NASA’s Cassini spacecraft had collected samples of ice spray shooting out of from Enceladus — one of 19 known moons of Saturn — during one of its frequent flybys.
The ice grains, scientists said, were originating from the “tiger stripe” surface fractures at the moon’s south pole and apparently have created the material for the faint E. Ring that traces its orbit around Saturn, the Daily Mail reported.
During three of Cassini’s passes through the plume in 2008 and 2009, the Cosmic Dust Analyser (CDA) on board measured the composition of freshly ejected plume grains.
The icy particles hit the detector’s target at speeds of up to 18 km-per-second, instantly vaporising them. The CDA separated the constituents of the resulting vapour clouds, allowing scientists to analyse them.
The ice grains found further out from Enceladus are relatively small and mostly ice-poor, closely matching the composition of the E. Ring. Closer to the moon, however, the Cassini observations indicate that relatively large, salt-rich grains dominate, the scientists reported in journal Nature.
Lead researcher Frank Postberg of the University of Heidelberg in Germany said: “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.”
“The study indicates that salt-poor particles are being ejected from the underground ocean through cracks in the moon at a much higher speed than the larger, salt-rich particles,” said co-author Sascha Kempf of the University of Colorado Boulder.
Ms. Kempf said: “The E. Ring is made up predominately of such salt-poor grains, although we discovered that 99 per cent of the mass of the particles ejected by the plumes was made up of salt-rich grains, which was an unexpected finding.
“Since the salt-rich particles were ejected at a lower speed than the salt-poor particles, they fell back onto the moon’s icy surface rather than making it to the E. Ring.”
According to the researchers, the salt-rich particles have an “ocean-like” composition that indicates most, if not all, of the expelled ice comes from the evaporation of liquid salt water rather than from the icy surface of the moon.
When salt water freezes slowly the salt is “squeezed out”, leaving pure water ice behind. If the plumes were coming from the surface ice, there should be very little salt in them, which was not the case, according to the research team.
The scientists believe that perhaps about 80 km beneath the surface crust of Enceladus a layer of water exists between the rocky core and the icy mantle that is kept in a liquid state by gravitationally driven tidal forces created by Saturn and several neighbouring moons.