Geophysical Impacts and Spectroscopic Identification of a Hydrous Iron Sulfate on Icy Worlds

Abstract Over geologic time‐scales, large volumes of exogenic sulfur ions from Io's plasma torus have been supplied to the surface of Europa and Ganymede, which, combined with recent interpretations of orbiter images, dynamical modeling, and surface‐subsurface exchange, suggests further sulfur transport into the interior of the icy worlds. These observations motivate mixed‐phase spectral modeling for interpreting orbiter spectroscopy data and determination of hydration states of candidate surface materials including hydrous sulfates. In this work, we present a combined experimental and theoretical study of the low temperature and high pressure vibrational spectral signature of the iron‐sulfate monohydrate endmember, szomolnokite (FeSO 4 ·H 2 O). By employing synchrotron Fourier‐transform infrared spectroscopy (FTIR) in the diamond anvil cell up to 23 GPa and down to 20 K, we explore the extreme range of pressure‐temperature domains relevant to icy environments throughout our solar system and beyond. Combined with our density‐functional theory quantum‐mechanics molecular dynamics results, we demonstrate that experimentally observed infrared features in the O‐H stretching region commonly associated with n H 2 O ( n > 1) hydration states can be attributed to a pure monohydrate without the need for pressure‐induced exsolved ice, other coexisting hydrous iron sulfates, or strong overtone and combination modes. We further discuss the possibility of lateral variations in density and shear properties on icy worlds associated with temperature variations and the high‐pressure phases of kieserite group monohydrated sulfates.