The dwarf planet Ceres, which is around 960 kilometers in size, orbits the outer edge of the asteroid belt – and is an oddity in many respects. It is much larger and more planet-like than the other asteroids, and its composition is also surprising: Ceres’ interior contains an unusually large amount of water ice and salts, although asteroids are actually rather dry and water-poor. There are even ice volcanoes on Ceres and places where salty brine from its interior emerges to the surface.
Where do Ceres’ organic spots come from?
And there is something else on Ceres: When NASA’s Dawn space probe reached the dwarf planet in 2015 and explored it more closely, its spectrometers detected some striking infrared signatures: In three larger craters – Ernutet, Inamahari and Urvara – there were spots with organic material in the form of aliphatic – chain-like – hydrocarbons. But where does this organic material, scattered over around 1,000 square kilometers, come from?
“Of course, it is reasonable to assume that Ceres’ unique cryovolcanism transported the organic material from its interior to the surface,” says co-author Andreas Nathues from the Max Planck Institute for Solar System Research (MPS) in Göttingen. To verify this, Nathues, lead author Ranjan Sarkar from the MPS and their team have now re-analyzed the data and camera recordings from the Dawn probe. They are also using artificial intelligence to detect possible additional deposits of organic material on Ceres.
Far away from cryovolcanoes and chimneys
The new analyses showed that organic material on Ceres is the exception rather than the rule. “Organic molecules are rare on Ceres,” says Sarkar. The occurrences are largely limited to the already known organic “spots” in the approximately 60-kilometer-wide Ernutet crater and its surroundings, as well as in the two other craters, Inamahari and Urvara. Other supposedly organic signatures turned out to be spectral signals of inorganic carbonates upon closer analysis, according to the team.
However, the camera recordings and data also showed the geological environment in which these organic deposits are located – and what is conspicuously missing there: “We found no evidence of current or previous volcanic or tectonic activity at any of the deposit sites: no trenches, ravines, volcanic domes or chimneys. In addition, there are no deep impact craters in the area,” reports co-author Martin Hoffmann from the MPS. The cryovolcanoes of Ceres, on the other hand, lacked any trace of organic material.
From space instead of from within?
According to the researchers, it is therefore very unlikely that the organic material comes from Ceres itself. “Even if an endogenous origin of the material would be plausible given the carbon-rich composition of Ceres and its long history of geological and geochemical changes, there are too many inconsistencies,” they explain. Neither the locations of the deposits nor their position within the craters are consistent with an origin in the subsurface of the dwarf planet.
Instead, Ceres must have “imported” these hydrocarbons, as the team explains. The most likely source of the organic material would be asteroids from the outer part of the main asteroid belt. These chunks often have a crust of carbon-rich material. In a comparatively slow collision with the dwarf planet, this material could have reached the surface of Ceres largely intact.
This would also explain why the sites are concentrated around the Ernutet crater. “The smaller spots in the Urvara, Inamahari and Vinotonus craters could have been scattered fragments of the same projectile that caused the Ernutet deposits,” write Sarkar and his colleagues. They hope that future studies will reveal more about the origin of Ceres’ organic spots, but also its interior. (AGU Advances, 2025; doi: 10.1029/2024AV001362 )
Source: Max Planck Institute for Solar System Research