Hypervelocity impacts as a source of deceiving surface signatures on iron-rich asteroids

Abstract

Several arguments point to a larger proportion of metal-rich asteroids than that derived from spectral observations, as remnants of collisional disruptions of differentiated bodies. We show experimentally that this apparent deficit may result from the coating of metallic surfaces by silicate melts produced during impacts of hydrated or dry projectiles at typical asteroid impact speeds. Spectral analysis of steel and iron meteorite targets after impact shows a profoundly modified optical signature. Furthermore, hydrated projectiles leave a 3-μm absorption hydration feature. This feature is thus consistent with a metallic surface and does not require an unusual low-speed impact. Unless systematizing radar measurements, ground-based spectral observations can be deceptive in identifying iron-rich bodies. The NASA Psyche mission rendezvous with Psyche will offer the unique opportunity both to measure the relative abundances of regolith and glassy coated surfaces and to substantially increase our understanding of impact processes and signatures on a metal-rich asteroid.

Observations, Meteorites, and Models: A Preflight Assessment of the Composition and Formation of (16) Psyche

“…These two scenarios should result in different surface distributions of silicates. For instance, deposition via impacts tend to leave silicates within the crater floors (an outcome also seen in impact experiments; Daly & Schultz, ; Libourel et al, ; Marchi et al, ), while endogenous silicates are expected to be more broadly distributed at the surface. As a result, the two scenarios could also lead to different stratifications of the silicates in the near‐surface, which can be exposed and probed via cratering.…”

“…Laboratory‐scale impact experiments show that craters in manufactured iron‐nickel ingots and iron meteorites exhibit a different morphology compared to craters in rocky targets. Notably, craters in metal have higher depth‐to‐diameter ratios and retain sharper rims than in rock, and their floors have characteristic “petal‐like” morphologies (e.g., Libourel et al, ; Marchi et al, ). These structures may or may not be present at much larger scales, such as those relevant to Psyche's observations.…”

“…In that case, its exterior may have sheets of its parental rock, fallen back after stripping, and as in all cases it will likely have minor chondritic material from subsequent impacts. In fact, the possible forms of silicate material delivered and processed by impacts have recently been added to by the discovery of glasses and foams from silicate impactors onto metal in laboratory experiments (Libourel et al, ).…”

Morphological Diversity of Impact Craters on Asteroid (16) Psyche: Insight From Numerical Models

“…In other words, the dotted line represents Equation with defined using Equation as before, but with the crater growth timescale (Equation ) calculated for an impactor mass, m , appropriate for a cm‐scale, laboratory impactor rather than a 10 km wide asteroid. The excellent agreement between the scaling relationship and the experimental data from impacts into Gibeon and iron ingot targets from Libourel et al (2019) and Marchi et al (2019) suggests that the scaling relationship is broadly applicable for impacts on an exposed iron target over a large range in impactor size.…”

“…(b) Scaled crater radius as a function of strength‐scaled impact size for iSALE simulations of 10 km impacts into iron and porous iron targets, at velocities between 1 and 25 km/s. (c) Scaled crater radius for iSALE simulations of 10 km impacts into iron targets, compared to scaled crater radius for lab scale impacts, from Libourel et al (2019) and Marchi et al (2019). The continuous line represents the corresponding scaling law for 10 km impacts and the dashed line represents the scaling law for cm‐sized impacts.…”