Methanol Near-Infrared Bands as a Probe of Irradiated Ices in Space

Cosmic rays, solar ions and UV photons induce changes in both the structure and composition of frozen volatiles trapped in solar system small bodies such as comets, Kuiper-belt objects (KBOs) and trans-neptunian objects (TNOs).

Recently, Stern et al. 2019 reported about the presence of methanol and water ice on the surface of (486958) 2014 MU₆₉ revealed by the New Horizons space probe. Both species were identified also in other small bodies, such as (55638) 2002 VE₉₅ and the centaur 5145 Pholus. Laboratory experiments that simulate the interaction between cosmic rays and frozen volatiles have shown that when carbon-bearing species are bombarded with keV-MeV ions, energetic processing determines the formation of both volatiles (such as CO, CO₂, H₂CO, etc.) and refractory species that may account for the visible and near-infrared reddening observed in various solar system small bodies.

Here we present laboratory experiments performed to contribute in the interpretation of observations of methanol-rich bodies. Frozen samples containing water and methanol were deposited at temperatures that are close to those measured on 2014 MU₆₉ and other TNOs. After the deposition, frozen volatiles were bombarded with 40 keV protons between a few and hundreds eV/molecule. Samples were analysed in the near-infrared range, where methanol exhibit an intense band at 2.27 microns and minor feature at 2.34 microns (4400 and 4280 cm^-1, respectively). The analysis revealed a different sensitivity to ion irradiation of the two features. In particular, we show that the ratio between the band areas evolves with increasing irradiation doses. We suggest that it could be used as a probe to discern between pristine and processed ices, as well as to establish upper limits for the 2.34 microns band when only the 2.27 microns band is observed.