August 2, 2011 | 5
“We are starstuff”, it’s a well-used phrase in popular astronomy (yes, we are. The nuclei of most heavy atoms in your body were forged long before our solar system existed, a million kilometers down inside the cores of long-since-gone massive stars). “We contain matter as old as the universe” (absolutely. Pretty much all the hydrogen nuclei – and even a few sneaky deuteriums – in your fleshy vessel are 13.7 billion years old, the leftovers of primordial nucleosynthesis). But what you probably haven’t heard before is “We all began with molecular hydrogen”. It doesn’t sound as dramatic but it’s correct, with a few minor caveats, and here’s why.
Most normal matter in the universe consists of hydrogen (75%) and helium (25%), everything else – absolutely everything else except for a tiny bit of lithium – amounts to a mere fraction of a percent that has taken many, many stellar generations to produce. But how do hydrogen and helium start making stars in the first place? It’s a critically important and still not fully answered question, but we think we have some pieces of the solution, and one those involves chemistry.
Pure hydrogen in the form of single atoms floating around in the cosmos is actually tricky stuff. To make an object like a star matter has to condense to high density. This process of shrinking and thickening is driven by the hydrogen’s own gravity, and a weeny bit of dark matter (on larger scales dark matter plays a much bigger role in pulling stuff together). For this to happen the gas mixture has to also initially cool off, lowering its pressure and letting gravity take over. But hydrogen atoms are horribly inefficient at cooling. Molecular hydrogen (H2) on the other hand is much better, since now the two protons and electrons have an axis – the molecule can rotate and even vibrate – which opens a new channel for getting rid of thermal energy. The story of how the first molecules of hydrogen form is a fascinating, but somewhat complex one for another day. Suffice to say that with the help of incredible new experiments here on Earth we’re homing in on the details.
H2 cooling allows the universe to make the first stars, it’s a dirty little secret – chemistry leads to stellar astrophysics. There’s more though. Once the universe has been polluted by heavier elements forged inside those stars it also begins to fill with a thin haze of molecules of all flavors. Interstellar space contains over 150 identified species of molecules, and likely far more than that exist out there. Some are simple, just a few atoms, others are more complex. They’re all over the place; in thick nebula, in proto-stellar systems, and in more tenuous warm gas between the stars. The great majority are also organic (in other words they involve carbon), and appear to have formed in-situ. But how does molecular chemistry take place in the sparse and frigid depths of space?
Reactions that build complex molecules in cold and thin gas typically need to be helped along, you can’t just put two single atoms on a blind date and hope for the best. Perhaps a stray ultraviolet photon comes by and strips an electron off an atom, making a chemically eager ion. But even better would be a real initiator, a chemical lighter fluid that deftly encourages atoms to combine. Nature, it turns out, has just the thing. While molecular hydrogen (H2) is the most abundant molecule in the universe, the next most abundant is the robust sounding “protonated molecular hydrogen”, or H3+. As the name implies, H3+ is ordinary old molecular hydrogen with an extra proton, making a stable but highly reactive (and acidic) structure.
H3+ is readily produced in interstellar space when cosmic ray particles (for example, fast moving electrons) ionize a regular H2 molecule to a positively charged H2+ which then pulls apart another H2 to grab that extra proton (along with another electron) and make H3+. And this is where the fun starts.
The majority of ion-molecule chemistry that takes place in interstellar environments begins with H3+, it is the mother of molecules. The illustration below (taken from work by Prof. Ben McCall at the University of Illinois at Urbana-Champaign) lays out just some of the chemical reaction chains that H3+ plays the pivotal role in.
It’s impressive. There’s a route to water, there’s a route to cyanide (a key ingredient for more complex carbon chemistry, including items like amino acids), and perhaps most critically there is a route at the top to arbitrarily long carbon chains.
So did we all begin with H3+? One of the most interesting things about protonated molecular hydrogen is that it was expected to exist and be important in the cosmos well before it was actually detected in the interstellar medium of our Galaxy. It also serves as a benchmark structure for quantum chemists. The detection was in part tricky because astronomers didn’t really know what the spectral signature would be for this molecule. Hard work on the problem eventually paid off and by the 1980′s H3+ was showing up in people’s data. Understanding the prevalence of H3+, the different environments it occurs in, and of course the role it plays at the root of interstellar chemistry is an ongoing business. But as progress is made it seems that H3+ really is a galaxy wide phenomenon, forming even in the environs of supernova, and planetary atmospheres (in Jupiter it is the dominant coolant of the ionosphere).
In order to make objects like rocky planets, and to coat them in water and basic carbon compounds, much is undoubtedly owed to the chemistry that takes place out in deep space. While none of those ancient, frigid, molecules of true interstellar origin necessarily end up intact on a planetary surface they nonetheless set the stage in nebular stellar nurseries for all that follows. From the formation of microscopic cosmic dust to larger chunks of material, chemistry is central. We don’t yet know all the pathways of matter agglomeration that take a molecule-rich cloud of gas and convert it into stars and planets, but I think it’s a pretty good bet that without H3+ things would turn out very differently.
Exactly how interstellar chemistry and proto-stellar chemistry influences the viability of planetary systems for harboring life is a huge and open question. But it does seem clear that two, and then three, hydrogen atoms may be all that stand between us and a rather empty and boring cosmos…