U. COLORADO-BOULDER (US) — Samples of icy spray ejected from a Saturn moon and collected by the Cassini spacecraft make a strong case for the existence of a subterranean saltwater ocean.
The spray shooting into space from Saturn’s moon Enceladus—one of 19 known moons of Saturn—originated from the so-called “tiger stripe” surface fractures at the moon’s south pole. The plumes apparently have created the material for the faint E Ring that traces the orbit of Enceladus around Saturn.
During three of Cassini’s passes through the plume in 2008 and 2009, the Cosmic Dust Analyser, or CDA, on board measured the composition of freshly ejected plume grains. The icy particles hit the detector’s target at speeds of up to 11 miles per second, instantly vaporizing them. The CDA separated the constituents of the resulting vapor clouds, allowing scientists to analyze them.
The study shows 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.
“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, lead author of a study published in Nature this month.
“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,” says study co-author Sascha Kempf of the Laboratory for Atmospheric and Space Physics at the University of Colorado-Boulder.
“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,” says Kempf. “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.”
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, say researchers.
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 researchers believe that perhaps 50 miles 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 neighboring moons, as well as by heat generated by radioactive decay.
According to the scientists, roughly 440 pounds of water vapor is lost every second from the plumes, along with smaller amounts of ice grains. Calculations show the liquid ocean must have a sizable evaporating surface or it would easily freeze over, halting the formation of the plumes.
“This study implies that nearly all of the matter in the Enceladus plumes originates from a saltwater ocean that has a very large evaporating surface,” says Kempf.
Salt in the rock dissolves into the water, which accumulates in a liquid ocean beneath the icy crust, according to the researchers. When the outermost layer of the Enceladus crust cracks open, the reservoir is exposed to space. The drop in pressure causes the liquid to evaporate into a vapor, with some of it “flash-freezing” into salty ice grains, which subsequently creates the plumes, the science team believes.
“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, European Space Agency’s project scientist for the Cassini-Huygens mission. “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.”
Researchers from the University of Potsdam, Open University, and the University of Stuttgart collaborated on the work.
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