Microstructural and Optical Evolution of Icy Moons Surface Analogues in Space Conditions

Enceladus is famous for its geyser-like plumes of water vapour and ice particles on its south pole, titled as ”tiger stripes”. They are predominantly released from fractures, and some of the ejected material falls back onto Enceladus’ surface, coating it with fresh ice grains that keep its albedo high [1]. Enceladus and other icy satellites like Europa, are believed to have salty subsurface oceans. Sodium salts such as NaCl, NaHCO₃ and Na₂CO₃ have been suggested as salts present in Enceladus’ [2,3] surface and ocean. Recently, Postberg et al. 2023 [4], re-examined Cassini's Cosmic Dust Analyzer data and denoted the presence of sodium phosphate salts in high amounts on Enceladus’s surface and ocean, Na₃PO₄ and Na₂HPO₄.At present we have missions like JUICE and Europa Clipper with the goals of studying in depth icy moons surfaces and its connections with their subsurface oceans. This work intends to support the interpretation of data from these missions, as well as to provide insights that might help shape the design and emphasis of future icy satellites investigations. We experimentally investigate energy balances and space weathering on Enceladus’ surface, with a focus on how contaminants affect energy transfer and structural evolution, processes that are also relevant to other icy moons with sub-surface oceans. We prepare granular intra-mixtures of water ice and salts, and insolate them in a pre-cooled vaccum chamber to analyse how they evolve overtime. In our experiments, we monitor temperature variations and light transmission through analogue surface materials during irradiation in the vacuum chamber. Additionally, we analyse microstructural and spectral changes by comparing each sample before and after solar exposure. This includes microscopic imaging to track surface evolution and reflectance spectroscopy to assess changes in optical properties. We also measure the hardness profile of the sample to understand structural evolution through sintering. References:[1] Porco, C. C., “Cassini Observes the Active South Pole of Enceladus”, Science, vol. 311, no. 5766, pp. 1393–1401, 2006. doi:10.1126/science.1123013.[2] Postberg, F., “Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus”, Nature, vol. 459, no. 7250, pp. 1098–1101, 2009. doi:10.1038/nature08046[3] Postberg, F., Schmidt, J., Hillier, J., Kempf, S., and Srama, R., “A salt-water reservoir as the source of a compositionally stratified plume on Enceladus”, Nature, vol. 474, no. 7353, pp. 620–622, 2011. doi:10.1038/nature10175.[4] Postberg, F., “Detection of phosphates originating from Enceladus's ocean”, Nature, vol. 618, no. 7965, pp. 489–493, 2023. doi:10.1038/s41586-023-05987-9.