2012A&A...544A..20M

Spectral analyses of trans-Neptunan objects (TNOs) and of the linked Centaurs, which are supposed to be among the most primitive minor bodies in the solar system, reveal some chemical and physical properties of their surface. To determine the surface composition of these objects and their surface evolution is essential for gaining clues on the conditions under which the solar system has been formed. Chemical composition and physical properties of the surface of three objects have been constrained by computing the depth of the absorption features of the spectra in the near-infrared, running spectral models based on radiative transfer theory in the [0.4-2.3]µm range, and analyzing new laboratory measurements of the spectral behavior of thin samples of H₂O-CH₃OH mixtures. Our investigations allow us to confirm the presence of CH₃OH ice on the surface of the Centaur (5145) Pholus and the resonant (55638) TNO 2002 VE_95_. It may also possibly be found on the classical TNO (120348) 2004 TY_364_. Our laboratory experiments indicate that the behavior of the methanol and water ice absorption bands is dependent on the ambient temperature and the dilution level of the mixture. These results also suggest that methanol may be diluted in water ice on the surface of the Centaur Pholus. Formation and destruction processes of methanol suggest that a part (at least) of the surface of these objects is younger than the solar system age. If confirmed, this shows that primordial ices could still be detected on the surface of objects that are submitted to irradiation and rejuvenation processes.