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A Science Strategy for the Exploration of Europa
TABLE 3.1 Galileo's Imaging Coverage of Europa's Surface
Percentage of Surface Covered
Galileo Europa Mission
1 to 4 km
0.1 to 1 km
20 to 100 m
6 to 20 m
NOTE: Includes actual coverage through encounter E-19 and expected coverage during encounter 1-25.
If all wavelengths are observed simultaneously, then the compositional information is free of such artifacts. Indeed, reflectance spectroscopic determination of surface composition works remarkably well for ices. The widely spaced and relatively deep absorption bands of different ices make unique identification of ices straightforward. Band shapes and depths allow determination of relative abundances with precisions and accuracy approaching 5%. This type of information, obtained on a global scale at a spatial resolution of 1 km, will allow models of surface origins to be tested and will help resolve the spectral contributions from water ice of varying textures and clarity in addition to the contributions from non-ice components.
In addition to conducting low-resolution global imaging and compositional mapping, selected regions should be targeted for coverage at higher spatial resolutions. For example, the Galileo Europa Mission (GEM) identified several regions exhibiting terrains that have been disrupted, presumably by internal activity, and that have a paucity of superposed impact craters, indicative of relative youth. These areas should be imaged at resolutions better than 50 m/pixel (with compositional mapping at 300 m/pixel) to determine if there have been changes on the surface that would be indicative of ongoing activity. Imaging at similar resolution is recommended for sites identified by GEM as being of high interest for exploration by future landers.
Orbital imaging should also be used to search for active eruptions such as geysers. Although GEM has so far failed to reveal current activity, viewing and lighting geometries precluded this type of observation on all but one brief encounter. Current estimates of the height to which geyser plumes would rise suggest that surface features produced by them would be about 15 to 20 km across; thus, images should have resolutions of 1 km or better for their detection.
GLOBAL MAPPING OF TOPOGRAPHY AND GRAVITY
Topography and gravity are basic geophysical data sets that provide information on the internal structure and dynamics of planetary bodies. Flybys of Europa during the nominal Galileo mission and GEM will yield information on the lowest-degree and lowest-order spherical harmonic contributions to the gravity field. The degree-two gravity field has been used to infer the moment of inertia of Europa and its layered internal structure.1,2 Only limited data exist on the very subdued topography of Europa. Its average shape, inferred from Galileo limb profiles, is not very different from that of a sphere and, given the precision of current measurements (>500 m), is indistinguishable from an object in hydrostatic rotational and tidal equilibrium (P. Thomas, Cornell University, private communication, 1999). A more precise determination of the radii of the three principal axes of the ellipsoid defining Europa's shape (accurate to approximately 100 m) would provide a crucial verification of the hydrostatic equilibrium assumed in the interpretation of the second-degree spherical harmonic gravity data. It would also provide an independent measure of CIMR2.
To proceed further in the exploration of Europa's interior after the completion of GEM requires determination of both the gravity field and topography of the satellite over the entire surface. This can be accomplished from an