Objectives: the patterns of moderately volatile element (MVE) depletion in the silicate portions of planetary bodies arise from two main processes: volatile loss (or incomplete condensation) and…

This is a $503,870 grant awarded by the National Aeronautics and Space Administration (NASA) to the University of California, Santa Cruz (UCSC) to study the patterns of moderately volatile element depletion in the silicate portions of planetary bodies. The research aims to investigate whether volatile loss from partially molten planetesimals can explain the observed elemental depletion patterns and whether conventional or "Grand Tack" accretion models are more consistent with the data. The project will combine models of planet growth and volatile loss with element partitioning experiments and calculations to explore the origin of these patterns in Earth, Mars, Vesta, and the Angrite Parent Body. The grant does not have a set-aside designation, reflecting UCSC's status as a large, research-focused public university. This work supports NASA's objectives related to the accretion of solar system bodies after the dissipation of the protoplanetary disk.

Objectives: the patterns of moderately volatile element (MVE) depletion in the silicate portions of planetary bodies arise from two main processes: volatile loss (or incomplete condensation) and core formation. The objective of this proposal is to combine models of planet growth and volatile loss with element partitioning experiments and calculations to explore the origin of the MVE patterns in earth, mars, vesta and the angrite parent body (APB). In particular, we will investigate whether volatile loss from partially molten planetesimals can explain the observations, and whether conventional or grand tack -style accretion models work better. Methods: we will begin with our existing core formation model coupled to dynamical accretion simulations. Successful simulations are required to match the non-volatile element concentrations (v, CR, CO, MO) and timings from the HF-w system for earth, mars and vesta. For earth, nucleosynthetic MO isotope measurements (indicating contributions from both reservoirs) must also be matched. We will also carry out high-pressure experiments to determine the partitioning of PB and AG. Successful core formation models will then be used with existing and our new partitioning data to predict the expected MVE concentrations (e.g. PB, AG, ZN, RB, GE, k, CU, NA, MN, LI, CO, NI). Discrepancies between the measured and predicted concentrations will indicate volatile loss or incomplete condensation. Here we will focus on the former as an end-member, based on isotopic evidence of loss of SI and MG. To model volatile loss, we will begin with the parameterization of sossi ET AL. (2019). This shows that volatile loss depends primarily on the melt temperature and the oxygen fugacity. The oxygen fugacity and growth histories of planetesimals are set by the core formation and accretion models the remaining variable is the temperature distribution. Initially we will treat this as a free parameter and use monte carlo simulations to determine what kinds of distributions are acceptable. Preliminary tests suggest a bimodal temperature distribution works well. We will then proceed to investigate the physics of melt production and volatile loss. We will track planetesimal growth and melting from impacts, to determine when and for how long magma is exposed at the surface. We will track the evolution of elemental and isotopic concentrations using the approach of young ET AL. (2019). The evolving size of the planetesimal is important to include as volatile loss shuts down for roughly moon-mass objects, and so too is the mixing of planetesimals having experienced different histories. The model timing of volatile loss is particularly important for the PD-AG and u-PB systems, which are chronometers similar to HF-w but sensitive to volatile loss as well as core formation. We will track the evolution of these isotopic systems for comparison with available measurements. Significance: this proposal will examine three big questions: whether volatile loss during planetesimal growth is responsible for the elemental depletion patterns what style of planetary accretion (e.g. grand tack VS. Conventional) is most consistent with the observations and when volatile loss happened during the accretion process. Even partial answers to these questions will sharpen our picture of how accretion proceeded in this solar system and will make predictions (e.g. AG isotopic signatures for mars) which are testable. Relevance: this project examines processes operating during the accretion of planetesimal- and embryo-size objects. IT is therefore relevant to the EW objectives, and specifically fits within "studies related to the accretion of solar system bodies after dissipation of the protoplanetary disk".