DIX Planetary Science Seminar

The Microwave Radiometer (MWR) instrument on the Juno spacecraft mapped the brightness temperature of Ganymede's ice shell at frequencies from 0.6 to 22 GHz. Based on the increasing brightness temperature with wavelength, a geothermal gradient of 1 K/km was derived, implying an upper bound of the thickness of Ganymede's ice shell to be 150 km (Brown et al., 2023). This constraint is based on assuming that except for a thin surface layer, this ice shell is of very high purity up to ∼ 20 km, the electrical skin depth for a pure ice at the longest wavelength channel of MWR. The presence of impurities that are more attenuating than pure ice at microwave frequencies will result in a decreased signal penetration depth, implying the signal originates from a shallower depth, and has consequences for the temperature gradient. There are multiple possible sources of impurities in the ice shell, both from above and from below, but we will argue that impacting bodies is likely to dominate. For most of solar system history, comets are the most likely source of impacts, which will emplace a surface layer that is a nearly homogeneous mix of the projectile and disrupted target material upon impact. In this work, we calculate the amount of impact material that will get mixed into ice at depth over the bombardment history of Ganymede, using estimates of impactor populations in the literature. We estimate the extent to which these impurities will affect the attenuation of the microwave signal, and discuss what these suggest for the bombardment history of Ganymede.

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Aluminum-phyllosilicates have been detected in upper stratigraphic units as the infrared spectrally dominant mineral in 100s–1000s of km2 exposures of Noachian (~3.7 Ga) rock across Mars. These units have been previously proposed to be analogous to terrestrial basalt weathering sequences with Fe/Mg-phyllosilicates below the Al-phyllosilicates. In some regions, these minerals appear associated with jarosite and alunite, sulfate minerals indicative of strongly acidic weathering conditions. To understand what these mineral assemblages reveal about Mars' aqueous history, we first analyzed the Nili Fossae region in detail, combining mineral maps generated from hyperspectral imagery from the CRISM instrument with digital elevation models and high-resolution imagery to understand the geologic contexts of these minerals. We find that Al-phyllosilicate (dominantly kaolinite) consistently occurs stratigraphically above Fe/Mg-phyllosilicate, but the two minerals have differing textures indicating that they did not form as part of a weathering sequence from a single protolith as expected. Layers in some Al-phyllosilicate deposits and its preferential association with sedimentary basins suggest that its protolith is reworked sedimentary material. We conclude that the protolith for the Al-phyllosilicate is most likely an Al/Si-rich (non-basaltic) volcanic ash deposit. We detected sparse jarosite closely associated with Al-phyllosilicate which we interpret to have formed from the interaction of Fe-enriched fluids with S in the Al-phyllosilicate protolith and an oxidizing atmosphere. The recent detection of Al-phyllosilicate-bearing float rocks in nearby Jezero Crater by the Perseverance Rover allows us to bridge the orbital and in-situ scales, and sample return could more conclusively determine processes and environmental conditions of Al-phyllosilicate formation in the Nili Fossae region. We will expand this analysis globally to understand what the formation of Al-phyllosilicate-bearing regions across Mars implies for the planet's geologic evolution.