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 (H2) ice. H2 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 H2-rich bodies plausibly form in the coldest dense cores of giant molecular clouds, where number densities are of order n ∼ 105, 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.

1I/‘Oumuamua as an N2 Ice Fragment of an exo‐Pluto Surface: I. Size and Compositional Constraints

“…For N 2 ice at a typical surface temperature of 25 K, this yields V jet = 80 m s −1 . This is similar to, but slightly different from, the jetting speed considered likely by Seligman and Laughlin (2020), who used…”

“…The abundant ices H 2 O and CO 2 are immediately ruled out as the main constituents of 'Oumuamua, as they are incapable of providing the necessary non-gravitational acceleration for any albedo, as was also found by Seligman and Laughlin (2020). Pure NH 3 and O 2 ices are less likely than H 2 O and CO 2 , and also ruled out as not capable of providing sufficient force.…”

“…and the mass-to-surface area ratio, denoted as η by Seligman and Laughlin (2020), is proportional to…”

“…here ξ 0 is the ratio of the area projected to the Sun to the total surface area. While ξ 0 was 0.19 over the (relatively short) epoch of the light curve observations (Seligman and Laughlin (2020)), we assume that the isotropic value of 0.25 applied over longer periods, especially given the tumbling rotation of 'Oumuamua (Fraser et al, 2018). The infrared emissivity of the ice is ϵ, which we assume is roughly 0.85, an appropriate value for N 2 ice (Stansberry et al, 1996).…”

“…While more volatile substances can be entrained in common ices like H 2 O and CO 2 , the release of these entrained volatiles is largely controlled by the sublimation of the less volatile parent ice (e.g., Fayolle et al, 2016). In this work we focus on a possibility that was overlooked by Seligman and Laughlin (2020): N 2 ice. They found that a surface composed primarily of N 2 ice would provide sufficient acceleration and, in contrast to hypothetical H 2 ice, significantly pure N 2 ice is observed in the solar system, on the surfaces of Pluto and Triton.…”