Download Icarus Geologic mapping of the Zal region of Io

Icarus 197 (2008) 354–367
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Geologic mapping of the Zal region of Io
Melissa K. Bunte ∗ , David A. Williams, Ronald Greeley
School of Earth and Space Exploration, Arizona State University, Box 871404, Tempe, AZ 85287, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
Received 6 November 2007
Revised 28 March 2008
Available online 10 May 2008
We produced a regional geologic map of the Zal region of Io’s antijovian hemisphere using Galileo
mission data. We discuss the geologic features, summarize the map units and structures that are present,
discuss the nature of volcanic activity, and present an analysis of the volcanic, tectonic, and gradational
processes that affect the region. The Zal region consists of five primary types of geologic materials:
plains, mountains, paterae floors, flows, and diffuse deposits. The flows and patera floors are similar, but
are subdivided based on uncertainties regarding emplacement environments and mechanisms. The Zal
region includes two hotspots detected by Galileo: one along the western scarp of the Zal Patera volcano
and one at the Rustam Patera volcano (name submitted to IAU). A third hotspot at the nearby At’am
Patera volcano (name submitted to IAU) is the source of diffuse and pyroclastic materials that blanket
north Zal Mons. The western bounding scarp of Zal Patera is the location of a fissure vent that is the
source of multiple silicate lava flows. The floor of Zal Patera has been partially resurfaced by dark lava
flows, although portions of the patera floor appear bright and unchanged during the Galileo mission. This
suggests that the floor did not undergo complete resurfacing as a flooding lava lake but does contain
a compound flow field. Mountain materials exhibit stages of degradation; lineated material degrades
into mottled material. We have explored the possibility that north and south Zal Mons were originally
one structure. We propose that strike-slip faulting and subsequent rifting separated the mountain units,
opened a fissure which serves as a vent for lava flow, and created a depression which, by further
extension during the rifting event, became Zal Patera. With comparison to other regional maps of Io,
this work provides insight into the general geologic evolution of Io.
© 2008 Elsevier Inc. All rights reserved.
Keywords:
Io
Volcanism
Geologic processes
1. Introduction
Voyager and Galileo data show that Io has diverse volcanic features and eruption styles (e.g., Smith et al., 1979a, 1979b; McEwen
and Soderblom, 1983; McEwen et al., 1998a, 2000a; Lopes-Gautier
et al., 2000; Keszthelyi et al., 2001; Lopes et al., 2004; Turtle et al.,
2004; Lopes and Spencer, 2006; Davies, 2007), which have been
mapped geologically (Schaber, 1980, 1982; Moore, 1987; Schaber
et al., 1989; Greeley et al., 1988; Whitford-Stark et al., 1991;
Crown et al., 1992; Williams et al., 2002, 2004, 2005, 2007). Sulfur, sulfur dioxide, and silicate materials interact in a complex way
to produce the wide variety of explosive and effusive deposits
(reviewed by Lopes and Williams, 2005, and Lopes and Spencer,
2006), as exemplified by Io’s diverse color palette (Geissler et al.,
1999, 2004).
The objective of this study was to investigate the geologic processes that formed the Zal region and to determine the importance
of explosive and effusive volcanic activity, tectonic influences, and
gradational processes in the evolution of the landforms. The ap-
*
Corresponding author. Fax: +1 480 965 8102.
E-mail address: [email protected] (M.K. Bunte).
0019-1035/$ – see front matter
doi:10.1016/j.icarus.2008.04.013
©
2008 Elsevier Inc. All rights reserved.
proach was to derive a stratigraphic context and geologic history
of the region by identifying flow units and surface features using planetary mapping techniques (e.g., Shoemaker and Hackman,
1962; Wilhelms, 1972, 1990).
2. Background
The Zal region (25–45◦ N, 65–85◦ W; Fig. 1) is located in Io’s
leading and antijovian hemispheres. The region consists of Zal Patera, a large (120 km wide × 197 km long) patera that has no
definite northern margin, north and south Zal Mons which border Zal Patera to the west and south (Radebaugh et al., 2001),
and Rustam Patera which is located west of south Zal Mons. Two
high resolution observations of the Zal region were obtained during the close Io flybys that occurred late in the Galileo extended
mission. The first was a near-terminator observation with a spatial resolution of 260 m/pixel focusing on Zal Montes in November
1999 (orbit I25; Fig. 2). Turtle et al. (2001) noted from an analysis of this mosaic that north Zal Mons is a smooth-topped layered
plateau (sloping to the west), with scalloped margins to the north
and west, a slumping margin to the southeast, and a linear scarp
to the east; shadow measurements indicate that north Zal Mons
rises 1.9 km above the floor of Zal Patera volcano to the imme-
Geologic mapping of the Zal region of Io
Fig. 1. Galileo SSI mosaic obtained during orbit I27 flyby in February 2000 (observation I27ZALTRM01). Zal Patera is located at 40.2◦ N, 74.5◦ W. Resolution is
335 m/pixel. North is up. Illumination is from the left. Major features in the Zal
region are noted.
Fig. 2. Galileo SSI mosaic obtained during orbit I25 flyby in November 1999 (observation I25TERM01a). Zal Montes is centered at 38.1◦ N, 77◦ W. Resolution is
260 m/pixel. North is up. Illumination is from the left.
diate east, and that its southern margin is cut by fractures. South
Zal Mons (Fig. 1) is a fractured mountain which rises 4.6 km above
the surrounding plains at its highest peak, has a steep scarp on the
west side, and lacks the smooth, plateau-like morphology of north
355
Zal Mons; it is thought to have been affected by extension due to
uplift or erosion along joint surfaces (Turtle et al., 2001).
The second high resolution observation of the Zal region was
also a near-terminator observation with a spatial resolution of
335 m/pixel and was obtained in February 2000 (orbit I27; Fig. 1).
This mosaic serves as the base for our map. Turtle et al. (2001) determined that the western margin scarp of Zal Patera has a height
of ∼0.5 km and that the Zal Montes and Patera complex was
an example of volcano-tectonic interaction on Io, as described by
Radebaugh et al. (2001), in which the ∼300 km long fracture that
separates north and south Zal Mons and that bounds Zal Patera to
the west is the source of the lavas flowing into Zal Patera. We further investigated this hypothesis in our mapping. The prominent
red diffuse deposits observed in distant Galileo observations were
noted by Turtle et al. (2001) to be associated with the fractures on
south Zal Mons rather than Zal Patera to the north.
The Zal region was also imaged as part of a color mosaic covering the antijovian hemisphere during the extended mission in
June 1999 (orbit C21). This observation (Fig. 3) obtained low-phase
angle (4◦ ) color coverage in the 756 nm, green, and violet filters at a spatial resolution of 1.4 km/pixel (McEwen et al., 2000a;
Keszthelyi et al., 2001). These data remain the best global-scale
color coverage obtained during the Galileo Mission. The Zal region
was imaged at very low resolution (∼5–20 km/pixel) by the Voyager spacecraft in 1979 (Fig. 4).
McEwen (1988) noted that, in the interim between the Voyager 1 and 2 flybys, a “dark material” was emplaced at Zal. Distant
Io imaging during the Galileo Nominal Mission (1996–1997) first
detected a hotspot at Zal in Near-Infrared Mapping Spectrometer
(NIMS) data in June 1996 (orbit G1; Lopes-Gautier et al., 1999). The
hot spot was not detected again until April (orbit G7) and May (orbit G8) of 1997 in Solid State Imager (SSI) eclipse images (McEwen
et al., 1998a) and June (orbit C9) and September (orbit C10) of 1997
in NIMS data (Lopes-Gautier et al., 1999). No active plume was observed at Zal; but fading of the Voyager-era dark deposit and the
appearance of a new red diffuse deposit were noted (McEwen et
al., 1998a; Lopes-Gautier et al., 1999).
Io is the most colorful body in the Solar System; the colors appear to indicate a variety of volcanic material compositions and
to give clues to the different emplacement mechanisms of those
materials. Thus, we attempt to use color as one of the major factors in identifying the surface materials at Zal. Simonelli et al.
(1997) and Geissler et al. (1999, 2000) studied the color of Io
in Galileo SSI and NIMS data. Dark (low albedo) materials occur
mainly as units confined to patera interiors, appear in an array
of colors from red to black, and are generally interpreted as silicate lava flows (Geissler et al., 1999). Most of these dark materials
correlate with active or recently active hot spots (Lopes-Gautier
et al., 1997, 2000; Lopes et al. 2001, 2004). Bright (intermediate
to high albedo) flow materials appear in an assortment of colors
from gray-white to yellow and light red; the colors may be indicative of variations in composition or in age and state of degradation
(Geissler et al., 1999; Kargel et al., 1999). These flows have been
interpreted as both sulfur-rich lava flows and degraded silicate
flows (Williams et al., 2002). Bright red diffuse materials are generally associated with active high-temperature plumes or hotspots
and are inferred to be pyroclastic deposits (McEwen et al., 1998a;
Geissler et al., 1999). White diffuse materials correspond to areas of SO2 frost (Carlson et al., 1997). Bright (yellow and green)
diffuse materials are located within or adjacent to white and
red diffuse materials and are thought to be composed mostly of
sulfur-dioxide (Carlson et al., 1997). The green hue of some diffuse
materials may be an effect of contamination by iron or silicates
(Geissler et al., 1999). Specific details on the compositional inferences of Io’s colors were published previously (Young, 1984;
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M.K. Bunte et al. / Icarus 197 (2008) 354–367
Fig. 3. Galileo color image of the Zal region; obtained during orbit C21 in June 1999 (merged with the I25 observation). Zal Patera is located at 40.2◦ N, 74.5◦ W. Resolution
is 1.4 km/pixel. North is toward upper left. Phase angle is 4◦ . Major features in the Zal region are noted. The white dot marks the location of a possible shield volcano. NASA
Photojournal PIA02527.
Geissler et al., 1999; Spencer et al., 2000; Geissler et al., 2001;
Williams et al., 2005).
NIMS spectral mapping (Carlson et al., 1997) shows that areas
which are bright (high albedo) at low phase angles are characterized by coarse-grained SO2 . This helps to explains color differences
between low and high phase angle images. Some features which
are visible at high phase angle are not visible at low phase angle (Geissler et al., 2001); the relative albedos of features appear
to change with phase angle as well. Thus, we examined images
that have similar phase angles to facilitate comparisons of relative
brightness. We use relative albedo of features in SSI images as a
qualitative descriptor to characterize units in addition to the color
interpretations discussed above.
3. Mapping techniques
Our geologic map of the Zal region of Io was produced using
the Galileo SSI orbit I27 mosaic (spatial resolution of 335 m/pixel;
Fig. 1). Interpretation of color units was made by examining the
relevant portion of the Galileo SSI C21 mosaic (spatial resolution
of 1.4 km/pixel; Fig. 3). To determine contact locations, we also
examined various projections of the Galileo SSI orbit I25 mosaic
(spatial resolution of 260 m/pixel; Fig. 2).
Geologic mapping of the Zal region of Io
Fig. 4. Best Voyager imaging of the Zal region (∼20 km/pixel). Extracted from global
Voyager mosaic PIA00318. Image centered at 38◦ N, 77◦ W. North is up.
We used the mapping approach and methodology of Wilhelms
(1972, 1990) that has been successfully applied to other regions of
Io (Williams et al., 2002, 2004, 2005, 2007). Our mapping focuses
on a small area of Io that appears to have related volcano-tectonic
features. Details on the mapping approach using Galileo data, including the constraints imposed by the multiple Galileo flybys, and
previous interpretations of the Galileo color data (e.g., Young, 1984;
Geissler et al., 1999, 2000, 2004) were described by Williams et
al. (2002, 2004, 2005, 2007). We define and characterize material
units based on landform morphology, albedo, and color (i.e., composition) and interpret the emplacement of the surface features
and the stratigraphic relationships between the volcanic and structural features.
Constraints on planetary mapping, in general, include the wide
range of phase angles and image resolutions (or lack thereof) that
must be used to produce the best possible map. In the case of the
Zal region, the low number of images that cover the region at high
resolution restricts the possibility of mapping detailed features and
structures. Also, the difference in resolution between the color and
grayscale images increases the difficulty of identifying morphological differences by composition/color. Materials within each unit
have similar compositions (combinations of silicates, sulfur, and
sulfur dioxide) and differ only by emplacement environment and
mechanism, morphology, or alteration (e.g. Lopes and Williams,
2005; Geissler et al., 2004).
4. Map units
Following Voyager-era maps (Schaber, 1980, 1982; Moore, 1987;
Schaber et al., 1989; Greeley et al., 1988; Whitford-Stark et al.,
1991; Crown et al., 1992), we define five types of material units
for the Zal region: mountain materials (i.e. tectonic massifs), plains
materials, patera floor materials, lava flow fields, and diffuse deposits. Detailed descriptions and interpretations of the material
units are given in Table 1. Origins of named features in the region are given in Table 2. Fig. 5 shows images of type localities for
each of the material units and their subunits. Fig. 6 shows our geologic map of the material units and structural features in the Zal
region, which are consistent with SSI-based maps of other regions
of Io (Williams et al., 2002, 2004, 2005, 2007).
4.1. Mountain/plateau materials
Mountain materials generally have colors and albedos similar to bright plains materials, and are often only visible in lowSun angle images where shadows highlight scarps, ridges, grooves,
and mountain peaks (Schenk et al., 2001; Turtle et al., 2001;
357
Jaeger et al., 2003). The mountain units appear to be embayed
by plains and flow materials and mantled by diffuse deposits. We
characterize four types of mountain materials: lineated (containing well defined ridges and grooves), mottled (containing lobes
and dome-like hills), undivided (mountain material that is characterized by aspects of both the mottled and lineated units, but
is dominated by neither), and plateau (similar to mottled material but occurs on flat-lying elevated topography). The lineations
on Io’s mountains correspond to topographically distinct ridges,
grooves, graben, scarps, or lineaments. The northern half of south
Zal Mons has lineations that are generally parallel to the trend of
the mountain ridge; the southern half has lineations that are generally perpendicular to that trend. North Zal Mons is a plateau
with some mottled mountain unit characteristics. The structure
is relatively flat with a slight slope to the south and exhibits
lobes and debris aprons on the downslope edge. The plateau lacks
lineations, though it has several grooves and depressions. Lineated mountains are interpreted as tectonically modified uplifted
crustal blocks, and mottled units are thought to be indicative
of displacement of crustal materials by mass wasting processes
that are likely the result of SO2 sapping (Moore et al., 2001;
Turtle et al., 2001).
4.2. Plains materials
Plains materials appear in SSI color images as yellow, graywhite, and red to red-brown color units. Most of the plains are
mantled by diffuse deposits, resulting in further color variations.
Plains materials appear to embay mountain units and to underlie flow and patera floor materials. The Zal region is dominated
by yellow and red-brown plains materials, although several local patches of white plains material occur adjacent to the patera.
Plains are interpreted as the silicate upper crust of Io with a mantle of sulfur- and/or SO2 -rich material (Bart et al., 2004; Keszthelyi
et al., 2004). The plains are formed by combinations of overlapping effusive flows, mass wasting of flow materials, SO2 sapping,
deposits from volcanic plumes containing SO2 and sulfur frosts,
and pyroclastic flows (McEwen et al., 2000b; Moore et al., 2001;
Schenk et al., 2001).
4.3. Patera floor materials
Patera floor materials are similar to flow materials with colors
ranging from yellow-orange to dark black. We define two subunits of patera floor materials: bright and dark materials. Patera
floor units have albedos and colors similar to those of the flow
units and are thought to be composed of the same materials as
the flow units. Traditionally, flow and patera floor materials are
mapped separately (Schaber, 1980, 1982; Crown et al., 1992) because there is uncertainty regarding the emplacement mechanisms
of patera floor materials (i.e., lava flows vs lava lakes). Patera floor
units are emplaced effusively within the bounding scarps of paterae, whereas flow units are emplaced outside the bounding scarps
of paterae but can flow downslope into paterae. The patera floor
materials may be superposed by flow materials. In the case of Zal
Patera, several dark flows are located within the bounding scarps
of the patera but do not cover the floor. Source vents for patera
floor materials are not always evident.
Color differences are often considered to be indicative of variations in composition, including sulfur and silicate mixtures (e.g.
Williams et al., 2002, 2004; Lopes et al., 2001). Zal Patera contains
a compound flow field which includes units of several different
colors but does not appear to be flooded by dark (i.e., fresh) flows.
Our mapping indicates that the units within Zal Patera are all
of similar silicate composition with minimal mantling by diffuse
deposits of sulfuric composition. In this case, we attribute color
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M.K. Bunte et al. / Icarus 197 (2008) 354–367
Table 1
Material units identified in the Zal region of Io
Material units
Description
Interpretation
Red diffuse material
(dr )
This unit mantles underlying materials and generally occurs as
asymmetric deposits around or along active vents or flows. It appears
red or orange.
Diffuse material may be explosively emplaced pyroclastic
deposits. The composition of red diffuse materials may be
short-chain sulfur or chlorides with contaminants (McEwen et al.,
1998a; Geissler et al., 1999; Spencer et al., 2000).
Bright diffuse material
(dw )
This unit mantles underlying materials and generally occurs as circular
deposits around active vents or flows. It appears green or yellow with
high albedo.
Diffuse material may be explosively emplaced pyroclastics. Bright
diffuse material may be largely composed of sulfur compounds
(Carlson et al., 1997).
White diffuse material
(dw )
This unit mantles underlying materials and generally occurs as circular
deposits around active vents or flows. It appears white with high albedo.
Diffuse material may be explosively emplaced pyroclastics. White
diffuse material may be largely composed of sulfur dioxide
(Carlson et al., 1997).
Dark diffuse deposits
(dd )
This unit mantles underlying plains and mountain materials. Its
morphology is digitate. It appears gray-white with low albedo.
Diffuse material may be explosively emplaced pyroclastics largely
composed of silicate ash. The digitate morphology and
topographic distribution indicate that this deposit is of pyroclastic
origin (Turtle et al., 2001).
Dark flow material (fd )
subunits: 1–3
Surfaces are dark, have lobate margins, and have lengths greater than
their widths. Contacts with surrounding terrains are generally sharp.
Flows appear to extend into topographic lows. Albedo variations and
superposition or cross-cutting indicate relative age compared to other
flows. Flows may extend into paterae. Subunits indicate relative age;
darker flows are younger.
Lava flows are flows of silicates and appear dark and lobate or
digitate. Albedo variations indicate age differences in flows. Flows
appear to be emplaced in the Promethean style (Keszthelyi et al.,
2001).
Bright flow material
(fb )
Surfaces are high albedo and appear bright yellow or white. Margins are
lobate and lengths are greater than widths. Contacts with surrounding
terrain are sharp. Flows extend into topographic lows. Albedo variations
and superposition or cross-cutting indicate relative age.
Flows may be of sulfur or sulfur dioxide composition (Williams
et al., 2001, 2002, 2005, 2007). Albedo variations indicate age
differences in flows. Flows appear to be emplaced in the
Promethean style.
Undivided flow material
(fu )
Terrain consists of bright and dark flows with a range of albedos.
Contacts with surrounding terrain are not distinct so that individual
flow units are not distinguishable.
Lava flow types are unclear. They are older silicate and sulfur-rich
flows than dark and bright flow materials.
Dark patera floor material
(pfd ) subunits: 1–3
Surfaces are black or dark gray with some variation in albedo. The
surfaces have distinct contacts with the surrounding terrain and are
found within paterae. Superposition indicates that subunit 3 is the
darkest and freshest material.
Dark floor materials are lavas (likely silicate lava flows); darker
surfaces are likely warm and recently emplaced. These materials
may be produced in lava lakes or be confined lava flows.
Bright patera floor
material (pfb )
Surfaces are bright red-orange and apparently smooth. The surfaces have
distinct contacts with the surrounding terrain and are found within
paterae but can occur beyond the rims of paterae.
Bright floor materials are lavas (likely sulfur-rich lava flows).
These materials may be produced in lava lakes or be confined
lava flows.
Red-brown-plains material
(prb )
The surface appears layered and exhibits colors of red and red-brown.
Albedo varies but is intermediate between that of yellow bright plains
and dark patera floors.
Red-brown plains are the silicate crust mantled by sulfur-rich
materials that have undergone alteration by radiation. This plains
material may be old lava flows that have not brightened to the
albedo of the bright plains material.
White bright plains
material (pbw )
The surface is layered and exhibits colors of white or white-gray. Albedo
varies but is higher that of yellow bright plains. White plains may be
mantled by white diffuse materials.
White plains are the silicate crust mantled by dominantly
SO2 -rich materials. The surface is possibly aged yellow bright
plains material with sulfur dioxide snow that is recrystallized
from plume fallout (Douté et al., 2001).
Yellow bright plains
material (pby )
The surface is layered and exhibits colors in shades of yellow. Albedo
varies but is intermediate between that of bright patera floors and
bright flows. Plains are often mantled by diffuse materials and contain
scarps and grooves.
Yellow bright plains are the silicate crust mantled by sulfur-rich
materials (Bart et al., 2004; Keszthelyi et al., 2004). Surface
deposits may be formed by volcanic plume fallout flow deposits.
Lineated mountain
material (ml )
Lineated mountains appear to be elevated blocks of bright plains but are
separated from the plains by a bounding scarp. They contain
well-defined grooves and ridges.
Mountains appear to be uplifted crustal blocks. Lineated
mountains exhibit little degradation or erosion by slumping
and/or other processes (Moore et al., 2001).
Undivided mountain
material (mu )
Undivided mountains appear to be elevated blocks of bright plains but
are separated from the plains by a bounding scarp. They contain few
grooves, ridges, and lineaments and may contain lobes or dome-like
hills.
Mountains appear to be uplifted crustal blocks. Undivided
mountains exhibit some signs of degradation and erosion by
slumping and/or other processes (Moore et al., 2001).
Mottled mountain
material (mm )
Mottled mountains appear to be elevated blocks of bright plains but are
separated from the plains by a bounding scarp. They generally do not
contain grooves, ridges, and lineaments but do contain lobes and
dome-like hills.
Mountains appear to be uplifted crustal blocks. Mottled
mountains exhibit lobes that result from slumping and mass
wasting (Moore et al., 2001).
Plateau material
(pl)
Mountains appear to be elevated blocks of bright plains but are
separated from the plains by a bounding scarp. They contain grooves,
ridges, and lineaments.
Mountains appear to be uplifted crustal blocks. Plateaus exhibits
depressions and grooves that result from sapping and scalloped
boundaries that may result from contact with warm materials
(Moore et al., 2001).
Letter symbols refer to the key for the geologic map of the region. Refer to Fig. 6. Albedo from Galileo SSI I27 and I25 observations (Figs. 1 and 2). Color from Galileo SSI C21
observation (Fig. 3). Images of type localities can be found in Fig. 5.
Geologic mapping of the Zal region of Io
359
Table 2
Origin of feature names in the Zal region
Feature
Center latitude
Center longitude
Origin of name
Zal Patera
Zal Montes
Rustam Patera
At’am Patera
40.2◦ N
38.1◦ N
34◦ N
41◦ N
74.5◦ W
77◦ W
74◦ W
84◦ W
Iranian Sun god
Iranian Sun god
Iranian hero, son of Zal
Mordvin (Russian) thunder god
Fig. 5. Color type examples of material units found in the Zal region of Io. Excerpts taken from Galileo SSI image PIA 02527 (Fig. 3) or Galileo SSI mosaic I27ZALTRM01 (Fig. 1).
Lineated mountain material (ml ), undivided mountain material (mu ), mottled mountain material (mm ), plateau material (pl), yellow bright plains material (pby ), white bright
plains material (pbw ), red-brown-plains material (prb ), dark patera floor material (pfd ), bright patera floor material (pfb ), dark flow material (fd ), bright flow material (fb ),
undivided flow material (fu ), red diffuse deposits (dr ), white diffuse deposits (dw ), bright diffuse deposits (db ), dark diffuse deposits (dd ).
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Fig. 6. Geologic map of the Zal region of Io. The map is based on observation I27ZALTRM01 (Fig. 1) at a resolution of 335 m/pixel. The southwest corner of the map is based on image PIA 02527 (Fig. 3) at a resolution of 1.4 km/pixel.
Map covers 25–45◦ N, 65–85◦ W.
Geologic mapping of the Zal region of Io
differences to differences in age between the units. We surmise
that the patera floor units are sufficiently aged to have brightened
in albedo due to degradation and to have changed hue by irradiation (Nash et al., 1986); thus, they appear brighter and more
yellow than other lavas within the patera.
Presently, Zal Patera does not resemble an active lava lake as
inferred for Loki (Rathbun et al., 2002; Howell and Lopes, 2007)
or Pele (Lopes et al., 2004); there is no Galileo data of Zal Patera
that indicates crustal cracking or foundering of lava crusts. Flow
materials of the same age do not cover the entire patera floor. The
lack of complete resurfacing within Zal Patera may indicate that
flow materials are not sufficiently mobile to cover the expanse of
the patera floor or that the patera has only been partially filled by
lavas.
4.4. Lava flow materials
We recognize three types of lava flow materials based on morphology, albedo, and color: bright flows, dark flows, and undivided
flows. Lava flows can embay the mountain units, are superposed
only by diffuse deposits, and can overly all other material units.
The lava flow units are characterized by a lobate and elongated
morphology, in which their flow lengths are greater than their
widths (Williams et al., 2002, 2004, 2005, 2007). Several of the
flow units appear to emanate from the fissure vent at the patera’s western scarp. Bright flows exhibit a variety of colors from
gray to orange and have a high albedo relative to the patera floor
materials. The albedo of bright flows is generally lower than the
surrounding and underlying plains. Dark flows appear dark black
and can have red, blue, and green tints. The albedo of dark flows
is extremely low compared to all other units. Flows with intermediate albedo and undefined contacts with other units are classified
as undivided flow materials.
Bright flows are thought to be sulfur-dominated, whereas dark
flows are thought to be silicate-dominated and associated with
active hotspots (Lopes et al., 2001, 2004; McEwen et al., 1997).
The color tint of dark flows is due to the mantling of the flows
by diffuse material or to variations in composition (see McEwen
et al., 1998b; Geissler et al., 1999; Williams et al., 2000). Subunits for the bright and dark flows indicate relative age of the
flows. In general, darker dark flows are younger and brighter bright
flows are younger than their counterparts (McEwen et al., 1998a;
Williams et al., 2002).
4.5. Diffuse deposits
Volcanic centers and tectonic fractures on Io release gases that
condense into precipitate, producing diffuse deposits that mantle
underlying topography. They exhibit a variety of colors that affect
the identification of surface units (Geissler et al., 1999). We recognize four general categories of diffuse deposits in the Zal region:
red, white, bright, and dark diffuse deposits.
Red diffuse deposits are associated with active volcanic centers and often with active plumes, and can (but do not often)
form circular rings (e.g. Pele; McEwen et al., 1998a). In the Zal region, they are associated with a scarp that trends NW-SE from the
eastern margin of north Zal Montes, along the western scarp of
Zal Patera, to the western margin of south Zal Montes (this setting also occurs in the case of Marduk: McEwen et al., 1998a).
This scarp appears to be the source of many of the lava flows
and a portion of the red diffuse deposits that are visible in the
Zal region (Turtle et al., 2001). Spencer et al. (2000) also speculate that several of the features identified as sapping points along
the eastern edge of north Zal Montes and the western face of
south Zal Montes could be the source of jets of red diffuse material. Red diffuse deposits are associated with explosive plume
361
activity resulting from short-chain sulfur (S3 , S4 ) produced by condensation and recrystallization of S2 gas (Spencer et al., 2000;
Kieffer et al., 2000). The composition of red diffuse materials may
also include chlorides with contaminants (McEwen et al., 1998a;
Geissler et al., 1999; Schmitt and Rodriguez, 2003).
White diffuse deposits are also associated with active plumes
and can form rings. White diffuse deposits have high albedo and
are composed largely of sulfur dioxide (Carlson et al., 1997). Bright
diffuse deposits are also high albedo and appear green or yellow. In the Zal region, they are found only within paterae (at Zal
and Rustam Paterae). These deposits may be largely composed of
sulfur-bearing compounds (Carlson et al., 1997). They may be the
result of jetting from the flow or patera floor materials. Dark diffuse deposits have a lower albedo than other diffuse deposits and
appear gray.
5. Discussion
The lack of impact craters in the Zal region (and Io overall)
prevents the stratigraphic correlation of map units by crater statistical methods. Johnson et al. (1979) estimate the extreme lower
limit resurfacing rate of Io to be 3 × 10−4 cm yr−1 . While crater
statistics cannot be used to suggest age relationships, stratigraphic
correlations can be made using superposition and cross-cutting relationships between material units. Observations of the Zal region
reveal that it is characterized by several types of volcanic activity
and features that indicate tectonic activity. Among these are multiple hotspots and volcanic vents, patera floor deposits, multiple
overlapping bright and dark effusive lava flows, a fissure vent, and
diffuse deposits which indicate plume activity. All of the surfaces
in each unit did not necessarily form at the same time.
Galileo images reveal a hot spot with surrounding red diffuse
materials on the western scarp of Zal Patera. These diffuse deposits
appear to have been emplaced since Voyager images were taken in
1979. Some diffuse deposits detected during the Voyager era had
faded by the time Galileo images were taken in 1997. Dark lava
flows appear to have erupted from the base of the fault scarp and
flowed east across the floor of Zal Patera (Radebaugh et al., 2001;
compare Figs. 3 and 4). High-temperature thermal emission was
detected by SSI along the fault scarp south of Zal Patera (McEwen
et al., 1998a; Keszthelyi et al., 2001), and additional emission was
detected at Zal Patera (Lopes et al., 2001). Images taken in February 2007 by the Long Range Reconnaissance Imager (LORRI) aboard
the New Horizons spacecraft suggest the presence of a small faint
plume that originates in the Zal region, although the exact location of the plume vent could not be determined at the resolution
of LORRI (Spencer et al., 2007a, 2007b).
It should be noted that some materials appear to change color
as they age (as surface materials are irradiated; Nash et al., 1986;
Kargel et al., 1999); for example, red diffuse deposits appear to
fade over a period of months (McEwen et al., 1998a; Turtle et al.,
2001; Geissler et al., 2004). Dark materials tend to brighten as they
are coated by diffuse deposits (Nash et al., 1986). Bright materials
may darken over time as a result of interaction with superposed
materials (Williams et al., 2004). Some dark patera floor materials may change from red to green hues as a result of reactions
with warm silicate materials (i.e., Pillan Patera; Phillips, 2000;
Keszthelyi et al., 2001). Each of these phenomena has been witnessed in different regions of Io (see Kargel et al., 1999); we note
that the same processes occur in the Zal region. As color is a major distinguishing characteristic in defining material units, we used
these kinds of surface changes to determine relative ages.
362
M.K. Bunte et al. / Icarus 197 (2008) 354–367
5.1. Hotspots
The number and location(s) of hotspots in the Zal region is unclear (McEwen et al., 1998a; Lopes-Gautier et al., 1999). Location
measurements place the hotspot(s) along the western bounding
scarp of Zal Patera, some as far south along the scarp as Rustam
Patera west of south Zal Mons. We consider Rustam Patera (see
Fig. 3) to be the most likely location for a hotspot, as it also appears to be the source of several flows and diffuse deposits. It is
possible that there is more than one hotspot along the scarp, especially as the scarp is the location of a fissure vent; there may also
be a hotspot within Zal Patera, though we consider this to be less
probable.
5.2. Mountain materials
North Zal Mons has previously been identified as layered plains
material and has been distinguished from other plains materials
only by tectonic uplift (Moore et al., 2001). We were able to map
several well-defined grooves, lobes, terraces, and debris aprons
that support the interpretation of Moore et al. (2001) that north
Zal Mons has been uplifted and tectonically modified then sloped,
scalloped, and leveled by SO2 sapping. The generally mottled appearance and the presence of debris aprons associated with the
southern portion of north Zal Mons suggests that this part of the
plateau has been severely degraded.
We were also able to map several circular depressions on each
of the main mountain structures; we mapped three small depressions on south Zal Mons and six prominent depressions along with
a major graben-like structure on north Zal Mons. The largest of
these depressions is no more than 10 km in diameter; the smallest
is on the order of 1–2 km in diameter. These depressions generally
occur at the bases of steep scarps or along deep fissures or grooves
(Geissler, 2003). Depressions in north Zal Mons are thought to be a
result of sapping, evidence of the mobilization of volatiles such as
sulfur or sulfur dioxide in the subsurface (Moore et al., 2001). We
agree with the interpretation that these depressions are not likely
of impact origin but are the result of sapping or weathering at the
ends of grooves (Moore et al., 2001).
Moore et al. (2001) also discussed how edges of mountain units
in the Zal region have different features that result from degrada-
tion; these features include sapping pits and depressions, scalloped
edges, and debris aprons. South Zal Mons does not exhibit many
of these features; south Zal Mons does have three sapping pits or
depressions but its prominent surface texture is a series of ridges
and grooves which has been defined as a lineated or sulcus texture
(Fig. 7). Several of the small unnamed mountain structures in the
region have a slightly lineated or grooved surface texture; another
of the small unnamed mountain structures exhibits a non-lineated
texture but is grooved. North Zal Mons exhibits a series of elongated grooves and sapping pits or depressions. One group of the
pits on north Zal Mons occurs as a series of 5 aligned pits. Portions
of north Zal Mons have the appearance of a large debris apron or
lobes of material that may have been mass-wasted. This lobe texture has been defined as a mottled texture (Fig. 7). We believe that
north and south Zal Mons may be characteristic of the progression
of the degradation effects that Moore et al. (2001) describe. As
lineated mountain material, south Zal Mons is the least degraded
of the mountain units. Degradation causes the destruction of the
lineated texture; perhaps this destruction is aided by mantling deposits infilling the grooves. Further degradation by mass wasting
or by SO2 sapping creates the depressions and the debris aprons
or lobes (i.e., Moore et al., 2001). In this scenario, north Zal Mons
is more degraded than south Zal Mons; the mottled lobes of the
southeast portion of north Zal Mons and the eastern portion of the
small mountain structure east of Zal Patera are the most degraded
of all mountain structures.
We speculate that the level of degradation of each of the mountain and plateau units is influenced greatly by the proximity to
active vents or warm materials. Moore et al. (2001) suggested
that the scalloped scarps of north Zal Mons may result from extended contact with warm silicate materials which can degrade
the mountain structure. The southern portion of north Zal Mons
and the eastern and southern boundaries of south Zal Mons are
not in contact with any warm materials; this lack of contact may
explain why the scarp boundaries in these locations are not scalloped. North Zal Mons is also mantled by diffuse materials whereas
south Zal Mons is largely unmantled. Perhaps diffuse deposits aid
in the mountain material degradation process.
Fig. 7. Enlarged views of Zal Montes. (Left) Enlarged view of south Zal Mons. Excerpt from Galileo SSI mosaic I27ZALTRM01 obtained during orbit I27 flyby in February 2000.
Arrow indicates the lineated texture that is characteristic of minimal degradation of the original tectonic structure. (Right) Enlarged view of north Zal Mons. Excerpt from
Galileo SSI mosaic I27ZALTRM01 obtained during orbit I27 flyby in February 2000. Arrows indicate the scalloped scarps (top arrow) that likely result from erosion by warm
silicate materials and the mottled texture (bottom arrow) that is characteristic of severe degradation. Scale bars are 100 m.
Geologic mapping of the Zal region of Io
363
5.3. Tectonism
Fig. 8. One possible original configuration of the major mountain structures in the
Zal region. Scale bar is 100 m.
Mountain units resemble tilted blocks bounded by steep scarps.
Schenk and Bulmer (1998) proposed that such mountains formed
by thrust faulting, which is driven by a global crustal compression
resulting from the high resurfacing rate of Io. Mountain formation,
in general, is thought to have a tectonic influence on the location
and morphology of some paterae (Radebaugh et al., 2001). (For
further review of models of mountain formation, please also see
McKinnon et al., 2001; Schenk et al., 2001; Turtle et al., 2001;
Jaeger et al., 2003.) The formation of the patera may be related
to the location of the mountain units in that the crust at the location of the patera is more susceptible to foundering due to the
weakness near the scarps. We propose that strike-slip faulting and
subsequent rifting separated the mountain units, opening a fissure
which serves as a vent for lava flow and creating a depression
which became Zal Patera and was partially infilled by patera floor
and flow materials. The depression itself may have resulted from
collapse as magmas were excavated or by further extension during
the rifting event. We have interpreted one possible configuration of
the main mountain units before such a rifting event (Fig. 8). Fig. 9
shows one possible progression of events to form Zal Patera as a
result of a faulting and rifting event. The mountain scarps serve as
boundaries for Zal Patera. The formation of Zal Patera occurred af-
Fig. 9. Possible sequence of events to create the current configuration of mountain structures and shape of Zal Patera. (a) Faulting or rifting event (orange lines) separates
the mountain units and possibly opens a fissure (oblong red feature). Lavas begin to erupt from the fissure. (b) Strike-slip faulting (arrows) causes north–south separation
of the mountain structures. Some east–west motion may occur during the rifting process. The rifting (arrows) may also create a depression into which the lavas begin to
flow; the depression is expanded both eastward and southward. (c) Further rifting allows for the expansion of the depression; thus, the oldest flow materials are pulled
eastward. (d) Further rifting expands Zal Patera to its current dimensions. The oldest flow materials are now located on the eastern side of the patera; young flows erupt on
the western side along the fissure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
364
M.K. Bunte et al. / Icarus 197 (2008) 354–367
Fig. 10. Enlarged view of Zal Patera. (Left) Excerpt from Galileo SSI image PIA 02527 obtained during orbit C21 in June 1999. Resolution is 1.4 km/pixel. North is toward upper
left. (Right) Excerpt from geologic map of Zal region (Fig. 6). The three episodes of deposition of patera floor materials are mapped as the yellow, orange, and light blue units.
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ter the uplift of the mountain units and before the emplacement
of the patera floor and flow materials. The flow materials have
been emplaced since the formation and partial infill of Zal Patera
by the patera floor materials.
5.4. Patera floor materials
Bright patera floor materials may be sulfur-dominated whereas
dark patera floor materials are silicate-dominated, though the explanation that the bright floor materials have simply been altered
from an original appearance similar to the dark floor materials to
their present color by mantling of sulfurous materials from plume
deposits is more likely at Zal (Nash et al., 1986). The materials
that cover portions of the floor of the patera appear to have emanated from the western scarp of the patera in a similar manner to
the fresh flow materials; however, the patera itself appears to have
widened as the rifting event which formed it continued. This extension moved the patera floor materials to their current location
away from the fissure vent (Figs. 9 and 10). The apparent linear
nature of the patera margins strongly supports structural controls
on magma ascent and emplacement (Jaeger et al., 2003).
5.5. Lava flow materials
Many of the dark flow units in the Zal region appear to emanate
from the western scarp of Zal Patera. The dark flow materials appear to flow preferentially along the line of the scarp that bounds
the western edge of Zal Patera and south Zal Mons. From this pattern and the observation that the freshest, darkest flows are found
only at the northern end of the scarp and at the hotspot, it can
be inferred that the scarp is the location of a fissure vent and that
the lavas are erupting along the fissure, as originally proposed by
Turtle et al. (2001). We conjecture that some portions of the flows
which are interpreted by albedo to be of the same flow unit but
are separated from the vent or the massive portion of the flow may
exhibit differences in viscosity between flows or indicate the presence of lava conduits that are not detectable at the resolution of
this image. These are tube-fed compound flows of the Promethean
eruption style (Keszthelyi et al., 2001) although the scarp vent may
exhibit lava fountaining in addition to the Promethean flow style.
The bright flow unit appears to originate from the hotspot vent
at Rustam Patera (see Fig. 6) which was detected by Galileo. Bright
flows have been interpreted as possible sulfur-rich flows (Williams
et al., 2001, 2002, 2004, 2005, 2007). If this is a sulfur-rich flow,
the magma chamber that supplies the vent might have undergone
change over time in order to supply both sulfur and silicate lavas;
the bright flow appears to superpose the undivided flow material
that appears to emanate from the same location. Alternatively, the
sulfur flow may represent secondary sulfur volcanism (i.e., heat
from silicate magma melts surrounding sulfur-rich rock; Greeley
and Fink, 1984). This flow may also be an older altered silicate
flow, but we deem this possibility unlikely due to the lower albedos of the undivided flow material.
5.6. Diffuse deposits
The digitate projections of the dark diffuse deposit that extend onto north Zal Mons suggest that the material was emplaced
explosively after the formation or uplift of north Zal Mons (see
Geissler et al., 2004; Fig. 4). Our mapping efforts support the original suggestion by Turtle et al. (2001) that the unit is a pyroclastic
material (see Figs. 1 and 3). The vent source of the material appears to be a shield volcano (see Figs. 2 and 3).
Some white diffuse material that superposes north Zal Mons is
a ring from an eruption at At’am Patera (41◦ N, 84◦ W) west of
north Zal Mons (Moore et al., 2001; Geissler et al., 2004). Moore
et al. (2001) also suggested that the material may be the result of
SO2 seepage from the base of north Zal Mons. Due to detection
of a small plume by LORRI aboard New Horizons, we propose that
the majority of diffuse deposits in the Zal region were supplied by
vertical plume activity rather than jets (associated with flow-front
plumes: Kieffer et al., 2000). We suggest that this plume may be
the source of the white diffuse materials that mantle north Zal
Mons.
5.7. Structural features and landforms
We have mapped a wide variety of structural features in the Zal
region, including several fault scarps, mountain ridges, grooves and
furrows in mountain units, lineaments, and sapping pits as well as
the patera depression itself. We also mapped five individual mountain structures; each one has a differently textured surface. Scarps
delineate both mountains and plains. Most grooves, lineaments,
and ridges are found in the mountain units. The plains contain
several scarps and one possible graben indicative of tectonic activity, possibly in the form of strike-slip faulting and rifting. None
Geologic mapping of the Zal region of Io
of these features are found within the flow fields, indicating that
the flows are younger than the tectonic activity. No positive relief
volcanic constructs such as domes, cones, or shields are resolvable
within the mapped region; however, one possible shield lies just
west of north Zal Mons.
5.8. Comparison with other regions
Our mapping efforts have aided in supporting previous ideas
about the materials in this region, including the degradation of
mountain structures (Moore et al., 2001) and possible tectonic
movement (Turtle et al., 2001). We were able to determine a geologic history for the region that can be compared to that of other
regions and that may be similar to global activity. We were able
to map small-scale flow lobes and to determine a stratigraphic
relationship between individual flow units from a single vent. Additional mapping results include the association of a source vent
for every flow unit in the region with the exception of the older
flows which make up the plains. Previous regional maps (Williams
et al., 2002, 2004, 2005, 2007) identified albedo differences between patera floor materials and associated only the major flows
with their respective sources. The highest resolution (186 m/pixel)
of the previous regional maps (Chaac-Camaxtli; Williams et al.,
2002) discussed the migration of vent positions as the process responsible for the changes in the paterae in the region. We have
speculated that Rustam Patera may have erupted two different
composition materials; a similar vent migration may explain the
presence of both silicate and sulfuric lavas near the patera.
6. Geologic history
We interpret that the plains materials were emplaced as multiple flows after the mountain and plateau units were uplifted; the
plains embay the mountain units. The mountain units were subsequently degraded by SO2 sapping, mass wasting, and contact with
diffuse deposits or warm silicates to form the lineated, undivided,
and mottled units; the degradation has taken place over the entire
formation history of the region.
The shield volcano west of north Zal Mons was likely formed after the emplacement of plains materials; its deposits overly plains
and mountain materials. The undivided flow materials were likely
emplaced after the uplift of the mountains units and are remainders of flows that were superposed by the bright flow units. The
undivided flow materials may represent a bright flow that is older
than the mapped bright flow; the radial pattern of exposed undivided flow suggests that its source is Rustam Patera.
The three paterae in the region formed after the uplift of the
mountain units; this claim is supported in that the bounding
scarps of Zal and Rustam Patera are aligned with the scarps of
the mountain units. Based on the albedo and color differences between the patera floor materials within Zal Patera, we infer that
there were at least three episodes of deposition of patera floor materials (three units of patera floor materials; Fig. 10). Perhaps early
in the formation history of Zal Patera, bright flows emanated from
the vent at Rustam Patera. Later, dark flows began to erupt from
the fissure vent at the western scarp of Zal Patera. Some of these
materials flowed into the patera floor, superposing the earlier emplaced patera floor materials. Lower albedo and darker color of the
flow units suggests that the flow materials are younger than the
patera floor materials. Assuming that both materials are compositionally similar, variations in albedos suggest that the bright patera
floor materials are older than all of the flow materials, but that the
youngest dark patera floor material may be younger than at least
one of the flow units (see Fig. 10 for an enlargement of the patera
floor).
365
Materials within Rustam Patera have an albedo lower than that
of all patera floor materials in Zal Patera and similar to that of the
youngest dark flows which are emanating from the fissure vent at
the western scarp of Zal Patera; this albedo comparison suggests
that Rustam Patera has likely been active more recently than Zal
Patera. We also suggest that the vent at Rustam Patera has been
active for a shorter period than At’am Patera or Zal Patera; this
is indicated by the less degraded lineated mountain material of
south Zal Mons and the more degraded mottled material at north
Zal Mons.
Bright diffuse deposits appear to form only a thin mantle over
the underlying flow materials; they were likely emplaced after one
or more of the flows or patera floor materials. The light mantling
may indicate that the diffuse materials are being emplaced by a
plume that has only been active since the emplacement of the
uppermost flow layers. The red and white diffuse materials cover
great expanses and appear to form a thicker mantle than the bright
diffuse deposits. These deposits may be the result of episodic
plume activity at plume sources that have been active since the
emplacement of the plains materials.
A trend in the progression of volcanic activity has become apparent due to the mapping results. As time passed, flows appear
to have progressed from high to low volume and from low to high
viscosity. We relate volume of material directly to the areal extent
of each unit; large areal extent infers high volume whereas small
areal extent infers low volume. Early in the history of formation
of the Zal region, flows were high volume and low viscosity (very
effusive or mobile). These high volume flows were degraded and
form the plains units. The bright and undivided flows appear to
have also been high volume and low viscosity, though they are perhaps lower volume and higher viscosity that the plains materials
were at the time of their emplacement. The patera floor materials
appear to be lower volume and perhaps higher viscosity than the
bright flows or plains materials. One factor that may contribute
to the apparent relative volume may be that these materials are
confined by the topography of the patera. Dark flow materials appear to be the lowest volume and highest viscosity of all materials.
There is a noticeable trend in volume decrease among the dark
flow subunits; the youngest subunits appear to be the lowest volume flows. The volume decrease between patera floor and flow
materials may be directly relating to the tectonic activity of the
region. As the faulting and rifting process began, high volumes of
material were erupted from the fissure vent that was created. As
the rifting process continued, lower volumes of lavas were erupted
from the fissure.
We have also speculated that the dark diffuse deposits that
mantle north Zal Mons were erupted from the same vent, possibly
a shield volcano, as the diffuse white materials that are deposited
in a circular ring mantling both north Zal Mons and the dark diffuse deposits. It is possible to infer from this superposition that
this vent has undergone a change in order to erupt progressively
more explosive materials.
7. Summary and conclusions
We have produced a geologic map of the Zal region of Io using Galileo images to better understand the volcanic activity of this
region. The primary types of material units (plains, flows, patera
floors, mountains, and diffuse deposits) first identified from Voyager images continue to serve as mappable units. Each of these
units occurs in the Zal region which is dominated by mountains,
a plateau, and two paterae, the larger of which contains a compound flow field. A third patera in the region is not included in
the map.
The Zal region exhibits various forms of volcanic activity, including Promethean style flows emanating from Rustam Patera and
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M.K. Bunte et al. / Icarus 197 (2008) 354–367
a fissure vent at the western bounding scarp of Zal Patera. Both silicate and sulfur-rich lava flows are likely present; individual flows
appear to be of different ages, i.e., in different stages of alteration due to their differing albedos and colors. Zal Patera includes
an active compound flow field rather than a flooding lava lake.
The detection of a plume by New Horizons (Spencer et al., 2007a,
2007b) confirms that the Zal region is currently active, producing
at least one small-scale Pele-type plume. The region also shows evidence of tectonic activity; the mountain units appear to be related
to one another although they each exhibit different degradation
features indicative of SO2 sapping and mass wasting. Morphologies
and cross-cutting relationships of several of the flows in the region
suggest that north Zal Mons was uplifted and modified.
Future work may include observations to determine if the
hotspot(s) at Zal, At’am, and Rustam Paterae are sporadic or periodic; this would require long term observations of Io from an
Io- or Jupiter-orbiting spacecraft. Additional research is also necessary to determine if paterae growth causes mountain separation or if tectonics plays a role in the building, movement, or
erosion of mountain units. We plan to observe and map paterae
with associated mountain units to determine if there is an erosional or tectonic influence of one structure upon the other. Mountain/Paterae complexes that we plan to map in the near future
include the Hi’iaka and Shamshu Complexes. Production, analysis,
and comparison of this regional map with maps of other regions
based on both Galileo and Voyager imaging may offer additional
details on geologic processes occurring on Io, a natural complement to the global mapping effort currently underway (Williams
et al., 2008). The global map will identify the number, distribution, and type of eruptive centers across Io’s surface. Regional maps
provide greater detail on the spatial and temporal nature of geologic processes on Io. Detailed maps of other regions may also help
to define emplacement and tectonic processes that may be active
globally.
Acknowledgments
This work was supported by NASA through the Outer Planets
Research Program and the Planetary Geology and Geophysics Program. We wish to thank reviewers David A. Crown and Gerald
W. Patterson, as well as Jani Radebaugh, for their helpful comments and suggestions.
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