Aliphatic Organic Compounds Detected on Ceres

An international team of planetary researchers has discovered the presence of aliphatic organic compounds — carbon-based building blocks that may have a role in the chemistry that creates life — on Ceres, the largest object in the main asteroid belt. And, according to the team, the organics are most likely native to the dwarf planet.

This false-color image shows the dwarf planet Ceres. Scientists use false color to examine differences in surface materials. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.

The discovery, reported in the journal Science, is the latest surprise turned up by NASA’s Dawn spacecraft, which entered orbit around Ceres almost two years ago.

Dawn has already found evidence of water-rich clay minerals on the dwarf planet’s surface as well as carbonate minerals indicative of active hydrothermal processes beneath its surface.

“Organic molecules are among the ingredients necessary for life. Some of the biggest questions in space and planetary science deal with the distribution of organics in the Solar System,” said Brown University Professor Carlé Pieters, co-investigator on the Dawn mission and co-author on the study.

“Where did they come from? How were they formed? How do they relate to the evolution of life? This discovery adds an important piece to the puzzle.”

“What we’ve found on Ceres is probably the most unambiguous detection of organics on any solar system body other than Earth.”

“We’ve collected meteorites on Earth with organic signatures, which makes us think their parent asteroids may have had organics. But until now we haven’t seen such definitive evidence on any asteroid.”

New data from Dawn show an unusually high concentration of organic matter close to a northern-hemisphere crater called Ernutet.

The distribution and characteristics of the organics seem to preclude association with any single crater.

The largest concentration appears to drape discontinuously across the southwest floor and rim of Ernutet and onto an older, highly degraded crater. Other organic-rich areas are scattered to the northwest.

This enhanced color composite image, made with data from NASA’s Dawn spacecraft, shows the area around Ernutet crater, which is about 32 miles (52 km) in diameter and is located in the northern hemisphere. The bright red portions appear redder with respect to the rest of Ceres. In the new study, planetary researchers found that these red areas around Ernutet are associated with evidence of organic material. Images taken using blue (440 nm), green (750 nm) and infrared (960 nm) spectral filters were combined to create the view. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.

“Data returned by Dawn’s Visible and InfraRed Mapping Spectrometer show a clear detection of an organic absorption feature at 3.4 micrometers on dwarf planet Ceres,” the researchers said.

“This signature is characteristic of aliphatic organic matter and is mainly localized on a broad region of 386 sq.miles (1,000 sq. km) close to the 32-mile (52 km) Ernutet crater.”

“The overall region is heavily cratered and appears to be ancient; however, the rims of Ernutet crater appear to be relatively fresh,” said co-author Dr. Simone Marchi, a senior research scientist at Southwest Research Institute.

“The organic-rich areas include carbonate and ammoniated species, which are clearly Ceres’ endogenous material, making it unlikely that the organics arrived via an external impactor.”

“There’s some reason to suspect that these formed internally within Ceres and were brought to the surface,” Prof. Pieters added.

“One of the other big surprises to come out of the Dawn mission had to do with strange-looking extremely bright areas we detected in another crater called Occator. Those small spots turned out to be concentrations carbonate minerals, probably extruded onto the surface along with some kind of volatile material. As the volatiles were lost into space, we were left with these carbonate deposits.”

“That finding clarified the idea that there are hydrothermal processes going on within or below the crust of Ceres. So it makes a well-integrated hypothesis that these organics could have been created by internal processes and later exposed on the surface as well.”

“That’s one good hypothesis, but it’s not the only one. It’s possible, for example, that these materials could have been delivered to Ceres as a cloud of debris from an external source traveling through the Solar System,” Prof. Pieters said.

“There are high-resolution data available from Dawn that provide the geologic context for these deposits. We’re looking at those data now, which will help us to pin down the origin of these materials.”

The findings don’t necessarily mean Ceres has or ever had life, but they do indicate a complex chemical environment on the dwarf planet, suggesting favorable environments for prebiotic chemistry.

“Even before we sent Dawn to Ceres, we knew by its low density that it probably had lots of water ice beneath its surface,” Prof. Pieters explained.

“Since then, Dawn has found clay minerals virtually everywhere on Ceres as well as ammoniated minerals and evidence of local hydrothermal activity.”

“Now we’ve added organics. Together, those are the essential ingredients for life.”

“However, it doesn’t mean life existed on Ceres. It’s kind of like baking a cake. You can have all the ingredients, but if you don’t put them together properly, you don’t end up with a cake. So there is still plenty of work to be done before we can start thinking about whether microbes were able to form on Ceres.”

“Altogether, it’s an exciting discovery and we’re certainly looking forward to investigating it further,” she said.

_____

M.C. De Sanctis et al. 2017. Localized aliphatic organic material on the surface of Ceres. Science 355 (6326): 719-722; doi: 10.1126/science.aaj2305