Saturn's moon Enceladus is to be closely examined for signs of life by the Cassini space probe. On Wednesday 28th October, it is to be steered through a fountain of icy spray that is spurting out of the ocean just beneath Enceladus’s icy shell—an ocean that might just have alien life-forms living in it. Analysing exactly what chemicals are in the spray would enable experts to find out whether the moon had the right “ingredients for life”: water, chemicals and energy. In December, Cassini will again fly by Enceladus, this time at a much higher altitude to measure the heat flowing from the moon's interior.

Cassini space probe will fly through a plume that is shooting out of Enceladus’s south polar region 49 kilometres (30 miles) above its icy surface. During the approach, instruments on board the craft will sample the spray and analyse the chemicals within it. The low flight path will enable Cassini to sample heavier molecules, including organic chemicals. The plume has already been seen erupting from one of the mysterious surface features on Enceladus known as "tiger stripes”: four 2-kilometre (1.2-mile) wide cracks, mint-green in colour, which are believed to be sites of regular volcanic activity on Enceladus.

The plume is fed by icy geysers which blast 250 kilograms (551 pounds) of water vapour, ice particles and chemicals into space at more than 2000 km/h (1240 mph). The geysers are thought to erupt from what scientists call hydrothermal vents: fissures (narrow openings) in the ocean bed where sea water seeping into the bedrock is heated to high temperatures and blasted upwards. In the Earth’s oceans, the minerals contained in the jets of water, known as “black smokers”, provide nutrients for bacteria and teeming colonies of life-forms growing up around them. Scientists are keen to find out whether the same might be found on Enceladus. Detecting certain special forms of hydrogen called “molecular hydrogen” in the plume would provide researchers with evidence not for life itself, but the likely existence of hydrothermal vents on Enceladus.