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

[Ron Baalke]

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