Necessarily cut from my piece in Nautilus on water and astrobiology was a paragraph of very early history, in which Christiaan Huygens anticipates this whole debate with eerie prescience. I hope it’s worth filling in that bit of the story here.
Galileo had looked at the moon and saw not the smooth, featureless sphere that Aristotelians believed in but mountains and valleys, their rugged topography picked out by the raking light of the Sun at the boundary where light meets darkness. Within just a couple of decades, writers and philosophers were starting to imagine journeying to this new world, much as Columbus had travelled to the Americas. The natural philosopher John Wilkins gave a factual account in his Discovery of a World in the Moone (1638), while the French soldier and writer Cyrano de Bergerac penned a satirical account of spaceflight in The States and Empires of the Moon, published posthumously in 1657. By the end of the century, scientists were starting to speculate about what the environments of these other worlds might be like.
In his posthumously published 1698 book Cosmotheoros, Huygens asserted that plants and animals on other planets must derive their “growth and nourishment” from “some liquid principle”. But he realized that water would freeze on Jupiter or Saturn, and so “Every planet therefore must have its waters of such a temper, as to be proportion’d to its heat”: Jupiter’s and Saturn’s “waters” must have a lower freezing point, and those of Venus and Mercury a higher boiling point. In other words, it isn’t too fanciful to say that Huygens was speculating that life on other planets might use non-aqueous solvents.
In my Nautilus article I veer towards the notion that there might be non-aqueous solvents for life. In my more technical article for the book Astrochemistry and Astrobiology (eds I. W. M. Smith, C. S. Cockell & S. Leach; Springer, Heidelberg, 2013), I equivocate rather more. It seems to me that this kind of Socratic dialogue (to be absurdly grandiose about it) is the best way of approaching the problem: one can make both cases, and it is hard to adduce any clear evidence at this point for which of them we should prefer. This is what I say in that latter piece:
“Attempts to enunciate the irreducible molecular-scale requirements for (as opposed to the emergent characteristics of) something we might recognize as life have been rather sporadic, and are often hampered by the difficulty of looking at the question through anything other than aqua-tinted spectacles. From the point of view of thinking about non-aqueous astrobiological solvents, a review of water’s roles in terrestrial biochemistry surely raises one key consideration straight away: it is not sufficient, in this context, to imagine a clear separation between the ‘molecular machinery’ and the solvent. There is a two-way exchange of behaviours between them, and this literally erases any dividing line between the biological components and their environment.
The key questions here are, then, necessarily vague. But the more we understand about the biochemical aspects of water, the less likely it seems that another solvent could mimic its versatility, sensitivity and responsiveness, for example to distinguish any old collapsed polypeptide chain from a fully functioning protein. It is perhaps this notion of responsiveness that emerges as the chief characteristic from a survey of water’s biological roles. It can be manipulated in three dimensions to augment the influence of biomolecules. It can receive and transmit their dynamical behaviours, and at the same time it can impose its own influence on solute dynamics so that some biomolecular behaviours become a kind of intimate conspiracy between solute and solvent. This adaptive sensitivity seems to facilitate the kind of compromise between structural integrity and reconfigurability that lies at the heart of many biomolecular processes, including molecular recognition, catalytic activity, conformational flexibility, long-range informational transfer and the ability to adapt to new environments. It is easy to imagine – but very hard to prove! – that such properties are likely to be needed in any molecular system with sufficient complexity to grow, replicate, metabolize and evolve – in other words, to qualify as living.
In these respects it does seem challenging to postulate any solvent that can hold a candle to water – not so much in terms of what it does, but in terms of the opportunities it offers for molecular evolution. This is by no means to endorse the dictum of NASA that astrobiologists need to ‘follow the water’. But hopefully it might sharpen the question of where else we might look.”