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Water vapor jets spewing from Saturn's icy moon, Enceladus. (NASA/JPL/Space Science Institute)
NASA’s Cassini spacecraft has gathered the strongest evidence yet for the existence of a huge underground salt water ocean inside one of Saturn’s 19 known moons, Enceladus.
The findings are published in Nature on June 23.
Flybys during the Cassini-Huygens mission to Saturn captured samples of saltwater shooting up from the surface of the moon. These plumes send out ice particles and water vapor into space, and were originally traced back to Enceladus by Cassini in 2005 to “tiger stripe” fractures in the moon’s south polar surface, that are contributing to the faint E Ring orbiting the moon.
In 2008 and 2009, Cassini passed through this plume, and used its Cosmic Dust Analyser (CDA), to examine the icy spray’s composition. Its particles are travelling at up to 11 miles per second, and vaporized upon hitting the detector.
The CDA revealed that the ice grains closest to the moon are relatively large and salt-rich, while those further away are small and ice-poor.
“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 at the Laboratory for Atmospheric and Space Physics in a press release.
“The E Ring is made up predominately of such salt-poor grains, although we discovered that 99 percent of the mass of the particles ejected by the plumes was made up of salt-rich grains, which was an unexpected finding.”
High resolution view of icy Enceladus, one of Saturn's moons, taken by Cassini on 14 July 2005, showing a bizarre mixture of softened craters and complex, fractured terrains. (NASA/JPL/Space Science Institute)
The slower salt-rich particles fall back to the moon’s icy surface whereas the faster salt-poor particles make it into the E Ring.
“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,” said lead author Frank Postberg of the University of Germany in the release.
The scientists believe that these salt-rich particles are evaporating from ocean water beneath the moon’s surface, perhaps 50 miles below, as it slowly freezes.
According to the scientists, the plumes lose around 440 pounds of water vapor plus smaller quantities of ice grains every second, suggesting the subsurface body of salt water is substantial. When exposed to space via surface cracks, the pressure drop causes evaporation from the internal ocean and some “flash-freezing” into salty ice grain 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,” said Nicolas Altobelli, ESA’s project scientist for the Cassini-Huygens mission in the release.
“This finding is therefore a crucial new piece of evidence showing that environmental conditions favorable to the emergence of life may be sustainable on icy bodies orbiting gas giant planets.”
