¹Johannes M.Meusburger,¹Martin Ende,¹Philipp Matzinger,¹Dominik Talla,¹Ronald Miletich,¹Manfred Wildner
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113459]
¹Institut für Mineralogie und Kristallographie, Althanstraße 14, 1090 Wien, Austria
Copyright Elsevier

The high-pressure behaviour of hydrated magnesium sulfate kieserite, MgSO₄⋅H₂O, has been investigated on isothermal compression at T = 295 K up to 8.3 GPa hydrostatic pressure. The crystal properties of synthetic endmember single crystals were investigated using a high-pressure diamond anvil cell by means of in-situ X-ray diffraction and vibrational spectroscopy methods. The experimental study reveals a second-order phase transition from the monoclinic (C2/c) α-phase to a triclinic (P 1¯) β-form at a transition pressure of 2.72 GPa. Elastic properties as determined from precise lattice parameters yield static elasticities as described by third-order Birch-Murnaghan equations of state with V₀ = 355.5(4) ų, K₀ = 48.1(5) GPa, K’ = 8.1(6) for the low-pressure polymorph (α-MgSO₄⋅H₂O), and V₀ = 355.8(1.8) ų, K₀ = 49.3(5.5) GPa, K’ = 4.8(1.0) for the high-pressure polymorph (β-MgSO₄⋅H₂O). The nature of the phase transition and its reversibility on pressure release make it seem unlikely that the β-polymorph can be recovered at surface conditions on any icy satellite, although in the context of impact events it is proposed to exist, but only on a limited time scale before re-transforming to α-MgSO₄⋅H₂O. With respect to the icy mantles of Ganymede and Callisto, the depth profile of Ganymede following the established thermal gradients suggest a stability field only for α-MgSO₄⋅H₂O being relevant to the presumable conditions in the icy mantle. In contrast, the depth profile for Callisto, as corresponding to maximum pressures of approximately 5 GPa, crosses the α-to-β-transition boundary and make the high-pressure polymorph a promising candidate rock-forming mineral for the deep icy mantle of the outermost Galilean moon. In particular the material parameters reported for the α and β form of MgSO₄⋅H₂O are fundamental to compute the icy mantle dynamics and accurately determine the radial density structure in models of Ganymede and Callisto.

¹Emily J.Foote,¹David A.Paige,²Michael K.Shepard,³Jeffrey R.Johnson,⁴Stuart Biggar
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113456]
¹University of California Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095-1567
²Bloomsburg University, 400 E. Second St., Bloomsburg, PA 17815, Bloomsburg, PA
³Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Road, 200-W230, Laurel, MD 20723-6005
⁴College of Optical Sciences, University of Arizona, 1630 E. University Blvd., P.O. Box 210094, Tucson, AZ 85721-0094
Copyright Elsevier

We have acquired a comprehensive laboratory bidirectional measurements of Apollo 11 and Apollo 16 lunar soil samples and have successfully fit photometric models to the laboratory data and have determined the solar spectrum averaged hemispheric reflectance as a function of incidence angle. The Apollo 11 (sample 10084) and 16 (sample 68810) soil samples are two representative end member samples from the Moon, dark lunar maria and bright lunar highlands. We used our solar spectrum averaged albedos in a thermal model and compared our model-calculated normal bolometric infrared emission curves with those measured by the LRO Diviner Lunar Radiometer Experiment. We found excellent agreement at the Apollo 11 site, but at the Apollo 16 site, we found that the albedos we measured in the laboratory were 33% brighter than those required to fit the Diviner infrared data. We attribute this difference at Apollo 16 to increased compaction and decreased maturity of the laboratory sample relative to the natural lunar surface, and to local variability in surface albedos at the Apollo 16 field area that are below the spatial resolution of Diviner.