[meteorite-list] Gullies on Vesta Suggest Past Water-Mobilized Flows

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 22 Jan 2015 17:07:11 -0800 (PST)
Message-ID: <201501230107.t0N17BgP023059_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?feature=4453

Gullies on Vesta Suggest Past Water-Mobilized Flows
Jet Propulsion Laboratory
January 21, 2015

Protoplanet Vesta, visited by NASA's Dawn spacecraft from 2011 to 2013,
was once thought to be completely dry, incapable of retaining water because
of the low temperatures and pressures at its surface. However, a new study
shows evidence that Vesta may have had short-lived flows of water-mobilized
material on its surface, based on data from Dawn.

"Nobody expected to find evidence of water on Vesta. The surface is very
cold and there is no atmosphere, so any water on the surface evaporates,"
said Jennifer Scully, postgraduate researcher at the University of California,
Los Angeles. "However, Vesta is proving to be a very interesting and complex
planetary body."

The study has broad implications for planetary science.

"These results, and many others from the Dawn mission, show that Vesta
is home to many processes that were previously thought to be exclusive
to planets," said UCLA's Christopher Russell, principal investigator for
the Dawn mission. "We look forward to uncovering even more insights and
mysteries when Dawn studies Ceres."

Dawn is currently in the spotlight because it is approaching dwarf planet
Ceres, the largest object in the main asteroid belt between Mars and Jupiter.
It will be captured into orbit around Ceres on March 6. Yet data from
Dawn's exploration of Vesta continue to capture the interest of the scientific
community.

Scully and colleagues, publishing in the journal "Earth and Planetary
Science Letters," identified a small number of young craters on Vesta
with curved gullies and fan-shaped ("lobate") deposits.

"We're not suggesting that there was a river-like flow of water. We're
suggesting a process similar to debris flows, where a small amount of
water mobilizes the sandy and rocky particles into a flow," Scully said.

The curved gullies are significantly different from those formed by the
flow of purely dry material, scientists said. "These features on Vesta
share many characteristics with those formed by debris flows on Earth
and Mars," Scully said.

The gullies are fairly narrow, on average about 100 feet (30 meters) wide.
The average length of the gullies is a little over half a mile (900 meters).
Cornelia Crater, with a width of 9 miles (15 kilometers), contains some
of the best examples of the curved gullies and fan-shaped deposits.

The leading theory to explain the source of the curved gullies is that
Vesta has small, localized patches of ice in its subsurface. No one knows
the origin of this ice, but one possibility is that ice-rich bodies, such
as comets, left part of their ice deep in the subsurface following impact.
A later impact would form a crater and heat up some of the ice patches,
releasing water onto the walls of the crater.

"If present today, the ice would be buried too deeply to be detected by
any of Dawn's instruments," Scully said. "However, the craters with curved
gullies are associated with pitted terrain, which has been independently
suggested as evidence for loss of volatile gases from Vesta." Also, evidence
from Dawn's visible and infrared mapping spectrometer and gamma ray and
neutron detector indicates that there is hydrated material within some
rocks on Vesta's surface, suggesting that Vesta is not entirely dry.

It appears the water mobilized sandy and rocky particles to flow down
the crater walls, carving out the gullies and leaving behind the fan-shaped
deposits after evaporation. The craters with curvy gullies appear to be
less than a few hundred million years old, which is still young compared
to Vesta's age of 4.6 billion years.

Laboratory experiments performed at NASA's Jet Propulsion Laboratory,
Pasadena, California, indicate that there could be enough time for curved
gullies to form on Vesta before all of the water evaporated. "The sandy
and rocky particles in the flow help to slow the rate of evaporation,"
Scully said.

The Dawn mission to Vesta and Ceres is managed by JPL, a division of the
California Institute of Technology in Pasadena, for NASA's Science Mission
Directorate, Washington. UCLA is responsible for overall Dawn mission
science.

For more information about Dawn, visit:

http://dawn.jpl.nasa.gov


Media Contact

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau at jpl.nasa.gov

2015-027
Received on Thu 22 Jan 2015 08:07:11 PM PST