Physical properties of (2) Pallas

Abstract

a b s t r a c tGround-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/pixel). 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°of [longitude, latitude] = [30°, À16°] in the Ecliptic J2000.0 reference frame, indicating a high obliquity of about 84°, leading to high seasonal contrast. The best triaxial-ellipsoid fit returns ellipsoidal radii of a ¼ 275 km; b ¼ 258 km, and c ¼ 238 km. From the mass of Pallas determined by gravitational perturbation on other minor bodies ð1:2 AE 0:3Þ Â 10 À10 M , [Michalak, G., 2000. Astron. Astrophys. 360, 363-374], we derive a density of 3:4 AE 0:9 g cm À3 significantly different from the density of C-type (1) Ceres of 2:2 AE 0:1 g cm À3 [Carry, B., Dumas, C., Fulchignoni, M., Merline, W.J., Berthier, J., Hestroffer, D., Fusco, T., Tamblyn, P., 2008. Astron. Astrophys. 478 (4), 235-244]. 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 $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.

Physical properties of the ESA Rosetta target asteroid (21) Lutetia

“…A complete description of the method can be found elsewhere , as well as an example of its application on (2) Pallas (Carry et al 2010).…”

“…The optical lightcurves bring indirect constraints on the shape of Lutetia, provided the albedo is homogeneous over its surface. Indeed, lightcurves are influenced by a combination of the asteroid shape 4 and albedo variation (see the discussion by Carry et al 2010, regarding the effect of albedo features on the shape reconstruction).…”

“…In this respect, our observing program with adaptive optics, allowing diffraction-limited observations from the ground with 10 m-class telescopes, has now broken the barrier that separated asteroids from real planetary worlds (e.g., Conrad et al 2007;Carry et al 2008;Drummond et al 2009;Carry et al 2010;Drummond et al 2010). Their shapes, topography, sizes, spins, surface features, albedos, and color variations can now be directly observed from the ground.…”

Density of asteroids

“…The recent recovery of the Almahata Sitta meteorite, originating from the impact of asteroid 2008 TC 3 on Earth in October 2008, indeed indicated that B-type could be associated with unusual Ureilite achondrites (Jenniskens et al 2009). Based on a comparison of the densities of (1) Ceres and (2) Pallas (used as archetypes for the definition of C and B taxonomic classes), Carry et al (2010a) had suggested that B-types were less hydrated than C-types; a hypothesis supported by the lack of signature of organic or icy material in their spectra (Jones et al 1990). …”

Physical properties of the ESA Rosetta target asteroid (21) Lutetia

“…Based on previously determined sizes of Lutetia, with diameters ranging from 96 km from IRAS (Tedesco et al 2002(Tedesco et al , 2004 to 116 km from radar (Magri et al 1999(Magri et al , 2007, Lutetia should have presented an apparent diameter of 0.10 , slightly more than twice the diffraction limit of the Keck Observatory 10 m telescope at infrared wavelengths (1−2 μm). Continuing our campaign (Conrad et al 2007;Drummond et al 2009;Carry et al 2010a) to study asteroids resolved with the world's large telescopes equipped with adaptive optics (AO), we have acquired more than 300 images of Lutetia, most from the 2008-09 season. An exceptionally good set of 81 images was obtained on 2008 December 2 with the Keck II telescope, which, despite the high sub-Earth latitude, yields a full triaxial ellipsoid solution from the changing apparent ellipses projected onto the plane of the sky by the asteroid.…”