Aims:The aim of this paper is to investigate the surface composition of the Transneptunian Object (TNO) Orcus.
Methods: High quality observations have been carried out with the new instrument SINFONI at the Very Large Telescope (VLT) of ESO.
Results: Crystalline water ice, and possibly ammonia ice, have been found from spectroscopic observations of the TNO Orcus between 1.4 and 2.4 μm.
Conclusions: The existence of such ices on the surface of Orcus may indicate a renewal mechanism on the surface and geological activity. The presence of ammonia on the surface of Orcus, if confirmed, could have important implications for the composition of the primitive solar nebula and the formation of the TNO population.