Constraints on the surface composition of Jupiter's moon Europa based on laboratory and spacecraft data.

摘要

Analysis of reflectance spectra of the Jovian satellite Europa returned by the Galileo Near-Infrared Mapping Spectrometer (NIMS) reveals the presence of distorted water absorption bands in the 1.0 to 3.0 mum spectral range. While the 1.04, 1.25, 1.5, and 2.0 mum absorption bands have been attributed to water by several researchers, the spectral behavior departs noticeably from that measured for ordinary water ice in the laboratory, especially for the 1.5 and 2.0 mum absorptions. A number of mechanisms have been proposed to explain this behavior, including gain size effects, the presence of water in a bound state such as water of hydration, scattering due to bubbles or pits in the surface ice, and additional surface materials. Suggested surface constituents include salts of varying hydration states, other hydrated minerals such as clays and zeolites, simple organics, and acids. This work evaluates the influence of scattering effects on the water ice absorption bands through laboratory measurements and numerical simulations, placing upper limits on many materials. Other effects which may be of relevance include radiation damage, thermal and mechanical stresses, macroscopic surface roughness, hydrogen bonding, temperature dependencies, and intimate mixing of surface grains.