The<i>Dawn</i>exploration of (4) Vesta as the ‘ground truth’ to interpret asteroid polarimetry

Abstract

The results of the in situ exploration of the asteroid (4) Vesta by the Dawn spacecraft open&#13;\nnew perspectives in the field of interpretation of remote-sensing polarimetric measurements of&#13;\nasteroids. (4) Vesta has long been known to be the only asteroid exhibiting a cyclic variation&#13;\nof the degree of linear polarization of the sunlight scattered by its surface, with a period which&#13;\nis synchronous with the object’s rotation. This variation must be the consequence of some&#13;\nheterogeneity of the asteroid’s surface, including regions characterized by different albedo,&#13;\nor composition, or regolith properties, or a combination of the above features. For a long&#13;\ntime, this kind of conclusion has remained essentially qualitative. Now, after the extensive&#13;\nexploration of Vesta’s surface by Dawn, it is possible to interpret the data set of polarimetric&#13;\nmeasurements of Vesta, including some unpublished data presented here for the first time, in&#13;\nterms of a correspondence between the degree of linear polarization and the variation of local&#13;\nproperties of the surface visible to ground-based observers during Vesta’s rotation, as seen at&#13;\ndifferent epochs and under different illumination conditions. This makes it possible to refine&#13;\nour knowledge of the empirical relation between polarization properties and albedo, which is&#13;\ncommonly used to derive the albedo from remote-sensing measurements of linear polarization&#13;\nof atmosphereless Solar system bodies.