Hypervelocity Impact Experiments in Iron‐Nickel Ingots and Iron Meteorites: Implications for the NASA Psyche Mission

Abstract

The National Aeronautics and Space Administration (NASA) Psyche mission will visit the 226‐km diameter main belt asteroid (16) Psyche, our first opportunity to visit a metal‐rich object at close range. The unique and poorly understood nature of Psyche offers a challenge to the mission as we have little understanding of the surface morphology and composition. It is commonly accepted that the main evolutionary process for asteroid surfaces is impact cratering. While a considerable body of literature is available on collisions on rocky/icy objects, less work is available for metallic targets with compositions relevant to Psyche. Here we present a suite of impact experiments performed at the NASA Ames Vertical Gun Range facility on several types of iron meteorites and foundry‐cast ingots that have similar Fe‐Ni compositions as the iron meteorites. Our experiments were designed to better understand crater formation (e.g., size, depth), over a range of impact conditions, including target temperature and composition. We find that the target strength, as inferred from crater sizes, ranges from 700 to 1,300 MPa. Target temperature has measurable effects on strength, with cooled targets typically 10–20% stronger. Crater morphologies are characterized by sharp, raised rims and deep cavities. Further, we derive broad implications for Psyche's collisional evolution, in light of available low resolution shape models. We find that the number of large craters (>50 km) is particularly diagnostic for the overall bulk strength of Psyche. If confirmed, the number of putative large craters may indicate that Psyche's bulk strength is significantly reduced compared to that of intact iron meteorites.

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

“…Available data of Psyche's shape indicate the presence of up to four cavities >50 km on its surface (Shepard et al, ; Viikinkoski et al, ). This is compatible with the number of such craters predicted by current collision models for a time span of 4.5 Ga and for a bulk strength of ~50 MPa (Marchi et al, ). This strength exceeds that of common hard rocks, such as basalts.…”

“…To first order, the morphology observed in lab impact experiments is primarily controlled by the high strength of metal (~500 MPa). This strength is typical for solid centimeter‐scale metallic objects, such as Fe‐Ni man‐made alloys or iron meteorites (Marchi et al, ). At larger scale, however, strength could be significantly reduced by the presence of porosity and fractures.…”

“…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.…”

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

“…A particular challenge is the selection of the appropriate measure of target strength Y , as this is poorly defined and unlikely to be well characterized by a single value (Holsapple, 2009). For this reason, it is common to eliminate one of the material‐specific constants K R 2 by subsuming it into the definition of Y to define an effective “cratering” strength (e.g., Holsapple, 1993; Marchi et al, 2019; Prieur et al, 2017). In principle, this allows to be determined empirically for a given target material (e.g., Marchi et al, 2019); however, in practice this approach requires an independent method to determine K R 1 , which is typically only possible with numerical simulations (e.g., Prieur et al, 2017).…”

“…Although both temperature and strain can be high near the impact point, neither of these terms is likely to vary significantly with impactor size. On the other hand, it is well documented that iron's yield strength rises significantly at high strain rates and recent laboratory experiments into iron targets (Marchi et al, 2019) have noted the importance of accounting for the effect of strain rate when extrapolating from laboratory‐scale crater measurements to planetary scales, or vice‐versa. For example, laboratory measurements of the quasistatic yield strength of iron meteorites give values between 200 and 400 MPa (Petrovic, 2001), however at high strain rates, for example, s −1 , the equivalent strength can almost double (Marchi et al, 2019).…”