Jovian System Data Analysis Program

Cover Page/Proposal Summary

ROSS-99    NRA 99-OSS-01

Date Due: 5/5/99

We will use radiative transfer models to determine the properties of water ice on Europa and map its distribution, using Galileo Europa Mission (GEM) NIMS observations as well as selected observations of the trailing hemisphere from the Galileo prime mission. The GEM observations provide us with spatial coverage and lighting geometries unavailable in earlier observations. We will derive ice particle sizes, mixing ratios in checkerboard and intimate grain mixtures with non-ice components, layer structures (particularly thin frost layers), and surface and subsurface temperatures (based on the variation of ice optical properties with temperature). We also should be able to identify the distribution and abundance of amorphous versus crystalline ice. Our approach involves detailed processing and analysis of public-domain NIMS data, and also uses PPR (for temperatures) and SSI (for sub- pixel component fractions) observations. We have developed many of our own methods to produce accurate radiometric calibrations of the NIMS measurements. We will use radiative transfer models to analyze the calibrated spectra. The separation of non-ice materials is key, and involves the characterization of their optical properties. Here we will be aided by similar studies on Europa's companion icy satellites, Callisto (where the ice and other materials appear spatially separated and easy to model) and Ganymede (where the variety of apparent non-ice materials invites comparison). The properties which can be determined have bearing on many of the important processes which occur on Europa, including segregation of icy cold-traps and non-icy lag deposits, radiation damage of the ice; mobility and transport of water molecules, as seen in grain coarsening with age and frost formation (perhaps amorphous); and issues of surface radiative balance and thermal inertia as evidenced by temperature and temperature gradients.