Letter to the Editor

Short-term variability on the surface of (1) Ceres^⋆,⋆⋆

A changing amount of water ice?

¹ LESIA–Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon, France
e-mail: davide.perna@obspm.fr
² Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
³ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (Roma), Italy
⁴ Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵ Université Paris Diderot – Paris 7, 4 rue Elsa Morante, 75013 Paris, France
⁶ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy

Received: 9 November 2014
Accepted: 24 January 2015

Abstract

Context. The dwarf planet (1) Ceres – next target of the NASA Dawn mission – is the largest body in the asteroid main belt. Although several observations of this body have been performed so far, the presence of surface water ice is still questioned.

Aims. Our goal is to better understand the surface composition of Ceres and to constrain the presence of exposed water ice.

Methods. We acquired new visible and near-infrared spectra at the Telescopio Nazionale Galileo (La Palma, Spain), and reanalyzed literature spectra in the 3-μm region.

Results. We obtained the first rotationally resolved spectroscopic observations of Ceres at visible wavelengths. Visible spectra taken one month apart at almost the same planetocentric coordinates show a significant slope variation (up to 3%/10³Å). A faint absorption centered at 0.67 μm, possibly due to aqueous alteration, is detected in a subset of our spectra. The various explanations in the literature for the 3.06-μm feature can be interpreted as due to a variable amount of surface water ice at different epochs.

Conclusions. The remarkable short-term temporal variability of the visible spectral slope and the changing shape of the 3.06-μm band can be hints of different amounts of water ice exposed on the surface of Ceres. This would agree with the recent detection by the Herschel Space Observatory of localized and transient sources of water vapor over this dwarf planet.