Ground-based high angular-resolution images of asteroid ( 2 ) Pallas at near-infrared wavelengths have been used to determine its physical properties ( shape , dimensions , spatial orientation and albedo distribution ) . We acquired and analyzed adaptive-optics ( AO ) J/H/K -band observations from Keck II and the Very Large Telescope taken during four Pallas oppositions between 2003 and 2007 , with spatial resolution spanning 32–88 km ( image scales 13–20 km/pix ) .
We improve our determination of the size , shape , and pole by a novel method that combines our AO data with 51 visual light-curves spanning 34 years of observations as well as archived occultation data . The shape model of Pallas derived here reproduces well both the projected shape of Pallas on the sky ( average deviation of edge profile of 0.4 pixel ) and light-curve behavior ( average deviation of 0.019 mag ) at all the epochs considered .
We resolved the pole ambiguity and found the spin-vector coordinates to be within 5 ^ { \circ } of [ long , lat ] = [ 30 ^ { \circ } , -16 ^ { \circ } ] in the Ecliptic J2000.0 reference frame , indicating a high obliquity of about 84 ^ { \circ } , leading to high seasonal contrast .
The best triaxial-ellipsoid fit returns ellipsoidal radii of a =275 km , b = 258 km , and c = 238 km .
From the mass of Pallas determined by gravitational perturbation on other minor bodies [ ( 1.2 \pm 0.3 ) \times 10 ^ { -10 } M _ { \odot } , Michalak 2000 , A & A , 360 ] , we derive a density of 3.4 \pm 0.9 g.cm ^ { -3 } significantly different from the density of C-type ( 1 ) Ceres of 2.2 \pm 0.1 g.cm ^ { -3 } [ Carry et al .
2008 , A & A , 478 ] .
Considering the spectral similarities of Pallas and Ceres at visible and near-infrared wavelengths , this may point to fundamental differences in the interior composition or structure of these two bodies . We define a planetocentric longitude system for Pallas , following IAU guidelines .
We also present the first albedo maps of Pallas covering \sim 80 % of the surface in K-band .
These maps reveal features with diameters in the 70 - 180 km range and an albedo contrast of about 6 % with respect to the mean surface albedo .