[meteorite-list] New Clues to Ceres' Bright Spots and Origins

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=4785

New Clues to Ceres' Bright Spots and Origins
Jet Propulsion Laboratory
December 9, 2015

Ceres reveals some of its well-kept secrets in two new studies in the 
journal Nature, thanks to data from NASA's Dawn spacecraft. They include 
highly anticipated insights about mysterious bright features found all 
over the dwarf planet's surface.

In one study, scientists identify this bright material as a kind of salt. 
The second study suggests the detection of ammonia-rich clays, raising 
questions about how Ceres formed.

About the Bright Spots

Ceres has more than 130 bright areas, and most of them are associated 
with impact craters. Study authors, led by Andreas Nathues at Max Planck 
Institute for Solar System Research, Göttingen, Germany, write that the 
bright material is consistent with a type of magnesium sulfate called 
hexahydrite. A different type of magnesium sulfate is familiar on Earth 
as Epsom salt.

Nathues and colleagues, using images from Dawn's framing camera, suggest 
that these salt-rich areas were left behind when water-ice sublimated 
in the past. Impacts from asteroids would have unearthed the mixture of 
ice and salt, they say.

"The global nature of Ceres' bright spots suggests that this world has 
a subsurface layer that contains briny water-ice," Nathues said.

A New Look at Occator

The surface of Ceres, whose average diameter is 584 miles (940 kilometers), 
is generally dark -- similar in brightness to fresh asphalt -- study authors 
wrote. The bright patches that pepper the surface represent a large range 
of brightness, with the brightest areas reflecting about 50 percent of 
sunlight shining on the area. But there has not been unambiguous detection 
of water ice on Ceres; higher-resolution data are needed to settle this 
question.

The inner portion of a crater called Occator contains the brightest material 
on Ceres. Occator itself is 60 miles (90 kilometers) in diameter, and 
its central pit, covered by this bright material, measures about 6 miles 
(10 kilometers) wide and 0.3 miles (0.5 kilometers) deep. Dark streaks, 
possibly fractures, traverse the pit. Remnants of a central peak, which 
was up to 0.3 miles (0.5 kilometers) high, can also be seen.

With its sharp rim and walls, and abundant terraces and landslide deposits, 
Occator appears to be among the youngest features on Ceres. Dawn mission 
scientists estimate its age to be about 78 million years old.

Study authors write that some views of Occator appear to show a diffuse 
haze near the surface that fills the floor of the crater. This may be 
associated with observations of water vapor at Ceres by the Herschel space 
observatory that were reported in 2014. The haze seems to be present in 
views during noon, local time, and absent at dawn and dusk, study authors 
write. This suggests that the phenomenon resembles the activity at the 
surface of a comet, with water vapor lifting tiny particles of dust and 
residual ice. Future data and analysis may test this hypothesis and reveal 
clues about the process causing this activity.

"The Dawn science team is still discussing these results and analyzing 
data to better understand what is happening at Occator," said Chris Russell, 
principal investigator of the Dawn mission, based at the University of 
California, Los Angeles.

The Importance of Ammonia

In the second Nature study, members of the Dawn science team examined 
the composition of Ceres and found evidence for ammonia-rich clays. They 
used data from the visible and infrared mapping spectrometer, a device 
that looks at how various wavelengths of light are reflected by the surface, 
allowing minerals to be identified.

Ammonia ice by itself would evaporate on Ceres today, because the dwarf 
planet is too warm. However, ammonia molecules could be stable if present 
in combination with (i.e. chemically bonded to) other minerals.

The presence of ammoniated compounds raises the possibility that Ceres 
did not originate in the main asteroid belt between Mars and Jupiter, 
where it currently resides, but instead might have formed in the outer 
solar system. Another idea is that Ceres formed close to its present position, 
incorporating materials that drifted in from the outer solar system - 
near the orbit of Neptune, where nitrogen ices are thermally stable.

"The presence of ammonia-bearing species suggests that Ceres is composed 
of material accreted in an environment where ammonia and nitrogen were 
abundant. Consequently, we think that this material originated in the 
outer cold solar system," said Maria Cristina De Sanctis, lead author 
of the study, based at the National Institute of Astrophysics, Rome.

In comparing the spectrum of reflected light from Ceres to meteorites, 
scientists found some similarities. Specifically, they focused on the 
spectra, or chemical fingerprints, of carbonaceous chondrites, a type 
of carbon-rich meteorite thought to be relevant analogues for the dwarf 
planet. But these are not good matches for all wavelengths that the instrument 
sampled, the team found. In particular, there were distinctive absorption 
bands, matching mixtures containing ammoniated minerals, associated with 
wavelengths that can't be observed from Earth-based telescopes.

The scientists note another difference is that these carbonaceous chondrites 
have bulk water contents of 15 to 20 percent, while Ceres' content is 
as much as 30 percent.

"Ceres may have retained more volatiles than these meteorites, or it could 
have accreted the water from volatile-rich material," De Sanctis said.

The study also shows that daytime surface temperatures on Ceres span from 
minus 136 degrees to minus 28 degrees Fahrenheit (180 to 240 Kelvin). 
The maximum temperatures were measured in the equatorial region. The temperatures 
at and near the equator are generally too high to support ice at the surface 
for a long time, study authors say, but data from Dawn's next orbit will 
reveal more details.

As of this week, Dawn has reached its final orbital altitude at Ceres, 
about 240 miles (385 kilometers) from the surface of the dwarf planet. 
In mid-December, Dawn will begin taking observations from this orbit, 
including images at a resolution of 120 feet (35 meters) per pixel, infrared, 
gamma ray and neutron spectra, and high-resolution gravity data.

Dawn's mission is managed by the Jet Propulsion Laboratory for NASA. 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


Media Contact

Elizabeth Landau
NASA's Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
Elizabeth.Landau at jpl.nasa.gov 

2015-365