Near-infrared Methanol Bands Probe Energetic Processing of Icy Outer Solar System Objects

Frozen methanol was detected in the outer solar system on the surfaces of the Centaur 5145 Pholus, the Trans-Neptunian Object (55638) 2002 VE₉₅, and more recently on (486958) Arrokoth. The icy surfaces of these objects are subjected to solar and cosmic ions that modify the physico-chemical properties of their surface. To study the effects of ion bombardment on methanol-rich surfaces, we performed experiments of ion irradiation of H₂O:CH₃OH mixtures and we monitored the evolution of the methanol near-infrared bands. We observed significant variations of the 2.34/2.27 μm methanol band ratios as a function of the irradiation dose. We then used the Arrokoth and Pholus spectra to test the 2.34/2.27 μm band ratio as a probe of irradiation of methanol-rich surfaces, and we estimated the timescales of processing by solar and cosmic ions. Our results indicate that solar energetic particles are the main drivers of changes in the near-infrared spectra of frozen surfaces in the outer solar system.