  During its year-long orbital mission, the Dawn spacecraft has mapped the
surface of main-belt asteroid Vesta multiple times at different spatial
resolutions and illumination and viewing angles. The onboard Framing Camera has
acquired thousands of clear filter and narrow band images, which, with the
availability of high-resolution global shape models, allows for a photometric
characterization of the surface in unprecedented detail. We analyze clear
filter images to retrieve the photometric properties of the regolith. In the
first part of the paper we evaluate different photometric models for the global
average. In the second part we use these results to study variations in albedo
and steepness of the phase curve over the surface. Maps of these two
photometric parameters show large scale albedo variations, which appear to be
associated with compositional differences. They also reveal the location of
photometrically extreme terrains, where the phase curve is unusually shallow or
steep. We find that shallow phase curves are associated with steep slopes on
crater walls and faults, as calculated from a shape model. On the other hand,
the phase curve of ejecta associated with young impact craters is steep. We
interpret these variations in phase curve slope in terms of physical roughness
of the regolith. The lack of rough ejecta around older craters suggests that
initially rough ejecta associated with impact craters on Vesta are smoothed
over a relatively short time of several tens of Myr. We propose that this
process is the result of impact gardening, and as such represents a previously
unrecognized aspect of Vesta space weathering (Pieters et al., 2012). If this
type of space weathering is common, we may expect to encounter this photometric
phenomenon on other main belt asteroids.
