**GEOLOGIC MAP OF THE**

**PHILUS SULCUS (Jg-4)**

**QUADRANGLE OF GANYMEDE**

By Scott L. Murchie and James W. Head

1989

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--1966, 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

> **Smooth material**---Intermediate to high albedo; geomorphically
> smooth at scale of Voyager resolution. Embays dark materials and some
> grooved materials. In places crosscut by grooves, groove pairs, and
> narrow groove lanes. *Interpretation:* Extrusive or possibly
> pyroclastic deposits of brine or brine-ice slurry having low silicate
> content; concentrated in topographic lows, especially downdropped
> blocks. Younger than some grooves but older than others
>
> **Grooved material**---High albedo. Subdued to conspicuous grooves 40
> km to several hundred kilometers long, commonly in parallel to
> subparallel sets with 5- to 50-km groove spacing. Individual grooves
> flat-floored to sinusoidal in cross section. Contacts with dark units
> commonly defined by diffuse-edged, intermediate-albedo, smooth
> materials. *Interpretation:* Groove sets are tectonic blocks of dark
> materials, downdropped and resurfaced by light smooth material, and
> subsequently fractured pervasively by narrow grabens, tension
> fractures, or both. Sinusoidal groove cross sections due in part to
> mass wasting
>
> **Mottled grooved material**---Overall intermediate albedo, having
> lighter patches at scale of tens of kilometers. Occurs as polygons
> several hundred kilometers in size, locally surrounded and partly
> dissected by throughgoing grooves or groove pairs and narrow groove
> lanes. *Interpretation:* Thinly resurfaced dark material fractured by
> groove formation

DARK MATERIALS

> **Grooved material**---Forms two low-albedo, polygonal areas having
> grooves spaced 10 to 50 km apart. Surrounded by light smooth or
> grooved materials. No conspicuous furrows. *Interpretation:* Degraded
> dark furrowed material, fractured by same tectonic processes
> responsible for formation of light grooved material
>
> **Lineated material**---Forms low- to intermediate-albedo, mottled
> areas between intersecting light-material bands. Irregular or
> sublinear to reticulate elongate depressions or albedo markings where
> observed at higher resolution. *Interpretation:* Dark furrowed
> material degraded by fracturing, mass wasting, or both
>
> **Furrowed material**---Low-albedo material dissected by
> light-material bands to form polygons as much as thousands of
> kilometers in size. Most densely cratered material in area, and only
> material containing large number of degraded craters and palimpsests.
> Morphology dominated by furrows, which are sets of parallel to
> subparallel arcuate to linear troughs 6 to 10 km wide and several
> hundred meters deep. Patches 20 to 100 km across have lower albedo.
> *Interpretation:* Oldest unit, consisting of mixture of ice and
> silicate, possibly resurfaced before formation of observed morphology;
> darkened by radiation, meteoritic contamination, and ablation of ices.
> Fractured by tectonic and giant impact processes to form furrows

CRATER MATERIALS

> **Material of bright craters**---Forms sharp-rimmed craters of high
> relief, some containing central peaks or central pits, and high-albedo
> ejecta surrounding these craters; ejecta range from rays (youngest
> ejecta) to
>
> diffuse-edged bright patches. More common in light than dark
> materials. Few patches of low-albedo ray material. *Interpretation:*
> Youngest impact craters; ejecta composed of frost-rich deposits. Dark
> patches may result from projectile contamination, low-albedo lag from
> devolatilized bright rays, or excavated dark material
>
> **Material of partly degraded craters**---Forms floors, internal
> structures, rims, and ejecta of craters having sharp rims and high
> relief. Albedo similar to target material, but dark floors common in
> small craters in light terrain (such as at lat 22° N., long 187°).
> Traces of bright ejecta on some rims and floors. *Interpretation:*
> Impact craters and ejecta of intermediate age, affected by long-term
> ablation of volatiles, mass wasting, and impact gardening
>
> **Material of degraded craters**---Forms craters having flat floors
> and subdued but resolvable rims. Ejecta commonly unresolved or furrows
> resolvable nearly to rim, suggesting thin ejecta. Albedo same as dark
> target material. *Interpretation:* Impact craters generally predating
> light materials; degraded by ablation of volatiles, mass wasting,
> impact gardening, and probably viscous relaxation

**Palimpsest materials**---Circular patches of intermediate to high
albedo

> **Material of basinlike palimpsest**---Hummocky, high-albedo surface
> with several low, concentric ridges and flatter central area.
> Surrounded by field of secondary craters. *Interpretation:* Impact
> basin whose high-relief features partly collapsed under own weight
> either immediately after impact because of low lithospheric strength
> or over time because of viscous relaxation
>
> **Material of subdued palimpsests**---Low- to intermediate-albedo
> surfaces in dark materials. Geomorphically subdued, having barely
> resolvable, low, arcuate ridges arranged roughly concentrically to
> center of patch. No secondary craters. Furrows of dark materials
> partly buried, found only in outer parts. *Interpretation:* Ancient
> impact craters whose high-relief features collapsed under own weight.
> Central parts are degraded basins; outer parts are ejecta

**Contact**---Dashed where approximately located; dotted where buried.
Includes boundaries between groove sets; these boundaries may terminate
without closure

**Graben or grabenlike structure**---Dotted where buried

**Major furrow or dark-terrain trough**

**Throughgoing conspicuous groove**---Commonly crosscuts or bounds
groove domains

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

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

**Albedo lineament or curvilineament**

**Crater rim crest**

**Subdued or buried crater rim crest**

**Central peak**

**Central pit---**Symbol outlines depression

**Central dome**

**Basin ring**

**Palimpsest ring**

**Bright crater rays**

**Dark crater rays**

**Secondary crater field**

**INTRODUCTION**

The Philus Sulcus quadrangle is located in the anti-Jovian hemisphere of
Ganymede, the third and largest of the Galilean satellites. The 180°
meridian defining the quadrangle's east boundary passes through the
anti-Jovian point at the equator, and the 270° meridian forming the
quadrangle's west boundary crosses the trailing point in the satellite's
orbit.

Ganymede consists of a mixture of rock and ice having an average density
of 1.93 g/cm3 (Smith and others, 1979a, b). The high albedo of the
satellite's surface (40 to 60 percent) and absorption features in its
spectrum suggest that water ice is the dominant surface material and
that it is intermixed with at least several percent of silicates and
minerals containing oxidized iron (Clark and others, 1986). The surface
is about equally divided between light and dark materials whose albedos
may differ as a result of slightly different silicate contents. Both
materials contain craters ranging in morphology from fresh (sharp rims
and bright rays) to degraded. Two types of tectonic structures are
pervasive, furrows and grooves. Furrows are linear to arcuate troughs,
commonly having raised rims, that occur solely in dark material. Large
areas of dark material, most of which appear furrowed where seen at high
resolution, are named regiones. Grooves are linear, U-shaped troughs
that are strongly concentrated in light materials but also occur in dark
materials; where they consistently crosscut the furrows. Bands of light
grooved material are named sulci. Within the sulci, grooves occur most
commonly in parallel to subparallel sets or domains having laterally
continuous groove orientation and morphology, and they form large areas
of grooved terrain. Two principal types of domains are represented in
the Philus Sulcus area (Murchie and Head, 1985b). Groove lanes are
elongate to linear and typically of light material; their grooves are
parallel to the domains' long axes. Grooved polygons are blocky to
polygonal in outline and of either light or dark material; their grooves
trend at oblique angles to the domains' long axes.

**GENERAL GEOLOGY**

The mapped units are material units subdivided on the basis of
geomorphology and pervasive structure. The main units are dark and light
materials. Dark materials are subdivided into dark grooved material
(unit **dg**), dark1ineated material (unit **dl**), and dark furrowed
material (unit **df**). Light materials are subdivided into light smooth
material (unit **ls**), light grooved material (unit **lg**), and
intermediate-albedo, mottled grooved material (unit **lmg**). Crater
materials are classified as material of bright, fresh craters (unit
**c3**), material of partly degraded craters (unit **c2**), material of
degraded craters (unit **c1**), and palimpsest materials. The latter are
high-albedo patches with subdued concentric ridges unit (**p3**) or
material that lacks well-preserved structure (unit **p2**). No highly
degraded palimpsests (unit **p1**) have been recognized in the Philus
Sulcus area, although they are found in other quadrangles.

The west half of the quadrangle was imaged only at low resolution by
Voyagers 1 and 2, and it is thus not geologically mappable using
existing data. It contains largely light materials, a few small areas of
dark materials, and some bright-rayed craters.

Of the mappable part of the quadrangle, containing about equal areas of
light and dark materials, the southeastern area consists mostly of two
large polygons of dark furrowed material, northern Marius Regio and the
northern part of southern Marius Regio, separated by Mashu Sulcus. Three
palimpsests 300 to 400 km in diameter are located in the
southeasternmost area. The northeastern part of the quadrangle contains
Ur Sulcus and Elam Sulci, several polygons of dark lineated

material, and three areas of dark furrowed material. The southwestern
part of the mapped area consists dominantly of light materials.

High-albedo, diffuse, bright materials are concentrated in the northern
part of the quadrangle. This observation is consistent with the
hypothesis that water-frost deposits are retained for longer periods of
time in the cold environment of the northern latitudes (Squyres, 1980a).
The diffuse deposits include both bright crater rays and part of the
north polar frost cap, a continuous, diffuse, bright deposit lacking
observable relief. The polar frost occurs north and west of northern
Marius Regio; it may have accumulated by cold-trapping of water ice
ablated by solar radiation (Purves and Pilcher, 1980) or by bombardment
by the Jovian radiation belts (Johnson, 1985), or it may be a relict
deposit formed at the time of emplacement of light materials (Shaya and
Pilcher,1984). The retention of albedo contrasts in spite of the polar
frost suggests either that only a very thin frost layer is present or
that frost is intermixed with and brightens the underlying regolith
(Helfenstein, 1986, p. 305-309). Crater materials south of the
frost-covered region are more commonly of low to intermediate albedo,
probably because of greater frost ablation and formation of a
silicate-enriched lag (Shoemaker and others, 1982).

DARK MATERIALS

Dark materials as a whole are older than light materials, as evidenced
by their higher crater density and superposition by light materials
(Smith and others, 1979a, b). Dark materials occur in two modes: (1) as
large polygons hundreds to thousands of kilometers across containing
sets of subparallel to parallel furrows, exemplified by Galileo Regio
and northern and southern Marius Regio; and (2) as smaller polygons 10
to 200 km across within large regions of light materials. In the smaller
polygons, furrows are highly degraded and replaced by grooves or
lineations.

Dark material in the large polygons is mapped as dark furrowed material
(unit **df**). Furrows are linear, curvilinear, or wavy troughs 6 to 10
km wide, commonly having raised rims; they are typically 50 to a few
hundred kilometers long. In the Philus Sulcus area they form three sets
having parallel to subparallel orientations at a scale of hundreds of
kilometers. The dominant set is composed of arcuate furrows (Smith and
others, 1979a; Lucchitta, 1980; Shoemaker and others, 1982; Casacchia
and Strom, 1984; Murchie and Head, 1987b). These furrows form a crudely
concentric pattern on a hemispheric scale; they are commonly spaced 10
to 30 km apart in Marius Regio and about 50 km apart in Galileo Regio
east of the map area. The concentric pattern and the location of a
faint, giant palimpsestlike feature at the center of the pattern (to the
southeast, outside the map area) suggest that the arcuate furrows
originated as ring fractures due to an impact (McKinnon and Melosh,
1980; Shoemaker and others, 1982). However, the arcuate furrows also
appear to have been modified by volcanism and extensional tectonism
(Murchie and Head, 1987b). Alternatively, Casacchia and Strom (1984)
suggested a purely tectonic origin for the arcuate furrows because of
extension over a large mantle upwelling.

Two additional furrow sets are superposed on the arcuate furrows in
Marius Regio. The first set is composed of furrows oriented
approximately orthogonal to the arcuate furrows. The orthogonal furrows
are widespread in Galileo Regio, east of the Philus Sulcus quadrangle,
as narrow troughs 10 to 50 km long; some conspicuous furrows reach
lengths of a few hundred kilometers (Casacchia and Strom, 1984). In
Marius Regio, the orthogonal furrows occur as single troughs 50 to 150
km long that crosscut the arcuate furrows. The second superposed furrow
set crosscuts the arcuate furrows at an oblique angle. These "oblique"
furrows belong to a larger system arranged radially to a point near lat
22° S., long 135° (Murchie and Head, 1987b).

Furrow sets provide zones of weakness that have influenced linear
boundaries of large polygons of dark furrowed material. These zones of
weakness are both parallel and perpendicular to the furrows (Murchie and
Head, 1985a, b), resulting in two pairs of zones of weakness. One pair
is composed of the arcuate and orthogonal furrows. The other pair
consists of the "oblique" furrows and furrows perpendicular to them.
Although no furrows with this perpendicular orientation are found in the
quadrangle, such furrows do occur in the southwestern part of the
Galileo Regio quadrangle (Murchie and Head, 1987b).

Although furrows within each set are morphologically similar within
Galileo Regio and also within Marius Regio, between the two regiones the
furrows differ somewhat in morphology, age, and orientation. Both the
arcuate and "oblique" furrows are shorter, less linear, and more closely
spaced in Marius Regio. The orthogonal furrows in Marius Regio are
consistently younger than the arcuate furrows, in contrast to their
commonly greater relative age in Galileo Regio. The orientation of the
arcuate furrows in the two regiones shifts abruptly by about 20° across
Nippur Sulcus. The shift in furrow orientation as well as regional
patterns of deformation in grooved materials suggest that the regiones
were offset by shear contemporaneous with deformation of the oldest
grooved materials (Murchie and Head, 1987a).

In their second mode of occurrence, dark materials form smaller polygons
tens to hundreds of kilometers across, located north of and within
Nippur Sulcus and west and south of Amon crater. In most of these
polygons, as well as in the highly dissected western extremity of
southern Marius Regio, furrows are degraded to shallow troughs or albedo
lineaments in a sublinear to reticulate pattern. The material of these
degraded polygons is mapped as dark lineated material (unit **dl**).
Although some mottling by higher albedo patches occurs within many small
dark polygons, the lineated material in the west margin of southern
Marius Regio is characterized by a lower albedo than that of dark
furrowed material immediately to the east. In addition, dark grooved
material (unit dg) occurs at lat 57° N., long 215°, having a groove
spacing of 20 to 30 km, and at lat 44° N., long 192°, having finer
grooves with a 10-km spacing.

Although Ganymede may have been bombarded twice as heavily as Callisto
by extra-Jovian objects (Passey and Shoemaker, 1982), crater density in
Ganymede's dark materials is about one-third of that on Callisto. This
observation may be attributed to a longer time required for Ganymede to
develop a lithosphere capable of supporting crater topography (Shoemaker
and others, 1982) or to widespread early resurfacing (Smith and
others,1979a; Helfenstein, 1986, p. 310-313). The observation is
consistent with Ganymede's higher silicate content and radiogenic heat
productivity.

LIGHT MATERIALS

Light materials dominate the sulci that separate large polygons of dark
furrowed material. Within the sulci are several small dark polygons that
are partly covered by light material. Craters in light material that are
believed to have penetrated to a depth of 1 km or more are interpreted
to have excavated underlying dark material (Schenk and McKinnon,1985).
Also, crater density in light materials is substantially less than it is
in dark materials. These observations suggest that light materials
consist of a layer several hundred meters to 1 km or more thick that
buries underlying, older, dark materials.

Smooth material (unit **ls**) embays topographic lows such as grooves
and crater floors, suggesting that it was emplaced as a low-viscosity
fluid, probably a silicate-poor brine or brine-ice slurry (Lucchitta,
1980). It occurs globally in three modes (Murchie and Head, 1985a), all
of which are found in the Philus Sulcus area. The first mode is
high-albedo deposits in the central parts of groove lanes, such as at
lat

26° N., long 194° and lat 48° N., long 202°. The second mode is
intermediate- to high-albedo, elongate to polygonal deposits, which are
outlined and at places also dissected by throughgoing grooves or groove
pairs; examples are at lat 50° N., long 190°; lat 23° N., long 205°; lat
23° N., long 232°; and lat 28° N., long 219°. The elongate,
intermediate-albedo, smooth areas within Philus Sulcus may also be of
this mode of occurrence. The third mode is diffuse-edged,
intermediate-albedo deposits on margins of dark polygons adjacent to
groove lanes, conspicuous grooves, or groove pairs. Such smooth material
is found at lat 38° N., long 210° and lat 37° N., long 201°; at other
locations it coalesces to form smooth polygons such as that at lat 33°
N., long 188°. Images of this third mode show no protruding dark areas
or gaps through which the smooth material apparently flowed, suggesting
either blanketing by pyroclastic ice or flooding of low-lying dark
materials.

Light materials that are pervasively grooved are mapped as grooved
material (unit **lg**) or mottled grooved material (unit **lmg**).
Grooves are linear to curvilinear troughs, U-shaped in cross section
(Squyres, 1981). Grooved materials contain intersecting groove domains,
and most have an intermediate to high albedo similar to that of smooth
material. The dominance of troughs in grooved materials has led to the
interpretation that they were formed by widespread extensional tectonism
(Smith and others, 1979a, b; Lucchitta, 1980; Golombek and Allison,
1981; Shoemaker and others, 1982; Squyres, 1982; Bianchi and others,
1986; Murchie and others, 1986). Individual grooves may be degraded
remnants of grabens or tension fractures; lithospheric thickness and
strain rate probably controlled the exact mode of origin of these
features (Parmentier and others, 1982; Squyres, 1982).

Orientations of groove sets are structurally controlled by the same two
pairs of furrow-related zones of weakness that define the boundaries of
large polygons of dark furrowed material (Murchie and Head, 1985a, b).
Bianchi and others (1986) have shown in their measurements of groove
orientation that the preferred orientation in this quadrangle is
approximately parallel to the orthogonal and "oblique" furrows.

Domains having groove sets with these orientations dominate Elam Sulci
and the southwestern part of the map area as well as Nippur, Mashu, and
Anshar Sulci. Philus Sulcus and grooves in the southern part of Ur
Sulcus are perpendicular to the "oblique" furrows. The dominant
northwest orientation of groove lanes in this quadrangle is part of a
larger global trend, in which most groove lanes and throughgoing grooves
form low angles to small circles centered on a proposed paleopole of
rotation near lat 70°-75° N., long 95° (Murchie and Head, 1986). This
global pattern was proposed to result from structural control of
groove-lane emplacement by relict tidal-despinning fractures that
predated the furrows, combined with control by the possibly impact
related furrows. Therefore, the dominant northwest groove-lane
orientation in the Philus Sulcus area may have resulted from control by
preexisting structures. Alternatively, Bianchi and others (1986)
proposed that the dominant orientation results from stresses due to
mantle convection.

The most conspicuous crosscutting relation of groove sets is the
truncation of many of them against major groove lanes. Two examples of
this relation are the contact of Nippur Sulcus with the groove domains
to its north and the contact of Philus Sulcus with the grooves to its
west. This relation appears to result from a confinement of groove-set
propagation by older zones of weakness underlying the groove lanes
(Golombek and Allison, 1981; Murchie and others, 1986). Despite the
greater age of the zones of weakness than of the groove sets terminating
against them, light materials within many of the groove lanes are
superposed on the truncated groove sets. This relation is best seen at
lat 45° N., long 195°, where the boundary of a groove set is only partly
buried by light material in Nippur Sulcus.

Two additional classes of crosscutting relations of groove domains are
particularly noteworthy. In the first class, groove lanes, throughgoing
grooves, and groove pairs crosscut interior parts of other groove
domains and segment them into smaller polygons. These relations are best
seen where the north-trending groove lane centered at lat 45° N., long
183° crosscuts a wider northeast-trending groove lane, isolating its
southwestern segment. In the second class, the groove set of a groove
lane or grooved polygon terminates within a bounding groove lane.
Examples are seen at the contact of the groove lanes at lat 51° N., long
201° and in southwestern Philus Sulcus. At lat 55° N., long 213°, the
groove set from one grooved polygon is also superposed on the edge of
another grooved polygon.

Clear evidence exists for temporal change in the regional orientation of
groove sets. This change is seen north of Nippur Sulcus, where the
oldest grooves (in Ur Sulcus) are oriented northeast. Crosscutting
relations indicate that younger domains (in Elam and Nippur Sulci) are
oriented progressively more northwest. A similar change in orientation
is seen west of Philus Sulcus, which trends north-northeast. Grooves in
the grooved polygon to the west, which are superposed on southwestern
Philus Sulcus, have a northwest orientation; Mashu Sulcus, which
crosscuts both features, is oriented west-northwest.

Lucchitta (1980) proposed that light grooved material develops in place
from dark materials and that intermediate stages of conversion of the
dark material may be found. Such intermediate stages occur in three
areas within this quadrangle. The first area is at lat 33° N., long
190°, where conspicuous throughgoing grooves and groove pairs outline
polygons 50 to 150 km long. Smooth material adjacent to the grooves has
coalesced to completely cover one of the polygons, although the polygons
to the north and south remain dark and their furrows are still
resolvable. The second area is northwest of Anshar Sulcus, where narrow
groove lanes and groove pairs outline small polygons of dark lineated
material. Several of these groove lanes and groove pairs continue east
of long 212°, within Anshar Sulcus, where the intervening polygons are
light and smooth. Feather-edged margins of light material mark the
transition from dark lineated material. Within Anshar Sulcus, the smooth
polygons are of the same size and polygonal shape as the dark polygons
to the northwest, and the intermediate area clearly consists of dark
blocks partly covered by light material. The third transitional area is
west of Anshar Sulcus (around lat 26° N., long 222°), where groove pairs
and narrow groove lanes outline polygons of two size scales. Two large
polygons, hundreds of kilometers across, are grooved and have an overall
intermediate albedo and light mottling at a scale of tens of kilometers.
These two polygons are mapped as mottled grooved material (unit
**lmg**). The smaller polygons, tens to a few hundreds of kilometers
across, are light and smooth, dark and lineated, or a mixture of the two
types. The dark polygons are partly resurfaced by the coalescence of
smooth deposits adjacent to the groove pairs and groove lanes.

Three consistent stratigraphic relations in these transitional areas
suggest the sequence of events responsible for the conversion in place
of dark furrowed to light grooved material: (1) the dissection of dark
furrowed material into polygons by throughgoing grooves, groove pairs,
or narrow groove lanes; (2) the coalescence of light smooth material
adjacent to these grooves to partly or completely resurface the
polygons; and (3) the superposition of grooves on the smooth deposits
without attendant refilling. For the throughgoing grooves to be the
sources of light smooth material and also to be superposed on it,
emplacement of the material must have occurred between groove-forming
events. Two additional observations on crosscutting relations of groove
domains are useful in determining the sequence of events during
grooved-terrain formation: Some groove sets are entirely confined within
grooved polygons, apparently by older fracture zones, and some groove

lanes appear to have formed in preexisting zones of weakness outlining
the polygons.

Based on the relations listed above, a sequence of events during
grooved-terrain formation has been proposed and tested by detailed
geologic mapping and crater-density measurements (Murchie and others,
1986). The sequence appears applicable to wide regions, including the
Philus Sulcus area. First, dark furrowed material was dissected by
throughgoing grooves or fracture zones to form polygonal blocks.
Adjacent blocks were offset vertically, and smaller blocks were
pervasively fractured to form lineated materials. Next, many downdropped
blocks and areas adjacent to the throughgoing grooves were flooded by
light smooth material, largely burying the throughgoing structures.
Groove sets mostly confined to single polygons by their marginal
fracture zones also developed at this time by intense fracturing,
forming grooved polygons. Finally, newer grooves and groove lanes were
superposed preferentially on the buried throughgoing zones.

CRATER MATERIALS

As elsewhere on Ganymede, craters in the Philus Sulcus area belong to
three geomorphic and albedo classes corresponding to degradational
states. Craters consisting of fresh, high-albedo material (unit **c3**)
have sharp rims, and many have preserved bright ejecta rays. The most
conspicuous bright ejecta are associated with the craters Geb and Amon.
Dark rays are uncommon; the only crater in light materials whose ejecta
are completely dark is the **c3** crater at lat 23° N., long 236°. The
high albedo of bright rays may result from small particle sizes of ray
material, from condensation as frost of impact-generated water vapor, or
from other processes (Chapman and McKinnon, 1986). The origin of dark
rays is uncertain, but they may result in part from contamination of
crater ejecta by fragments of impacting projectiles (Conca, 1981).

Partly degraded crater material (unit **c2**) has undergone only minor
degradation of crater rims and ejecta, but it has an albedo close to
that of background materials. Rays are not visibly associated with these
**c2** craters; they apparently have ablated or been impact-gardened
into the regolith (Shoemaker and others, 1982). Material of degraded
craters (unit **c1**) is characterized by subdued rims, barely
recognizable ejecta, flat floors, and low albedo close to that of the
dark materials on which it occurs.

Palimpsests, the most degraded craters, are found in the southeastern
part of the map area and are 300 to 450 km in diameter. Two palimpsests
(unit **p2**) in dark furrowed material have an albedo slightly higher
than that of target material and display partly to completely buried
furrows. Because the furrows are not entirely buried in the distal parts
of the palimpsests, the material there cannot be significantly thicker
than the relief of the furrows, a few hundred meters or less (Shoemaker
and others, 1982). The third palimpsest (unit **p3**) is on a contact
between light and dark materials, has a higher albedo than does unit
**p2**, possesses degraded remnants of basin rings, and is surrounded by
a field of secondary craters. Palimpsests are interpreted to be craters
whose high-relief features collapsed as a result either of low strength
of the young lithosphere at the time of their formation or of viscous
relaxation with time (Smith and others, 1979a, b; Parmentier and Head,
1981; Passey and Shoemaker, 1982).

Crater forms progress with size from simple bowl shapes to craters with
central peaks, to craters with central pits, to multi-ringed basins
(Passey and Shoemaker, 1982). A trend toward increasingly complex
craters of the same size also exists in craters of greater ages. The
latter progression probably occurs because, as Ganymede lost its
internal heat and its early lithosphere thickened, craters of a given
size formed with less complex morphologies and greater relief.
Central-peak craters in the quadrangle range in size from 15 to 30 km,
central-pit craters from less than

40 km to 120 km. The only basin having well-preserved rings, Geb, has an
outer ring 120 km in diameter.

Rims of many craters are markedly polygonal to hexagonal; examples are
at lat 37° N., long 181° and lat 24° N., long 195°. Generally, at least
one linear rim segment or segments on opposite sides of the crater are
parallel to the arcuate furrows, suggesting some structural control of
crater shape by furrows.

Buried or highly degraded craters and craters that are crosscut by
grooves are uncommon in light materials, but several buried craters are
suggested by albedo curvilineaments or arcuate ridges having subdued
relief. Examples are found at lat 58° N., long 207°; lat 44° N., long
185°; lat 23° N., long 194°; and lat 37° N., long 183°. At the
resolution of Voyager images, it is difficult to be certain that these
craters are buried and not in fact simply greatly flattened. Degraded or
partly buried craters at lat 39° N., long 219° and lat 37° N., long 183°
are in light materials but are crosscut by grooves, suggesting that a
substantial period of time passed between resurfacing and the end of
groove formation. The **c2** craters at lat 24° N., long 195°; lat 63°
N., long 192°; and lat 62° N., long 199° (Agrotes) are superposed partly
on light material and partly on dark material, but no pronounced change
in rim morphology is seen at the contacts, as might be the case if a
large contrast existed in the viscosities of dark and light materials.
No craters on dark materials are crosscut by arcuate furrows, but the
degraded crater at lat 42° N., long 203° is crosscut by an "oblique"
furrow.

**GEOLOGIC HISTORY**

The first recognizable event in the Philus Sulcus area was the formation
of a lithosphere, which is inferred to have been fractured by tidal
despinning. Before any of the present surface features were formed,
extensive resurfacing or viscous relaxation may have obliterated a
Callisto-like crater population and formed dark-terrain surfaces.
Subsequently, arcuate furrows formed as part of a hemispheric system,
possibly by modification of ring fractures created by a giant impact.
For some time impact craters were formed on the arcuate furrows and
began to degrade because of mass wasting or viscous relaxation. Then
widely spaced single furrows were superposed on the arcuate furrows in
orthogonal and oblique directions. Crosscutting relations to the east in
Galileo Regio (Casacchia and Strom,1984) indicate that the "oblique"
furrows were emplaced subsequent to the orthogonal ones. Cratering and
degradation of high-relief features continued; the largest craters
collapsed or viscously relaxed to leave little more than albedo patches
as palimpsests.

In the map area, light grooved and smooth materials developed in place
from dark furrowed material by a general three-stage process: (1)
dissection of furrowed material into polygonal blocks, many of which
developed lineated surfaces; (2) extensive resurfacing by silicate-poor
brine or slush commonly followed by pervasive groove formation; and (3)
superposition of throughgoing grooves and groove lanes.
Contemporaneously with the earliest stages of groove formation, shear
offset of large lithospheric blocks may have disrupted preexisting
structures.

Crater formation continued, and by the end of light-material
emplacement, the lithosphere had strengthened enough to support remnants
of basin structures. What was a basin south of Nippur Sulcus did not
collapse completely; its form is transitional between a basin and a
palimpsest (Shoemaker and others, 1982). This structure was the last
major impact feature in the quadrangle to be affected severely by
palimpsest-formation processes. The last features to form were fresh
craters, whose rays have not yet ablated or been impact-gardened into
the regolith.

Ganymede's geologic history must be understood in terms of its thermal
history and the development of its lithosphere. Five aspects of its
thermal and lithospheric histories are particularly relevant. First, as
Ganymede's internal temperature

evolved, the thickness of the lithosphere changed in response to the
evolving lithospheric thermal gradient (Consolmagno and Lewis, 1976;
Parmentier and Head, 1979; Shoemaker and others, 1982). Lithospheric
thickness, as estimated from studies of crater relaxation, increased
from a few kilometers to 10 km at the time of furrow formation to 35 km
at the time of groove formation. Alternatively, lithospheric thickness
as estimated from the study of tectonic features may have decreased from
5 to 10 km during furrow formation to 2 to 5 km during groove formation
(Golombek and Banerdt, 1986, 1987). Second, through the time of groove
formation, the calculated Rayleigh number for the mantle is high enough
that vigorous convection is assumed to have occurred, but it would have
created only several bars or less of stress (Squyres and Croft, 1986).
Third, several mechanisms may have caused a global volumetric increase
that would have placed the entire lithosphere in tensional stresses tens
to hundreds of bars in magnitude. Proposed mechanisms include thermal
expansion (Zuber and Parmentier, 1984), differentiation (Squyres,
1980b), and mantle phase changes (Shoemaker and others, 1982). Fourth,
early in the history of groove formation, stresses that may have been
responsible for left-lateral offset of Galileo and Marius Regiones may
have affected much of the quadrangle. Fifth, the formation of the basin
Gilgamesh (Osiris quadrangle, centered at lat 59° S., long 124°), late
in the history of groove formation, possibly reoriented Ganymede from a
paleopole of rotation at lat 70°--75° N., long 95°. Distortion of the
reorienting satellite would have created several bars of stress (Murchie
and Head, 1986). Thickening of the lithosphere would have allowed crater
forms of higher relief to have been supported, and it would have
affected the style of fracturing and faulting due to tectonic activity
(Squyres,1982). Although global expansion possibly provided the dominant
source of stress for groove formation, the resulting uniform tension
would have added to the deviatoric stresses resulting from convection,
global reorientation, and especially from large-scale shear. The summed
stresses, acting upon relict zones of weakness (furrows and possibly
tidal despinning fractures), may have created the regionally dominant
groove orientations such as those seen in the Philus Sulcus area

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