Only a handful of objects outside Earth host organic molecules, and they’re mostly the small, frozen wastelands of comets and asteroids. According to a new paper published today in Science, researchers have added a new planetary body to that club: Ceres.
The discovery, made using NASA’s Dawn spacecraft, raises intriguing questions about the dwarf planet’s internal evolution, said Simone Marchi, a senior research scientist at the Southwest Research Institute in Boulder, Colo., and a coauthor on the new paper. For example, are hydrothermal processes creating these molecules, or were they brought by impacting asteroids?
And “if it is true that this organic material was formed on Ceres, then another compelling question emerges,” Marchi said. “Is it possible that during those processes, molecules that are really important for life were synthesized as well?”
Spectral Signs
Although it’s not quite clear exactly what molecules the researchers spotted, they’re confident that they’ve found long chains of hydrogen and carbon, perhaps similar to the hydrocarbons kerite and asphaltite.
They know this thanks to Dawn’s Visual and Infrared Spectrometer. When sunlight hits Ceres, some of the surface materials absorb the light while others reflect it. Using the spectrometer, the researchers can examine the reflected infrared light to determine the material’s most likely composition. This determination is possible because different compounds have their own absorption and reflection characteristics. Although there are classes of materials with similar spectral responses, scientists can nonetheless estimate what the analyzed material is likely to be.
When looking at the infrared data from in and around Cere’s Ernutet crater, the researchers found a dip in the reflected light at wavelengths of 3.4 micrometers, which meant that these wavelengths were being absorbed. And the most likely type of compound that absorbs light at this wavelength is organics, Marchi said, specifically, those with long chains of hydrogen and carbon bonds.
Seeded or Grown?
Finding organics led to more questions: Was this organic material formed on Ceres, or was it delivered by an asteroid or comet that hit Ceres’s surface? After all, telescope observations and up-close scrutiny have revealed that comets and some asteroids house organic material. However, several factors make it likely that the organic material did, indeed, form on Ceres, Marchi said.
First, Dawn’s observations of organic material aren’t limited to impact craters. True, the data do reveal highly concentrated organic material very close to the 50-kilometer-wide Ernutet crater, which might point to impact delivery. But the researchers also found the organic material 200 kilometers away, distributed in a manner that doesn’t necessarily associate it with any particular impact crater, Marchi said.
Second, the organic molecules that Dawn found are likely relatively fragile. Heat generated by an impact would likely have destroyed them, Marchi said, giving more credence to the idea that they formed on Ceres.
Finally, impactors generally contain less organic material—about 4% of their composition—than what the scientists have found locally on Ceres’s surface. The researchers found that the area around the Ernutet crater contains 5%–10% organic material, so unless some process is causing Ceres to lose its inorganics, impact delivery seems less likely.
Internal Processes
Before finding these organics, Dawn scientists were already impressed by Ceres’s wide variety of complex molecules. The dwarf planet seems to be “a chemical factory,” Marchi said, referring to the ammonia-rich clays, the salts, and the carbonates they’ve previously spotted on Ceres’s surface. Just last year, researchers found these molecules in Ceres’s famous bright spots in Occator crater.
Scientists are so intrigued by these molecules, as well as the newly found organics, because researchers rarely find this kind of material on comets and asteroids. On those smaller bodies, scientists generally find more “primordial” and less complex material, Marchi said. So the molecules’ complexity implies that Ceres’s surface has undergone some kind of alteration.
On Earth, scientists find carbonates and ammoniated molecules in hydrothermal environments, where hot water comes into contact with rock. The presence of these, plus the newly discovered organics, adds further evidence that hydrothermal systems may be found on Ceres, Marchi said. It could even be evidence of an ancient—or current—subsurface ocean.
Implications for Life?
“Finding organic compounds on Ceres hints on the possible presence of prebiotic chemistry on Ceres,” said Michael Küppers, a planetary scientist at the European Space Agency who wasn’t involved with the research. It also “demonstrates that Ceres—in addition to Mars, Europa, Titan and Enceladus—is another candidate that could potentially harbor life.”
Although these organic molecules are thought to be important for life, their presence alone doesn’t mean that life once existed or still exists on Ceres, Marchi said. But these findings do help scientists better understand the distribution of organic material throughout the solar system, which will ultimately help them better understand how life could evolve elsewhere.
—JoAnna Wendel (@JoAnnaScience), Staff Writer