Identifying Interstellar Object Impact Craters

Abstract

The discoveries of two interstellar objects (ISOs) in recent years have generated significant interest in constraining their physical properties and the mechanisms behind their formation. However, their ephemeral passages through our solar system permitted only incomplete characterization. We investigate avenues for identifying craters that may have been produced by ISOs impacting terrestrial solar system bodies, with particular attention toward the Moon. A distinctive feature of ISOs is their relatively high encounter velocity compared to asteroids and comets. Local stellar kinematics indicate that terrestrial solar system bodies should have experienced of order unity ISO impacts exceeding 100 km s−1. By running hydrodynamical simulations for projectiles of different masses and impact velocities up to 100 km s−1, we show how late-stage equivalence dictates that transient crater dimensions alone are insufficient for inferring the projectile’s velocity. On the other hand, the melt volume within craters of a fixed diameter may be a potential route for identifying ISO craters, as faster impacts produce more melt. This method requires that the melt volume scales with the energy of the projectile while the crater diameter scales with the point-source limit (subenergy). Given that there are probably only a few ISO craters in the solar system at best, and that transient crater dimensions are not a distinguishing feature for impact velocities, at least up to 100 km s−1, identification of an ISO crater proves a challenging task. Melt volume and high-pressure petrology may be diagnostic features once large volumes of material can be analyzed in situ.

The Interstellar Interlopers

“…Therefore, we are unlikely to find evidence for craters formed by interstellar projectiles or for interstellar meteorites in our collections. Even if the flux were higher, it is unclear how to distinguish a crater formed by an interstellar impact from one formed by a Solar System projectile (Cabot & Laughlin 2022).…”

Lunar ore geology and feasibility of ore mineral detection using a far-IR spectrometer

“…The discoveries of 'Oumuamua (from the Pan-STARRS survey; Meech et al, 2017) and comet 2I/Borisov (Guzik et al, 2020) have prompted intensive study of the number density, composition, and origin of ISOs. Motivated by an encouragingly high encounter rate of ISOs, up to about seven per year, that pass within 1 AU of the Sun, Cabot and Laughlin (2022) have proposed an alternative route to characterize ISOs by identifying their impact craters on terrestrial solar system bodies. For example, the molten and vaporized projectile matter may mix with impact-modified target rock (impactites) and impart telltale chemical signatures.…”

“…The secondary goals are 1) demonstration of mineralogical differences in the ore mineralization between the surfaces of the lunar mare and the anorthositic highlands, 2) verification of differences in ore mineralization between the lunar circumpolar areas and other areas, 3) providing new insights into the geology of KREEP areas enriched in K, REE, and P, located on the basalt mare's margins (Jolliff et al, 2000), which can be characterized for ore mineral content but also apatite (robust spectral features between 25 and 30 µm), as well as 4) identifying the potential interstellar object (ISO) craters on the Moon's surface (Cabot and Laughlin, 2022) if those are significantly enriched in metals.…”