^1,2Tuan H. Vu1, Robert Hodyss, ^1,2Mathieu Choukroun, ^1,2Paul V. Johnson
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²NASA Astrobiology Institute

The composition of Europa’s subsurface ocean is a critical determinant of its habitability. However, our current understanding of the ocean composition is limited to its expression on the surface. This work investigates experimentally the composition of mixed sodium–magnesium–sulfate–chloride solutions when frozen to 100 K, simulating conditions that likely occur as ocean fluids are emplaced onto Europa’s surface. Micro-Raman spectroscopy is used to characterize phase composition of the frozen brines at 100 K. Our results show that solutions containing Na+, Cl−, Mg2+, and ${{\mathrm{SO}}_{4}}^{2-}$ preferentially crystallize into Na2SO4 and MgCl2 hydrated minerals upon freezing, even at elevated [Mg2+]/[Na+] ratios. The detection of epsomite (MgSO4•7H2O) on Europa’s surface, if confirmed, may thus imply a relatively sodium-poor ocean composition or a radiolytic process that converts MgCl2 to MgSO4 as suggested by Brown & Hand. The formation of NaCl on the surface, while dependent upon a number of factors such as freezing rate, may indicate an ocean significantly more concentrated in sodium than in magnesium.

Reference
Vu TH, Hodyss R, Choukroun M, Johnson PV (2016) CHEMISTRY OF FROZEN SODIUM–MAGNESIUM–SULFATE–CHLORIDE BRINES: IMPLICATIONS FOR SURFACE EXPRESSION OF EUROPA’S OCEAN COMPOSITION. The Astrophysical Journal (Letters) 816, L26
Link to Article [http://dx.doi.org/10.3847/2041-8205/816/2/L26]

¹S. Xu, ²M. Jura, ³P. Dufour, ²B. Zuckerman
¹European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching, Germany
²Department of Physics and Astronomy, University of California, Los Angeles CA 90095-1562, USA
³Institut de Recherche sur les Exoplanètes (iREx), Université de Montréal, Montréal, QC H3C 3J7, Canada

We report high-resolution spectroscopic observations of WD 1145+017—a white dwarf that was recently found to be transitted by multiple asteroid-sized objects within its tidal radius. We discovered numerous circumstellar absorption lines with linewidths of ~300 km s−1 from Mg, Ca, Ti, Cr, Mn, Fe, and Ni, possibly from several gas streams produced by collisions among the actively disintegrating objects. The atmosphere of WD 1145+017 is polluted with 11 heavy elements, including O, Mg, Al, Si, Ca, Ti, V:, Cr, Mn, Fe, and Ni. Evidently, we are witnessing the active disintegration and subsequent accretion of an extrasolar asteroid.

Reference
Xu S, Jura M, Dufour P, Zuckerman B (2016) EVIDENCE FOR GAS FROM A DISINTEGRATING EXTRASOLAR ASTEROID. The Astrophysical journal (Letters), 816 L22
Link to Article [http://dx.doi.org/10.3847/2041-8205/816/2/L22]