Analysis of near-IR spectra of 1 Ceres and 4 Vesta, targets of the Dawn mission

Abstract

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 0.7–2.5 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 2.0–3.8 range. The 3 μm 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 2.0–4.1 μm range and the presence of the 3.06 μm absorption feature confirmed. Laboratory measurement of ion-irradiated organics and ices suggest that the 3.06 μm feature can be reproduced with a linear mixture of crystalyne ice and residues of ion-irradiated asphaltite.