Composition of KBO (50000) Quaoar

International audienceAims: The objective of this work is to investigate the physical properties of objects beyond Neptune - the new frontiers of the Solar System - and in particular to study the surface composition of (50 000) Quaoar, a classical Transneptunian (or Kuiper Belt) object. Because of its distance from the Sun, Quaoar is expected to have preserved, to a degree, its original composition. Our goals are to determine to what degree this is true and to shed light on the chemical evolution of this icy body. Methods: We present new near-infrared (3.6 and 4.5 mum) photometric data obtained with the Spitzer Space Telescope. These data complement high resolution, low signal-to-noise spectroscopic and photometric data obtained in the visible and near-infrared (0.4-2.3 mum) at VLT-ESO and provide an excellent set of constraints in the model calculation process. We perform spectral modeling of the entire wavelength range - from 0.3 to 4.5 mum by means of a code based on the Shkuratov radiative transfer formulation of the slab model. We also attempt to determine the temperature of H2O ice making use of the crystalline feature at 1.65 mum. Results: We present a model confirming previous results regarding the presence of crystalline H2O and CH4 ice, as well as C2H6 and organic materials, on the surface of this distant icy body. We attempt a measurement of the temperature and find that stronger constraints on the composition are needed to obtain a precise determination. Conclusions: Model fits indicate that N2 may be a significant component, along with a component that is bright at lambda>3.3~mum, which we suggest at this time could be amorphous H2O ice in tiny grains or thin grain coatings. Irradiated crystalline H2O could be the source of small-grained amorphous H2O ice. The albedo and composition of Quaoar, in particular the presence of N2, if confirmed, make this TNO quite similar to Triton and Pluto