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