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

[Ron Baalke]

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