2005A&A...436.1113V

We obtained high signal to noise spectra of the two targets of the Dawn mission, 4 Vesta and 1 Ceres from observations carried out in remote control between the Observatoire de Paris-Meudon and the NASA Infrared Telescope Facility on Mauna Kea. 4 Vesta was observed in the 0.7-2.5µm spectral region at three different rotational phases in order to i) determine the mineral composition; ii) understand the spectral variations across the surface. Vesta was also observed in the 2.0-3.8µm range. The 3µm absorption feature was not detected, implying the absence of OH and/or H2O-bearing minerals on the asteroid surface at the latitude of our observations. The spectrum of 1 Ceres was obtained in the 2.0-4.1µm range and the presence of the 3.06µm absorption feature confirmed. Laboratory measurement of ion-irradiated organics and ices suggest that the 3.06µm feature can be reproduced with a linear mixture of crystalyne ice and residues of ion-irradiated asphaltite.