Monday, January 10, 2011
The ten most important questions for astrobiology: Number 8
Is there a relationship between interstellar chemistry and life?
There are a number of layers to this question and to potential avenues in answering it. First, as with anything in this universe most of what's out there in the great beyond is simply hydrogen and helium (curse you Big Bang and expanding universe!). Hydrogen chemistry, while surprising complex, doesn't produce a big toolkit, and helium as we all know is mostly chemically inert. The formation of molecular hydrogen and molecular hydrogen ions does nonetheless seem to play a central role in cooling processes in the young universe that help gravity collapse matter to make the first stars. However, the kind of chemistry we're really interested in is that which takes the traces of heavy elements produced by those stars and their descendants and makes complex molecular species.
Rather remarkably, of the 150 plus interstellar molecules that we have thus far identified, and there are many more to go, then the great majority involve carbon. Carbon chemistry - organic chemistry - seems to be ubiquitous in the universe. We can even find it in the accretion disks around super-massive black holes. Why is this so? Carbon is just the right kind of atom to make all sorts of chemical links to other atoms, including nice single, double and triple covalent bonds. Particularly handy is its ability to covalently bond to itself, creating almost limitless chains or polymers. Carbon-based structures like polycyclic aromatic hydrocarbons built from benzene rings are also particularly tough and can handle a wide range of interstellar radiation environments, from hot to cold.
We, and all the life we know of, are of course based on carbon chemistry. Planets and moons in our solar system have a lot of carbon chemistry on their surfaces. Asteroids and comets can be positive smorgasbords of rich organic molecules. The circumstellar and proto-planetary disks of nascent star systems reveal, to put it crudely, a ton of carbon chemistry going on. It's a carbon, carbon world after all.
So, do we have an answer to question 8? Does interstellar chemistry have a direct bearing on the chemistry that ends up on planetary surfaces? Well, this is still open to debate. Chemistry in interstellar space, or even thicker nebula, is a long-winded, low temperature process. In the clumps of nebula or molecular clouds that end up making new stars and planets things speed up, but conditions may also undo the ancient mix of molecules brewing across a galaxy. Does it matter? Some evidence suggests that when low-mass stars are forming they may not emit enough ultraviolet light to crack open the tough interstellar molecules that would otherwise set in motion whole chemical chains. So unless interstellar space provided a rich starter mix then things might reach a bottleneck in terms of early chemistry. Of course the surface of a young rocky planet is likely a hot, rough, and chemically productive place - but perhaps the initial chemical wash it receives does actually matter, possibly even for dictating the chirality or handedness of biological molecules. Can old interstellar molecules make it through the chaos of a forming solar system and end up on young planetary surfaces? We don't know, but isotopic evidence suggests that at least some meteorites here on Earth could contain molecules that formed in the fearsome chill of interstellar space rather than more locally.
The complete answer to number 8 is therefore still elusive. New astronomical instruments like Herschel and ALMA will help open up the chemical universe further, and tucked away in a number of labs are remarkable new experiments that seek to reproduce interstellar carbon chemistry and learn more about its pathways and efficiencies. What there can be no doubt about is that the predominant chemistry of the universe as a whole is organic, it just remains to be seen which bits link the most directly to life on a planet.