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Ceres' Temporary Atmosphere Linked to Solar Activity
Jet Propuslion Laboratory
April 6, 2017

Scientists have long thought that Ceres may have a very weak, transient atmosphere, but mysteries lingered about its origin and why it's not always present. Now, researchers suggest that this temporary atmosphere appears
to be related to the behavior of the sun, rather than Ceres' proximity
to the sun. The study was conducted by scientists from NASA's Dawn mission
and others who previously identified water vapor at Ceres using other observatories.

"We think the occurrence of Ceres' transient atmosphere is the product
of solar activity," said Michaela Villarreal, lead author of the new study
in the Astrophysical Journal Letters and researcher at the University
of California, Los Angeles.

Ceres is the largest object in the asteroid belt that lies between Mars
and Jupiter. When energetic particles from the sun hit exposed ice and
ice near the surface of the dwarf planet, it transfers energy to the water molecules as they collide. This frees the water molecules from the ground, allowing them to escape and create a tenuous atmosphere that may last
for a week or so.

"Our results also have implications for other airless, water-rich bodies
of the solar system, including the polar regions of the moon and some asteroids," said Chris Russell, principal investigator of the Dawn mission, also at UCLA. "Atmospheric releases might be expected from their surfaces, too, when solar activity erupts."

Before Dawn arrived in orbit at Ceres in 2015, evidence for an atmosphere
had been detected by some observatories at certain times, but not others, suggesting that it is a transient phenomenon. In 1991, the International Ultraviolet Explorer satellite detected hydroxyl emission from Ceres,
but not in 1990. Then, in 2007, the European Southern Observatory's Very
Large Telescope searched for a hydroxide emission, but came up empty.
The European Space Agency's Herschel Space Observatory detected water
in the possible weak atmosphere, or "exosphere," of Ceres on three occasions, but did not on a fourth attempt.

As Dawn began its thorough study of Ceres in March 2015, scientists found ample evidence for water in the form of ice. The spacecraft's gamma ray
and neutron detector (GRaND) has found that the uppermost surface is rich
in hydrogen, which is consistent with broad expanses of water ice. This
ice is nearer to the surface at higher latitudes, where temperatures are lower, a 2016 study published in the journal Science found. Ice has been detected directly at the small bright crater called Oxo and in at least
one of the craters that are persistently in shadow in the northern hemisphere. Other research has suggested that persistently shadowed craters are likely
to harbor ice. Additionally, the shapes of craters and other features
are consistent with significant water-ice content in the crust.

Because of this evidence for abundant ice, many scientists think that
Ceres' exosphere is created in a process similar to what occurs on comets, even though they are much smaller. In that model, the closer Ceres gets
to the sun, the more water vapor is released because of ice sublimating
near or at the surface.

But the new study suggests comet-like behavior may not explain the mix
of detections and non-detections of a weak atmosphere.

"Sublimation probably is present, but we don't think it's significant
enough to produce the amount of exosphere that we're seeing," Villarreal
said.

Villarreal and colleagues showed that past detections of the transient atmosphere coincided with higher concentrations of energetic protons from
the sun. Non-detections coincided with lower concentrations of these particles.

What's more, the best detections of Ceres' atmosphere did not occur at
its closest approach to the sun. This suggests that solar activity, rather than Ceres' proximity to the sun, is a more important factor in generating
an exosphere.

The research began with a 2016 Science study led by Chris Russell. The
study, using GRaND data, suggested that, during a six-day period in 2015, Ceres had accelerated electrons from the solar wind to very high energies.

In its orbital path, Ceres is currently getting closer to the sun. But
the sun is now in a particularly quiet period, expected to last for several more years. Since their results indicate Ceres' exosphere is related to
solar activity, study authors are predicting that the dwarf planet will
have little to no atmosphere for some time. However, they recommend that
other observatories monitor Ceres for future emissions.

Dawn is now in its extended mission and studying Ceres in a highly elliptical orbit. Engineers are maneuvering the spacecraft to a different orbital
plane so that Ceres can be viewed in a new geometry. The primary science objective is to measure cosmic rays to help determine which chemical elements lie near the surface of Ceres. As a bonus, in late April, the sun will
be directly behind Dawn, when the spacecraft is at an altitude of about
12,300 miles (20,000 kilometers). Ceres will appear brighter than before
in that configuration, and perhaps reveal more secrets about its composition and history.

The Dawn 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://www.nasa.gov/dawn

http://dawn.jpl.nasa.gov

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

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