The Cassini spacecraft's found the strongest evidence yet that Saturn's moon Enceladus is concealing a large, underground saltwater ocean - raising hopes for the possibility of life on similar moons.
Cassini discovered water plumes jetting from the 'tiger stripes' of the surface in 2005 and was recently able to fly directly through and sample them.
During three of Cassini's passes through the plumes in 2008 and 2009, its Cosmic Dust Analyser measured the composition of freshly ejected plume grains. These hit the detector at up to 11 miles per second, and vaporizing instantly and allowing scientists to analyze them.
The team found that ice grains further out from Enceladus were relatively small and mostly ice-poor, closely matching the composition of the E Ring, which traces the orbit of Enceladus around Saturn.
Closer to the moon, however, the Cassini observations indicate that relatively large, salt-rich grains predominate.
"There currently is no plausible way to produce a steady outflow of salt-rich grains from solid ice across all the tiger stripes other than the salt water under Enceladus' icy surface," says Frank Postberg of the University of Germany.
The team believes that deep beneath Enceladus’ surface, perhaps 80 km down, there is a layer of water between the rocky core and the icy mantle. This is kept liquid by tidal forces from Saturn and some neighbour moons, as well as by the heat generated by radioactive decay.
Salt in the rock dissolves into the water, which accumulates beneath the icy crust - and is exposed to space when the outermost layer cracksopen. The drop in pressure causes the liquid to evaporate, with some flash-freezing into salty ice grains, creatingthe plumes.
Around 200 kg of water vapour is lost every second in the plumes, with smaller amounts of ice grains. According to the team’s calculations, the water reservoirs must have large evaporating surfaces; otherwise, they'd freeze over, stopping the plumes.
"Enceladus is a tiny icy moon located in a region of the outer Solar System where no liquid water was expected to exist, because of its large distance from the sun," says Nicolas Altobelli, ESA’s project scientist for the Cassini-Huygens mission.
"This finding is therefore a crucial new piece of evidence showing that environmental conditions favourable to the emergence of life may be sustainable on icy bodies orbiting gas giant planets."