VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis

Vol. 654
10. Planets and planetary systems

VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis

Until recently, detailed, kilometer-accurate, 3D shape information of asteroids has been limited to bodies visited by spacecraft missions. This situation has changed with the advent of imagers operating at the ~20 mas diffraction limit in the optical, such as SPHERE/ZIMPOL at the VLT. Vernazza et al. report the final results of an ESO Large Programme in which 42 large main belt asteroids have been imaged — out of which 20 of the 23 asteroids have a diameter larger than 200 km — and down to objects as small as ~ 100 km. In addition to remarkable results reported previously on several individual objects, including the evidence for craters that testify of family-forming catastrophic collisions, these observations provide a unique dataset on asteroid shapes and densities (combining the measured volumes with available mass estimates). In particular, they reveal that the departure from smooth shapes is inversely correlated with the objects' volume-equivalent radius; furthermore, there is a bimodality in shape ("spherical" versus "elongated" objects) associated with the rotation period, and a bimodality in density (< 2.2 g cm-3 and > 2.7 g cm-3) reflecting volatile-rich and volatile-poor bodies. Most objects, and especially the volatile-rich ones, appear to be located close to the Maclaurin equilibrium sequence. Finally the low density of some of the C- and P/D-type asteroids may point to an origin in the transneptunian disk.