The universe teems with complex organic molecules

Asteroids are less pristine than comets, having often endured heating and the effects of liquid water. But these effects can produce a dramatic new organic complexity. For decades, scientists have known that meteorites called chondrites, which come from asteroids, contain an amazing diversity of organic molecules. The Murchison meteorite, which fell in Australia in 1969, contains more than 96 different amino acids. Life uses only 20 or so. Osiris-Rex and Hayabusa2 confirmed that asteroids Bennu and Ryugu are complex like those meteorites. And at least some of this complexity seems to have come before the asteroids themselves: A preliminary analysis of the Bennu specimen suggests that it retained organic matter, including polycyclic aromatic hydrocarbons, from the protoplanetary disk.

The chemistry of life?

Organic molecules on early Earth took a remarkable new step in complexity. They in a certain way they organized themselves in something alive. Some hypotheses for the origins of life on Earth involve a starting kit of organic matter from space. The “PAH world” hypothesis, for example, posits a stage in the primordial soup that was dominated by polycyclic aromatic hydrocarbons. From this slurry emerge the first genetic molecules.

In general, understanding how complex organics form in space and end up on the planets could give a better idea of ​​whether life has also arrived on other worlds. If the raw material for life on Earth formed in the interstellar medium, the material for life must 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 hunting 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 already 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 the atmosphere for organic molecules, like the upcoming Dragonfly mission to Saturn’s moon Titan.

However, it is difficult to determine whether a given organic molecule is it a biosignature or not. If scientists were to find organic molecular assemblies quite complex, that would be enough to convince at least some researchers that we have found life in another world. But as comets and asteroids reveal, the lifeless world is complex in its own right. Compounds thought to be biosignatures have been found on lifeless rocks, such as the dimethyl sulfide Hänni recently identified on 67P.