Acceleration of 1I/'Oumuamua from radiolytically produced H in HO ice.

In 2017, 1I/'Oumuamua was identified as the first known interstellar object in the Solar System. Although typical cometary activity tracers were not detected, 'Oumuamua showed a notable non-gravitational acceleration. So far, there has been no explanation that can reconcile these constraints. Owing to energetic considerations, outgassing of hyper-volatile molecules is favoured over heavier volatiles such as HO and CO (ref. ). However, there are theoretical and/or observational inconsistencies with existing models invoking the sublimation of pure H (ref. ), N (ref. ) and CO (ref. ). Non-outgassing explanations require fine-tuned formation mechanisms and/or unrealistic progenitor production rates. Here we report that the acceleration of 'Oumuamua is due to the release of entrapped molecular hydrogen that formed through energetic processing of an HO-rich icy body. In this model, 'Oumuamua began as an icy planetesimal that was irradiated at low temperatures by cosmic rays during its interstellar journey, and experienced warming during its passage through the Solar System. This explanation is supported by a large body of experimental work showing that H is efficiently and generically produced from HO ice processing, and that the entrapped H is released over a broad range of temperatures during annealing of the amorphous water matrix. We show that this mechanism can explain many of 'Oumuamua's peculiar properties without fine-tuning. This provides further support that 'Oumuamua originated as a planetesimal relic broadly similar to Solar System comets.