Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown@nasa.gov
Jia-Rui C. Cook
Jet Propulsion Laboratory, Pasadena,
Calif.
818-354-0850
jccook@jpl.nasa.gov
WASHINGTON -- NASA's Dawn spacecraft has
revealed the giant asteroid Vesta has its own version of ring around the
collar. Two new papers, based on observations from the low-altitude mapping
orbit of the Dawn mission, show volatile, or easily evaporated, materials have
colored Vesta's surface in a broad swath around its equator.
The volatiles were released from
minerals likely containing water. Pothole-like features mark some of the
asteroid's surface where the volatiles boiled off. Dawn did not find actual
water ice at Vesta. However, it found evidence of hydrated minerals delivered
by meteorites and dust in the giant asteroid's chemistry and geology. The findings
appear Thursday in the journal Science.
One paper, led by Thomas Prettyman, the
lead scientist for Dawn's gamma ray and neutron detector (GRaND) at the
Planetary Science Institute in Tucson, Ariz., describes how the instrument
found signatures of hydrogen, likely in the form of hydroxyl or water bound to
minerals in Vesta's surface.
"The source of the hydrogen within
Vesta's surface appears to be hydrated minerals delivered by carbon-rich space
rocks that collided with Vesta at speeds slow enough to preserve their volatile
content," said Prettyman.
A complementary paper, led by Brett
Denevi, a Dawn participating scientist at the Johns Hopkins University Applied
Physics Laboratory in Laurel, Md., describes the presence of pitted terrain
created by the release of the volatiles.
Vesta is the second most massive member
of our solar system's main asteroid belt. Dawn was orbiting at an average
altitude of about 130 miles (210 kilometers) above the surface when it obtained
the data. Dawn left Vesta on Sept. 5 EDT (Sept. 4) and is on its way to a
second target, the dwarf planet Ceres.
Scientists thought it might be possible
for water ice to survive near the surface around the giant asteroid's poles.
Unlike Earth's moon, however, Vesta has no permanently shadowed polar regions
where ice might survive. The strongest signature for hydrogen in the latest
data came from regions near the equator, where water ice is not stable.
In some cases, space rocks crashed into
these deposits at high speed. The heat from the collisions converted the
hydrogen bound to the minerals into water, which evaporated. Escaping water
left holes as much as six-tenths of a mile (1 kilometer) wide and as deep as
700 feet (200 meters). Seen in images from Dawn's framing camera, this pitted
terrain is best preserved in sections of Marcia crater.
"The pits look just like features
seen on Mars, and while water was common on Mars, it was totally unexpected on
Vesta in these high abundances," said Denevi. "These results provide
evidence that not only were hydrated materials present, but they played an
important role in shaping the asteroid's geology and the surface we see
today."
GRaND's data are the first direct
measurements describing the elemental composition of Vesta's surface. Dawn's
elemental investigation by the instrument determined the ratios of iron to
oxygen and iron to silicon in the surface materials. The new findings solidly
confirm the connection between Vesta and a class of meteorites found on Earth
called the Howardite, Eucrite and Diogenite meteorites, which have the same
ratios for these elements. In addition, more volatile-rich fragments of other
objects have been identified in these meteorites, which supports the idea the
volatile-rich material was deposited on Vesta.
The Dawn mission is managed by NASA's
Jet Propulsion Laboratory for the Science Mission Directorate in Washington.
The spacecraft is as a project of the Discovery Program managed by NASA's
Marshall Space Flight Center in Huntsville, Ala. The University of California,
Los Angeles, is responsible for overall mission science. Orbital Sciences
Corporation of Dulles, Va., designed and built the spacecraft.
The framing cameras that saw the pitted
terrain were developed and built under the leadership of the Max Planck
Institute for Solar System Research, Katlenburg-Lindau, Germany, with
contributions by the German Aerospace Center (DLR) Institute of Planetary
Research, Berlin, and in coordination with the Institute of Computer and
Communication Network Engineering, Braunschweig. The framing camera project is
funded by NASA, the Max Planck Society and DLR. The gamma ray and neutron
detector instrument was built by Los Alamos National Laboratory, N.M., and is
operated by the Planetary Science Institute.
To view new images and for more
information about Dawn, visit http://www.nasa.gov/dawn.
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