Selection of chemical species for Europa’s surface using Galileo/NIMS

1G. Cruz Mermy,1,2F. Schmidt,3T. Cornet4, I. Belgacem,4N. Altobelli
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115379]
1Université Paris-Saclay, CNRS, GEOPS, 91405, Orsay, France
2Institut Universitaire de France (IUF), France
3Aurora Technology BV for ESA, Netherlands
4European Space Agency (ESA), European Space Astronomy Centre (ESAC), Spain
Copyright Elsevier

Europa’s surface shows evidence of active resurfacing which can be explained by either endogenic or exogenic processes. Apart from water ice, several compounds have been proposed to account for the potential complex chemistry that could take place if a connection with the subsurface salty ocean would 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. We now have access to optical constants and laboratory spectra of synthetic chemical compounds such as hydrated sulfates and chlorinated salts under Europa’s surface conditions.

In this study, we test for the first time the relevance of 15 potential endmembers on a Galileo/NIMS observation of a dark lineament of the Trailing Anti-Jovian hemisphere using a realistic radiative transfer modelling and a robust Bayesian inference framework. We consider an intimate mixture of 3, 4 and 5 endmembers and perform a fitting procedure on all possible combinations among the list of 15 compounds in the 1.0 – 2.4

m spectral range. At the end, 5000 combinations have been tested and analyzed. The parameters we are fitting for are the surface roughness, the volumetric abundances and the grain size. Given the fact that NIMS noise level and absolute calibration are uncertain, we have distinguished two scenarios: assuming a pessimistic signal-to-noise ratio (SNR) of 5 and an optimistic SNR of 50. For each scenario, we use several criteria to highlight the contribution of each endmember and thus identify the most relevant compounds.

We first show that no combinations using only 3 endmembers are able to accurately reproduce the observation. Then, when considering 4 and 5 endmembers together and assuming the SNR=50, there is 21 and 153 combinations that evenly reproduce the observation. From this result, we discuss the relevance of each endmember using criteria such as the Root-Mean-Square (RMS) deviation of the best-fits, the spectral contribution at each wavelength, the occurrences distribution of the selected best-fits and the numerical abundances retrieved by the fitting procedure. By grouping criteria together, we show that only sulfuric acid octahydrate and water ice appear as essential compounds. Then, hydrated sulfates are in general preferred over others compounds. This result is only valid for a lineament of the Trailing Anti-Jovian hemisphere and can be possibly extended to other lineaments in the same hemisphere. However, we show that chlorinated salts but also ammonium sulfate mascagnite ((NH4)2SO4), sodium chloride (NaCl) and even magnetite (Fe3O4) are not excluded, their contributions from the selected criteria are just less pronounced than hydrated sulfates. Nevertheless, it is clear that it is not possible to correctly distinguish between these endmembers, and even less between endmembers with similar chemical composition, at these wavelengths. We therefore propose for future investigation to either consider a mixture that would include only one representative of each chemical compound, rather than using all possible endmembers and overfitting the data.

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