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.
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.
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.