 |
| This perspective view of Marcia crater on the giant asteroid Vesta shows the most spectacularly preserved example of "pitted terrain," an unexpected discovery in data returned by NASA's Dawn mission [PHOTO: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/JHUAPL] |
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.