We thank K. Tanaka and L. Kestay (USGS-Flagstaff) and four anonymous referees for constructive reviews of this manuscript. This research was carried out for the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA under the Mars Program Office, including JPL contracts 1449884 (R.M.E.W.) and 1273887 (Malin Space Science Systems). Work in France was carried out with funding from the Centre National d'Etudes Spatiales. Work in the UK was funded by the UK Space Agency. Work in Denmark was funded by the Danish Council for Independent Research/Natural Sciences (FNU grants 12-127126 and 11-107019) and the TICRA Foundation. Work in Germany was partly funded by Deutsche Forschungsgemeinschaft grant GO 2288/1-1. Data in this manuscript are available from the NASA Planetary Data System. This is PSI contribution 603.