(50000) Quaoar: Surface composition variability

Aims. The goal of this work is to investigate the composition of the surface of (50000) Quaoar and its spatial variability. Methods. We present new continuous spectra from the visible to near-IR (0.3−2.3 μm) obtained with the X-Shooter instrument at the VLT-ESO at four different epochs on the surface of Quaoar. The data represent the highest spectral resolution data ever obtained for this object and the first near-IR dataset acquired in a single exposure over the entire wavelength range. They are complemented by previously published photometric observations obtained in the near-IR (3.6, 4.5 μm) with the Spitzer Space Telescope, which provide an extra set of constraints in the model calculation. Spectral modelling was performed for the entire wavelength range by means of a code based on the Shkuratov radiative transfer formulation and of an updated value of albedo obtained from recent Herschel observations. Results. We obtained compositional information for different observed areas that can cover about 40% of the assumed rotational period of 8.84 h. Our analysis helps to prove the presence of CH4 and C2H6, as previously reported, along with indications of the possible presence of NH3·H2O. New evidence of N2 is inferred from the shift in the CH4 bands. The albedo at the two Spitzer bands suggests there may be CO diluted in N2, and CO2 for one of the surface locations. Conclusions. The spectral similarities indicate the overall homogeneity of the surface composition of one hemisphere of Quaoar, while some subtle variations are apparent when modelling. The presence of NH3·H2O would support the idea that Quaoar’s surface may be relatively young