Asteroids are less pristine than comets, often having experienced heat and the effects of liquid water. But these effects can produce spectacular new organic complexity. For decades, scientists have known that meteorites called chondrites, which come from asteroids, contain a staggering diversity of organic molecules. The Murchison meteorite, which fell in Australia in 1969, contains more than 96 different amino acids. Life only uses twenty of them. Osiris-Rex and Hayabusa2 confirmed that the asteroids Bennu and Ryugu are as complex as these meteorites. And at least some of this complexity appears to have arisen before the asteroids themselves: preliminary analysis from the Bennu sample suggests that it retained organic matter, including polycyclic aromatic hydrocarbons, from the protoplanetary disk.
The chemistry of life?
Organic molecules from early Earth reached a new and remarkable level of complexity. They somehow they organized themselves into something alive. Some hypotheses about the origins of life on Earth involve a starter kit containing organic matter from space. The “PAH world” hypothesis, for example, postulates a primordial soup stage dominated by polycyclic aromatic hydrocarbons. From this mush came the first genetic molecules.
In general, understanding how complex organic materials form in space and end up on planets could give us a better idea of how life appeared on other worlds as well. If the raw materials for life on Earth were formed in the interstellar medium, the substance of life would have to be everywhere in the universe.
For now, such ideas remain largely untestable. But because life itself represents a new level of organic complexity, astrobiologists are searching for complex organic matter as a possible biosignature, or sign of life, on other worlds in our solar system.
The European Space Agency’s Juice mission is already on its way to study Jupiter and three of its icy moons, and NASA’s Europa Clipper mission launched to one of those moons, Europa, in October. Both will use onboard instruments to search for organic molecules in the atmosphere, much like the upcoming Dragonfly mission to Saturn’s moon Titan.
However, it is difficult to determine whether a given organic molecule is a biosignature or not. If scientists could discover sufficiently complex organic molecular assemblies, it would be enough to convince at least some researchers that we have found life on another world. But as comets and asteroids reveal, the non-living world is complex in its own right. Compounds considered biosignatures have been discovered on lifeless rocks, such as dimethyl sulfide that Hänni’s team recently identified on 67P.
