Amorphous and Crystalline Ice on the Galilean Satellites: A Balance Between Thermal and Radiolytic Processes

Details Amorphous and Crystalline Ice on the Galilean Satellites: A Balance Between Thermal and Radiolytic Processes Amorphous and Crystalline Ice on the Galilean Satellites: A Balance Between Thermal and Radiolytic Processes

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....8069h&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #8069

Computer Science

Scientific paper

Water ice is abundant on the three outermost Galilean Satellites of Jupiter, Europa, Ganymede, and Callisto. We use near-infrared spectra returned by the Near-Infrared Mapping Spectrometer on the Galileo spacecraft to study the crystalline order of the surface ice on all three satellites. The lattice order of ice is known to be dependent on its condensation temperature and rate, its temperature history, and its radiation environment. The surface temperature of these satellites is in a range where ice will crystallize over very short time. New amorphous ice can be created from vapor deposition, but a more likely source is the disruption of crystalline ice by particle radiation, which is strong and widespread throughout the Galilean satellites. We envision a balance between thermal crystallization and radiolytic disruption, where low temperatures and high radiation fluxes would favor amorphous ice on Europa, and lower radiolytic fluxes and higher temperatures would favor crystalline ice on Callisto. We use two features in the spectrum of water ice to probe lattice order, the temperature-sensitive band at 1.65 μm, which disappears altogether in warm ice and in cold amorphous ice, and the fundamental O-H-stretch near 3.1 μm (appearing as a reflection peak), which has a stronger and more detailed spectrum for crystalline ice. The 1.65-μm feature arises from ˜1 mm below the surface, while the 3.1-μm Fresnel reflection peak comes effectively from zero depth. Our analysis is limited to spatial averages on Europa and Callisto, because of radiation induced noise, and low water ice abundance, respectively. For Ganymede, we have completed significant pixel-level mapping of the shape of the 3.1-micron peak across its surface. The results show that the ice on Europa appears to be uniformly amorphous at the surface and fully crystalline at ˜1mm depth. The ice on Callisto is uniformly crystalline at the surface and mostly crystalline at depth. Imperfect fits in the 1.65-micron region imply either some amorphous ice, or perhaps warmer ice at ˜1mm depth on Callisto. The mapping of Ganymede has revealed large variations in the shape of the 3.1-μm feature, resulting in complex spatial patterns of amorphous, crystalline, and mixed ices at the surface. It is possible to find areas of mostly crystalline ice at both equatorial and polar latitudes, and the same for amorphous ice, but the overall distribution of amorphous ice is greater in the polar latitudes and on the trailing side, implying some control by the expected particle radiation fluxes.

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