Asteroid 16 Psyche: Shape, Features, and Global Map

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).

A numerical approach using a finite element model to constrain the possible interior layout of (16) Psyche

“…This variation indicates that Psyche does not have a constant composition across the surface. The recent works (e.g., Shepard et al (2021) and Ferrais et al (2020)) also support this conclusion. In addition, Sanchez et al (2016) and Takir et al (2016) showed the presence of orthopyroxene and hydrated silicates on the surface although they do not provide strong constraints on abundances of silicates.…”

“…However,if the asteroid is a pure metal or dominantly metal-rich object, the most puzzling physical parameter is its bulk density. While the density of iron meteorites is ∼7.5 g cm −3 (Smyth & Mccormick 1995), the recent estimates of the Psyche's bulk density are all close to ∼4.0 g cm −3 (Carry 2012;Hanuš et al 2017;Drummond et al 2018;Viikinkoski et al 2018;Shepard et al 2021;Siltala & Granvik 2021;Elkins-Tanton et al 2020), which is remarkably lower than the expected value. This inconsistency may be understandable if Psyche is a rubble-pile asteroid having a very high porosity (> ∼50%).…”

“…If the pure-metallic structure with high porosity is not the case for Psyche, we may interpret that this object has mixed silicates and metal. Shepard et al (2017Shepard et al ( , 2021 showed that radar albedo has significant variations across the entire surface as Psyche rotates. While the highest albedo follows that of M-type asteroids, the lowest value (∼0.2) is within the high radar albedo ranges of S-and C-type asteroids.…”

Distinguishing the Origin of Asteroid (16) Psyche

“…Lutetia also has lower radar albedo than Psyche has (Shepard et al. 2015 , 2021 ). In contrast to the limited data available from the Lutetia flyby, the Psyche mission instrument package will provide much greater detail needed to identify meteorite analogs.…”

“… 2016 ) and large-scale radar albedo (Shepard et al. 2021 ) that would be spatially resolvable by GRNS if they correspond to chemical differences.…”