Synthesized microstructures and reflectance spectra of solids in the ice Ih–MgCl2•12H2O system: Implications for Europa

Abstract

The existence of hydrated MgCl₂ on Europa's surface has been suggested by remote sensing spectrometry, but this interpretation is limited by the low signal-to-noise ratio of the data and a lack of relevant laboratory-derived spectra within spectral libraries. Furthermore, the compaction state of Europa's surface remains poorly constrained, yet there is a lack of laboratory-measured spectra for dense specimens that are necessary to understand endmember behavior of a dense surface. We acquired vis-near-infrared reflectance spectra at wavelengths from 0.35 to 2.5 μm for synthetic specimens in the ice Ih–MgCl₂•12H₂O system. Reflectance spectra were measured for dense water-ice aggregates with different grain sizes and were used to assess the influence of density on spectral signature. In general, large-grained aggregates exhibited decreased albedo, broadening of absorption bands, and decreased absorption band depth. Additionally, reflectance spectra of powdered MgCl₂•12H₂O were measured as a function of particle size and were observed to contain spectral features distinct from other hydration states (MgCl₂•nH₂O, n = 1,2,4,6,8). Particle size dependency observed in the spectra of powdered MgCl₂•12H₂O mirrored grain size dependent trends observed in the spectra of dense water-ice aggregates. We also measured the spectra of eutectic (20.6 wt% MgCl₂) and porphyritic hypoeutectic (7.7 and 16.4 wt% MgCl₂) dense ice Ih–MgCl₂•12H₂O aggregates. We argue that the microstructures of synthesized eutectic and hypoeutectic aggregates are suitable to represent texture likely existent in Europa's shell and are appropriate for use in mechanical testing where texture needs to be carefully considered.