Magma ocean crystallization models that track fO2 evolution can reproduce the D/H ratios of both the Earth and Mars without the need for exogenous processes. Fractional crystallization leads to compositional evolution of the bulk oxide components. Metal-saturated magma oceans have long been thought to contain negligible ferric iron oxide (Fe3+O1.5), but recent work suggests they may contain near-present-day Fe3+ concentrations. We model the fractional crystallization of Earth and Mars, including Fe2+ and Fe3+ as separate components. We use two independent equations of state (Deng, Armstrong EOS) to calculate Fe3+ partition coefficients for lower mantle minerals and compare results of fractional crystallization for different magma ocean configurations for both Earth and Mars. We calculate the oxygen fugacity (fO2) at the surface as the systems evolve and compare them to constraints on the fO2 of the last magma ocean atmosphere from D/H ratios. For Earth, we find that Fe3+ must behave incompatibly in the lower mantle to match the D/H constraint for whole mantle models, but shallow magma ocean models also provide reasonable matches to the constraints. For Mars, both EOSs produce near identical results but cannot match the D/H constraints on last fO2 unless the magma ocean begins with less than 50% of the predicted Fe3+. This model shows that Fe3+ partitioning has a measurable effect on magma ocean atmosphere redox state, which is not a static quantity but evolves throughout the magma ocean’s lifetime. We highlight the need for additional experimental constraints on ferric iron partitioning.

Anorthosites that comprise the bulk of the lunar crust are believed to have formed during solidification of a Lunar Magma Ocean (LMO) in which these rocks would have floated to the surface. This early flotation crust would have formed a thermal blanket over the remaining LMO, prolonging solidification. Geochronology of lunar anorthosites indicates a long timescale of LMO cooling, or re-melting and re-crystallization in one or more late events. To better interpret this geochronology, we model LMO solidification in a scenario where the Moon is being continuously bombarded by returning projectiles released from the Moon-forming giant impact. More than one lunar mass of material escaped the Earth-Moon system onto heliocentric orbits following the giant impact, much of it to come back on returning orbits for a period of 100 Myr. If large enough, these projectiles would have punctured holes in the nascent floatation crust of the Moon, exposing the LMO to space and causing more rapid cooling. We model these scenarios using a thermal evolution model of the Moon that allows for production (by cratering) and evolution (solidification and infill) of holes in the flotation crust that insulates the LMO. For effective hole production, solidification of the magma ocean can be significantly expedited, decreasing the cooling time by more than a factor of 5. If hole production is inefficient, but shock conversion of projectile kinetic energy to thermal energy is efficient, then LMO solidification can be somewhat prolonged, lengthening the cooling time by 50% or more.

Neutron spectroscopy has become a standard technique for remotely measuring planetary surface compositions from orbital spacecraft around various planets. Measurements have successfully been carried out at the Moon, Mars, Mercury, and the asteroids Vesta and Ceres. The NASA Psyche mission is planning to make neutron measurements to characterize the composition of the M-class asteroid (16) Psyche. Earth-based remote sensing measurements allow for a wide range of Fe concentrations, ranging from ~25 wt.% to 90 wt.%, and geochemically plausible Ni concentrations range from 0 wt.% to 10 wt.% or higher. To prepare for the analysis of Psyche neutron data, we have developed a new principal component analysis framework using four neutron energy ranges of thermal, low-energy epithermal, high-energy epithermal, and fast neutrons. With this analysis framework, we have demonstrated that the neutron measurements can uniquely distinguish variations of metal-to-silicate fraction, Ni, and hydrogen compositions. The strongest principal component is that of metal-to-silicate; the second strongest is Ni variations; the third is hydrogen variations. The validity of this framework can be first tested during a Mars gravity assist prior to arrival at Psyche.