths of conspicuous grooves indicates that at least
some grooves are vents for extrusive materials (Murchie and others,
1986), suggesting that groove morphologies are shaped by cryovolcanic
processes as well.

The light materials differ from the dark materials in the nature of the
associated structural features, in albedo, and in time of emplacement.
Like the dark materials, the light materials were emplaced over an
extended period of time in a series of discrete events, but mostly after
the formation of the dark materials (Shoemaker and others, 1982). The
reasons for the change in tectonic style and the production of lighter
and apparently "cleaner" water cryomagmas during light-material
emplacement are poorly understood. Depending on the nature of the
darkening agent in Ganymede's cryomagmas, the lighter cryomagmas may
have been produced as a result of a change in source regions; a change
in primary magma composition through exhaustion of carbon-bearing ices;
a slowing of the rate of cryomagma accumulation or velocity of
migration, reducing the load capacity for suspended solids; movement
through a thickening crust of relatively clean ice, resulting in the
pickup of fewer dark xenoliths; or possibly some combination of these
phenomena.

CRATER AND PALIMPSEST MATERIALS

Impact landforms on Ganymede exhibit a broader range of morphologies
than is found on the terrestrial planets, although the most abundant
crater forms on Ganymede are similar to those of the terrestrial
planets. These forms include simple bowl-shaped craters; complex craters
with flat floors, central peaks or pits, and poorly developed rim
terraces; and multiring structures. Simple craters (\<7 km diameter) are
near the limit of resolution and too small to be mapped separately. Most
of the mapped craters are pit craters (complex craters with central pits
instead of peaks). Ombos Facula is the only structure in the map area
that is analogous to terrestrial multiring basins. On the basis of
comparison with albedo patterns associated with other impact basins on
Ganymede observed at comparable and lower sun-elevation angles, Ombos
Facula is interpreted as an impact basin with an outer rim, a bright
inner ring, and a bright central smooth plain (unit **csm**) of probable
re-frozen impact melt (Croft, 1983a). The sun angle at Ombos is too high
to determine if the plain is elevated as in Hathor basin (lat 70° S.,
long 268°) or domed as in Ilus basin (lat 12° S., long 111°). Degraded
craters (unit **c1**) have broken or obscured crater rims. Most of the
mapped craters are partly degraded (unit **c2**), having relatively
sharp rims but lacking rays or high-albedo ejecta deposits. Dark and
light rayed craters (unit **c3**) are the freshest.

Crater forms that are unlike those on the terrestrial planets are also
mapped. Large "moat" or "anomalous pit" craters differ from typical pit
craters in having a topographically subdued rim and a conspicuous
inward-facing scarp, ridge ring, or peak ring on the crater floor
surrounding an abnormally large central dome (Croft, 1983a). A
moderately fresh moat crater is located near lat 16° S., long 223°, and
a heavily degraded one is near lat 3° N., long 228°. Palimpsests are
subcircular, moderately bright albedo patches with poorly developed
concentric ridge systems around an irregularly shaped, smooth central
plain. Two palimpsests (unit **p2**) occur within the map area: Busiris
Facula and an unnamed palimpsest at lat 4° S., long 222°. The center of
Busiris Facula is obscured by a superposed rayed crater, but the central
plain of smooth material (unit **ps**) is seen in the center of the
southern palimpsest. Two conspicuous dark-floored grooves cut across the
somewhat mottled annulus outside the ridge structure of Busiris Facula.
If

the dark-floored grooves are extensional structures as is commonly
assumed, then the dark floor material originates from the lower side
walls, below the bright surface layer. If so, the bright palimpsest
materials of the outer part of the albedo patch are probably thin and
superficial, at most only a few hundred meters thick. Palimpsests are
usually interpreted as viscously relaxed impact basins (for example,
Passey and Shoemaker, 1982, and Guest and others, 1988). However, Croft
(1983a) pointed out that the internal structures of multiring impact
basins on Ganymede could not have been erased or altered by viscous
relaxation to the substantially different internal structures
characteristic of palimpsests. Thus the peculiar features of palimpsests
are probably intrinsic to the freshly formed structures, and if they are
impact basins, they either formed under unusual conditions such as
high-velocity impacts (Croft, 1983a), or were modified by endogenic
processes.

Dark-ray craters are common in this quadrangle, interspersed irregularly
among the light-ray craters. The albedos of the rays and continuous
ejecta of different craters within the quadrangle differ remarkably.
Combinations include dark rays and dark ejecta (Antum, Mir), dark rays
and light ejecta (northeast of Ombos), and rays and ejecta that are
light on one side of a crater and dark on the other (Tammuz). The
albedos of the dark rays lighten and darken noticeably as they
alternately cross strips of light and dark terrain, respectively. The
rays are also much less conspicuous on light materials than on dark. The
darkening agent in the rays of dark-ray craters may be material from
impactors of unusually dark, perhaps rocky, composition. Alternatively,
the dark rays may contain material excavated from local deposits of dark
material on the surface or at depth (Hartmann, 1980; Schenk and
McKinnon, 1985). Ice sputtering that produces lag deposits may make dark
rays more conspicuous over time (Conca, 1982). Dark rays also occur
around some craters on the Moon, where they are interpreted as having
been excavated from buried layers of dark material (Schultz and Spudis,
1979). The striking light- and dark-rayed crater Tammuz straddles a
boundary between light and dark terrains; its dark rays emanate from the
part of the crater on dark terrain and its light rays from the part on
light terrain. This configuration is most easily explained by excavation
of materials having different albedos.

Several patches of dark mantling material are mapped. Most are
associated with impact craters and are probably dark ejecta. However,
one conspicuous dark streak near lat 5° S., long 219° has no obvious
source crater. It is not associated with any visible surface feature,
volcanic or tectonic, and it has no apparent topographic expression; its
origin is problematic.

**GEOLOGIC HISTORY**

Comparison of Ganymede with neighboring Callisto suggests that the
original surface in the map area was primitive and heavily cratered. It
was then buried by cryovolcanic flows to minimum depths of a few
kilometers. The initial flows, perhaps rising through undifferentiated
crust, were probably "muddy," containing small amounts of suspended
rocky solids or dissolved carbon-bearing molecules. Eruptions occurred
at many discrete sites over an extended period of time. Cryomagmas
originating from different source regions and ascending through
different conduits containing different impurities probably caused the
regional differences in albedo observed in the dark cratered terrain.
The flows apparently had relatively low viscosity: they cover large
areas (implying large flow distances) and have no discernible relief at
the edges or between the center and the edges of albedo patches.

The next major event was the formation of the concentric furrow system
in the Galileo and Marius Regiones, either by impact (Passey and
Shoemaker, 1982; Schenk and McKinnon, 1987) or by internal stresses
(Casacchia and Strom, 1984). The resurfacing rates on the dark terrain
were so much higher than the crater flux that the furrow system formed
on a surface virtually free of craters (Smith and others, 1979a). If the
furrow system originally extended into the map area, as suggested by the
meandering troughs, it

was subsequently largely buried by continued flooding. Alternatively,
the furrow system may not have extended significantly into the map area,
perhaps because of a locally thicker lithosphere associated with
Southwest Marius Regio (Croft and Goudreau, 1987). After formation of
the furrow system, extrusion and emplacement of dark materials decreased
significantly. Moderately heavy cratering continued, but with
progressively decreasing intensity. The palimpsests and the radial
trough system formed during this time.

An episode of regional fracturing, subsidence, and flooding around the
periphery and in the interior of Southwest Marius Regio followed,
producing the first deposits of light materials. Cratering continued at
a reduced rate, forming Ombos Facula and other impact structures.
Localized tectonism then resulted in the linear depressions on the
lineated dark material in the southeast corner of the map area. We know
that tectonism both preceded and followed the later stages of
emplacement of light materials, because (1) the lineated dark material
is largely superposed by light material whose grooves trend in sharply
different directions than do the linear depressions, indicating
tectonism before emplacement of some light materials; (2) linear
depressions near lat 14° S., long 222° can be traced unbroken from light
material westward through a strip of dark material and into the light
units of Kishar Sulcus, indicating that here tectonism and groove
formation occurred after the emplacement of light materials.

The final stages of flooding formed the youngest deposits of smooth
light material, particularly along a few of the larger conspicuous
grooves. The string faculae then formed. The latest events in the map
area were impacts producing partly degraded craters (unit **c2**) and
rayed craters (unit **c3**) whose ejecta are superposed upon all other
features.

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