Galileo’s Near InfraRed Mapping Spectrometer (NIMS) took several observations of Europa in the 2-5 micro-m wavelength region. Hydrated sulfuric acid, specifically sulfuric-acid-octahydrate or SAO, was detected in significant abundance in the trailing hemisphere of Europa (Carlson et al. 2005). While spectral fitting analysis by Carlson et al. (2005) derived quantitative estimates for the amount of SAO, the uncertainties on the abundances were not provided. Moreover, the model did not include crystalline ice, which is also found on the surface of Europa. Trailing hemisphere spectra also showed evidence of CO2 and SO2 features in the 3-5 micro-m wavelength regime (Hansen & McCord 2008). Apart from tentative detection, compositional analysis via spectral fitting has not been performed for CO2 and SO2. Using cryogenic optical constants of amorphous water ice, crystalline water ice, SAO, CO2 and SO2 that exist in literature, we are performing a thorough compositional analysis of select Galileo/NIMS spectra of trailing hemisphere of the Europa. We employ a novel Bayesian spectral inversion framework (schematic figure attached), wrapped around a bidirectional reflectance model (Hapke 1981, 2012). A Bayesian framework generates millions of models and allows us to quantify detection significances of the species included in the model, statistically constrain parameters like species abundances and average grain-sizes and explore degenerecies of solution. Currently, our analysis framework is being validated on synthetic Galileo/NIMS data that we generated using the reflectance model and noise characteristics of the NIMS instrument.
Carlson, R.W., Anderson, M.S., Mehlman, R., Johnson, R.E., 2005, Icarus, Europa Icy Shell 177, 461-471
Hansen, G.B., McCord, T.B., 2004, Journal of Geophysical Research: Planets 109
Hapke, B., 1981, Journal of Geophysical Research: Solid Earth 86, 3039-3054
Hapke, B., 2012, Cambridge Core