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Europan macula - Possible origins

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Europan macula - Possible origins

Abstract

Voyager images of the surface of Europa revealed many diverse features, including bright and dark linea, triple bands, gray bands, ridges, pits, and dark spots termed maculae. Seen at Voyager resolution, maculae are circular to irregularly shaped, low albedo patches with little additional morphologic character. Maculae occur in both bright and mottled terrains, commonly in association with dark linea. These intriguing dark patches are high priority for the Galileo Solid State Imaging experiment.
EUROPAN MACULA: POSSIBLE ORIGINS. J.M. Moore
1
, K.C. Bender
2
, R.J. Sullivan
2
, R. Greeley
2
, A.S. McEwen
3
, B.R.
Tufts
3
, J.W. Head III
4
, R.T. Pappalardo
4
, M.J.S. Belton
5
, and the Galileo SSI Team (
1
NASA Ames, MS 245-3, Moffett Field,
CA 94035;
2
Geology Dept., Arizona State University, Tempe, AZ 85287;
3
LPL, University of Arizona, Tucson, AZ 85721;
4
Geological Sciences Dept, Brown University, Providence, RI 02912;
5
NOAO, Box 26732, Tucson, AZ 85717)
Voyager images of the surface of Europa revealed many
diverse features, including bright and dark linea, triple bands,
gray bands, ridges, pits, and dark spots termed maculae
(Smith et al., 1979; Lucchitta and Soderblom, 1982). Seen at
Voyager resolution, maculae are circular to irregularly
shaped low albedo patches with little additional morphologic
character. Maculae occur in both bright and mottled terrains,
commonly in association with dark linea. These intriguing
dark patches are high priority for the Galileo Solid State
Imaging (SSI) experiment.
On Dec. 19, 1996 the Galileo spacecraft made its first close
encounter with Europa. During this encounter three images
were taken of a macula located at 334°W, 16°S. This feature
was identified as an interesting target on low resolution (6.9
km/pixel) images taken earlier in the mission as a 100 km
diameter low albedo spot . The low altitude of the December
encounter permitted high resolution (120 m/pixel) imaging
of much of the feature. Morphologically, the macula can be
divided into three zones. The inner zone is approximately 50
km in diameter and is characterized by a rugged surface. The
middle zone encircles the central rough zone and is
characterized by ridges, smooth areas, and several small
depressions. The outer zone is dominated by two large,
concentric, continuous graben, additional smaller graben,
numerous fractures, and a generally smooth surface. Graben
widths (~800 m) were used to estimate the depth of the
mechanical discontinuity at ~500 m, and indicate a fractured
uppermost surface layer in contrast to previous assumptions
(Golombek and Banert, 1990). Just outside the macula, to
the southwest, are two large depressions (each ~10 km
across). Two prominent ridges intersect the macula, one
from the WNW, the other from the ESE. Both ridges are
modified in the portions that cross the different zones of the
macula, indicating they either predate the formation of the
macula or that their own formation was affected by the
presence of the macula. Additionally, there are numerous
small depressions, often with raised rims, found in the area
surrounding the macula, some of which form pit chains
oriented radially to the macula center.
These images provide the first look at the detailed
morphology of a specific macula, enabling interpretations of
this feature's mode of formation. For example, the radial pit
chains could represent secondary craters, suggesting an
impact (exogenic) origin. On the other hand, the pit chains
could be collapse pits, and a thermal or tectonic (endogenic)
origin might be more likely. Detailed examination of macula
morphology and relations to the surrounding terrain, and
comparison to similar features seen elsewhere in the solar
system enables testing of possible modes of formation for
this type of feature on Europa. Preliminary comparisons can
be made with palimpsests on Ganymede (exogenic) and
small coronae on Venus (endogenic).
Exogenic Hypothesis: The Palimpsest Analog
Palimpsests were identified on Ganymede and Callisto (e.g.,
Passey and Shoemaker, 1982), and possibly Europa
(Lucchitta and Soderblom, 1982). Initial Voyager-based
work characterized these features on Ganymede and Callisto
as roughly circular, high albedo patches or spots with little or
no relief. Their centers are usually smooth but may become
texturally rough around the peripheries. Palimpsests are
generally thought to represent impact "scars" whose
morphology either: a) developed by viscous relaxation of the
original impact crater with initially "classic" morphology
(e.g., Passey and Shoemaker, 1982); or b) promptly formed
as its presently appears due to unusual (non-brittle) target
properties with little change since the impact event (e.g.,
Greeley et al., 1982). Alternatively, they could be of
endogenic origin (e.g., Squyres, 1980), but the predominant
interpretation of these features remains exogenic.
Work using Voyager images of Ganymede Greeley et al.
(1982) and Schenk (1996) favor the present appearence of
palimpsests as representing the original morphology; in
which a liquid slurry, or slurry with solid chunks, was
ejected at the time of impact, forming a smooth marginal
plateau unit and inner fill while producing either no or else a
highly modified and subtle crater rim. Detailed mapping by
Schenk (1996) on Ganymede of a number of complex and
pedestal craters as well as the penepalimpsest (a palimpsest
formed in a slightly more brittle target) Nidaba (centered at
19°N, 124°W) that exhibits secondary craters, a marginal
smooth facies, and an identifiable rim was used to scale the
crater rim diameters of other palimpsests based on the
diameter of the feature as a whole.
If the edge of the 50 km diameter inner rugged zone of the
macula on Europa corresponds to a crater rim, then the
scaling model of Schenk (1996) would predict the marginal
smooth unit to extend outward some 25 to 30 km. The zone
of the macula in which patches of smooth terrain occur is
coincident with this marginal unit. Strings of pits radial to
the macula that could be secondaries are seen beyond the
zone containing smooth terrain, just where Schenk's scaling
relationship would predict secondaries to be found. The
concentric graben preseumably formed subsequent to the
imediate effects of the putative impact as they cut the smooth
terrain. The graben may have formed by the slow inward
flow of warm ice at depth as an initial central mass deficit
was filled (see McKinnon and Melosh, 1980). The
morphology and distribution of facies composing the macula
appear consistent with a palimpsest-style impact origin. If
so, this macula and ganymedan palimpsests may represent
interpretable and scalable examples of primary landform
suites formed by impact into liquid slurry-rich targets on
airless icy satellites.
Lunar and Planetary Science XXVIII
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EUROPAN MACULA: POSSIBLE ORIGINS J.M. Moore and the Galileo SSI Team
Endogenic Hypothesis: The Venus Corona Analog
Coronae are a class of features on Venus typified by a
concentric annulus of tectonic features (Pronin and Stofan).
Generally circular in planform, coronae on Venus range in
size from 60 to 2600 km diameter. The tectonic annulus may
be comprised of extensional features, compressional features
or a mix of the two. The annulus width varies greatly, but
generally widens with increased overall corona size. The
interiors of most coronae on Venus are typically smooth
plains (volcanic deposits); which represent either the pre-
existing surface or new flows associated with corona
formation. Volcanic flows originating at corona tectonic
features, such as annular fractures, are associated with many
corona. The topographic expression of coronae ranges from
domes to plateaus to plateaus with interior lows to rimmed
depressions. Most workers agree that coronae on Venus are
the surface manifestations of mantle plumes/diapirs (Stofan
et al, 1992; Squyres et al, 1992; Janes et al., 1992).
Broadly similar in size and tectonic structure to small
venusian coronae, the macula on Europa differs mainly in the
rugged relief of its interior. If we hypothesize that the
feature represents the surface expression of diapiric activity
(similar to Venus), then the nature of the material (ice) and
its behavior under various thermal conditions controls the
resultant interior morphology. In this scenario a plume of
ductile ice rises toward the surface, perhaps originating over
a silicate hot spot. This mechanically lifts the surface and
thermally alters the surface ice grain size. Grain size
variations can account for up to a 15% albedo variation
(Dozier, 1989). Heating would be greatest above the plume,
which may explain why this feature is dark. In this model,
the annulus of graben was produced by hoop tension forces
exerted about the periphery during the initial phase of
doming. The material forming the smooth patches may have
erupted at this time along the border of the inner zone and
flooded low spots along the flanks of the dome. The rugged
nature of the center may result from compression as heat is
withdrawn and the dome subsides. Pre-existing structures in
the area are likely removed or subdued by thermal and
mechanical effects of the diapiric activity. However, major
features, such as ridges, could be more "resistant" or able to
withstand the thermal and mechanical effects to some degree.
The numerous depressions found around the macula may be
collapse structures related to thermo-karst or areas where
material has been sublimated.
Conclusions
Even with the new high resolution images, both exogenic
and endogenic hypotheses of origin for this macula remain
viable. The morphology of the macula itself, and its relation
to the surrounding terrain can be explained by either impact
or diapiric activity. Multi-spectral data (from NIMS and
later SSI imaging) may provide information that could better
delineate one or the other hypothesis, if the composition can
be positively identified as deriving from either subsurface
materials or cometary/asteroidal material. Comparison with
other maculae on Europa at a variety of resolutions may
show a range of morphologies of this type of feature and
possibly indicate that maculae are formed by different modes
(i.e. are not a single class of feature).
References:
Dozier, J (1989) Remote Sens. Environ., 28, 9-22;
Golombek , M.P. and W.B. Banert (1990) Icarus, 83, 441-
452; Greeley, R., et al. (1982) Satellites of Jupiter, D,
Morrison ed., p.p. 340-378; Janes, D., et al., 1992, JGR, 97,
16055-16067; Lucchitta, B.K. and L.A. Soderblom (1982)
Satellites of Jupiter, D, Morrison ed., p.p. 521-555;
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Lunar and Planetary Science XXVIII
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Article
The Galileo mission has returned the first high-resolution (21 m/pixel) images of the surface of Europa. These images reveal structures with morphologies reminiscent of those seen on terrestrial sea ice. Although it is premature to make one-to-one analogies between sea ice and Europa's surface, a review of the types of surface features commonly formed on Earth and of various sea-ice processes can provide insight into the complex geology of Europa. For example, deformation of terrestrial sea ice results from winds, tides, and currents and from thermally induced stresses; the resulting features include fractures ranging in width from millimeters to kilometers, pressure ridges, shear ridges, and rafted ice. Potential agents of deformation on Europa are more likely to be limited to tidal flexing and possibly convection, but could produce similar features and perhaps account for the ridges and fractures seen in many areas. Subtle differences in albedo and color in terrestrial sea ice result from differences in ice thickness and grain size, attributed to factors such as the rate of ice-crystal growth, water turbulence, age of the ice, and deformation. Similar factors could account for differences observed in the bright icy plains of Europa. Moreover, salts in both the solid form and as brine vary in concentration and composition as a function of space and time on Earth, leading to differences in density and the strength of ice sheets. Salts are also suspected in the europan ice and could lead to similar differences, enhancing the creation of topographic relief from density contrasts and the formation of fractures from brittle failure of the ice. Differences in the environments between Europa and terrestrial sea ice in terms of parameters such as temperature, gravity, time, and ice compositions suggest caution in drawing direct analogies. Future work by the planetary and sea-ice communities must include understanding the terrestrial processes sufficiently for extrapolation to Europa.
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Golombek, M.P. and W.B. Banert (1990) Icarus, 83,
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