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Orcus is an intermediate-size 1000km-scale Kuiper Belt Object in 3:2mean-motion resonance with Neptune, in an orbit very similar to that of Pluto.We present visible and near-infrared photometry and spectroscopy obtained withthe Keck 10m-telescope and the Gemini 8m-telescope . We confirm the unambiguousdetection of crystalline water ice as well as absorption in the 2.2\mu mregion. Both in the visible and near-infrared Orcus' spectral properties appearto be homogeneous over time (and probably rotation) at the resolutionavailable. From Hapke radiative transfer models involving intimate mixtures ofvarious ices we find for the first time that ammonium (NH+4) and traces ofethane (C2 H6), which are most probably solar irradiation products of ammoniaand methane, and a mixture of methane and ammonia (diluted or not) are the bestcandidates to improve the description of the data with respect to a simplewater ice mixture (Haumea type surface). The possible more subtle structure ofthe 2.2\mu m band(s) should be investigated thoroughly in the future for Orcusand other intermediate size Plutinos to better understand the methane andammonia chemistry at work, if any. We investigated the thermal history of Orcuswith a new 3D thermal evolution model. Simulations over 4.5 x109 yrs with aninput 10% porosity, bulk composition of 23% amorphous water ice and 77% dust,and cold accretion show that even with the action of long-lived radiogenicelements only, Orcus should have a melted core and most probably suffered acryovolcanic event in its history which brought large amounts of crystallineice to the surface. The presence of ammonia in the interior would strengthenthe melting process. The crystalline water ice possibly brought to the surfaceby a past cryovolcanic event sbe detectable after several billion years despitethe irradiation eects, as demonstrated by recent laboratory experiments.

Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? A portrait of Plutino (90482) Orcus