Impact of irradiated methane ice crusts on compositional interpretations of TNOs

Context. Minor bodies in the outer solar system show a wide variety of spectral colors related to both composition and surface processing, e.g. cosmic ion irradiation. Aims. We investigate the effect of an irradiation mantle on the reflectance spectrum in the V-NIR (visible-near-infrared) range. In particular we investigate the condition needed for a weathering crust to mask the presence of water ice. Methods. We start from laboratory experiments of ion irradiated methane ice to study the optical properties of the hydrocarbon residue by-product of the space weathering process. We compare the real and imaginary index of refraction with those of Titan and ice tholins and with those of amorphous carbon. We use the estimated optical constants to model a layered configuration using the Hapke theory, varying the thickness and grain size of the modeled crust. Results. We find that a relatively thin (tens of microns) crust of irradiated methane by-products can mask the presence of water ice bands in the spectrum, while a much larger layer of tholins would be required to produce a similar effect. We also estimate the conditions for the detection of water ice on trans-Neptunian objects (TNOs). We discuss the results in view of the astrophysical timescale