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.
Cradle of life?
So what do we make of these confusing observations, which come from the visible and infrared mapping spectrometer on the Dawn spacecraft?
The authors argue that the organics are unlikely to come from the impact of another body with Ceres, because the specific nature of the organic compounds that have been detected implies they would have been degraded or destroyed by the high temperatures of the collision. It is also likely that collision with another body would have mixed any organics with the surface material, not leaving them concentrated in the way that they are.
So instead the authors infer that the compounds are probably indigenous to Ceres. This is strengthened by the fact that the molecules are found together with carbonates and clays containing ammonia. These have been observed in many regions of Ceres, and are believed to be produced by hydrothermal processes (reactions involving heated water) on the dwarf planet – something we know can also produce organic material on Earth.
Indeed, the data show that carbonates and clays are higher in abundance around Ernutet than the surrounding landscape. Hydrothermal processes, such as those that occur at hot springs on Earth, might have been active in Ceres’ past, when the asteroid was warmer at depth than it is now, leading to the formation of the organics. But this also means that the mechanism that brought the minerals to the surface at Ernutet – and nowhere else – is unknown.
The combination of hot water and organic material is extremely exciting. Once you have an environment conducive to the production of organic materials – especially one that also contains the nitrogen-bearing clay minerals which are known to catalyse other reactions – it may not be a step too far to posit that Ceres had (and maybe still has) all the ingredients essential for formation of the chemicals that, on Earth, eventually led to the origin of life.
Ernutet is the Egyptian goddess of fertility or nourishment. Wouldn’t it be wonderful if finding organic molecules in a crater named after her was the first indication of a non-terrestrial cradle of life?