Using data from NASA’s Dawn spacecraft about Ceres’ gravity and topography, mission scientists have found that the dwarf planet is ‘differentiated,’ which means that it has compositionally distinct layers at different depths. The findings were published this week in the journal Nature.
This artist’s concept shows a diagram of how the inside of Ceres could be structured, based on data about the dwarf planet’s gravity field from NASA’s Dawn spacecraft. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.
Ceres’ gravity field is measured by monitoring radio signals sent to the Dawn orbiter, and then received back on Earth, by NASA’s Deep Space Network.
Using these signals, Dawn scientists can measure the spacecraft’s speed to a precision of 0.004 inches (0.1 mm) 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.
The researchers 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 a partially differentiated body, with a rocky core overlaid by a volatile-rich shell.
The scientists also have found that, as they suspected, Ceres is much less dense than Earth, the Moon, giant asteroid Vesta and other rocky bodies in the 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,” said study’s lead author Dr. Ryan Park, from NASA’s Jet Propulsion Laboratory.
“Earth, with its metallic core, semi-fluid mantle and outer crust, has a more clearly defined structure than Ceres.”
The team 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, they 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 co-author Dr. Carol Raymond, also from NASA’s Jet Propulsion Laboratory.
“That, combined with the new density structure, tells us that Ceres experienced a complex thermal history.”
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R.S. Park et al. A partially differentiated interior for (1) Ceres deduced from its gravity field and shape. Nature, published online August 4, 2016; doi: 10.1038/nature18955
