Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p31d..02s&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P31D-02
Mathematics
Logic
[5422] Planetary Sciences: Solid Surface Planets / Ices, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5475] Planetary Sciences: Solid Surface Planets / Tectonics
Scientific paper
Voyager left us with the impression that Europa is a flat world where topography is limited to “a few hundred meters at most”, with the subtle implication that Europa’s floating ice shell is rather thin and incapable of supporting significant relief. This perspective persists even today but is demonstrably false. Topographic mapping is a key element of understanding the geologic evolution and geophysical structure of the ice shell. Although the Galileo orbiter was unable to obtain global stereo mapping of the surface, high-resolution digital elevation models (DEM) of small scattered areas are possible from the limited high resolution mapping acquired during the mission. Supplementing these data are DEMs based on shape-from-shading (or PC) techniques. In many cases, these maps coincide and the PC data can be used to significantly improve the resolution of the stereo DEMs to ~ pixel scale. Here I review the nature of the available topographic data, emphasizing that a true global topographic datum does not exist for Europa (limb profiles provide useful information but are too sparse to provide meaningful baselines). Roughly 20% of Europa is mapped. Most imaging sites were targeted in the blind with regard to topography. Despite this, many of the DEM sites, if not the majority, show relief of 500 meters or more. At least three sites have been identified with relief of 800 to 1000 m. These take the form of broad undulating plains or discrete scarp-bounded plateaus. Several additional plateaus are observed from shadow measurements in unmapped regions. Accounting for the limited mapping coverage, it is likely that between 50 and 10 such areas of high relief are scattered across the surface. Normal faults 350-800 m high as well as several 500-km-long rift graben are also observed. The most distinctive known depression is the 300-m-deep dark reddish spot Castalia Macula. The deepest relief is found not in such isolated depressions but in arcuate troughs 300-500 meters deep and associated closed depressions at least 1 km deep (for a total dynamic range of 2 km). These form two concentric rings and are attributed to polar wander (Schenk et al., 2008). Some of these features have been used to model the thickness of Europa’s lithosphere, but the ductile base of the ice shell makes determination of the total shell thickness obscure. The topographic expression of Europa is proving to be very complex: the fact that these faults, depressions and plateaus exist and persist indicates that the ice shell is thick enough to support significant topography.
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