[meteorite-list] Dawn Maps Ceres Craters Where Ice Can Accumulate

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=6560

Dawn Maps Ceres Craters Where Ice Can Accumulate
Jet Propulsion Laboraotry
July 8, 2016

Scientists with NASA's Dawn mission have identified permanently shadowed 
regions on the dwarf planet Ceres. Most of these areas likely have been 
cold enough to trap water ice for a billion years, suggesting that ice 
deposits could exist there now.

"The conditions on Ceres are right for accumulating deposits of water 
ice," said Norbert Schorghofer, a Dawn guest investigator at the University 
of Hawaii at Manoa. "Ceres has just enough mass to hold on to water molecules, 
and the permanently shadowed regions we identified are extremely cold 
-- colder than most that exist on the moon or Mercury."

Permanently shadowed regions do not receive direct sunlight. They are 
typically located on the crater floor or along a section of the crater 
wall facing toward the pole. The regions still receive indirect sunlight, 
but if the temperature stays below about minus 240 degrees Fahrenheit 
(minus 151 degrees Celsius), the permanently shadowed area is a cold trap 
-- a good place for water ice to accumulate and remain stable. Cold traps 
were predicted for Ceres but had not been identified until now.

In this study, Schorghofer and colleagues studied Ceres' northern hemisphere, 
which was better illuminated than the south. Images from Dawn's cameras 
were combined to yield the dwarf planet's shape, showing craters, plains 
and other features in three dimensions. Using this input, a sophisticated 
computer model developed at NASA's Goddard Space Flight Center, Greenbelt, 
Maryland, was used to determine which areas receive direct sunlight, how 
much solar radiation reaches the surface, and how the conditions change 
over the course of a year on Ceres.

The researchers found dozens of sizeable permanently shadowed regions 
across the northern hemisphere. The largest one is inside a 10-mile-wide 
(16-kilometer) crater located less than 40 miles (65 kilometers) from 
the north pole.

Taken together, Ceres' permanently shadowed regions occupy about 695 square 
miles (1,800 square kilometers). This is a small fraction of the landscape 
-- much less than 1 percent of the surface area of the northern hemisphere.

The team expects the permanently shadowed regions on Ceres to be colder 
than those on Mercury or the moon. That's because Ceres is quite far from 
the sun, and the shadowed parts of its craters receive little indirect 
radiation.

"On Ceres, these regions act as cold traps down to relatively low latitudes," 
said Erwan Mazarico, a Dawn guest investigator at Goddard. "On the moon 
and Mercury, only the permanently shadowed regions very close to the poles 
get cold enough for ice to be stable on the surface."

The situation on Ceres is more similar to that on Mercury than the moon. 
On Mercury, permanently shadowed regions account for roughly the same 
fraction of the northern hemisphere. The trapping efficiency -- the ability 
to accumulate water ice -- is also comparable.

By the team's calculations, about 1 out of every 1,000 water molecules 
generated on the surface of Ceres will end up in a cold trap during a 
year on Ceres (1,682 days). That's enough to build up thin but detectable 
ice deposits over 100,000 years or so.

"While cold traps may provide surface deposits of water ice as have been 
seen at the moon and Mercury, Ceres may have been formed with a relatively 
greater reservoir of water," said Chris Russell, principal investigator 
of the Dawn mission, based at the University of California, Los Angeles. 
"Some observations indicate Ceres may be a volatile-rich world that is 
not dependent on current-day external sources."

The findings are available online in the journal Geophysical Research 
Letters.

Dawn's mission is managed by NASA's Jet Propulsion Laboratory 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, 
Alabama. UCLA is responsible for overall Dawn mission science. Orbital 
ATK Inc., in Dulles, Virginia, designed and built the spacecraft. The 
German Aerospace Center, Max Planck Institute for Solar System Research, 
Italian Space Agency and Italian National Astrophysical Institute are 
international partners on the mission team. For a complete list of mission 
participants, visit:

http://dawn.jpl.nasa.gov/mission

More information about Dawn is available at the following sites:

http://dawn.jpl.nasa.gov

http://www.nasa.gov/dawn

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

Written by Elizabeth Zubritsky
NASA's Goddard Space Flight Center

2016-180