A new geologic map of the asteroid Vesta has been published by Arizona State University planetary scientist David
Williams and his colleagues. It's online in the
December special issue of the journal Icarus on The Geology of Vesta.
They propose a chronostratigraphic scheme and geologic timescale for Vesta with four periods based on major impact events. The conclude that the proposed vestan geologic timescale is comparable to those developed for other airless terrestrial planets, and Vesta’s youngest period is not based on rayed craters, due to different space weathering.
Ref: D.A. Williamsa, R. Jaumannb, H.Y. McSween Jr., S. Marchie, N. Schmedemann, C.A. Raymond, C.T. Russell, "The chronostratigraphy of protoplanet Vesta,"Icarus, Volume 244, December 2014, Pages 158–165