Image: Deconvolved image of Ceres. The rotational axis is aligned with vertical. The dark feature with a bright spot in its center serves as reference meridian for mapping (see CeresMap.png). The bright pixels on Ceres edge are artifacts from deconvolution.

Although Ceres is the largest main-belt asteroid and was the first to be discovered (by G.Piazzi in 1801), its physical properties are still not well understood. While it is expected to have retained a large amount of primordial water ice in its interior, many questions about the composition of its surface and sub-surface layers, the properties of its regolith, its degree of differentiation, etc, remain unanswered.

We used state-of-the-art adaptive optics instrumentation available at the Keck observatory, Mauna Kea, Hawaii, to image the surface of Ceres with a spatial resolution of ~30km. Our observations were carried out during the 2002 opposition of Ceres in the near-infrared J/H/K-bands, at a wavelength range particularly well adapted to investigate the composition and properties of planetary surfaces. We produced albedo maps covering 80% of the asteroid, which appears to display a wealth of 40 to 160km large geological features with intensity in reflected light varying by ~ 12% across the surface (see attachment). We believe that these variations could be due to particular terrain features, as well as differences in their surface composition and/or degree of alteration by space weathering effects (ageing of surface due to interaction of solar wind, micrometeorites impacts, etc).

We also derived measurements of the dimension and shape of Ceres, which can be considered as an oblate spheroid of radii a=481km and b=447km (incertitude of +/-14 km). The direction of its spin axis in a J2000 coordinate referential is right ascension=287 degrees and declination=69 degrees (5 degrees of incertitude). These two results are in agreements with earlier reports made by Thomas et al. (Nature 2005) from the analysis of Hubble Space Telescope observations.

Additional spatially resolved spectroscopic observations are needed to investigate further the properties of Ceres surface. Such programs will help in the preparation of the NASA DAWN mission, which will reach Ceres in 2015 and explore, from orbit, the properties of this intriguing solar system body. We are currently planning complementary observations of Ceres during its next opposition of November 2007, using a suite of adaptive optics instruments at the Very Large Telescope of the European Southern Observatory in Chili.

The images can be downloaded from:

http://www.sc.eso.org/~cdumas/CeresImage.png and http://www.sc.eso.org/~cdumas/CeresMap.png

Contact Information:

Benoit Carry
benoit.carry@free.fr
Paris-Meudon Observatory

Christophe Dumas
cdumas@eso.org
+56 8 219 3150
European Southern Observatory

M. Fulchignoni (Paris-Meudon Observatory)
T. Fusco (Onera)
W. Merline (Southwest Research Institute, Boulder, Colorado)