**GEOLOGIC MAP**

**OF IO**

By

David A. Crown, Ronald Greeley,

Robert A. Craddock, and Gerald G. Schaber

1992

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--2209, as part of the Atlas of Jovian Satellites, 1:15,000,000
> 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**

PATERA FLOOR MATERIAL

pf **Patera floor material**---Smooth, generally mottled deposits of low
to high albedo in floors of small to large, steep-walled, circular
depressions; some deposits associated with observed thermal anomalies.
In some areas, such as at Euboea Fluctus (lat 46° S., long 350°), the
depressions are elongate and appear to be bounded by subparallel,
curvilinear scarps. Type area: lat 53° S., long 344° (Creidne Patera).
*Interpretation:* Volcanic flows, pyroclastic deposits, sulfur-rich
fumarolic deposits, and possibly active or extinct lava lakes in
calderas or fissures; materials may be composed of sulfur, sulfur
compounds, or silicates

CONE MATERIAL

c **Cone material**---Forms raised rims surrounding some occurrences of
patera floor material. Type area: lat 35° N., long 307° (Amaterasu
Patera). *Interpretation:* Rim deposits of silicate pyroclastic
materials around volcanic vent, or possibly high-yield-strength material
emplaced as lava flow or erosional remnant of layered plains

FLOW MATERIALS

fh **Hummocky material**---Massive, thick deposits with grooved surfaces
and lobate distal scarps of considerable relief. Type area: lat 2° N.,
long 318° (Carancho Patera). *Interpretation:* Viscous lava flows
consisting of silicates and possibly sulfur or sulfur compounds;
alternatively, lavas extruded at low effusion rates

ff **Fissure material**---Elongate to broad, lobate deposits of low to
moderate albedo; originated from linear to curvilinear fissures. Type
area: lat 45° S., long 352° (Euboea Fluctus). *Interpretation:* Consists
of silicates, sulfur, or both erupted from linear vent. Low-albedo
deposits overlie older, higher albedo deposits

fd **Digitate material**---Mottled deposit of apparently moderate relief
associated with circular depression and having low-albedo, fingerlike
flows emanating from its margins. One occurrence mapped at lat 12° S.,
long 303° (Kibero Patera). *Interpretation:* Lava flows, pyroclastic
flows, or both, consisting of silicates, sulfur, or both

fs **Shield material**---Forms circular feature with outer scarp and
central depression. Topographic data obtained by photoclinometry
indicate high relief (Moore and others, 1986). Type area: lat 30° S.,
long 246°. *Interpretation:* Forms shield volcano composed of silicates
and possibly sulfur

ft **Tholus material**---Forms circular, shield-shaped features with
well-defined outer scarp and central depression. One occurrence at lat
19° S., long 324° has an elongate flow unit extending from near its
central depression for more than 200 km to the south. Type area: lat 16°
S., long 349° (Inachus Tholus). *Interpretation:* Tholi are low-relief,
"discoid," shield volcanoes characterized by central eruptions of lava
flows consisting of silicates, sulfur, or both; origin of basal scarp
unknown

fl **Lobate material**---Broad to lobate, low- to moderate-albedo,
overlapping deposits of low relief commonly associated with
moderate-sized to large vents. Type area: lat 19° N., long 275°
(Daedalus Patera).

*Interpretation:* Lava flows, pyroclastic flows, or both, consisting of
silicates, sulfur, or both; appears to overlie plains-forming material
(unit **fpf)**

fpf **Plains-forming material**---Extensive, moderate- to high-albedo,
low-relief deposits commonly originating from moderate-sized to large
(\>100 km) vents; may appear mottled. Individual flow units from a
specific vent commonly cannot be identified within occurrence of this
unit. Type area: lat 20° N., long 300° (Loki Patera). *Interpretation:*
Lava flows, pyroclastic flows, or both, consisting of silicates, sulfur,
or both

fp **Patera material**---Long, narrow, overlapping, low- to high-albedo
deposits of moderate relief that commonly originated from vents less
than about 50 km across. Unit forms widespread, low-relief shield
volcanoes, typically mapped as shield materials and undivided flow
materials by Schaber and others (1989) . Individual flows from specific
vents commonly identifiable. Type area: lat 8° S., long 325° (Ra
Patera). *Interpretation:* Flows of silicates, sulfur, or both; may
include pyroclastic material. Low-albedo deposits apparently overlie
older, high-albedo deposits; many flows appear channelized

f **Undivided material**---Overlapping deposits of varied albedo with
mottled appearance; no discernible relation with specific occurrence of
patera floor material. To northeast, precise boundaries of large
complexes of unit are unclear due to low resolution of images. Type
area: lat 20° S., long 287°. *Interpretation:* Lava flows, pyroclastic
flows, or both, consisting of silicates, sulfur, or both; may include
patera material (unit **fp**), lobate material (unit **fl**),
plains-forming material (unit **fpf**), and fissure material (unit
**ff**). Flow deposits cannot confidently be traced to source vent(s)

> PLAINS MATERIALS

p **Interpatera material**---Vast, generally flat deposits of uniform
albedo between paterae; contain rare grabens and other linear features;
no flow features visible. Type area: lat 47° S., long 290°.
*Interpretation:* Flood lavas, pyroclastic deposits, plume fallout, or
combination thereof

pla **Layered material**---Forms extensive, elevated, layered plains of
moderate albedo, generally flat surfaced; bounded by scarps or faults;
some grabens and ridges. Where associated with mountain materials, as at
Euboea Montes, unit forms series of small plateaus rising from plains to
base of mountain; in several areas, as at Nemea Planum, unit makes up
many small, isolated plateaus. Unit equivalent to plateau material of
Moore (1987), Schaber and others (1989), and Whitford-Stark and others
(1990). Type area: lat 43° S., long 245°. *Interpretation:* Flood lavas,
pyroclastic deposits, or both, isolated as plateaus by erosional scarps
or faults

> MOUNTAIN MATERIALS

mg **Grooved material**---Forms rugged, elevated terrain having many
parallel linear depressions. Type area: lat 48° S., long 339° (Euboea
Montes). *Interpretation:* Tectonically deformed ancient silicate crust
or extremely viscous silicate lava flows, possible association with
tectonism and local reheating due to thermal anomaly in Creidne Patera
(Schaber and others, 1989)

ms **Smooth material**---Forms elevated, relatively smooth terrain
protruding above present surface. Type area: lat 48° S., long 337°
(Euboea Montes). *Interpretation:* Part of ancient silicate crust or
volcanic construct(s)

m **Undivided material**---Forms rugged to undulatory elevated terrain
protruding above present surface; many linear to curvilinear depressions
and scarps. Commonly bordered by layered plains (unit **pla**) and
interpatera plains (unit **p**) materials; in some areas appears embayed
by adjacent units. Type area: lat 48° S., long 333° (Euboea Montes).
*Interpretation:* Ancient silicate crust or volcanic construct(s);
detectable volcanic vents rare; tectonically deformed

**Contact**---Dashed where approximately located or inferred. Includes
boundaries between flow materials of same type but from different source
vents; boundaries may terminate without closure

**Fault**---Bar and ball on downthrown side

**Graben**---Dashed where inferred. One symbol may represent several
grabens in areas of high concentration in layered plains material

**Scarp**---Line at top of scarp, hachures point downslope; dashed where
inferred; used as contact in places

**Ridge crest**

**Linear to curvilinear feature of unknown origin**

**Steep-walled circular depression**---Dashed where approximately
located or inferred

**INTRODUCTION**

Io, the innermost of the Galilean satellites of Jupiter, is one of the
most geologically active bodies in the Solar System. It is dominated by
volcanism and shows only minor surface modification by tectonic
processes or erosion. Impact craters are not seen at the resolution of
Voyager images, indicating the relative youth of the surface (Smith and
others, 1979) and its rapid resurfacing by volcanic eruptions (Johnson
and others, 1979). As reviewed by Nash and others (1986), most of our
knowledge of Io is based on data obtained by Voyagers 1 and 2, which
encountered Io in 1979 and imaged about 35 percent of its surface (fig.
1). Other information was obtained by Pioneer 10, which flew past
Jupiter in 1973, and from Earth-based observations and theoretical
considerations (Matson and others, 1981; Pearl and Sinton, 1982; Goguen
and Sinton, 1985; Lunine and Stevenson, 1985). The general geology of Io
was described by Masursky and others (1979), Schaber (l980, l982) and
Nash and others (1986). Volcanic aspects have been discussed by Carr and
others (1979), Johnson and others (1984, 1988), McEwen and Soderblom
(1983), McEwen and others (1985, 1989), and Carr (1986).

**MAPPING TECHNIQUES**

Four areas of Io have been mapped geologically at larger scales: the
Maasaw Paterzi area (lat 35° S. to 48° S., long 335° to 350°) at
1:1,000,000 scale (Moore, 1987); the Ra Patera quadrangle (lat 5° N. to
20° S., long 290° to 340°) at 1:2,000,000 scale (Greeley and others,
1988); the Ruwa Patera quadrangle (lat 50° N. to 50° S., long 270°
westward to about long 40°) at 1:5,000,000 scale (Schaber and others,
1989); and the Lerna (south polar) region (lat 45° S. to 90° S.) at
1:5,000,000 scale (Whitford-Stark and others, 1990). The present
photogeologic mapping was completed by compilation and synthesis of
these maps and of work in progress by C.A. Wood in the Colchis Regio
quadrangle and by extension into unmapped areas. Several of the units
and contact placements on the 1:15,000,000 map differ from those of the
larger scale maps. All photogeologic mapping of Io is based on Voyager 1
and 2 images whose resolutions range from [\<]{.underline} 0.5 km to
\>20 km/pixel. (See resolution diagram.)

Conventional planetary photogeological mapping techniques (Wilhelms,
1972) were combined with techniques developed for mapping terrestrial
volcanoes. However, the lack of systematic topographic data hindered
interpretations, especially in defining superposition relations and flow
directions. Individual volcanic centers were identified and their
associated flows were mapped. Types of volcanoes and vents were defined
by their morphologies to provide a basis for interpreting styles of
volcanism and the general volcanic history. As in mapping terrestrial
volcanoes, mapping on Io is complicated because flows from adjacent
vents coalesce and interfinger, making it difficult to distinguish flow
sequences and relative ages of volcanic centers. Anastomosing flows from
single vents also complicate mapping of volcanic units. On Io, these
problems are compounded by the existence of plume deposits and flows
that are derived from great distances and that may interweave with flow
units from local sources. Despite these difficulties, many flows and
other volcanic units are identified and, where possible, traced to their
source vents.

**GENERAL GEOLOGY**

lo has an overall density of 3.57 gm cm-3 (Burns, 1986, p. 14), which
suggests a silicate composition, and a diameter of 3,642 km (Gaskell and
others, 1988), similar to the diameter of Earth's moon. Earth-based
observations show that Io is

the reddest object in the Solar System in the ultraviolet to visible
region of the spectrum. Its poles are relatively dark and its equatorial
region is bright; the leading hemisphere is brighter than the trailing
hemisphere. (Io is in synchronous rotation in its orbit around Jupiter.)

The discovery of active volcanoes on Io is one of the highlights of
Solar System exploration. Consideration of the forced eccentricity of
Io's orbit between those of Jupiter and Europa and the resulting
frictional heat generated within Io led Peale and others (1979) to
predict active volcanism on Io. Subsequent analysis of Voyager images by
Morabito and others (1979) showed the presence of spectacular volcanic
eruption plumes driven by internal heat as high as 300 km above the
surface.

Nine active volcanic plumes were observed during the Voyager 1 encounter
(table 1; fig. 2). Their geometric and thermal characteristics suggest
two different eruption styles; large, Pele-type plumes were emplaced by
short-lived (days to weeks), high temperature (\~650 K) eruptions,
whereas the more common, smaller, Prometheus-type plumes were generated
by longer lived (years), lower temperature (\<400 K) eruptions (McEwen
and Soderblom, 1983). For Pele-type eruptions, velocities as high as 1
km s-l carried sulfurous gases and solids as high as 300 km above the
surface. Pyroclastic material was deposited at distances as far as 700
km from its source vents. Of the nine active plumes observed during the
Voyager 1 flyby, one had ceased to be active 4 months later when Voyager
2 imaged Io, and deposits from two new plumes were observed. The
volcanic plumes are driven by internal heating, possibly by a geyserlike
mechanism involving sulfur dioxide (SO2), sulfur, or both (Kieffer,
1982). Volcanic resur facing rates on Io are estimated to be between
10-3 and 10 cm yr-l (Johnson and others, 1979; Johnson and Soderblom,
1982) The upper limit is consistent with rates of surface renewal
necessary to account for an absence of impact craters.

In general, the average natural color of Io's surface is pale yellow
(Young, 1984, 1985; McEwen, 1988), but Io displays a large degree of
spectral variability which, when enhanced (particularly in the violet
and near ultraviolet) results in various shades of red, yellow, orange
and brown. Many of these colors have been attributed to sulfur (Sagan,
1979) or to anhydrous mixtures of sulfur allotropes, SO2 frost, and
sulfurous salts of sodium and potassium (Fanale and others, 1979;
McEwen, 1988; McEwen and others, 1988). Pieri and others (1984)
described colors for some of the Ra Patera flow units and suggested that
these flows may be sulfur allotropes or sulfur-rich compounds; however,
Young (1984, 1985) argued that the images used by Pieri and others
(1984) do not represent the true colors on Io and do not indicate the
presence of sulfur allotropes.

Considerable controversy has arisen regarding the composition of lava
flows on Io. Spectral data indicate the presence of sulfur and
sulfur-rich compounds (Sagan, 1979; Sill and Clark, 1982, p.175--195).
However, these materials may be only thin deposits that mantle silicate
flows (Carr and others, 1979). The morphology of the vents and flow
materials is, for the most part, typical of silicate volcanism observed
on Earth. Sulfur exhibits unusual rheological properties: its viscosity
increases as temperature decreases from 320 to 185 °C, then decreases
rapidly while cooling to 160 °C; below that temperature the viscosity
increases again. This rheologic behavior, as observed in experimental
sulfur flows, led Greeley and Fink (1981) and Fink and others (1983) to
suggest that the morphologies of some sulfur or sulfur-rich flows may be
distinguishable from silicate flows. In addition, Greeley and others
(1984) suggested that the spectral data may indicate the presence of
secondary deposits of sulfur, such as those associated with terrestrial
fumaroles. Several lines of evidence suggest silicate volcanism on Io
(Clow and Carr, 1980; Johnson and others, 1988), which would introduce
sufficient heat to melt any sulfur that is present and to produce local
sulfur flows. These flows may be

composed of sulfur that melts at a low temperature (\~115 °C) and is
very fluid. Analyses of sulfur's thermal properties and
temperature-dependent rheology suggest that sulfur flows could retain
enough heat to flow long distances, especially if insulated by a chilled
crust (Fink and others, 1983). Although mapping of the high-resolution
images, as presented here, may shed light on some of these problems, it
is unlikely that the controversy will be resolved until better
compositional data are obtained.

MATERIAL UNITS

The imaged surface of Io can be subdivided into geologic units of four
principal classes: mountain materials, plains materials, volcanic flows,
and patera floor material (Schaber, 1980, 1982). All but the mountain
materials are attributed to volcanic processes; mountains may be formed
of crustal materials not necessarily related directly to volcanism. In
addition, various possible erosional features and tectonic structures
have been mapped.

*Smooth, grooved, and undivided mountain materials* (units ms, mg, and
m, respectively) form high-standing blocks of rugged relief that can
exceed 100 km in length. Mountains rise more than 9 km above the
surrounding surface and appear to be evenly distributed in latitude and
longitude (Schaber, 1982). Embayment relations with the surrounding
plains suggest that mountains comprise the oldest materials on Io.
However, Whitford-Stark and others (1990) suggested that the formation
of mountain materials may have occurred more recently, because mountain
material overlaps layered plains material (called plateau material by
Whitford-Stark and others, 1990) at Euboea Montes. (See also Mouginis
Mark and others, 1984.) In addition, some mountains, such as Haemus
Montes (south polar region), appear to be associated with recently
active vents. Clow and Carr (1980) have analyzed the physical properties
of sulfur and showed that, on Io, features such as steep scarps, if
composed of sulfur, are unstable when their heights exceed 1 km. On the
basis of this analysis, mountains and other features of significant
relief are interpreted to be composed of silicate materials (Clow and
Carr, 1980; Carr, 1986).

Plains materials of two types are the most widespread units observed in
images of Io. The natural colors of these materials are pale yellow to
shades of greenish gray and yellow. However, the spectral variations of
the plains (at violet and near-ultraviolet wavelengths) indicate a wider
range of colors, shown as combinations of red, orange, yellow, green,
and brown on false-color images. Units in the polar areas tend to be
darker (Masursky and others, 1979). *Layered plains material* (unit pla)
forms smooth, flat surfaces containing grabens and exhibiting scarps as
high as 1,700 m, which indicate a composition other than pure sulfur
(Clow and Carr, 1980). Although they occur throughout the mapped area,
the most conspicuous layered plains are found in the south polar region.
Transection by faults, overlap by the interpatera plains unit, and
evidence of erosion suggest a complex geologic history. *Interpatera*
plains material (unit p) is characterized by smooth surfaces of
intermediate albedo with low scarps. This unit constitutes about 40
percent of the area observed by Voyagers 1 and 2. Interpatera plains
material was interpreted by Schaber (1982) as fallout deposits from
volcanic plumes, interbedded with fumarolic materials and near-vent
flows.

McCauley and others (1979) noted that mesas, hills, and remnants of
subdued scarps beneath layers of younger plains material suggest
multiple cycles of erosion and deposition. These disrupted, "etched"
surfaces and their irregular "fretted" margins cannot be attributed to
fluvial or eolian processes, given the conditions of the near-surface
environment on Io. In addition, because of their relative youth, ionic
bombardment from Jupiter cannot account for these features. McCauley and

others (1979) proposed a sapping mechanism in which liquid SO2 is the
dominant erosional agent. A hydrostatic condition could be established
if the crust were fractured; molten SO2 would be driven toward the
surface, and, at the triple point for SO2, part of the liquid would
begin to crystallize and the system would expand, forming SO2 vapor.
Upon reaching the surface at or near a vent, the solid-fluid mixture
could be released at a velocity as high as 350 m s-l. At this velocity,
the SO2 "snow" could be sprayed as far as 70 km from the vent. The many
bright patches on Io may have formed by this mechanism. McCauley and
others (1979) suggested that newly formed scarps would be rapidly eroded
by undercutting and slumping, leading to the formation of irregular,
unstable margins. Withdrawal of support from beneath the solid crust
would cause collapse, generating irregular surfaces. This sequence of
processes would continue until the local source of SO2 was depleted.
Although the surface morphology of many plains regions appears to be
erosional, the presence of debris deposits at the bases of scarps, which
would be likely in a sapping environment, has not yet been confirmed
(Whitford-Stark and others, 1990). Alternatively, thermal erosion by
lava flows could form the scarps and erosional remnants observed in
layered plains material in the south polar region, as proposed by
Mouginis-Mark and others (1984). Whitford-Stark and others (1990)
suggested that the layered plains material (referred to as plateau
material) consists of stratified pyroclastic deposits that have been
modified by a combination of sapping and fluid and thermal erosion.

In addition to eruptive plumes resulting from explosive volcanism,
several different types of volcanic flow materials have been identified
on Io. *Patera flow material* (unit fp) forms narrow, sinuous flows as
much as 300 km long that radiate from central vents and accumulate to
form low-relief shield volcanoes. Other abundant flows are composed of
plains-forming flow material (unit fpf); these flows are massive, extend
as far as 700 km, and emanated from moderate-size to large calderas. The
morphology of this unit suggests that the materials were relatively
fluid and were erupted at high rates of effusion. Lobate flow material
(unit fl) and di\~itateflow material (unit fd) have been suggested to
have morphologies similar to those of experimental sulfur flows (Greeley
and Fink, 1981; Fink and others, 1983). Alternatively, apparent
stratigraphic relations indicate that the lobate and plains-forming flow
materials could be related and that the lobate flows represent currently
active or younger plains-forming flow materials.

Other flow materials on Io (Carr and others, 1979) have accumulated to
form *tholus* (unit ft), *shield* (unit fs), *fissure* (unit ff), and
*hummocky* (unit fh) *materials*. Most features identified as shields
are inferred as such from their planimetric form, but at least one
feature that has a classic shield profile has been noted (Moore and
others,1986). (Although the large-scale topography of Io is known
(Gaskell and others, 1988), the lack of systematic, local topographic
information prohibits assessment of more detailed morphology.) Schaber
(1982) noted that shieldlike volcanoes tend to be limited to the region
between lat 45° S. and 30° N., coinciding approximately with the
equatorial band where active volcanic plumes are found (Strom and
Schneider, 1982).

*Cone material* (unit c), which forms raised rims typically surrounding
occurrences of patera floor material in equatorial regions of Io, may
also have been emplaced as volcanic flows. Due to its apparent relief
and its proximity to a vent, the material would presumably have had a
high yield strength. Alternatively, these rim deposits may be
pyroclastic materials or erosional remnants of the layered plains.

*Patera floor material* (unit pf) includes all units that can be related
directly to paterae, interpreted as calderas, and to other vents of
various types (including fissure vents). More than 300 vents of all
types have been identified (Masursky and

others, 1979). Patera floor material constitutes about 5 percent of the
total mapped area. Calderas on Io average about 48 km across in the
equatorial region and 65 km across in the south polar region (Schaber,
1982), where, although generally larger, they are less common than in
lower latitudes. Calderas are more randomly distributed on Io than on
Earth, the Moon, or Mars, where the local and global tectonic frameworks
control the locations of volcanic vents. Vent distribution on Io
indicates that the locations of vents and related "hot spots" (table 2)
are not influenced by strongly patterned convection cells.

**STRUCTURE**

Calderas exhibit complex morphologies suggestive of multiple eruptions
and enlargements by wall collapse. Calderas range in depth from 0 km
(vents level with the surrounding surface) to more than 2 km
(Schaber,1982). The relatively high, steep walls of some calderas may
indicate that they are composed dominantly of silicate materials (Clow
and Carr, 1980). A correlation between low-albedo calderas, recent or
active volcanism, and thermal anomalies (hot spots) detected by Voyager
1 infrared observations has been observed (McEwen and others, 1985). The
spectral properties of these dark calderas are consistent with a variety
of sulfur-rich materials, including molten sulfur (Nelson and others,
1983). Quenched sulfur allotropes are redder than these low-albedo
features and would not exist at the high temperatures associated with
hot spots (McEwen and others, 1985). Voyager 1 and 2 images show changes
that occurred in the 4-month interval between encounters; parts of the
lava lake at Loki Patera may have crusted over, while new vents appear
to have developed in other areas (Terrile and others, 1981; McEwen,
1988). Loki Patera has been suggested to be the primary source of excess
thermal radiation evident in eclipses of Io (Sinton and Kaminski, 1988).

For a planetary object that is so volcanically active, Io displays
relatively few features that can be ascribed to tectonic processes
except in the polar layered plains. Some scarps in Nemea Planum and near
Babbar Patera may represent faults. These and other scarps are not
associated with any obvious volcanic vents, nor do they form patterns
diagnostic of largescale tectonic deformation. Grabens are typically not
found on the younger units, suggesting that tectonic forces on Io were
of greater magnitude in the past, that much of the surface is very young
and has not yet undergone deformation, or that only with age do the
surface materials become strong enough to deform by brittle failure
rather than ductile flow (Whitford-Stark and others, 1990). Another
feature, whose sharp boundaries suggest a tectonic origin (McEwen and
others, 1985), is the "south pole ring" (table 2), an area of low albedo
and elevated temperatures about 1,000 km in diameter.

**GEOLOGIC HISTORY**

Because most of the units exposed on the surface are relatively young,
it is difficult to derive a geologic history for Io based on mapping. Io
is assumed to have accreted at the same time as the other satellites in
the Jovian system. Together with the various young flows observed,
volcanic plumes indicate a highly active body that has been chemically
differentiated through geologic time. On the basis of observations of
current activity, a mass equivalent to the entire mass of Io is
estimated to have been recycled in its lifetime (Johnson and Soderblom,
1982). Volatiles such as water and carbon dioxide would have been lost
early in Io's geologic history, and most heavier materials would have
sunk to form a core. Sulfur and various sulfur compounds, mobilized by
higher temperature silicate magmas, are constantly recycled, forming the
complex surface observed today.

**ACKNOWLEDGMENTS**

This work was supported by the National Aeronautics and Space
Administration through a grant from the Office of Planetary Geosciences.
Discussions with Laurie L. Watson contributed to the synthesis of
pre-existing maps and the definition of geologic units. Maa-ling Chen
and Sue Selkirk assisted in the preparation of the map for publication.
A preliminary version of the map was reviewed by G. Bawden, K. Edgett,
E. Paisley, K. Ring, and A. Skypeck of Arizona State University. This
work has been improved after scientific reviews by B. Lucchitta, A.
McEwen, J. Whitford-Stark, and C. Wood and through the helpful comments
of Doris Weir of the U.S. Geological Survey.

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Only the named features referred to in text or tables are identified on
this map. See U.S. Geological Survey (1987) for identification of all
other named features.