Evidence that 1I/2017 U1 (`Oumuamua) was composed of molecular hydrogen ice

Darryl Seligman; Gregory Laughlin

doi:10.3847/2041-8213/ab963f

paper

Evidence that 1I/2017 U1 (`Oumuamua) was composed of molecular hydrogen ice

arXiv:2005.12932 · doi:10.3847/2041-8213/ab963f

Abstract

`Oumuamua (I1 2017) was the first macroscopic ( $l \sim 100 m$ ) body observed to traverse the inner solar system on an unbound hyperbolic orbit. Its light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that 'Oumuamua experienced non-gravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. We show that all of `Oumaumua's observed properties can be explained if it contained a significant fraction of molecular hydrogen (H $_{2}$ ) ice. H $_{2}$ sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining `Oumuamua's shape. Back-tracing `Oumuamua's trajectory through the Solar System permits calculation of its mass and aspect ratio prior to encountering the Sun. We show that H $_{2}$ -rich bodies plausibly form in the coldest dense cores of Giant Molecular Clouds, where number densities are of order $n \sim 1 0^{5}$ , and temperatures approach the $T = 3 K$ background. Post-formation exposure to galactic cosmic rays implies a $τ \sim 100$ Myr age, explaining the kinematics of `Oumuamua's inbound trajectory.

Accepted for publication in ApJL, 8 pages, 2 figures. All software is available at https://github.com/DSeligman/Oumuamua_Hydrogen

Evidence that 1I/2017 U1 (`Oumuamua) was composed of molecular hydrogen ice

Abstract

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