Abstract
Abstract We develop a shape model of asteroid 16 Psyche using observations acquired in a wide range of wavelengths: Arecibo S-band delay-Doppler imaging, Atacama Large Millimeter Array (ALMA) plane-of-sky imaging, adaptive optics (AO) images from Keck and the Very Large Telescope (VLT), and a recent stellar occultation. Our shape model has dimensions 278 (−4/+8 km) × 238(−4/+6 km) × 171 km (−1/+5 km), an effective spherical diameter D eff = 222-1/+4 km, and a spin axis (ecliptic lon, lat) of (36°, −8°) ± 2°. We survey all the features previously reported to exist, tentatively identify several new features, and produce a global map of Psyche. Using 30 calibrated radar echoes, we find Psyche’s overall radar albedo to be 0.34 ± 0.08 suggesting that the upper meter of regolith has a significant metal (i.e., Fe–Ni) content. We find four regions of enhanced or complex radar albedo, one of which correlates well with a previously identified feature on Psyche, and all of which appear to correlate with patches of relatively high optical albedo. Based on these findings, we cannot rule out a model of Psyche as a remnant core, but our preferred interpretation is that Psyche is a differentiated world with a regolith composition analogous to enstatite or CH/CB chondrites and peppered with localized regions of high metal concentrations. The most credible formation mechanism for these regions is ferrovolcanism as proposed by Johnson et al. (2020).
Highlights
Asteroid 16 Psyche is the largest Tholen (1984) M-class asteroid and a target of the NASA Discovery mission Psyche (Elkins-Tanton et al 2017, 2020)
Visible and near-infrared spectra (Bell et al 1989), optical polarimetry (Dollfus et al 1979), thermal observations (Matter et al 2013), and radar observations (Ostro et al 1985; Magri et al 2007a; Shepard et al 2008, 2017) all support the hypothesis that its surface to near-surface is dominated by metal
We examine topographical and optical albedo features discussed in previous work (Shepard et al 2017; Viikinkoski et al 2018; Ferrais et al 2020), produce a global map, and overlay it with radar albedo values, a proxy for metal concentrations in the upper meter of the regolith
Summary
Asteroid 16 Psyche is the largest Tholen (1984) M-class asteroid and a target of the NASA Discovery mission Psyche (Elkins-Tanton et al 2017, 2020). Recent studies of Psyche have converged to size estimates (effective diameter) between 220 and 230 km, and to spin poles within a few degrees of (36°, −8°) (ecliptic lon, lat) (Shepard et al 2017; Drummond et al 2018; Viikinkoski et al 2018; Ferrais et al 2020). Given this size, mass estimates have led to a consensus that Psyche’s overall bulk density is between 3400 and 4100 kg m−3 (Elkins-Tanton et al 2020), which places constraints on its internal structure, composition, and possible origin.
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