Composition of KBO (50000) Quaoar

Aims. 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 μm) 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 μm) 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 μm 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 μm. Results. We present a model confirming previous results regarding the presence of crystalline H₂O and CH₄ ice, as well as C ₂H₆ 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 < 3.3 μm, 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.