Looking for life in the Solar System

The search began on Mars. In 1976 two American spacecraft called Viking landed there, each carrying three experiments built to find living microbes in the soil. The soil did something odd and unexpected when it was fed and watered, but it was chemistry. Neither lander found a clear sign of life. For years afterwards the search turned to an easier question: was Mars ever a place where life could have lived?

It was. Mars once had rivers and lakes. In July 2024 the rover Perseverance, working in a dried-up river valley, came on this rock. It is about a metre long, and it is covered in small pale spots with dark rims. Inside the spots are two minerals that, on Earth, are often left behind by microbes feeding. They can also form with no life at all. NASA called it a potential sign of life, which means: worth every test there is. The rover has kept a piece.

Further out, the best hopes are oceans. This is Europa, a moon of Jupiter, and what you see is ice. Under it is thought to be a salt sea holding more than twice the water of all the Earth’s oceans. The spacecraft Europa Clipper left the Earth in October 2024 and reaches Jupiter in April 2030. It will pass close over Europa 49 times. It is not going to look for life. It is going to find out whether life could live there.

At Saturn, the little moon Enceladus sprays its ocean into space. Jets of ice and vapour leave cracks near its south pole at 400 metres a second, and the Cassini spacecraft flew through them. It found salts, and compounds of carbon, and tiny grains of a mineral that forms only where hot water meets rock. That is all three things life needs, on a moon 500 kilometres across.

Titan, Saturn’s biggest moon, is the strangest of the four. This is its surface, photographed in January 2005 by the European probe Huygens. It is 179 degrees below freezing. The pebbles are thought to be water ice, frozen as hard as rock. Titan has thick air, and rain, rivers and seas, but they are of liquid methane. Life there, if there is any, would have to be nothing like ours. A flying machine called Dragonfly is due to set off for Titan in 2028 and arrive in 2034.

Only one of these four places, Mars, lies in the band round the Sun where sunshine could keep water liquid. The others are far out in the cold, and their water is kept liquid by heat from inside. That discovery changed the search. A world does not need to be like the Earth, or even near its star, to be worth a look.