Acceleration of 1I/'Oumuamua from radiolytically produced H₂ in H₂O ice

In 2017, 1I/'Oumuamua was identified as the first known interstellar object in the Solar System¹. Although typical cometary activity tracers were not detected^2-6, 'Oumuamua showed a notable non-gravitational acceleration⁷. So far, there has been no explanation that can reconcile these constraints⁸. Owing to energetic considerations, outgassing of hyper-volatile molecules is favoured over heavier volatiles such as H₂O and CO₂ (ref. ⁹). However, there are theoretical and/or observational inconsistencies¹⁰ with existing models invoking the sublimation of pure H₂ (ref. ⁹), N₂ (ref. ¹¹) and CO (ref. ¹²). Non-outgassing explanations require fine-tuned formation mechanisms and/or unrealistic progenitor production rates^7,13-15. Here we report that the acceleration of 'Oumuamua is due to the release of entrapped molecular hydrogen that formed through energetic processing of an H₂O-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 H₂ is efficiently and generically produced from H₂O ice processing, and that the entrapped H₂ is released over a broad range of temperatures during annealing of the amorphous water matrix^16-22. We show that this mechanism can explain many of 'Oumuamua's peculiar properties without fine-tuning. This provides further support³ that 'Oumuamua originated as a planetesimal relic broadly similar to Solar System comets.