Huygens probe lands on Titan

A European space probe has landed on Titan, the largest moon of Saturn, sending back the first pictures from its surface.

The Huygens probe touched down on 14 January after a descent of around two and a half hours through Titan's thick, hazy atmosphere. It is the most distant landing ever made by a spacecraft.

A strange world

Pictures taken during the descent show a landscape that looks strangely familiar, with what appear to be river channels, shorelines and hills. On the surface, the probe found rounded pebbles, thought to be made of water ice, on ground that scientists described as having the consistency of wet sand or clay.

Titan is bigger than the planet Mercury and has a thick atmosphere made mainly of nitrogen. Its surface temperature is around minus 180 degrees Celsius, so cold that methane, a gas on Earth, can exist as a liquid.

Scientists believe the channels may have been carved by liquid methane, and that it may rain methane on Titan.

A long journey

Huygens was carried to Saturn by NASA's Cassini spacecraft, which was launched in 1997 and arrived in orbit around Saturn last year. The probe separated from Cassini on Christmas Day and spent three weeks travelling to Titan.

The mission is a joint project between NASA, the European Space Agency and the Italian space agency. Huygens was built by the European Space Agency, and British scientists and engineers played an important part in designing its instruments.

A glitch

Not everything went to plan. One of the two radio channels used to send data from Huygens to Cassini was not switched on because of a programming error, and some of the pictures were lost. But scientists say they still have a wealth of information.

The probe continued sending data from the surface for more than an hour before Cassini moved out of range.

Clues to the early Earth

Scientists are interested in Titan because its atmosphere is rich in organic chemicals, the building blocks of life. They believe it may resemble the early Earth before life began, preserved in deep freeze.

The European Space Agency's director of science described the landing as a great achievement for Europe and for science.

The Cassini spacecraft will continue to study Saturn, its rings and its moons for several years.