SAMUEL CABOT'S RESEARCH SITE

CHARACTERIZING INTERSTELLAR OBJECTS

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An illustration of the first discovered interstellar object, `Oumuamua. At present, there is no consensus regarding the object's composition or origins. Observations have ruled out typical minor body classifications such as comets and asteroids.

Interstellar objects (ISOs) are an enigmatic population of free-floating minor bodies in the Milky Way. The first discovered ISO, 1I/`Oumuamua, was discovered in 2017. It exhibited non-gravitational acceleration during the outbound portion of its trajectory; however, followup observations did not detect a coma comprised of volatiles typical for Solar System comets. If outgassing was indeed responsible for the acceleration, these observations suggest `Oumuamua must have been comprised of a never-before-seen exotic ice that, following sublimation, does not produce transitions visible to Spitzer (e.g. Nitrogen or Hydrogen). The second discovered ISO, 2I/Borisov was cometary in nature. However, it was significantly enriched in CO relative to water ice, which makes it a fascinating outlier compared to most Solar System comets. 

In Levine et al. (2021), I helped assess the production of nitrogen ice fragments from collisions analogous to Kuiper Belt Objects (KBOs) striking Pluto. Desch & Jackson (2021) proposed such collisions as responsible for forming 'Oumuamua and similar Interstellar Objects (ISOs). We found that impacts are generally of insufficient speed to eject lightly-shocked nitrogen fragments. The ejecta produced by Grady-Kipp fragmentation are also considerably smaller than current estimates for 'Oumuamua's diameter. These findings prompted us to revisit the possibility that `Oumuamua was comprised of CO ice (Seligman et al. 2021). We found consistency between the energetics constrained by the Spitzer upper-limits on outgassed CO and `Oumuamua's non-ballistic trajectory, and that a bursty-outgassing model may satisfy the otherwise self-conflicting observed features.

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Simulated KBO Impact on Pluto (left) using iSALE 2D (e.g. Collins 2014) against predictions from shock wave-interference theory (Melosh 1984). Figure from Levine et al. (2021).

In Levine et al. (2021), I helped assess the production of nitrogen ice fragments from collisions analogous to Kuiper Belt Objects (KBOs) striking Pluto. Desch & Jackson (2021) proposed such collisions as responsible for forming 'Oumuamua and similar Interstellar Objects (ISOs). We found that impacts are generally of insufficient speed to eject lightly-shocked nitrogen fragments. The ejecta produced by Grady-Kipp fragmentation are also considerably smaller than current estimates for 'Oumuamua's diameter. These findings prompted us to revisit the possibility that `Oumuamua was comprised of CO ice (Seligman et al. 2021). We found consistency between the energetics constrained by the Spitzer upper-limits on outgassed CO and `Oumuamua's non-ballistic trajectory, and that a bursty-outgassing model may satisfy the otherwise self-conflicting observed features.

Assuming that ISOs follow a velocity distribution matching that of nearby stars, a non-trivial fraction should encounter the Solar System at speeds exceeding 100 km/s, which is faster than virtually any impact that could occur between a Kuiper-Belt Object and a terrestrial planet or moon. I investigated the prospects for identifying the fastest impacts on the Moon (Cabot & Laughlin 2022) as an independent route to characterizing ISOs. While crater morphology is strongly degenerate between the projectile's speed, angle of approach, and size, geophysical alterations to the target material may reveal an anomalously high impact speed. These features include enhanced melt and vapor products, as well as high-pressure mineralogy. Identification will require large-scale characterization of the lunar regolith.

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Simulated impacts of icy projectiles into basaltic targets, representative of ISO impacts on terrestrial bodies like the Moon. Faster impacts can generate considerably higher pressures upon impact, which has ramifications for melt volume production and high-pressure mineral states.