What life needs

Everything known about life comes from one place. This is it. Nobody has yet found a living thing, or the trace of one, anywhere else. So to guess where else life might be, the only thing to do is to ask what life here cannot do without, and then go looking for worlds that have it.

The list is short. First, liquid water. The chemistry of every living thing on Earth happens in water. Second, the right ingredients: six common elements, carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur, from which the molecules of life are built. Third, a supply of energy to keep the chemistry going. And then time, a great deal of it.

For a long time people assumed that energy had to mean sunlight. Then, in 1977, scientists exploring the sea floor near the Galápagos Islands found cracks where warm water, heated by molten rock below, seeps out. Round them, in total darkness, were crowds of creatures never seen before. Two years later, further north in the Pacific, they found vents pouring out water at more than 350 degrees. This is a vent of that hotter kind, on the floor of the Atlantic. These live on the chemicals in the hot water. No sunlight is needed.

Heat is no bar either. This is the Grand Prismatic Spring in Yellowstone, in the United States, about 110 metres across. The blue middle is the hottest part. The bands of orange and yellow round it are not rock. They are living mats of microbes. In springs like this one, some microbes thrive only in water above 80 degrees.

This animal is a water bear, shown by an electron microscope. A big one is about a millimetre long. In 2007 the European Space Agency sent about 3,000 of them into orbit and opened them to space for 10 days, with no air at all. The man who led the experiment reported that the emptiness of space was no problem for them. Some were also left in the Sun’s full ultraviolet light. Most of those died, but a few came home alive.

None of these creatures can do without liquid water. Dried, frozen or scorched, the toughest of them can wait, but they cannot grow until water returns. That is why the search for life elsewhere is first of all a search for water. And the hottest that anything is known to grow is 122 degrees, a microbe taken from a sea-floor vent and grown in a laboratory under high pressure.