Evidence that 1I/2017 U1 (‘Oumuamua) was Composed of Molecular Hydrogen Ice

Abstract

‘Oumuamua (I1 2017) was the first macroscopic ( l ∼ 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 nongravitational 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  ∼ 10 5 , and temperatures approach the T  = 3 K background. Post-formation exposure to galactic cosmic rays implies a τ  ∼ 100 Myr age, explaining the kinematics of ‘Oumuamua’s inbound trajectory.