Silicon and oxygen are potential light elements in Earth's core because their stronger affinity to metal observed with increasing temperature posits that significant amounts of both can be incorporated into the core. It was proposed that an Fe–Si–O liquid alloy could expel SiO2 at the core‐mantle boundary during secular cooling, leaving the core with either silicon or oxygen, not both. This was recently challenged in a study showing no exsolution but immiscibility in the Fe–Si–O system. Here we investigate the liquidus field of Fe–Si and Fe–O binaries and Fe–Si–O ternaries at core‐mantle boundary pressures and temperatures using ab initio molecular dynamics. We find that the liquids remain well mixed with ternary properties identical to mixing of binary properties. Two‐phase simulations of solid SiO2 and liquid Fe show dissolution at temperatures above 4100 K, suggesting that SiO2 crystallization as well as liquid immiscibility in Fe–Si–O is unlikely to occur in Earth's core.

Seismic measurements made as part of the InSight mission (Banerdt et al., 2020) indicate that Mars' core is large and light with radius and mean density ranging between 1,790 and 1,870 km and 5.7-6.3 g/cm 3 , respectively (Drilleau et al., 2022;Durán et al., 2022;Khan et al., 2022;Stähler et al., 2021). Relative to pure liquid Fe (Kuwayama et al., 2020), this implies a density deficit far in excess of that observed in Earth's core (Birch, 1964). Taken at face value, it requires the incorporation of substantial amounts of light elements (LEs) into the martian core during the early stages of planetary formation (e.g., Khan et al., 2022;Steenstra & van Westrenen, 2018). Previous work that relied mostly on analyses of martian meteorites and cosmochemical arguments (e.g., Lodders

Sodium chloride is expected to be found on many of the surfaces of icy moons like Europa and Ganymede. However, spectral identification remains elusive as the known NaCl-bearing phases cannot match current observations, which require higher number of water of hydration. Working at relevant conditions for icy worlds, we report the characterization of three “hyperhydrated” sodium chloride (SC) hydrates, and refined two crystal structures [2NaCl·17H 2 O (SC8.5); NaCl·13H 2 O (SC13)]. We found that the dissociation of Na + and Cl − ions within these crystal lattices allows for the high incorporation of water molecules and thus explain their hyperhydration. This finding suggests that a great diversity of hyperhydrated crystalline phases of common salts might be found at similar conditions. Thermodynamic constraints indicate that SC8.5 is stable at room pressure below 235 K, and it could be the most abundant NaCl hydrate on icy moon surfaces like Europa, Titan, Ganymede, Callisto, Enceladus, or Ceres. The finding of these hyperhydrated structures represents a major update to the H 2 O–NaCl phase diagram. These hyperhydrated structures provide an explanation for the mismatch between the remote observations of the surface of Europa and Ganymede and previously available data on NaCl solids. It also underlines the urgent need for mineralogical exploration and spectral data on hyperhydrates at relevant conditions to help future icy world exploration by space missions.