Hydrocarbon seas on Saturn's moon could be a solvent for biological molecules.
Are those dark patches on Titan really oceans, fed by rivers of liquid ethane? And if so, what are the fish like?
The extraordinary images sent from Saturn's giant moon by the Huygens spacecraft should make speculation about life in liquids other than water more than a scientific parlour game.
In fact, a lifeless Titan would point to a gap in our understanding of carbon-based molecules, says chemist Steven Benner of the University of Florida in Gainesville. Organisms should be comfortable in a hydrocarbon ocean, he says.
Water is a serious nuisance. Steven Benner , University of Florida, Gainesville
This possibility has previously been given short shrift. NASA has mostly explored places thought to contain liquid water, either now or in the past. It has sent craft to Mars, where ancient rivers seem to have carved the surface. And it has staged fly-bys of Jupiter's icy moon Europa, thought to have a watery ocean below its frozen surface.
Earth demonstrates the logic in this. Life is found just about everywhere there is water and a source of energy, and water seems a prerequisite for every form of life. This makes some scientists pessimistic about life on Titan: "There is no chance for life on the surface because it is too cold and there is no liquid water," says François Raulin, a scientist working on the European Space Agency's Huygens mission.
But does life depend on water? Or could it be that Earth life has evolved to suit its watery home? Anything we might recognize as life probably needs a liquid solvent to transport molecules and bring them together. But who says the solvent must be water?
Benner and his colleagues argue in Current Opinion in Chemical Biology that water-free environments on other worlds might fulfil the conditions for life1. Liquid ammonia is rather similar to water: it dissolves molecules with electrically charged parts, including carbon-based (organic) ones. On Earth, ammonia boils at -33°C; but there are many places in the Solar System where it could exist in liquid state, such as the clouds of Jupiter.
Other worlds could support exotic solvents: all of the gas giants might contain patches of dense, liquid-like hydrogen in their atmospheres, and Venus has clouds composed of droplets of sulphuric acid.
But Titan looks like the best candidate for non-aqueous life. It seems to have rivers and oceans, and its sticky surface is apparently made partly from organic molecules. There are nitrogen-containing organic compounds called nitriles in its atmosphere, which, it has been suggested, could react with water ice to form a rich blend of organic ingredients for possible life forms2.
Non-aqueous solvents such as hydrocarbons can support complex organic reactions, Benner points out. In fact, organic chemists usually prefer them to water, which is reactive and can interfere with delicate chemical processes.
One of the puzzles about the origin of life on Earth is why the first biological molecules were not torn apart by reactions with water. Life evolving in hydrocarbon liquids would not have this problem. "Water is a serious nuisance," Benner says: because of its reactivity, "the human genome survives only because it is constantly being repaired."
Even on Earth, many of the chemical reactions of life take place without water, catalysed by enzymes with water-repellent pockets. And many enzymes work perfectly well in the oily, water-free environment inside cell walls.
Relatively weak bonds, called hydrogen bonds, give terrestrial biomolecules, such as the DNA double helix, the crucial ability to stick together and then separate.
But water molecules form hydrogen bonds too, so groups of molecules bound by hydrogen bonds can fall apart rather easily in water. "In ethane," says Benner, "a hypothetical form of life would be able to use hydrogen bonding more."
So it's not obvious that water is special, apart from the fact that it exists in large quantities on Earth. "If life is an intrinsic property of chemical reactivity," Benner concludes, "life should exist on Titan. We need to go back, with a lander that can survive for weeks, not minutes."
But we'll have to wait a long time: so far, NASA has no firm plans for a return mission.
BennerS. A., RicardoA. & CarriganM. A. Curr. Opin. Chem. Biol., 8. 672 - 689 (2004).
HudsonR. L. & MooreM. H. Icarus, 172. 466 - 478 (2004).
University of Florida, Gainesville
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Ball, P. Titanic life may bloom without water. Nature (2005). https://doi.org/10.1038/news050131-2