[meteorite-list] What's Inside Ceres? New Findings from Gravity Data

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 21 Aug 2016 19:26:11 -0700 (PDT)
Message-ID: <201608220226.u7M2QBe8009141_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?feature=6583

What's Inside Ceres? New Findings from Gravity Data
Jet Propulsion Laboratory
August 3, 2016

In the tens of thousands of photos returned by NASA's Dawn spacecraft,
the interior of Ceres isn't visible. But scientists have powerful data
to study Ceres' inner structure: Dawn's own motion.

Since gravity dominates Dawn's orbit at Ceres, scientists can measure
variations in Ceres' gravity by tracking subtle changes in the motion
of the spacecraft. Using data from Dawn, scientists have mapped the variations
in Ceres' gravity for the first time in a new study in the journal Nature,
which provides clues to the dwarf planet's internal structure.

"The new data suggest that Ceres has a weak interior, and that water and
other light materials partially separated from rock during a heating phase
early in its history," said Ryan Park, the study's lead author and the
supervisor of the solar system dynamics group at NASA's Jet Propulsion
Laboratory, Pasadena, California.

Ceres' gravity field is measured by monitoring radio signals sent to Dawn,
and then received back on Earth, by NASA's Deep Space Network. This network
is a collection of large antennas at three locations around the globe
that communicate with interplanetary spacecraft. Using these signals,
scientists can measure the spacecraft's speed to a precision of 0.004
inches (0.1 millimeters) per second, and then calculate the details of
the gravity field.

Ceres has a special property called "hydrostatic equilibrium," which was
confirmed in this study. This means that Ceres' interior is weak enough
that its shape is governed by how it rotates. Scientists reached this
conclusion by comparing Ceres' gravity field to its shape. Ceres' hydrostatic
equilibrium is one reason why astronomers classified the body as a dwarf
planet in 2006.

The data indicate that Ceres is "differentiated," which means that it
has compositionally distinct layers at different depths, with the densest
layer at the core. Scientists also have found that, as they suspected,
Ceres is much less dense than Earth, the moon, giant asteroid Vesta (Dawn's
previous target) and other rocky bodies in our solar system. Additionally,
Ceres has long been suspected to contain low-density materials such as
water ice, which the study shows separated from the rocky material and
rose to the outer layer along with other light materials.

"We have found that the divisions between different layers are less pronounced
inside Ceres than the moon and other planets in our solar system," Park
said. "Earth, with its metallic core, semi-fluid mantle and outer crust,
has a more clearly defined structure than Ceres," Park said.

Scientists also found that high-elevation areas on Ceres displace mass
in the interior. This is analogous to how a boat floats on water: the
amount of displaced water depends on the mass of the boat. Similarly,
scientists conclude that Ceres' weak mantle can be pushed aside by the
mass of mountains and other high topography in the outermost layer as
though the high-elevation areas "float" on the material below. This phenomenon
has been observed on other planets, including Earth, but this study is
the first to confirm it at Ceres.

The internal density structure, based on the new gravity data, teaches
scientists about what internal processes could have occurred during the
early history of Ceres. By combining this new information with previous
data from Dawn about Ceres' surface composition, they can reconstruct
that history: Water must have been mobile in the ancient subsurface, but
the interior did not heat up to the temperatures at which silicates melt
and a metallic core forms.

"We know from previous Dawn studies that there must have been interactions
between water and rock inside Ceres," said Carol Raymond, a co-author
and Dawn's deputy principal investigator based at JPL. "That, combined
with the new density structure, tells us that Ceres experienced a complex
thermal history."

Dawn's mission is managed by JPL for NASA's Science Mission Directorate
in Washington D.C. 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

2016-201
Received on Sun 21 Aug 2016 10:26:11 PM PDT