The Psyche Light Elements Investigation

Abstract

Light elements, such as C, S, Si, O, C, and H, are thought to be present in Earth’s liquid-Fe outer core. These elements lower melting temperatures, thereby allowing the core to remain in liquid state at high pressure and influencing magnetic and geodynamic processes. However, the identity and abundance of the light elements in the cores of terrestrial planets and how they were delivered to these cores is not well known. The NASA Psyche mission will travel to and explore (16) Psyche, which may be the metal-rich core of a differentiated planetesimal exposed by collisional stripping. If so, the Psyche mission could provide a direct assessment of the light element content of an asteroidal core, allowing comparisons to the inferred composition of planetary cores and the parent bodies of the magmatic iron group meteorites. In particular, Earth’s high-pressure core formed gradually (over ∼100 Myr), in a multistage process, under increasingly oxidizing conditions, whereas the cores of planetesimals formed quickly (within 10 Myr) at low pressure, likely in chemical equilibrium with their mantles. The trace element systematics and mineral composition of magmatic iron meteorites indicate the presence of C, P, and S in planetesimal cores prior to solidification. Such elements would have played a role in core dynamics, including dynamo generation. Their low solubility combined with the immiscibility of their mineral precipitates would have resulted in their separation from Fe upon crystallization and their eruption onto the surface of a stripped core (via ferrovolcanism). The Psyche spacecraft will detect their elemental, mineral, and magnetic signatures with the payload instruments, which include a Gamma Ray and Neutron Spectrometer, a Multispectral Imager, and a Magnetometer. Additional constraints on interior composition and processes influenced by light elements will be provided by Psyche’s gravity and geomorphology investigations. We provide a brief introduction to the topic of light elements along with prospects for (16) Psyche. While we emphasize core formation processes, we also consider other possibilities for the origin and evolution of this metal-rich body.