Sometimes, I think scientists are just that little bit too modest. A new paper in Science has a humdinger of a title: “Localized aliphatic organic material on the surface of Ceres”. It doesn’t exactly trip off the tongue and may not even seem that important. But what the researchers have discovered is a huge deal. They’ve found organic compounds – the kind of molecules from which life on Earth originated – on the surface of Ceres, the solar system’s largest asteroid.
For people like me who study asteroids, finding organic molecules is not necessarily surprising. It has been known for over 200 years that meteorites (which are fragments from asteroids) contain a wide range of organic compounds. And Ceres was selected as a target for the Dawn mission precisely because it was hoped that organic material would be found. So why am I so excited over the discovery? The significance is in the first two words of the title: “Localised aliphatic”.
Let’s start with “localised”. The molecules were found in a specific place on the surface – around the crater Ernutet (at a latitude of 50°N and a longitude of 45.5°E). There are two possible origins for the organic compounds on Ceres. Either they have always been there, native to the asteroid, and part of the primitive material from which Ceres (and the rest of the solar system) formed. Or the organics were added later, through impact from comets, other asteroids or interplanetary dust. In either case, organic material should be distributed more or less uniformly over the surface, not be clustered in a specific place. The significance of the observation is not so much the finding of organic compounds at Ernutet, but not finding them everywhere.
Let’s move on to the second term: aliphatic. Organic molecules are broadly divided into two major types: aromatic and aliphatic. In the former, carbon atoms are arranged in rings that can build up into vast networks of molecules. In contrast, aliphatic compounds are chains of carbon atoms. And we know that aromatic compounds are generally sturdier and more resistant to radiation and heat than aliphatic molecules with the same number of carbon atoms.
On an active asteroid surface, like Ceres, it would be more likely that aromatic compounds survived than aliphatic. This is also reflected in the most carbon-rich of meteorites, where aromatic compounds are by far the more abundant component in the intimate mixture of aromatic and aliphatic organics that they contain. However, the organic molecules that have been detected on Ceres are complex aliphatic compounds that seem to be almost tar-like in nature.