A Comprehensive Revisit of Select Galileo/NIMS Observations of Europa

Ishan Mishra,Paul Helfenstein,R W Carlson,Ryan J Macdonald,Nikole Lewis,Kevin P Hand,Jonathan Lunine

doi:10.3847/psj/ac1acb

Ishan Mishra, Paul Helfenstein + Show 5 more

Open Access

https://doi.org/10.3847/psj/ac1acb

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Abstract

Abstract The Galileo Near Infrared Mapping Spectrometer (NIMS) collected spectra of Europa in the 0.7–5.2 μm 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 has 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), CO2, and SO2; (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 CO2 or SO2 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 μm Galileo NIMS data.

Highlights

Europa’s young surface and geological features like chaos terrains suggest an active exchange of materials between the surface and subsurface ocean (Carr et al 1998; Moore et al 2009; Kattenhorn & Prockter 2014; Trumbo et al 2019a)
The Galileo Near Infrared Mapping Spectrometer (NIMS) collected spectra of Europa in the 0.7-5.2 μm wavelength region, which have been critical to improving our understanding of the surface composition of this moon
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

Summary

Introduction

Europa’s young surface and geological features like chaos terrains suggest an active exchange of materials between the surface and subsurface ocean (Carr et al 1998; Moore et al 2009; Kattenhorn & Prockter 2014; Trumbo et al 2019a). Spectroscopy in the near-infrared has helped reveal the majority of species identified on Europa’s surface (Carlson et al 2009), including hydrated sulfuric acid (Carlson et al 1999a, 2005), hydrated sulfates (McCord et al 1998; Dalton 2007), chlorinates (Strazzulla 2011; Ligier et al 2016) and oxidants (Hansen & McCord 2008; Hand & Brown 2013). Serendipitous observations of Europa by Juno (Filacchione et al 2019; Mishra et al 2021) have provided a new window into this world, the Galileo NIMS dataset still holds great potential to further improve our understanding of Europa’s surface composition, by the application of new, comprehensive methods, as elucidated below

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