D.A. Williams^a, R. Jaumann^b,c, H.Y. McSween Jr.^d, S. Marchi^e, N. Schmedemann^c, C.A. Raymond^f, C.T. Russell^g

^aSchool of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA
^bDLR, Institute of Planetary Research, Berlin, Germany
^cFreie Universität Berlin, Institut für Geowissenschaften, Germany
^dDepartment of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee, 37996-1410, USA
^eSolar System Exploration Research Virtual Institute, Southwest Research Institute, Boulder, Colorado, USA
^fNASA JPL, California Institute of Technology, Pasadena, California, USA
^gUCLA, Los Angeles, California, USA

In this paper we present a time-stratigraphic scheme and geologic time scale for the protoplanet Vesta, based on global geologic mapping and other analyses of NASA Dawn spacecraft data, complemented by insights gained from laboratory studies of howardite-eucrite-diogenite (HED) meteorites and geophysical modeling. On the basis of prominent impact structures and their associated deposits, we propose a time scale for Vesta that consists of four geologic time periods: Pre-Veneneian, Veneneian, Rheasilvian, and Marcian. The Pre-Veneneian Period covers the time from the formation of Vesta up to the Veneneia impact event, from 4.6 Ga to >2.1 Ga (using the asteroid flux-derived chronology system) or from 4.6 Ga to 3.7 Ga (under the lunar-derived chronology system). The Veneneian Period covers the time span between the Veneneia and Rheasilvia impact events, from >2.1 to 1 Ga (asteroid flux-derived chronology) or from 3.7 to 3.5 Ga (lunar-derived chronology), respectively. The Rheasilvian Period covers the time span between the Rheasilvia and Marcia impact events, and the Marcian Period covers the time between the Marcia impact event until the present. The age of the Marcia impact is still uncertain, but our current best estimates from crater counts of the ejecta blanket suggest an age between ∼120-390 Ma, depending upon choice of chronology system used. Regardless, the Marcia impact represents the youngest major geologic event on Vesta. Our proposed four-period geologic time scale for Vesta is, to a first order, comparable to those developed for other airless terrestrial bodies.

Reference
Williams DA, Jaumann R, McSween Jr. HY, Marchi S, Schmedemann N, Raymond CA and Russell CT (in press) The chronostratigraphy of protoplanet vesta. Icarus
[doi:10.1016/j.icarus.2014.06.027]
Copyright Elsevier

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