Back in the early days of my pre-Corante blog, I wrote a piece about some other kinds of chemistry that might be used in living systems. There's now a wonderful one-stop review for all sorts of speculations on this topic, which incorporates everything I've ever thought of and plenty more. Steven Benner at the University of Florida, who my fellow Corantean Carl Zimmer has interviewed, and two co-workers (here's his research group) published "Is There a Common Chemical Model for Life in the Universe?" in Current Opinion in Chemical Biology late last year. (here's the abstract; I can't find the full text available yet on the Web.)
I can't say enough good things about this article. This is the sort of topic I've enjoyed thinking about for years, but there were still plenty of things in this review that had never occurred to me. Benner goes over the likely requirements for life as we know it, life as we'd probably recognize it, and life upon which we can barely speculate. As a chemist, he's particularly strong on discussions of the types of bonds that could best form the complex molecules that chemical-metabolism-based life needs. Energetic considerations - how much chemical bond energy is available, how soluble the materials are, how reactive they are at the various temperatures involved - are never far from his mind.
He devotes sections to ideas about living systems without chemical solvents (gas clouds, solid states) and the more familiar solvent-based chemistry. There's plenty of water out there in the universe - which is why bad movies about aliens coming to drain our oceans are so laughable - and it's natural enough that we should concentrate on water-based life. But there's plenty of ammonia out there, too, along with methane, sulfuric acid, and other potential solvents like the supercritical dihydrogen found in the lower layers of gas giant planets.
So, is all this stuff out there? Is life something that is just going to happen to susceptible chemical systems, given enough time? If so, which ones are susceptible? Benner's thoughts are, I think, best summed up by his take on Titan:
"Thus, as an environment, Titan certainly meets all of the stringent criteria outlined above for life. Titan is not at thermodynamic equilibrium. It has abundant carbon-containing molecules and heteroatoms. Titan's temperature is low enough to permit a wide range of bonding, covalent and non-covalent. Titan undoubtedly offers other resources believed to be useful for catalysts necessary for life, including metals and surfaces.
This makes inescapable the conclusion that if life is an intrinsic property of chemical reactivity, life should exist on Titan. Indeed, for life not to exist on Titan, we would have to argue that life is not an intrinsic property of the reactivity of carbon-containing molecules under conditions where they are stable. Rather, we would need to believe that either life is scarce in these conditions, or that there is something special, and better, about the environment that Earth presents (including its water)."
As for me, I can't wait to find out. I want Titan rovers, Jupiter and Saturn dirigibles, Venusian atmosphere sample return, instrument-laden miniature submarines melting down through the ice on Europa and Enceladus: the lot. How much of this will I ever get a chance to see in my lifetime? Current betting is running to "none of it, damn it", but things can change. Depends on how easily and cheaply we can get payloads up to (and out of) Earth orbit.