Measuring the surface abundance of iron and nickel on the asteroid (16) Psyche in the presence of large solar particle events

This work investigates a novel signature for measuring the Ni/Fe ratio on the asteroid (16) Psyche that is robust against interference from large Solar Particle Events. NASA's Psyche mission launched on October 13th, 2023, and is headed to investigate this M-type asteroid. A primary science requirement for the Psyche gamma-ray spectrometer is to measure the absolute surface abundance of Ni and Fe. In particular, the Ni/Fe ratio will help test the hypothesis that (16) Psyche is a metal-rich body, possibly a remnant core from a failed planetesimal. However, Solar Particle Events can activate iron in the spacecraft, as well as the body of Psyche itself, disrupting the measurement of the surface abundance of iron for six months or more. Such an event happened during NASA's MESSENGER mission in orbit around Mercury on June 4, 2011, precluding further mapping of iron for the remainder of the mission. A similar event at Psyche could adversely affect mission science goals and/or prolong operation. Given the expected high abundance of Fe at Psyche, this paper proposes an alternative signature that relies on gamma rays from ⁵⁴Fe rather than ⁵⁶Fe. Although ⁵⁴Fe has a lower natural abundance than ⁵⁶Fe (5.8% vs 91.7%, respectively), ⁵⁴Fe is much less susceptible to interference from activation and would allow measurements of the surface abundance of iron to resume within days after a large Solar Particle Event. In addition, ⁵⁸Ni is shown not to be susceptible to interference from activation, thus making the ⁵⁸Ni/⁵⁴Fe ratio a robust alternative signature in the presence of Solar Particle Events.