A key question pertaining to Europa’s habitability is whether hydrothermal activity could be sustained for long periods of time, enabling redox and nutrient exchange between the ocean and rocky interior [e.g. 1, 2]. Europa’s early ocean, if formed during differentiation, could have been infused with gases [3]. A consequence of this initial infusion is that clathrate hydrates may have been stable within the ocean. These clathrates could then rise to the bottom of the ice shell, or blanket the seafloor, depending on their density relative to the ocean. Accumulations of floating and sinking clathrates would affect the geological and thermal evolution of Europa because of their high heat capacity and low thermal conductivity compared to ice Ih, but sinking clathrates could also inhibit chemical exchange between the ocean and the rocky interior. We calculate the stability and density of CH4 and CO2 clathrates, and predict the volumes precipitated at the seafloor or accumulated at the base of the ice shell, for ocean compositions evolved from the interior of Europa during metamorphism on the path towards formation of a metallic core [3]. For a chemically reduced ocean derived from heating a mix of chondritic material near Jupiter [4], plus cometary volatiles, ~2 x 10^7 km^3 of methane clathrates form. These are less dense than the ocean (Fig. 1), and float to the base of the ice shell. However, for a CO2-rich ocean derived from CI or CM chondrites, ~3 x 10^8 – 2 x 10^9 km^3 of CO2 clathrates could form, i.e., sufficient feedstock to form a 13–77 km global layer on the seafloor. A salty ocean (e.g. 10 % MgSO4) or a warm seafloor (316 K) may be needed to prevent the accumulation of a CO2 clathrate blanket (Fig. 1), although the blanketing effect would thin the equilibrium thickness of the clathrate layer to ~500 m for allowable heat fluxes (~50 mW/m^2). [1] Vance, S. et al. (2007). Astrobiology, 7(6), 987–1005. https://doi.org/10.1089/ast.2007.0075 [2] Klimczak, C. et al. (2019). 50th Lunar. Planet Sci. Conf., Abstract #2132, p. 2912. https://ui.adsabs.harvard.edu/abs/2019LPI….50.2912K [3] Melwani Daswani, M. et al. (2021). A metamorphic origin for Europa’s ocean (preprint). https://doi.org/10.1002/essoar.10507048.1 [4] Desch, S. J. et al. (2018). Astrophys. J., Suppl. Ser., 238(1), 11. http://dx.doi.org/10.3847/1538-4365/aad95f

The five large moons of Uranus are important targets for future spacecraft missions. To motivate and inform the exploration of these moons, we model their internal evolution, present-day physical structures, and geochemical and geophysical signatures that may be measured by spacecraft. We predict that if the moons preserved liquid until present, it is likely in the form of residual oceans less than 30 km thick in Ariel, Umbriel, Titania, and Oberon. The preservation of liquid strongly depends on material properties and, potentially, on dynamical circumstances that are unknown. Miranda is unlikely to preserve liquid until present unless it experienced tidal heating a few tens of million years ago. The triaxial shapes estimated from Voyager 2 data for Miranda and Ariel further support the prospect that these moons are internally differentiated with a rocky core and icy shell. We find that since the thin residual layers may be hypersaline, their induced magnetic fields could be detectable by future spacecraft-based magnetometers. However, if the ocean is maintained primarily by ammonia, and thus well below the water freezing point, then its electrical conductivity may be too small to be detectable by spacecraft. Lastly, our calculated tidal Love number (k2) and dissipation factor (Q) are consistent with the Q/k2 values previously inferred from dynamical evolution models. In particular, we find that the low Q/k2 estimated for Titania supports the hypothesis that Titania currently holds an ocean.

The shergottite family of meteorites shows a remarkable petrographic and geochemical variety, revealing information about mantle processes and basalt formation on Mars. Northwest Africa (NWA) 11115, found in Morocco in 2016, is one of the newest meteorites in this family. We report bulk-rock major and trace element abundances of NWA 11115, bulk oxygen isotope systematics, and the petrography and mineralogy of a thick section, and compare the geochemistry of this recent find to other martian rocks. NWA 11115 is an olivine-phyric shergottite with an enriched rare earth element pattern, and shares similarities with NWA 1068, another enriched olivine-phyric shergottite. The large (< 2.5 mm) olivine phenocrysts are likely to be cumulates, similar to NWA 1068. However, the abundant maskelynite (~30 vol. %) in NWA 11115 places the bulk chemistry somewhat closer to the basaltic shergottites. We suggest that NWA 11115 is genetically linked to NWA 1068, perhaps crystallizing slightly above in the same cumulate pile. NWA 11115 contains one of the lowest K/Th ratios among the martian meteorites (K/Th = 2987 ± 810), and far lower than the surface of Mars (K/Th = 5300). Finally, while NWA 11115 contains abundant (~0.4 vol. %) fracture-filling calcite (presumably from hot desert alteration during its terrestrial residence), diagnostic bulk element mass ratios were not indicative of the presence of terrestrial alteration (Th/U ≈ 4.09, Sr/Nd ≈ 12.26, K/La ≈ 526.94, Ce/Ce* ≈ 1.01).