A team of researchers has used data from the Dawn spacecraft to piece together clues about the interior of the dwarf planet Ceres. The new data indicates that while Ceres, which is the largest body in the asteroid belt, was once warm enough for water to have shifted internally, those temperatures were never high enough for an iron core to separate from the rest of the dwarf planet's interior.

Measuring gravity

The new information comes in part from an estimate of Ceres' moment of inertia, a measure of a body’s resistance to being spun on its axis. A body's moment of inertia depends on two factors. First is the variation of its gravity field over its surface: even though Ceres is roughly spherical, its gravitational strength isn’t uniform. These variations can’t be measured from Earth, though.

The second factor is whether Ceres’ gravity is strong enough to collapse it into a roughly spherical shape, bringing the internal forces into balance with each other. This state is called hydrostatic equilibrium, and it can only be estimated if researchers can determine Ceres’ precise precession rate, which is too small to observe from Earth.

Since it was in orbit around Ceres, the Dawn spacecraft was able to measure both the gravity variations and the dwarf planet’s shape more precisely than ever before. Based on its shape, the researchers determined that Ceres is indeed in hydrostatic equilibrium. Its shape is largely controlled by what’s going on inside—since Ceres doesn’t experience any significant tidal forces, the main force acting to deform it is its rotation.

As for the gravity variations, that might seem like a hard thing to measure. But NASA was able to do so by measuring variations in Dawn’s motions as it orbited the dwarf planet. This is accomplished by monitoring radio signals sent back and forth between Dawn and NASA’s Deep Space Network. The differences in signal arrival time allowed researchers to determine Dawn’s speed to within 0.1 millimeters per second, from which gravitational variations could be determined.

These gravity variations differ from those of other bodies such as Vesta, the second-largest body in the asteroid belt. On Vesta, the variations mostly come from surface features, but on Ceres, the map of gravitational variations doesn’t match the one predicted from the surface features. Instead, Ceres’ internal structure must be the primary factor.

What’s inside?

Since these features measured by Dawn are largely controlled by Ceres’ internal structure, the researchers could use them to learn about that structure.

The data suggests that Ceres is partially differentiated, meaning it’s not uniform throughout but has different compositions and densities at different depths. Differentiation happens because, in a molten body, heavier material goes to the bottom because it's denser. But Ceres is not quite as differentiated as other worlds. According to Ryan Park, the study's lead author:

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

The study confirms Ceres is a low-density body (Earth, the Moon, and Vesta are all higher-density) that contains light materials like water ice. That ice is one of the materials that rose to the surface because of differentiation.

Dawn also found that Ceres’ mantle material is being displaced by the weight of the mountains on the dwarf’s surface. While this effect is seen elsewhere in the Solar System, including on Earth, this is the first time it’s been detected on Ceres. This displacement indicates that there’s a lower-viscosity layer somewhere deep within Ceres, probably due to the dwarf planet being hotter near its core.

Complex history

The new data, combined with previous research on Ceres, sheds light on the asteroid’s past. Water must have been moving around during the body’s formation, for one thing, and the interior never heated up enough to melt silicates and form a metallic core. “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 Park.

Water, the researchers think, likely played a role in preventing Ceres from becoming fully differentiated. “We know from previous Dawn studies that there must have been interactions between water and rock inside Ceres,” said Carol Raymond, one of the paper’s authors and Dawn's deputy principal investigator. “That, combined with the new density structure, tells us that Ceres experienced a complex thermal history.”

Clearly Ceres’ internal heat, along with the partial differentiation that took place, make the dwarf planet's history more complex than researchers had expected. This work will help in the understanding of other large icy and rocky bodies, which were present in much higher numbers during the Solar System's formation.

Nature, 2015. DOI: doi:10.1038/nature18955 (About DOIs)