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A New Dawn

Since 17 July 2011, NASA's spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684) use Dawn's observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687) report on the asteroid's overall geometry and topography, based on global surface mapping. Vesta's surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690) report on Vesta's complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694) describe two giant impact basins located at the asteroid's south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697) present the mineralogical characterization of Vesta, based on data obtained by Dawn's visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet.

Abstract

Dawn’s global mapping of Vesta reveals that its observed south polar depression is composed of two overlapping giant impact features. These large basins provide exceptional windows into impact processes at planetary scales. The youngest, Rheasilvia, is 500 kilometers wide and 19 kilometers deep and finds its nearest morphologic analog among large basins on low-gravity icy satellites. Extensive ejecta deposits occur, but impact melt volume is low, exposing an unusual spiral fracture pattern that is likely related to faulting during uplift and convergence of the basin floor. Rheasilvia obliterated half of another 400-kilometer-wide impact basin, Veneneia. Both basins are unexpectedly young, roughly 1 to 2 billion years, and their formation substantially reset Vestan geology and excavated sufficient volumes of older compositionally heterogeneous crustal material to have created the Vestoids and howardite–eucrite–diogenite meteorites.

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References and Notes

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Published In

Science
Volume 336 | Issue 6082
11 May 2012

Submission history

Received: 12 April 2012
Accepted: 20 April 2012
Published in print: 11 May 2012

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Acknowledgments

The authors thank D. Bogard, E. Asphaug, and H. J. Melosh for helpful discussions and comments and the NASA Dawn at Vesta Participating Scientist program for support. We thank the Dawn team for the development, cruise, orbital insertion, and operations of the Dawn spacecraft at Vesta. A portion of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Dawn data are archived with the NASA Planetary Data System.

Authors

Affiliations

Paul Schenk* [email protected]
Lunar and Planetary Institute, Houston, TX 77058, USA.
David P. O’Brien
Planetary Science Institute, Tucson, AZ 85719, USA.
Simone Marchi
NASA Lunar Science Institute, Boulder, CO 12489, USA.
Robert Gaskell
Planetary Science Institute, Tucson, AZ 85719, USA.
Frank Preusker
Institute of Planetary Research, Deutsches Zentrum für Luft- und Raumfahrt, Berlin 80302, Germany.
Thomas Roatsch
Institute of Planetary Research, Deutsches Zentrum für Luft- und Raumfahrt, Berlin 80302, Germany.
Ralf Jaumann
Institute of Planetary Research, Deutsches Zentrum für Luft- und Raumfahrt, Berlin 80302, Germany.
Debra Buczkowski
Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA.
Thomas McCord
Bear Fight Institute, Wintrop, WA 98862, USA.
Harry Y. McSween
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA.
David Williams
School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.
Aileen Yingst
Planetary Science Institute, Tucson, AZ 85719, USA.
Carol Raymond
Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
Chris Russell
Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA.

Notes

*
To whom correspondence should be addressed. E-mail: [email protected]

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