Near-infrared mapping and physical properties of the dwarf-planet Ceres

Aims. We study the physical characteristics ( shape, dimensions, spin axis direction, albedo maps, mineralogy) of the dwarf-planet Ceres based on high angular-resolution near-infrared observations. Methods. We analyze adaptive optics J/H/K imaging observations of Ceres performed at Keck II Observatory in September 2002 with an equivalent spatial resolution of similar to 50 km. The spectral behavior of the main geological features present on Ceres is compared with laboratory samples. Results. Ceres' shape can be described by an oblate spheroid ( a = b = 479.7 +/- 2.3 km, c = 444.4 +/- 2.1 km) with EQJ2000.0 spin vector coordinates alpha(0) = 288 degrees +/- 5 degrees. and delta(0) = +66 degrees +/- 5 degrees Ceres sidereal period is measured to be 9.07410(-0.00014)(+0.00010) h. We image surface features with diameters in the 50-180 km range and an albedo contrast of similar to 6% with respect to the average Ceres albedo. The spectral behavior of the brightest regions on Ceres is consistent with phyllosilicates and carbonate compounds. Darker isolated regions could be related to the presence of frost.