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 System. Although typical cometary activity tracers were not detected, `Oumuamua exhibited a significant non-gravitational acceleration. To date there is no explanation that can reconcile these constraints. Due to energetic considerations, outgassing of hyper-volatile molecules is favored over heavier volatiles like H2O and CO2. However, there are are theoretical and/or observational inconsistencies with existing models invoking the sublimation of pure H2 , N2, and CO. 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 which 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. 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.Comment: Author's version; 23 pages, 3 figure

CO diffusion and desorption kinetics in CO2_2 ices

Diffusion of species in icy dust grain mantles is a fundamental process that shapes the chemistry of interstellar regions; yet measurements of diffusion in interstellar ice analogs are scarce. Here we present measurements of CO diffusion into CO$_2$ ice at low temperatures (T=11--23~K) using CO$_2$ longitudinal optical (LO) phonon modes to monitor the level of mixing of initially layered ices. We model the diffusion kinetics using Fick's second law and find the temperature dependent diffusion coefficients are well fit by an Arrhenius equation giving a diffusion barrier of 300 $\pm$ 40 K. The low barrier along with the diffusion kinetics through isotopically labeled layers suggest that CO diffuses through CO$_2$ along pore surfaces rather than through bulk diffusion. In complementary experiments, we measure the desorption energy of CO from CO$_2$ ices deposited at 11-50 K by temperature-programmed desorption (TPD) and find that the desorption barrier ranges from 1240 $\pm$ 90 K to 1410 $\pm$ 70 K depending on the CO$_2$ deposition temperature and resultant ice porosity. The measured CO-CO$_2$ desorption barriers demonstrate that CO binds equally well to CO$_2$ and H$_2$O ices when both are compact. The CO-CO$_2$ diffusion-desorption barrier ratio ranges from 0.21-0.24 dependent on the binding environment during diffusion. The diffusion-desorption ratio is consistent with the above hypothesis that the observed diffusion is a surface process and adds to previous experimental evidence on diffusion in water ice that suggests surface diffusion is important to the mobility of molecules within interstellar ices