Thursday, September 01, 2005

Life and Water

In February I wrote a post on whether water was required for extra-terrestrial life, and whether NASA should be chasing water in it's search for aliens.

So what is needed for the generation of life de novo (from scratch)?

First we need a basic concept of what life is. For now lets just say that life is a self replicating entity. So the "search for life" could be substituted for "the search for self replicating entities". If life is going to start, it will most probably start with the simplest "entity", a molecule - most probably a polymer. Our quest then is "the search for self replicating polymers (and their descendants)" .

So what should we look for? Or what conditions are necessary for polymers to form?

Oxygen? Actually Oxygen would be a great impediment. Complicated molecules need to form large polymers. Oxygen, which likes to steal electrons from other molecules, would prevent the sharing of electrons (i.e. the covalent bonds) that are required for polymer formation. All geologic evidence points to the earth's original environment as being reducing (i.e. the opposite of oxidizing). Free oxygen first appeared on our planet after photosynthesis evolved. Are they asserting that photosynthesis (or a similar process) evolved on Mars? That is unlikely.

Water? Well from humans' experience with polymer chemistry water is not important. In fact most chemists prefer to synthesize polymers in non-aqueous (i.e. non-water) solutions. Water, like other polar solvents, is very reactive and can limit the types of polymers found. So if we were to look for polymers, we would have a better chance in an environment that had non-polar solvents as the principal medium such as the methane oceans of titan (left). But as Stanley L. Miller and Harold C. Urey showed, polymers can self assemble to a certain degree in water (as what most probably happened on earth).

Do we need liquids of any sort? Since liquids are a good medium for chemical reactions, they would help, however polymers could also form in gaseous environments ...

So why are we searching for life on Mars? There is not much liquid. If there was any it was water. And the planet has an oxidative environment (as there is free oxygen and little evidence of reduced carbon). Besides mars is one big desert.

A more apt target would be a chemically rich planet with lots of activity ... like Venus.

I had initiated a vicious debate with some but now it seems that many are arriving at the same conclusion I had made (and I'm sure made by many others). So why do some insist that water is special? From an article in last week's edition of Nature on a conference on the subject held in Varenna, Italy:
Water has many properties that seem indispensable for the functioning of proteins and cells. It is an excellent solvent for ions, for example it's crucial for nerve signaling, enzymatic processes, biomineralization and the behavior of DNA. It is also a master of weak intermolecular interactions such as hydrogen bonds and hydrophobic forces. The latter play a central role in protein folding and protein‚ąíprotein interactions, whereas the former often act as bridges between protein binding sites and their substrates. And water's ability to absorb and lose heat without undergoing a large temperature change provides thermal cushioning, shielding cells and organisms from wild temperature swings.
But then others such as Steven Brenner point out:
... water is generally not a good solvent for doing organic chemistry which is, in the end, what life is all about. For one thing, water is rather reactive, tending to split apart the bonds that link the building blocks of biomolecules together. It readily breaks peptide bonds, for example, as well as many of the bonds in nucleic acids, such as RNA. "The structure of RNA screams 'I did not arise in water!'" Benner asserts. He says that in about four out of five cases, synthetic organic chemists will avoid using water as a solvent.
Having said that the article reports:
"I think it is perfectly possible that at least elements of relevant biochemistry can be persuaded to work in a completely non-aqueous environment," says physicist John Finney at University College London. Finney points to evidence that enzymes can work in 'dry' air, where they hold on to only the barest coatings of water molecules, and even in non-aqueous solvents.

What are possible candidates?

  • Liquid Ammonia
  • Formamide
  • Liquid Hydrogen
  • Liquid Nitrogen
  • Liquid Methane
So what conclusions did the conference attendees make?
[T]he participants in Varenna generally agreed that life on Earth is adapted to water rather than the other way round. "Life on Earth itself is fine-tuned to water a consequence of it evolving in close association with the medium," says Finney. "To put it the other way is perhaps to put the cart before the horse."