Microphysics of Europa’s surface using Galileo/NIMS

Europa’s surface shows evidence of active resurfacing, which can be explained by endogenic processes. Apart from water ice, several chemical compounds have been proposed to account for the potential complex chemistry that could take place if a connection with the subsurface salty ocean were to occur. Previous spectroscopic studies that investigated the surface composition were limited by the number of compounds to consider due to the unavailability of laboratory measurements.

In a previous study (Cruz-Mermy et al., 2023), we tested for the first time the relevance of all possible combinations of 15 potential endmembers on a Galileo/NIMS observation of a dark lineament of the trailing anti-jovian hemisphere using realistic radiative transfer modeling and a robust Bayesian inference framework. This previous article was focused on the composition.

We report here on a follow-up exploratory study based on the same NIMS spectrum to determine robust microphysical characteristics of the surface. In the previous study, we considered an optimistic scenario corresponding to a 2% uncertainty in reflectance levels, equivalent to an SNR of 50. Based on this scenario, we showed that out of approximately 5,000 model combinations, only 174 provided a satisfactory fit to the selected NIMS spectrum. In this study, we focus on these best-fitting models and perform a statistical analysis on the estimated posterior distributions of the volume abundances and grain sizes for each of the 15 endmembers using all model combinations in which they are found. We show that despite the high number of potential good fits, a strong constraint on the volume abundance and grain size of the two mandatory compounds (sulfuric acid octahydrate – SAO – and water ice) can be drawn. We also show that these properties vary very little from one mixture to another, meaning we can successfully derive maps of these microphysical properties using a whole NIMS hyperspectral cube. Results on the hydrated sulfates and chlorinates show weaker but still sufficiently constrained volume abundances and grain sizes. Using a proxy such as the numerical abundance that links the volume abundance to the grain size to quantify the spectral contribution of the compounds allows a good representation of the surface for these compounds. However, it is still impossible to discriminate between these compounds by analyzing the microphysical properties obtained. The constraint on the estimated parameters would still allow us to derive the spatial distribution of these microphysical properties if the actual composition of the surface can be determined by other methods. Finally, we demonstrate that no constraint can be made about surface roughness with spectral analysis only.