  The Galileo Near Infrared Mapping Spectrometer (NIMS) collected spectra of
Europa in the 0.7-5.2 $\mu$m wavelength region, which have been critical to
improving our understanding of the surface composition of this moon. However,
most of the work done to get constraints on abundances of species like water
ice, hydrated sulfuric acid, hydrated salts and oxides have used proxy methods,
such as absorption strength of spectral features or fitting a linear mixture of
laboratory generated spectra. Such techniques neglect the effect of parameters
degenerate with the abundances, such as the average grain-size of particles, or
the porosity of the regolith. In this work we revisit three Galileo NIMS
spectra, collected from observations of the trailing hemisphere of Europa, and
use a Bayesian inference framework, with the Hapke reflectance model, to
reassess Europa's surface composition. Our framework has several quantitative
improvements relative to prior analyses: (1) simultaneous inclusion of
amorphous and crystalline water ice, sulfuric-acid-octahydrate (SAO), CO$_2$,
and SO$_2$; (2) physical parameters like regolith porosity and
radiation-induced band-center shift; and (3) tools to quantify confidence in
the presence of each species included in the model, constrain their parameters,
and explore solution degeneracies. We find that SAO strongly dominates the
composition in the spectra considered in this study, while both forms of water
ice are detected at varying confidence levels. We find no evidence of either
CO$_2$ or SO$_2$ in any of the spectra; we further show through a theoretical
analysis that it is highly unlikely that these species are detectable in any
1-2.5 $\mu$m Galileo NIMS data.
