**DESCRIPTION OF MAP UNITS**

PLAINS MATERIALS

sp **Smooth plains**---Occupies areas of slightly higher albedo and
lower crater frequency than surrounding materials. Found at base of some
scarps in northeastern part of Valhalla multiring system and as an
isolated spot within the cratered plains. Appears to embay surroundings.
Interpretation: Possible resurfacing material, may be composed of water
ice or slurries

lp **Light plains**---Forms areas of slightly higher albedo than
surrounding materials. Usually circular to slightly elliptical in
outline. Crater frequency typically slightly less than on surrounding
plains. Interpretation: Crater scar originating by the viscous
relaxation of a transient impact crater and its rim material

cp **Cratered plains**---Most heavily cratered unit. Average albedo 0.2.
Forms most of Callisto\'s surface. Interpretation: Ancient crustal
material, consisting of ice, rock, and dust, brecciated by extensive
impact cratering and gardening

MULTIRING STRUCTURE MATERIALS

Valhalla Formation

vi **Inner facies**---Higher albedo than surrounding material. Lacks
structural features. Elliptical in outline with lobate margins in some
areas. Occurs in center of Valhalla multiring system. Interpretation:
Material representing the relaxed transient crater formed by the impact
that created the Valhalla multiring system or excavated subsurface
material emplaced by impact-related processes (or both)

vo **Outer facies**---Material transected by multiple discontinuous
ridges concentric with Valhalla. Lower crater frequency than surrounding
materials. Interpretation: Possibly the ejecta blanket from Valhalla
impact. Ridges part of overall multiring system formed by impact

Asgard Formation

ai **Inner facies**---Higher albedo than surrounding material. Possibly
a few interior structural features at outer margin. Elliptical in
outline, occurs in center of Asgard multiring system. Interpretation:
Material representing the relaxed transient crater formed by the impact
that created the Asgard multiring system or excavated subsurface
material emplaced by impact-related processes (or both)

ao **Outer facies**---Material transected by multiple discontinuous
inward-facing concentric scarps. Crater frequency apparently less than
in surrounding materials. Interpretation: Possibly the ejecta blanket
from Asgard impact. Scarps part of multiring system formed by impact

CRATER MATERIALS

\[Craters \>60 km in diameter are mapped. Interpretation: Formed by
impact. Degree of degradation is assumed to approximate relative age (c1
oldest to c3 youngest)\]

c3 **Class three**---Characterized by sharp topographic detail, bright
rays, or extensive non-ray ejecta material

c2 **Class two**---Characterized by moderately to fairly sharp
topographic detail, complete rims, and lacking visible ejecta material

c1 **Class one**---Characterized by low topographic relief, usually
incomplete rims, and lacking visible ejecta material

> **Contact**---Dashed where inferred, queried where uncertain
>
> **Ridge** **crest**
>
> **Trough**---Queried where uncertain
>
> **Scarp base**---Barb points downslope
>
> **Crater rim**---Showing crest (\>60 km diameter)
>
> **Chain craters**---Indicates length and location of chain (does not
> relate to size or number of individual craters)

INTRODUCTION

Callisto is the outermost of the Galilean satellites of Jupiter. With
diameters ranging from 3,530 to 5,262 km, densities ranging from 1.86 to
3.57 g/cm3, and surface temperatures of about 120 K, the properties of
the Galilean satellites contribute important information about planetary
formation. Consequently, they occupy an important niche in comparative
studies for assessing geologic processes and planetary histories.

Prior to exploration of the Jovian system by spacecraft, Earth-based
observations gave some indication as to the general characteristics of
Callisto and its surface. The 4,840-km-diameter Callisto has a
synchronous prograde orbit around Jupiter and thus keeps the same
hemisphere oriented toward Jupiter. This geometry gives rise to the
terms \"Jovian hemisphere\" (for the side facing Jupiter), \"anti-Jovian
hemisphere\", \"leading hemisphere\" (for the side facing the direction
of the satellite\'s orbit around Jupiter), and \"trailing hemisphere.\"
The density of Callisto is 1.86 g/cm3, making it the least dense of the
Galilean satellites and suggesting that it has the greatest proportion
of water ice to silicate materials in its interior. Telescopic
observations show that, although Callisto has the lowest albedo (0.2) of
the Galilean satellites, it is nearly twice as bright as Earth\'s Moon
(0.12). As with Ganymede and Europa, spectral analysis of Callisto\'s
surface indicates extensive ice exposed on or near its surface (Roush
and others, 1990). The low albedo is consistent with a rocky surface,
and studies have shown that over 50% of the visible surface may be
non-ice material (Calvin and Clark, 1991). In addition, a slight
spectral variation exists between the leading and trailing hemispheres.
The hemispheric spectral asymmetry can be attributed to larger grain
sizes on the trailing hemisphere (Calvin and Clark, 1993).

Callisto was observed through the Voyager 1 and 2 spacecraft in 1979
(Smith and others, 1979a, b). The photographic coverage of Callisto is
incomplete, particularly coverage between longitudes 270° and 310°, and
south of latitude 55° S. Additionally, only very poor resolution
coverage (\>20 km/pxl) was obtained south of 10° N., between longitudes
50° and 160°. Image resolution is as high as 2.3 km per line pair for
some Voyager 1 images and as high as 3.9 km per line pair for some
Voyager 2 images. In both sets of images, solar incidence angles range
from 10° to 90°. The variable and mostly poor image resolution, sun
angles higher than about 45°, and smearing by spacecraft motion make the
image set difficult to analyze. In addition, bright frosts obscure
topographic features in the north polar region (Spencer and Maloney,
1984). Whether bright frosts also occur in the south polar region is
unknown.

Conventional photogeologic techniques for mapping planets (Wilhelms,
1990) were employed for recognizing and mapping material units,
structures, and stratigraphic relations. Photogeologic units were
identified based on albedo, crater density, surface features, and
cross-cutting relations. All craters larger than 60 km in diameter were
mapped and classified according to their stage of degradation. Because
the poor resolution makes crater counting difficult and correlation of
counts between areas uncertain, qualitative estimates of crater
frequency rather than quantitative counts were used to gain insight into
relative unit ages. The surfaces seen thus far on Callisto seem to be
remarkably simple for such a large satellite.

**GENERAL GEOLOGY**

Surface Characteristics

The physical and compositional characteristics of Callisto's surface
were determined primarily from photometric measurements, polarimetric
observations, radiometry, and microwave, radar, and spectral reflectance
data, as reviewed by

Veverka and others (1986) and Clark and others (1986). The albedo is 0.2
for the heavily cratered surface and 0.4 for bright, youthful crater
material. As discussed by McKinnon and Parmentier (1986), near-infrared
absorption bands indicate the presence of water ice. The surface is
probably composed of a mixture of ice and dust or rocky material. Clark
and others (1986) found that the reflectance spectra of Callisto
indicate patches of relatively pure water ice as well as patches of the
surface that are ice-free. Other studies of reflectance spectra (Roush
and others, 1990; Calvin and Clark, 1991) show that Callisto\'s surface
can be modeled using an ice-magnetite-serpentine mixture. Reassessment
of the observed spectra for Callisto by Roush and others (1990) using
such a mixture suggests that ice may account for 37--43 weight percent
on the leading hemisphere surface and 26--37 weight percent on the
trailing hemisphere surface. Calvin and Clark (1993) note that
variations in the ice grain size is sufficient to match the
leading/trailing hemispheric spectral variation and that the surface is
likely an ice-rock mixture of 35% pure rock and 65% ice rock intimately
mixed.

At least four processes (Shoemaker and Wolfe, 1982) probably have
modified Callisto\'s surface and contributed to the development of
regolith:

1\. \"gardening\" by meteoroid impact and secondary debris from large
craters,

2\. contamination by infalling meteoritic debris,

3\. transfer or loss of volatile constituents by sublimation at low
latitudes with possible redeposition at high latitudes, and

4\. sputtering by collision of high energy ions and the Jovian
magnetosphere.

Models for regolith generation developed for Ganymede\'s icy crust
(Shoemaker and Wolfe, 1982) can be extrapolated broadly to Callisto and
used to predict a regolith thickness of a few meters or more. However,
the predominantly ice-rich regolith may anneal itself with time,
although the physics and rate of such a process have not been fully
assessed.

McKinnon and Parmentier (1986) noted that the leading hemisphere of
Callisto is darker than the trailing hemisphere in visible and
ultraviolet reflectance spectra. This difference could be attributed to
enhanced micrometeoroid implantation and gardening to darken the
surface. This interpretation is supported by Buratti (1995) and Calvin
and Clark (1993). Alternatively, Schenk and McKinnon (1986) proposed
that the impact of D-type asteroids and comets could be the source of
the dark material. Pang and others (1983) suggested that the trailing
hemisphere has been brightened by the formation of an ice film on the
surface.

CRATER MORPHOLOGY

Craters occur on Callisto in various morphologies, including
bowl-shaped, flat floor, central peak, central pit, central pit with
dome, and multiring (Passey and Shoemaker, 1982; Schenk, 1993). Most
craters larger than 20 km in diameter have a central pit, and most over
60 km in diameter---the size mapped here---have a central pit with a
dome (Schenk, 1993). Passey and Shoemaker (1982) reported that most
craters of all sizes on Callisto and Ganymede are highly flattened as a
consequence of topographic relaxation of the ice crust by viscous or
plastic flow; however, fresh complex craters on Callisto and Ganymede
may have formed \~65% shallower than lunar craters, apparently due to
lower effective strength of ice relative to rock (Schenk, 1991) rather
than to having been modified due to relaxation over time. Several
studies (Strom and others, 1981; Woronow and Strom, 1981; Chapman and
McKinnon, 1986; McKinnon and Parmentier, 1986) indicate that Callisto
has a dearth of craters larger than 60 km in diameter and none larger
than 150 km, in contrast to craters on the Moon, Mars, and Mercury. This
dearth is clearly confirmed by our geologic map. Possibly,
long-wavelength forms in the crust relax to a greater extent than
short-wavelength forms, so larger craters

were created but are no longer recognizable. For very large craters to
disappear from the surface, relaxation could have been slow over a
period of time or a rapid collapse could have occurred due to
instability of a thin, weak lithosphere at the time of impact (McKinnon
and Melosh, 1980; Greeley and others, 1982; Croft, 1983; Chapman and
McKinnon, 1986).

In addition to primary impact craters, Callisto has at least eight
catenae, or chains, interpreted as secondary craters. The individual
craters of a catena are morphologically similar to primary craters of
the same size (Passey and Shoemaker, 1982). The fact that some catenae
are radial to multiring systems suggest that those catenae are formed by
secondary impacts from large primary impacts. For catenae where no
source craters or systems have been identified, Melosh and Schenk (1993)
suggest the catenae were formed by the impact of comets split into
pieces by the stress of Jupiter\'s gravitational field (as was seen with
comet Shoemaker-Levy/9) (Schenk and others, 1996).

MULTIRING SYSTEMS

Five multiring and two single-ring systems with diameters greater than
200 km have been identified on Callisto (table 1). They are
characterized by concentric or subconcentric, discontinuous arcuate
ridges, troughs, or scarps (mapped by structural symbols) around a
relatively featureless central plain. Impact origins are inferred for
the ring systems because of their overall similarity to deeper
multiringed impact basins on other planets and satellites (Remsberg,
1981; Melosh, 1982; Thomas and Masson, 1985; McKinnon and Parmentier,
1986); the brighter central plains may represent emplacement of
subsurface material. Deposits and central plains have been mapped at the
relatively well imaged Valhalla and Asgard basins. The presence of
scarps and secondary crater chains (catenae) that can be traced back to
the centers of the ring systems are also consistent with planetary
impact-basin morphology. However, other modes of origin for multiring
systems have been proposed, including the rise of a molten diapir (Hale
and others, 1980) and the subsidence of a lithospheric mass (Wood,
1981).

Crater frequencies within multiring systems are slightly lower than
those of the surrounding plains, suggesting that the systems, though
formed early in Callisto's history, are younger than the heavily
cratered plains. The faint, degraded appearance of some of the smaller
multiring systems indicates they may be older than the Valhalla and
Asgard systems.

The ring systems give insight into Callisto\'s lithosphere at the time
of formation. Ring number, spacing, and morphology are considered to be
functions of lithospheric thickness and strength as well as crater
diameter (McKinnon and Melosh, 1980), and asymmetries in ring structure
may reflect preexisting variations in the properties of the lithosphere,
such as thickness, composition, and the presence of old impact scars
(Schenk and McKinnon, 1987). For example, a 1,100-km-wide zone of
heavily cratered plains northeast of Valhalla has been modified by
scarps concentric with Valhalla, whereas the modified zone is only about
500 km wide in other sectors (Remsberg, 1981) and consists mainly of
troughs. Contemporaneous with or possibly soon after the formation of
the Valhalla scarps, limited resurfacing by planar material occurred.
This smooth plains material (unit sp) may be cleaner ice derived from
Callisto\'s interior. If so, it may come from depths of 20 to 30 km, the
inferred thickness of the lithosphere at the time of Valhalla\'s
formation (McKinnon and Parmentier, 1986).

Structures associated with the multiring systems of Asgard and Valhalla
extend beyond the material units of these systems. Surrounding Asgard,
the cratered plains (unit cp) have been modified by inward-facing scarps
and troughs in the northwest sector. Around Valhalla, the cratered
plains have been modified by the development

of multiple, discontinuous troughs (possibly graben) and outward-facing,
concentric scarps in the northeast sector. These scarps transect craters
of the cratered plains (McKinnon and Melosh, 1980) and probably
developed at the same time as the structures within the material units
of the Valhalla Formation. A widely accepted interpretation is that the
outer troughs and scarps were formed by inward flow of the lithosphere
toward a collapsing transient cavity that was about as deep as the
lithosphere was thick, \~20 km (McKinnon and Melosh, 1980; Melosh, 1982;
Chapman and McKinnon, 1986; McKinnon and Parmentier, 1986).

**STRATIGRAPHY**

A single geologic unit of cratered plains (unit cp) appears to be fairly
uniform in albedo and crater density and covers most of the imaged part
of Callisto. Its superposed craters range in size from the limit of
resolution to 150 km in diameter. In some areas modification of the
cratered plains by light plains (unit lp) or smooth plains (unit sp)
material is apparent. The light plains material is characterized by its
higher albedo and typically occurs as elliptical to irregular patches.
Most light plains have fewer superposed craters than the surrounding
cratered plains. Some crater chains appear to begin at the margin of a
patch of light plains, suggesting that the light plains patch is the
remnant of an old crater. Smooth plains material is limited in areal
extent and appears to embay or fill craters. The presence of smooth
plains material at the base of scarps, where faulting may penetrate to
considerable depth, and the lobate outline of most patches suggest it
may be material erupted onto the surface (Remsberg, 1981; Melosh, 1982;
Stooke, 1990; Schenk, 1992). This material may be contemporaneous with
(Remsberg, 1981) or younger than the Valhalla Formation.

ASGARD FORMATION

The Asgard Formation includes an inner and outer facies of material
centered in the Asgard multiring system. The inner facies (unit ai)
forms an elliptical central plain about 380 km by 300 km across. It has
a higher albedo and an apparently lower crater frequency than that of
the Callisto-wide cratered unit. A few scarps within the southern
portion are its only structures. The outer facies (unit ao) surrounds
the central plain and extends 300 km to 500 km from its margin. The
outer facies is characterized by multiple, discontinuous, inward-facing
scarps and a single trough. Its crater frequency also appears to be less
than that of the surrounding cratered plains.

VALHALLA FORMATION

The Valhalla Formation, also divided into an inner and an outer facies,
consists of material of the Valhalla multiring system. The inner facies
(unit vi), a plain measuring 450 km by 600 km across, is characterized
by higher albedo than the surroundings, few superposed craters, and a
lack of structural or tectonic features. The surrounding outer facies
(unit vo) is characterized by multiple, discontinuous, arcuate to
sinuous structures concentric with the central plain. It extends for 530
km to 750 km from the margin of the inner facies and includes many
sinuous ridges. A few troughs (possibly graben) are found in the eastern
margin of the outer facies and several outward-facing scarps are found
on the northeast margin. The crater frequency of the outer facies is
less than that of the surrounding cratered plains (Passey, 1982).

CRATER MATERIALS

All recognized materials of craters 60 km and larger are mapped here and
assigned to three geologic units based on degree of apparent
degradation. Craters range from fresh and rayed to very subdued and
indistinct. The identification of ejecta is based primarily on albedo.
The presence of bright halos around craters indicates the presence of
impact-related materials, either original bright (ice?) ejecta or frosts
trapped by a rough ejecta blanket. Because of the lack of frost near the
equator, craters with rough ejecta not resolvable in the images and
showing no albedo variation from the surrounding material may have been
classified as c2, whereas the presence of frosts around identical
craters at high latitudes may have led to a c3 classification. However,
ages inferred from degradation state are questionable, because
degradation is controlled by the lithosphere, which appears to have
changed with time (Passey and Shoemaker, 1982) and which also may vary
by location. Crater retention ages are affected by crater relaxation,
which is controlled by the lithosphere at the time of impact. Small
craters are less affected by relaxation due to their shorter wavelength
form and may appear more pristine than larger craters that are actually
younger.

**GEOLOGIC HISTORY**

The geologic evolution of Callisto has been considered by Cassen and
others (1980), Passey and Shoemaker (1982), and others. The satellite
accreted from a mix of volatiles and silicates to become a water-rich
rocky object. Heating from various sources, including accretion and
radionuclides, may have led to differentiation and the formation of a
silicate-ice core. However, Callisto may not have differentiated at all
or may retain an undifferentiated mantle below an icy crust and above a
rocky core (Schubert and others, 1986). Very early in its history the
lithosphere probably thickened rapidly, as indicated by the retention of
many craters on the surface. At any given location progressively larger
craters were retained as the lithosphere thickened and strengthened
(Passey and Shoemaker, 1982). No craters greater than 150 km in diameter
have been retained on the present day surface of Callisto. The size
range of craters within the cratered plains, and in particular the lack
of very large craters, may reflect not only the lower effective strength
of ice and the viscous relaxation of old large craters (Passey and
Shoemaker, 1982) but possibly the unique flux of objects within the
Jovian system (Passey, 1982; Woronow and Strom, 1981). Subsequent
modification of this initial surface has produced the various units and
structures superposed on the globally extensive cratered plains.

Early in Callisto\'s history, large impacts probably produced the ring
systems and light plains. Crater retention ages, based on a flux model,
are 4.04 ± 0.14 ¥ 109 years for the Asgard multiring system and 3.96 ±
0.12 ¥ 109 years for Valhalla (Passey, 1982). Age relations among units
on Callisto are difficult to determine because the multiring systems are
not globally widespread deposits useful as time markers.

The youngest features on Callisto are the bright rayed craters; crater
retention ages for the regions in which rayed craters occur range in age
from 1.1 ¥ 109 years in the leading hemisphere to 3.7 ¥ 109 years near
the antapex (Passey and Shoemaker, 1982).

In summary, except for the young craters (unit c3), most of the activity
on Callisto occurred very early in its history.

ACKNOWLEDGMENTS

We wish to thank Ken Edgett for his input and Paul Schenk for his
insightful review.

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**Table 1.** *Ring systems \>200 km diameter on Callisto*

\[\*Outer ring diameter differs from material-unit diameter. References:
1, Passey

and Shoemaker, 1982; 2, Schenk and McKinnon, 1987\]

Latitude Longitude Outer ring diameter Ring type Reference

16° N 57° \~4,000 km\* multi Valhalla (1)

30° N 139° \~1,640 km\* multi Asgard (1)

46° S 33° \~600 km multi 1

3° S 230° \~600 km single 43° N 136° \~550 km multi 1

36° N 0° \~300 km multi 2

22° S 340° \~300 km single