Asteroids are less pure than comets, which often survive heating and the effects of liquid water. But these effects can produce remarkable new organic complexity. For decades, scientists have known that meteorites called chondrites, which come from asteroids, contain a surprising variety of organic molecules. The Murchison meteorite, which fell in Australia in 1969, contains more than 96 different amino acids. Life only takes 20 or so. Osiris-Rex and Hayabusa2 confirmed that asteroids Bennu and Ryugu are as complex as meteorites. And at least some of this complexity seems to have arisen before the asteroids themselves: A preliminary analysis of the Bennu sample suggests that it remains organic material, including polycyclic aromatic hydrocarbons, from the protoplanetary disk.

The Chemistry of Life?

The organic molecules of the early Earth took on a new, remarkable step in complexity. they somehow organized themselves to a living thing. Other hypotheses for the origin of life on Earth include a starter kit of organic material from space. The “PAH world” hypothesis, for example, posits a stage in the primordial soup dominated by polycyclic aromatic hydrocarbons. From this slurry came the first genetic molecules.

In general, understanding how complex organics form in space and end up on planets can give us a better idea of ​​whether life has also arisen on other worlds. If the raw materials for life on Earth were formed in the interstellar medium, the stuff of life must be everywhere in the universe.

So far, such ideas remain untested. But because life itself represents a new level of organic complexity, astrobiologists are looking for complex organics as a possible biosignature, or sign of life, on other worlds in our solar system.

The European Space Agency’s Juice mission is on its way to study Jupiter and three of its icy moons, and NASA’s Europa Clipper mission launched toward one of those moons, Europa, in October. Both will use onboard instruments to search atmospheres for organic molecules, as will the upcoming Dragonfly mission to Saturn’s moon, Titan.

But it is difficult to determine whether a given organic molecule a biosignature or not. If scientists can find complex enough organic molecular assemblages, that will be enough to convince some researchers that we have found life on another world. But as comets and asteroids reveal, the inanimate world is complex in its own right. Compounds believed to be biosignatures have been found in lifeless rocks, such as Hänni’s dimethyl sulfide group recently identified in 67P.