Life beyond Earth
Mars was once warmer and wetter than it is today. Pictures sent back by ESA's Mars Express and other spacecraft show huge channels that look like dry river beds. Where did the water go?
A small amount is locked up in the polar ice caps. Some escaped into space. But much of it is frozen into the soil and rock.
It is possible that life began on Mars billions of years ago when it was warm and wet. Today, the surface is too cold and dry to support life. However, some scientists think that simple life, such as bacteria, may still exist deep underground.
In a few years' time, ESA is planning to send a rover to search for Martian life. This will be followed by a mission to return rock and soil for scientific study.
Apart from Mars, life may exist in ice-covered oceans on some of the moons of Jupiter. Titan, the largest moon of Saturn, is also a very interesting place.
Scientists think Titan is like the young Earth – only much colder. In January 2005, ESA's Huygens probe landed on this icy, smog-covered satellite. It found dry river channels and lakes that are sometimes filled with liquid methane.