[meteorite-list] Where is the Ice on Ceres? New NASA Dawn Findings

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=6703

Where is the Ice on Ceres? New NASA Dawn Findings
Jet Propulsion Laboratory
December 15, 2061

At first glance, Ceres, the largest body in the main asteroid belt, may 
not look icy. Images from NASA's Dawn spacecraft have revealed a dark, 
heavily cratered world whose brightest area is made of highly reflective 
salts -- not ice. But newly published studies from Dawn scientists show 
two distinct lines of evidence for ice at or near the surface of the dwarf 
planet. Researchers are presenting these findings at the 2016 American 
Geophysical Union meeting in San Francisco.

"These studies support the idea that ice separated from rock early in 
Ceres' history, forming an ice-rich crustal layer, and that ice has remained 
near the surface over the history of the solar system," said Carol Raymond, 
deputy principal investigator of the Dawn mission, based at NASA's Jet 
Propulsion Laboratory, Pasadena, California.

Water ice on other planetary bodies is important because it is an essential 
ingredient for life as we know it. "By finding bodies that were water-rich 
in the distant past, we can discover clues as to where life may have existed 
in the early solar system," Raymond said.

Ice is everywhere on Ceres

Ceres' uppermost surface is rich in hydrogen, with higher concentrations 
at mid-to-high latitudes -- consistent with broad expanses of water ice, 
according to a new study in the journal Science.

"On Ceres, ice is not just localized to a few craters. It's everywhere, 
and nearer to the surface with higher latitudes," said Thomas Prettyman, 
principal investigator of Dawn's gamma ray and neutron detector (GRaND), 
based at the Planetary Science Institute, Tucson, Arizona.

Researchers used the GRaND instrument to determine the concentrations 
of hydrogen, iron and potassium in the uppermost yard (or meter) of Ceres. 
GRaND measures the number and energy of gamma rays and neutrons emanating 
from Ceres. Neutrons are produced as galactic cosmic rays interact with 
Ceres' surface. Some neutrons get absorbed into the surface, while others 
escape. Since hydrogen slows down neutrons, it is associated with fewer 
neutrons escaping. On Ceres, hydrogen is likely to be in the form of frozen 
water (which is made of two hydrogen atoms and one oxygen atom).

Rather than a solid ice layer, there is likely to be a porous mixture 
of rocky materials in which ice fills the pores, researchers found. The 
GRaND data show that the mixture is about 10 percent ice by weight.

"These results confirm predictions made nearly three decades ago that 
ice can survive for billions of years just beneath the surface of Ceres," 
Prettyman said. "The evidence strengthens the case for the presence of 
near-surface water ice on other main belt asteroids."

Clues to Ceres' inner life

Concentrations of iron, hydrogen, potassium and carbon provide further 
evidence that the top layer of material covering Ceres was altered by 
liquid water in Ceres' interior. Scientists theorize that the decay of 
radioactive elements within Ceres produced heat that drove this alteration 
process, separating Ceres into a rocky interior and icy outer shell. Separation 
of ice and rock would lead to differences in the chemical composition 
of Ceres' surface and interior.

Because meteorites called carbonaceous chondrites were also altered by 
water, scientists are interested in comparing them to Ceres. These meteorites 
probably come from bodies that were smaller than Ceres, but had limited 
fluid flow, so they may provide clues to Ceres' interior history. The 
Science study shows that Ceres has more hydrogen and less iron than these 
meteorites, perhaps because denser particles sunk while brine-rich materials 
rose to the surface. Alternatively, Ceres or its components may have formed 
in a different region of the solar system than the meteorites.

Ice in permanent shadow

A second study, led by Thomas Platz of the Max Planck Institute for Solar 
System Research, Gottingen, Germany, and published in the journal Nature 
Astronomy, focused on craters that are persistently in shadow in Ceres' 
northern hemisphere. Scientists closely examined hundreds of cold, dark 
craters called "cold traps" -- at less than minus 260 degrees Fahrenheit 
(110 Kelvin), they are so chilly that very little of the ice turns into 
vapor in the course of a billion years. Researchers found deposits of 
bright material in 10 of these craters. In one crater that is partially 
sunlit, Dawn's infrared mapping spectrometer confirmed the presence of 
ice.

[Images from NASA's Dawn spacecraft]

This movie of images from NASA's Dawn spacecraft shows a crater on Ceres 
that is partly in shadow all the time. Such craters are called "cold traps." 
Dawn has shown that water ice could potentially be preserved in such place 
for very long amounts of time. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

This suggests that water ice can be stored in cold, dark craters on Ceres. 
Ice in cold traps has previously been spotted on Mercury and, in a few 
cases, on the moon. All of these bodies have small tilts with respect 
to their axes of rotation, so their poles are extremely cold and peppered 
with persistently shadowed craters. Scientists believe impacting bodies 
may have delivered ice to Mercury and the moon. The origins of Ceres' 
ice in cold traps are more mysterious, however.

"We are interested in how this ice got there and how it managed to last 
so long," said co-author Norbert Schorghofer of the University of Hawaii. 
"It could have come from Ceres' ice-rich crust, or it could have been 
delivered from space."

Regardless of its origin, water molecules on Ceres have the ability to 
hop around from warmer regions to the poles. A tenuous water atmosphere 
has been suggested by previous research, including the Herschel Space 
Observatory's observations of water vapor at Ceres in 2012-13. Water molecules 
that leave the surface would fall back onto Ceres, and could land in cold 
traps. With every hop there is a chance the molecule is lost to space, 
but a fraction of them ends up in the cold traps, where they accumulate.

'Bright spots' get names

Ceres' brightest area, in the northern-hemisphere crater Occator, does 
not shine because of ice, but rather because of highly reflective salts. 
A new video produced by the German Aerospace Center (DLR) in Berlin simulates 
the experience of flying around this crater and exploring its topography. 
Occator's central bright region, which includes a dome with fractures, 
has recently been named Cerealia Facula. The crater's cluster of less 
reflective spots to the east of center is called Vinalia Faculae.

"The unique interior of Occator may have formed in a combination of processes 
that we are currently investigating," said Ralf Jaumann, planetary scientist 
and Dawn co-investigator at DLR. "The impact that created the crater could 
have triggered the upwelling of liquid from inside Ceres, which left behind 
the salts."

Dawn's next steps

Dawn began its extended mission phase in July, and is currently flying 
in an elliptical orbit more than 4,500 miles (7,200 kilometers) from Ceres. 
During the primary mission, Dawn orbited and accomplished all of its original 
objectives at Ceres and protoplanet Vesta, which the spacecraft visited 
from July 2011 to September 2012.

Dawn's mission is managed by JPL 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

2016-321