Methanol on red TNOs: A link between early composition and irradiation history

Abstract

Solid methanol is detected on small icy bodies of the outer solar system, namely trans-Neptunian objects (TNOs). Interestingly, all objects with methanol detections also display a strong positive spectral slope in the visible to near-infrared range, commonly attributed to complex organics, that could be formed by space weathering of methanol ice. We question the relationship between this red color and methanol content by performing irradiation experiments on methanol ices, pure and mixed with water, to simulate the ion bombardment of TNO surfaces. To compare the experimental spectral evolution under irradiation to observations, we focused our investigations on the normalized reflectivity gradient between the J and R photometric bands (JR slope) and the methanol to water abundance ratio post-irradiation. We show that ion irradiation triggers the destruction of methanol and the spectral reddening of the ices. Moreover, we find that JR slopes depend on both irradiation dose and initial ice composition. To account for red slopes alongside detectable methanol ice, we conclude that an irradiation gradient needs to be present at the surface of methanol-rich objects. However, methanol needs to be at least as abundant as water in the initial icy mixture, which suggests an early segregation of the icy species. We provide an estimation of the irradiation doses received at different depths of methanol-rich TNO surfaces, consistent with surface exposures of $\sim$4.5 Gyrs.