Do aliens get thirsty?
It makes sense that life on a wet planet like ours evolved to depend on water. But is water really that special? Could life on other planets get by without it?
Living organisms are pretty extraordinary, but get any one of us under a microscope and we're all pretty much the same: bags of water with chemicals floating around in them. They're called cells, and they're the basis of every life form from bacteria up. And those cells couldn't exist without the physical and chemical qualities of water.
All the processes that make living cells tick — getting energy from food, splitting in half to make new cells and doing the odd repair job — are just chemical reactions. And like all chemical reactions they can only happen if the molecules involved bump into one another, which is where water comes in.
Most of the chemicals involved in cell chemistry are either solids (like proteins or salts) or gases (like oxygen). Chemistry doesn't work so well in these states — the atoms in solids haven't got enough energy to move around, and gases are so spread out the odds of a decent collision are slim. But dissolved in liquid water all those molecules are constantly jostling around ready to react.
Some reactions happen whenever the right molecules bump into one another. But the big reactions — like breaking down food for energy, or manufacturing molecules according to the rules in a cell's genes — need enzymes to get going.
Enzymes run the show as far as living tissues go, but they'll only work under very specific conditions. Basically unless they're soaking in nice warm, slightly salty water they're duds. Put an enzyme in the wrong kind of liquid or at the wrong temperature and it unravels in a very bad hair kind of way.
So water's a great meet 'n' greet medium for all the molecules that dissolve in it. But just as important as the things that water can dissolve are the things it can't dissolve — fats.
Fats and water repel one another, which makes fats fantastic for separating different pockets of watery stuff in living things. And that's just the kind of organisational system you need to make cells that can be assembled and reorganised into complex things like us.
The fatty membranes around the outside of cells stop the water inside them from leaking out, and vice versa. And the smaller fatty membranes inside subdivide cells into different sections where different reactions and processes can happen. Without the mutual repulsion of water and fats, life on this planet couldn't have evolved beyond soup.
Water for power
But we don't just need water to organise our cells and make our enzymes look good. Water is essential for the energy systems that power all life on Earth.
Living things get their energy from food. Plants make their own food through photosynthesis, and animals eat other things. And neither process could take place without water.
Plants are like floppy green solar power stations. Instead of solar panels they use water, carbon dioxide and chlorophyll to trap energy from the sun. They rejig the atoms in H2O and CO2 to form sugar (C6H12O6), trapping solar energy in the new chemical bonds. That sugar (glucose) gets made into other complex molecules, like proteins, fats and bigger carbs, which we non-plant types refer to as 'food'.
And while plants use water to make food, animals use water to help tear apart food molecules so we can get access to that recycled solar power.
Water's pretty special all right. It's essential for building and powering every living thing on Earth. But does that mean all life needs water to exist?
Life, but not as we H2O it …
Two other liquids that are pretty common in space are often touted as possible candidates for supporting non-watery life.
Liquid ammonia (NH3) dissolves a lot of the same kinds of compounds that water does. And Saturn's moon Titan has entire oceans of methane (CH4). They're liquids, so they tick the first box as water substitutes. But they fall well short of ideal candidates for life support.
For starters, the reason they're liquids in space is that space is bloody cold. Once the mercury gets above -33°C ammonia boils into a gas, so it could only be the basis for life in places that make the Arctic look balmy.
But that's nothing compared to methane. The oceans of Titan would evaporate if temperatures ever topped -164°C. Life there would be only slightly warmer than life in liquid nitrogen. Cold climates are one thing, but the kind of reactions that make life tick just wouldn't happen on any useful kind of timescale at those temperatures — there just isn't enough energy.
Even if temperature wasn't a problem, there's no way enzymes could do their job in these liquids. Enzymes can't cope with water that's a bit too acidic, salty or hot — they would literally unravel in ammonia or methane.
So as far as supporting the kind of chemistry that runs life on Earth, ammonia and methane don't cut it. Liquid water is still the prize for those searching for signs of life in the skies.
But if life here has taught us anything it's that almost anything goes. And with a sample size of just one planet to go by, science is keeping an open mind in its lookout for liquids that can support life. As unlikely as they might be, we haven't completely ruled out frigid life forms that smell like cleaning products.
Thanks to Associate Professor Terry Walsh from the Institute of Health Biomedical Innovation at Queensland University of Technology.
Published 08 June 2010