Asteroid (16) Psyche: Evidence for a silicate regolith from spitzer space telescope spectroscopy

Asteroid (16) Psyche is a unique, metal-rich object belonging to the M taxonomic class. It may be a remnant protoplanet that has been stripped of most silicates by a hit-and-run collision. Because Psyche offers insight into the planetary formation process, it is the target of NASA's Psyche mission, set to launch in 2023. In order to constrain Psyche's surface properties, we have carried out a mid-infrared (5-14 pm) spectroscopic study using data collected with the Spitzer Space Telescope's Infrared Spectrograph. Our study includes two observations covering different rotational phases. Using thermophysical modeling, we find that Psyche's surface is smooth and likely has a thermal inertia Gamma = 5-25 Em(2)/K/s(1/2) and bolometric emissivity epsilon = 0.9, although a scenario with epsilon = 0.7 and thermal inertia up to 95 j/m(2)/K/s(1/2) is possible if Psyche is somewhat larger than previously determined. The smooth surface is consistent with the presence of a metallic bedrock, which would be more ductile than silicate bedrock, and thus may not readily form boulders upon impact events. From comparisons with laboratory spectra of silicate and meteorite powders, Psyche's 7-14 pm emissivity spectrum is consistent with the presence of fine-grained (< 75 pm) silicates on Psyche's surface. We conclude that Psyche is likely covered in a fine silicate regolith, which may also contain iron grains, overlying an iron-rich bedrock. (C) 2017 Elsevier Inc. All rights reserved.