Asteroid (16) Psyche: evidence for a silicate regolith from Spitzer Space Telescope spectroscopy

1Zoe A. Landsman, 2Joshua P. Emery, 1Humberto Campins, 3Josef Hanuš, 4Lucy F. Lim, 5Dale P. Cruikshank
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.11.035]
1Department of Physics, University of Central Florida, 4111 Libra Drive, PS 430, Orlando, FL, 32826, United States
2Earth and Planetary Science Department, Planetary Geosciences Institute, University of Tennessee, 306 EPS Building, 1412 Circle Dr, Knoxville, TN, 37996, United States
3Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, Prague, 18000, Czech Republic
4NASA Goddard Space Flight Center, Mail Code 691, Greenbelt, MD, 20771, United States
5NASA Ames Research Center, Mail Stop 245-6, Moffett Field, CA, 94035, United States
Copyright Elsevier

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 μm) 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 Γ = 5–25 J/m2/K/s1/2 and bolometric emissivity ϵ= 0.9, although a scenario with ϵ=0.7 and thermal inertia up to 95 J/m2/K/s1/2 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 7–14μm emissivity spectrum is consistent with the presence of fine-grained ( < 75μm) 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.

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