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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 ∼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 α 0 = 288• ± 5• and δ 0 = +66 • ± 5• . Ceres sidereal period is measured to be 9.074 10 +0.000 10 −0.000 14 h. We image surface features with diameters in the 50-180 km range and an albedo contrast of ∼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.

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