By Jonathan Amos

Science reporter, BBC News

The plumes have been one of the great discoveries of the Cassini mission

Nasa's Cassini spacecraft has obtained strong evidence that Saturn's tiny moon Enceladus retains liquid water. The probe has detected sodium salts in the vicinity of the satellite, which appear to spew from its south pole. Liquid water that is in prolonged contact with rock will leach out sodium - in exactly the same way as Earth's oceans have become salty over time. Scientists tell Nature magazine that the liquid water may reside in caverns just below the surface of the moon. If confirmed, it is a stunning result. It means the Saturnian satellite may be one of the most promising places in the Solar System to search for signs of extraterrestrial life. "We need three ingredients for life, as far as we know - liquid water, energy and the basic chemical building blocks - and we seem to have all three at Enceladus, including some fairly complex organic molecules," commented John Spencer, a Cassini scientist from the Southwest Research Institute, Boulder, Colorado. "That's not to say there is life on Enceladus but certainly the 'feedstock' is there for life to use if it does exist," he told BBC News. ENCELADUS - AN ACTIVE MOON OF SATURN Enceladus experiences tidal contortions as it orbits its parent planet This energy is producing a "hotspot" at the satellite's southern pole Big cracks (L) are 100 degrees warmer than the surrounding ice surface These tiger stripes are the source of immense plumes (R)

Scientists have been looking for sodium near Enceladus since the discovery in 2005 that this 500km-wide moon was active and hurling water vapour and ice particles into space. The vapour and ice particles emerge in super-fast jets from a series of "warm" surface cracks referred to as "tiger stripes" because of their resemblance to the big cat's coat markings. Researchers speculated that the jets could be being fed by a large sub-surface body of liquid water, even an ocean. But the best indicator remained frustratingly elusive. If it existed, such a mass of water in contact with rock deep within Enceladus would acquire a range of dissolved salts over time and these ought to be detectable in the jets by Earth telescopes. Indeed, sodium (which in Earth's oceans forms the dominant sea salt, sodium chloride) is one of the easiest elements for observatories to spot in space. Please turn on JavaScript. Media requires JavaScript to play. However, even mighty telescopes like the Keck on Mauna Kea in Hawaii could never see sodium when they looked towards Enceladus. The latest Cassini data appears to solve this conundrum. The Nasa spacecraft has been flying through Saturn's outer E ring which is sustained by the constant stream of material coming up from the tiger stripes. Using its Cosmic Dust Analyser (CDA), Cassini has analysed thousands of ice grains and directly "tasted" the missing salt - principally sodium chloride and sodium bicarbonate ("baking soda"). The amounts, though, are tiny - less than 2% of the mass of the sampled grains. The low abundance helps explain why the telescopes had overlooked the salt. The fact that the sodium is bound into the water-ice molecules also effectively hides its light signature from the observatories' instruments. However, scientists say the Cassini and telescope observations taken together give hints about what the water reservoir on Enceladus might look like. HOW THE 'PLUMBING' MIGHT LOOK Tidal heating melts ices to produce a large sub-surface sea Liquid water in contact with the rocky core dissolves minerals The salty water moves up through the mantle to large caverns The bubbling reservoirs slowly vaporise and make ice grains This material is drawn up through cracks and shot out into space Please turn on JavaScript. Media requires JavaScript to play. The popular picture that is now emerging is of a very deep mass of water pressed up against the moon's rocky core and which is dissolving the salts. Water from this sub-surface sea is then working its way up to shallower reservoirs through a network of faults in Enceladus's ice mantle. Scientists envisage misty caverns just below the tiger stripes where some of the water vaporises free of sodium and some of it becomes frozen into the small grains detected by Cassini. "Water droplets are probably lifted by gas bubbles in the water (like the spray you see above sparkling water)," said Nature author Frank Postberg, a CDA scientist with the Max Planck Institute for Nuclear Physics, Heidelberg, Germany. "These aerosol-droplets shock freeze and conserve the liquid composition. Then they are accelerated upwards through the cracks in the ice crust by the up-streaming vapour." Critically, the whole process cannot be too energetic otherwise the salt would be blown into space in a way that would be visible to Earth telescopes. A previous suggestion that the jets are geyser-like phenomena is dead. "This idea of slow evaporation from a deep cavernous ocean is not the dramatic idea that we imagined before, but it is possible given both our results so far," said Professor Nicholas Schneider, whose telescopes team has a companion paper to Postberg's in Nature. But the Colorado University-Boulder scientist also cautioned that the presence of sub-surface water was not yet proven fact. Several other explanations for the jets were equally plausible, he said. "It could still be warm ice vaporising away into space. It could even be places where the crust rubs against itself from tidal motions and the friction creates liquid water that would then evaporate into space," he said. "These are all hypotheses but we can't verify any one with the results so far." The Cassini-Huygens mission is a co-operative project of Nasa, the European Space Agency (Esa) and the Italian space agency (Asi). Jonathan.Amos-INTERNET@bbc.co.uk



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