NASA Dawn Mission Reveals Secrets of Large Asteroid

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Jia-Rui Cook 818-354-0850
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Dwayne Brown 202-358-1726
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News release: 2012-132 May 10, 2012

NASA Dawn Mission Reveals Secrets of Large Asteroid

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2012-132&cid=release_2012-132

PASADENA, Calif. – NASA's Dawn spacecraft has provided researchers with the first orbital
analysis of the giant asteroid Vesta, yielding new insights into its creation and kinship with
terrestrial planets and Earth's moon.

Vesta now has been revealed as a special fossil of the early solar system with a more varied, diverse
surface than originally thought. Scientists have confirmed a variety of ways in which Vesta more
closely resembles a small planet or Earth's moon than another asteroid. Results appear in today's
edition of the journal Science.

"Dawn's visit to Vesta has confirmed our broad theories of this giant asteroid's history, while helping
to fill in details it would have been impossible to know from afar," said Carol Raymond, deputy
principal investigator at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Dawn's residence at
Vesta of nearly a year has made the asteroid's planet-like qualities obvious and shown us our
connection to that bright orb in our night sky."

Scientists now see Vesta as a layered, planetary building block with an iron core – the only one
known to survive the earliest days of the solar system. The asteroid's geologic complexity can be
attributed to a process that separated the asteroid into a crust, mantle and iron core with a radius of
approximately 68 miles (110 kilometers) about 4.56 billion years ago. The terrestrial planets and
Earth's moon formed in a similar way.

Dawn observed a pattern of minerals exposed by deep gashes created by space rock impacts, which
may support the idea the asteroid once had a subsurface magma ocean. A magma ocean occurs when
a body undergoes almost complete melting, leading to layered building blocks that can form planets.
Other bodies with magma oceans ended up becoming parts of Earth and other planets.

Data also confirm a distinct group of meteorites found on Earth did, as theorized, originate from
Vesta. The signatures of pyroxene, an iron- and magnesium-rich mineral, in those meteorites match
those of rocks on Vesta's surface. These objects account for about 6 percent of all meteorites seen
falling on Earth.

This makes the asteroid one of the largest single sources for Earth's meteorites. The finding also
marks the first time a spacecraft has been able to visit the source of samples after they were
identified on Earth.

Scientists now know Vesta's topography is quite steep and varied. Some craters on Vesta formed on
very steep slopes and have nearly vertical sides, with landslides occurring more frequently than
expected.

Another unexpected finding was that the asteroid's central peak in the Rheasilvia basin in the
southern hemisphere is much higher and wider, relative to its crater size, than the central peaks of
craters on bodies like our moon. Vesta also bears similarities to other low-gravity worlds like
Saturn's small icy moons, and its surface has light and dark markings that don't match the predictable
patterns on Earth's moon.

"We know a lot about the moon and we're only coming up to speed now on Vesta," said Vishnu
Reddy, a framing camera team member at the Max Planck Institute for Solar System Research in
Germany and the University of North Dakota in Grand Forks. "Comparing the two gives us two
storylines for how these fraternal twins evolved in the early solar system."

Dawn has revealed details of ongoing collisions that battered Vesta throughout its history. Dawn
scientists now can date the two giant impacts that pounded Vesta's southern hemisphere and created
the basin Veneneia approximately 2 billion years ago and the Rheasilvia basin about 1 billion years
ago. Rheasilvia is the largest impact basin on Vesta.

"The large impact basins on the moon are all quite old," said David O'Brien, a Dawn participating
scientist from the Planetary Science Institute in Tucson, Ariz. "The fact that the largest impact on
Vesta is so young was surprising."

Launched in 2007, Dawn began exploring Vesta in mid-2011. The spacecraft will depart Vesta on
August 26 for its next study target, the dwarf planet Ceres, in 2015.

Dawn's mission to Vesta and Ceres is managed by JPL for NASA's Science Mission Directorate in
Washington. Dawn is a project of the directorate's Discovery Program managed by NASA's
Marshall Space Flight Center in Huntsville, Ala. UCLA is responsible for overall Dawn mission
science. Orbital Sciences Corp. in Dulles, Va., designed and built the spacecraft. The German
Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency
and the Italian National Astrophysical Institute are international partners on the mission team. The
California Institute of Technology in Pasadena manages JPL for NASA.

For images and videos related to the findings, visit:
http://www.nasa.gov/mission_pages/dawn/news/dawn20120510.html .

For more information about the Dawn mission, visit: http://www.nasa.gov/dawn and
http://dawn.jpl.nasa.gov .

-end-


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