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Ice in Ceres' Shadowed Craters Linked to Tilt History
Jet Propulsion Laboratory
March 22, 2017

Dwarf planet Ceres may be hundreds of millions of miles from Jupiter,
and even farther from Saturn, but the tremendous influence of gravity
from these gas giants has an appreciable effect on Ceres' orientation.
In a new study, researchers from NASA's Dawn mission calculate that the
axial tilt of Ceres -- the angle at which it spins as it journeys around
the sun -- varies widely over the course of about 24,500 years. Astronomers consider this to be a surprisingly short period of time for such dramatic deviations.

Changes in axial tilt, or "obliquity," over the history of Ceres are related to the larger question of where frozen water can be found on Ceres' surface, scientists report in the journal Geophysical Research Letters. Given conditions

on Ceres, ice would only be able to survive at extremely cold temperatures
-- for example, in areas that never see the sun.

"We found a correlation between craters that stay in shadow at maximum obliquity, and bright deposits that are likely water ice," said Anton
Ermakov, postdoctoral researcher at NASA's Jet Propulsion Laboratory, Pasadena, California, and lead author of the study. "Regions that never
see sunlight over millions of years are more likely to have these deposits."

Cycles of Obliquity

Throughout the last 3 million years, Ceres has gone through cycles where
its tilt ranges from about 2 degrees to about 20 degrees, calculations indicate.

"We cannot directly observe the changes in Ceres' orientation over time,
so we used the Dawn spacecraft's measurements of shape and gravity to precisely reconstruct what turned out to be a dynamic history," said Erwan Mazarico, a co-author at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The last time the dwarf planet reached a maximum tilt, which was about
19 degrees, was 14,000 years ago, researchers said. For comparison, Earth
is tilted 23.5 degrees. This significant tilt causes our planet to experience seasons: The northern hemisphere experiences summer when it is oriented
toward the sun, and winter when it's pointed away from the sun. By contrast, Ceres' current tilt is about 4 degrees, so it will not have such strong seasonal effects over the course of a year there (which is about 4.6 Earth years).

How Obliquity Relates to Ice

When the axial tilt is small, relatively large regions on Ceres never
receive direct sunlight, particularly at the poles. These persistently shadowed regions occupy an area of about 800 square miles (2,000 square kilometers). But when the obliquity increases, more of the craters in
the polar regions receive direct exposure to the sun, and persistently shadowed areas only occupy 0.4 to 4 square miles (1 to 10 square kilometers). These areas on Ceres' surface, which stay in shadow even at high obliquity, may be cold enough to maintain surface ice, Dawn scientists said.

These craters with areas that stay in shadow over long periods of time
are called "cold traps," because they are so cold and dark that volatiles
-- substances easily vaporized -- that migrate into these areas can't
escape, even over a billion years. A 2016 study by the Dawn team in Nature Astronomy found bright material in 10 of these craters, and data from
Dawn's visible and infrared mapping spectrometer indicate that one of
them contains ice.

The new study focused on polar craters and modeled how shadowing progresses
as Ceres' axial tilt varies. In the northern hemisphere, only two persistently shadowed regions remain in shadow at the maximum 20-degree tilt. Both
of these regions have bright deposits today. In the southern hemisphere,
there are also two persistently shadowed regions at highest obliquity,
and one of them clearly has a bright deposit.

Shadowed Regions in Context

Ceres is the third body in the solar system found to have permanently
shadowed regions. Mercury and Earth's moon are the other two, and scientists believe they received their ice from impacting bodies. However, Mercury
and the moon do not have such wide variability in their tilts because
of the stabilizing gravitational influence of the sun and Earth, respectively. The origin of the ice in Ceres' cold traps is more mysterious -- it may
come from Ceres itself, or may be delivered by impacts from asteroids
and comets. Regardless, the presence of ice in cold traps could be related
to a tenuous water atmosphere, which was detected by ESA's Herschel Space Observatory in 2012-13. Water molecules that leave the surface would fall
back onto Ceres, with some landing in cold traps and accumulating there.

"The idea that ice could survive on Ceres for long periods of time is important as we continue to reconstruct the dwarf planet's geological
history, including whether it has been giving off water vapor," said Carol Raymond, deputy principal investigator of the Dawn mission and study co-author,

based at JPL.

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@jpl.nasa.gov

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