Visible to Near‐Infrared Reflectance Spectroscopy of Asteroid (16) Psyche: Implications for the Psyche Mission's Science Investigations

Abstract

The NASA Psyche mission will explore the structure, composition, and other properties of asteroid (16) Psyche to test hypotheses about its formation. Variations in radar reflectivity, density, thermal inertia, and visible to near‐infrared (VNIR) reflectance spectra of Psyche suggest a highly metallic composition with mafic silicate minerals (e.g., pyroxene) heterogeneously distributed on the surface in low abundance (<10 vol.%). The Psyche spacecraft's Multispectral Imager is designed to map ≥80% of the surface at high spatial resolution (≤20 m/pixel) through a panchromatic filter and provide compositional information for about ≥80% of the surface using seven narrowband filters at VNIR wavelengths (∼400–1,100 nm) and at spatial scales of ≤500 m/pixel. We analyzed 359 reflectance spectra from samples consistent with current uncertainties in Psyche's composition and compared them to published reflectance spectra of the asteroid using a chi‐square test for goodness of fit. The best matches for Psyche include iron meteorite powder, powders from the sulfide minerals troilite and pentlandite, and powder from the CH/CBb chondrite Isheyevo. Comparison of absorption features support the interpretation that Psyche's surface is a metal‐silicate mixture, although the exact abundance and chemistry of the silicate component remains poorly constrained. We convolve our spectra to the Imager's spectral throughput to demonstrate preliminary strategies for mapping the surface composition of the asteroid using filter ratios and reconstructed band parameters. Our results provide predictions of the kinds of surface compositional information that the Psyche mission could reveal on the solar system's largest M‐type asteroid.

Space Weathering Effects in Troilite by Simulated Solar‐Wind Hydrogen and Helium Ion Irradiation

“…The Gamma Ray and Neutron Spectrometer (GRNS) will measure bulk elemental abundances in the upper 10–100 cm across large‐footprint (∼tens of km diameter) regions of Psyche's surface beginning in Orbit D, with the ability to detect sulfur with 15% measurement precision if it is present at >3 wt% abundance (Peplowski et al., 2018). The Multispectral Imager will infer surface mineralogy using eight filter bands in the visible to near‐infrared (∼400 to ∼1,100 nm) built into twin CCD cameras (Dibb & Bell, 2018). Changes in surface micro‐roughness and chemistry may both play a role in altering the optical reflectance, slope, and albedo of troilite and other minerals present on the surface of Psyche (Yon & Pieters, 1987).…”

A Post‐Launch Summary of the Science of NASA's Psyche Mission

“…In this scenario, the mantle of the proto‐Psyche impactor would be stripped and accreted by the target body, while the metal‐rich core material may escape and survive as a runner to become what we see as Psyche today (e.g., Gabriel & Cambioni, 2023). The erosion of proto‐Psyche's silicate mantle in this way could explain the observed lack of spectral features consistent with Fe‐bearing olivine (a typical mantle material) on Psyche's surface and the lack of a dynamical family linked to Psyche (Binzel et al., 1995; Burbine et al., 1996; Dibb et al., 2023; Sanchez et al., 2017).…”

“…The Multispectral Imager is a block‐redundant pair of digital charge coupled device cameras with 148‐mm focal length optics that will obtain polychromatic and narrow‐band color images of the asteroid at pixel scales ranging from 35 down to ∼3.8 m·pixel −1 (assuming nominally planned orbital altitudes) (Bell et al., 2016; Dibb et al., 2023). The Imager will observe the surface through a nadir‐pointing, broadband clear filter, through seven color filters across the 427–1,025 nm wavelength region optimized to identify silicate and sulfide minerals, and through the clear filter at multiple look angles to produce DTMs using image‐based stereo photogrammetry or stereo photoclinometry.…”

“…However, it is still not known whether M‐type asteroids formed in the inner or outer solar system (Elkins‐Tanton et al., 2022). This story becomes more intriguing if the M‐types are linked to metal‐rich CB chondrites (e.g., Dibb et al., 2023). These meteorites have the youngest measured chondrules and it has been suggested that CBs formed from late‐stage protoplanetary collisions when the solar nebula was presumably close to dissipation and Jupiter was near its full size (Krot et al., 2005).…”

“…This abundance in Ni could be detected by GRNS and the variation of FeO in silicates would be detectable by the Multispectral Imager. Finally, sulfide minerals such as oldhamite and troilite (FeS) exhibit changes in spectral slope at visible wavelengths that can be characterized by multispectral data (Dibb et al., 2023). To accomplish this objective, the Psyche mission will determine whether Psyche's surface has a global average of >10 wt.% oldhamite and <4 wt.% Ni (indicating reducing conditions) or >12 wt.% Ni (indicating oxidizing conditions) using the Multispectral Imager and GRNS.…”