CERES SURFACE-PROPERTIES BY HIGH-RESOLUTION IMAGING FROM EARTH

Direct imaging of Ceres has been performed using an adaptive optics system, developed by Laserdot (France), Paris Observatory (Meudon, France), ONERA (France), and ESO. The diffraction limit of the 3.6-m ESO telescope (La Silla, Chile) was obtained at near infrared wavelengths in the K, L ′, and M bands (0.13, 0.25, and 0.30 arcsec, respectively). Several physical parameters for Ceres are deduced from these observations. The rotation is prograde (consistent with previous observations). The pole orientation is λ = 332 ± 5° and β = 70 ± 15°, implying a low obliquity with respect to Ceres orbital plane. The estimated thermal inertia of the regolith is 3.8 ± 1.4 × 104 erg cm-2 sec-1/2K-1. The measured temperature of the warmest area is 235 ± 4 K. The value of the mean radius, 484 ± 20 km, is consistent with earlier values obtained by stellar occultation and thermal infrared photometry. No albedo variations can be detected in reflected light and the center-to-limb distribution clearly departs from a Lambertian distribution. © 1993 by Academic Press, Inc.