Last year, scientists with NASA's Dawn mission announced the detection of organic material, carbon-based compounds that are necessary components for life, exposed in patches on the surface of the dwarf planet Ceres. Now, a new analysis of Dawn data by Brown University researchers suggests that those patches may contain a much higher abundance of organic compounds than originally thought.
The findings, recently published in Geophysical Research Letters raise intriguing questions about how those organic compounds came to the surface of Ceres, and the methods used in the new study could also provide a template to interpret data for future missions, researchers say.
"What this article shows is that you can be really different results that depend on the type of organic material you use to compare and interpret Ceres data," said Hannah Kaplan, a postdoctoral researcher at the Southwest Research Institute who led the research while completing his Ph.D. in Brown. "That's important not only for Ceres, but also for missions that will soon explore asteroids that may also contain organic material."
Organic molecules are the building blocks of life. Its detection in Ceres does not mean that life exists or exists there; Non-biological processes can also give rise to organic molecules. But since life as we know it can not exist without organic material, scientists are interested in how it is distributed through the solar system. The presence of organic material in Ceres poses intriguing possibilities, particularly because the dwarf planet is also rich in water ice, and water is another necessary component of life.
The original discovery of organic compounds in Ceres was made using Visible and Infrared (VIR) Spectrometer in the Dawn spacecraft, which entered orbit around the dwarf planet in 2015. By analyzing the patterns in which sunlight interacts with The surface – looking carefully at what wavelength are reflected and which are absorbed – scientists can get an idea of the compounds present in Ceres. The VIR instrument collected a signal consistent with organic molecules in the Ernutet crater region in the northern hemisphere of Ceres.
To get an initial idea of how abundant these compounds could be, the original research team compared the Ceres VIR data with laboratory reflectance spectra of organic material formed on Earth. Based on that standard, the researchers concluded that between six and 10 percent of the spectral signature they detected in Ceres could be explained by organic matter.
But for this new research, Kaplan and his colleagues wanted to reexamine that data using a different standard. Instead of relying on Earth's rocks to interpret the data, the team turned to an extraterrestrial source: meteorites. It has been shown that some meteorites, pieces of carbonaceous chondrites that have fallen to Earth after being ejected from primitive asteroids, contain organic material that is slightly different from what is commonly found on our own planet. And Kaplan's work shows that the spectral reflectance of extraterrestrial organic compounds is different from that of terrestrial elements.
"What we find is that if we model the Ceres data using extraterrestrial organic compounds, which may be a more appropriate analogue than those found on Earth, then we need much more organic matter in Ceres to explain the strength of the spectral absorption that we see there, "Kaplan said. "We estimate that up to 40 to 50 percent of the spectral signal we see in Ceres is explained by organics, which is a big difference compared to the six to 10 percent reported earlier on the basis of organic terrestrial compounds."
If the concentration of organics in Ceres is really that high, it raises a series of new questions about the source of that material. There are two competitive possibilities of where the Ceres organics may have come from. They could have been produced internally in Ceres and then exposed on the surface, or they could have been released to the surface by an impact of a comet or asteroid rich in organic matter.
This new study suggests that if organic products are delivered, then the high potential concentrations of organic compounds would be more consistent with the impact of a comet rather than an asteroid. It is known that comets have a significantly higher internal abundance of organic compounds compared to primitive asteroids, potentially similar to the 40 to 50 percent figure suggested by this study for these locations in Ceres. However, the heat of an impact is likely to destroy a substantial amount of the organic compounds of a comet, so it is not clear whether these high abundances could even be explained by a cometary impact.
The alternative explanation, that organic compounds formed directly in Ceres, raises questions as well. The detection of organic compounds has so far been limited to small patches in the northern hemisphere of Ceres. Such high concentrations in such small areas require an explanation.
"If the organic substances are made in Ceres, then you probably still need a mechanism to concentrate it in these specific places or at least to preserve it in these places," said Ralph. Milliken, associate professor in the Department of Terrestrial, Environmental and Planetary Sciences of Brown and co-author of the study. "It is not clear what that mechanism might be, Ceres is clearly a fascinating object, and understanding the history and origin of organic compounds in these places and elsewhere in Ceres will probably require future missions that can analyze or return samples."
For now, researchers hope this study will be useful in informing upcoming return sample missions to near-Earth asteroids that are also thought to harbor aquatic minerals and organic compounds. The Japanese spacecraft Hayabusa2 is expected to arrive at the asteroid Ryugu in several weeks, and NASA's OSIRIS-REx mission will arrive at the asteroid Bennu in August. Kaplan is currently a member of the scientific team with the OSIRIS-REx mission.
"I believe that the work that was done in this study, which included new laboratory measurements of important early meteorite components, can provide a framework for better interpreting asteroid data and making links between observations of spacecraft and samples in our collection of meteorites, "said Kaplan. "As a new member of the OSIRIS-REx team, I am particularly interested in how this could be applied to our mission"