Ceres is the largest body in the asteroid belt. Because Ceres is small, there was not enough gravitational energy when it formed to heat the interior. Virginia Tech Professor Scott King and colleagues investigated whether heat generated by the decay of radiogenic elements can power the tectonism, ice-volcanism, and evidence for past hydrothermal activity that have been documented by NASA’s Dawn mission. Using computer modeling, they found a planet-scale asymmetric instability (one hemisphere up, one hemisphere down) forms as a small spherical body heats due to the decay of radiogenic elements within the interior. They showed that this planet-scale instability can explain many puzzling features on Ceres, including the high topographic plateau, fracture zones, and the absence of large craters.
This illustration models the topography of Ceres from NASA’s Dawn mission, with green and blue colors. Image credit: King et al., doi: 10.1029/2021AV000571.
NASA’s Dawn mission was the first spacecraft to visit Ceres, the largest asteroid-belt object with a radius of approximately 470 km (292 miles).
Ceres has many features commonly associated with active, icy bodies including: hydrothermal, cryovolcanic, and tectonic features.
The dwarf planet likely formed between 3 and 5 million years after the formation of calcium-aluminum-rich inclusions — the oldest solids that formed in the Solar System — and partially differentiated into a body with a rocky interior and crust composed of rock, salts, clathrates, water ice, and possibly organic rich material.
The detection of material rich in sodium carbonate and ammonium salts at multiple sites on Ceres’ surface suggests that an ocean may have been present in the uppermost 10s of kilometers.
“For a long time, our view of Ceres was fuzzy,” said lead author Professor Scott King, a geoscientist in the Department of Geosciences at Virginia Tech.
Professor King and his colleagues wanted to know how a body as small as Ceres could generate the heat needed to power that kind of geological activity and account for the surface features picked up by Dawn.
Through modeling, they found that the decay of radioactive elements within Ceres’ interior could keep it active.
“The collision between objects that form a planet creates that initial heat. Ceres, by contrast, never got big enough to become a planet and generate heat the same way,” Professor King said.
To learn how it could still generate enough heat to power geologic activity, the researchers used theories and computational tools previously applied to bigger planets to study Ceres’ interior, and they looked for evidence that could support their models in the Dawn data.
Their model of the dwarf planet’s interior showed a unique sequence: Ceres started out cold and heated up because of the decay of radioactive elements such as uranium and thorium — which was alone enough to power its activity — until the interior became unstable.
“What I would see in the model is, all of a sudden, one part of the interior would start heating up and would be moving upward and then the other part would be moving downward,” Professor King said.
That instability could explain some of the surface features that had formed on Ceres, as revealed by the Dawn mission.
The large plateau had formed on only one side of Ceres with nothing on the other side, and the fractures were clustered in a single location around it.
The concentration of features in one hemisphere signaled to the team that instability had occurred and had left a visible impact.
“It turned out that you could show in the model that where one hemisphere had this instability that was rising up, it would cause extension at the surface, and it was consistent with these patterns of fractures,” Professor King said.
Based on the team’s model, Ceres didn’t follow a planet’s typical pattern of hot first and cool second, with its own pattern of cool, hot, and cool again.
“What we’ve shown in this paper is that radiogenic heating all on its own is enough to create interesting geology,” Professor King said.
The authors see similarities to Ceres in Uranus’ moons Ariel, Miranda, and Titania and Saturn’s moons Rhea, Dione, and Tethys.
With additional improvements to the model, they look forward to exploring their interiors as well.
“Some of these moons are not too different in size from Ceres. I think applying the model would be really exciting,” Professor King said.
The team’s work was published in the journal AGU Advances.
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Scott D. King et al. Ceres’ Broad-Scale Surface Geomorphology Largely Due To Asymmetric Internal Convection. AGU Advances, published online May 17, 2022; doi: 10.1029/2021AV000571
