NASA JPL-Caltech | Cassini Solstice Mission | 2011 June 22
NASA's Cassini spacecraft has discovered the best evidence yet for a large-scale saltwater reservoir beneath the icy crust of Saturn's moon Enceladus. The data came from the spacecraft's direct analysis of salt-rich ice grains close to the jets ejected from the moon.
Data from Cassini's cosmic dust analyzer show the grains expelled from fissures, known as tiger stripes, are relatively small and predominantly low in salt far away from the moon. But closer to the moon's surface, Cassini found that relatively large grains rich with sodium and potassium dominate the plumes. The salt-rich particles have an "ocean-like" composition and indicate that most, if not all, of the expelled ice and water vapor comes from the evaporation of liquid salt water. The findings appear in this week's issue of the journal Nature.
"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 salt water under Enceladus's icy surface," said Frank Postberg, a Cassini team scientist at the University of Heidelberg, Germany, and the lead author on the paper. When water freezes, the salt is squeezed out, leaving pure water ice behind. If the plumes emanated from ice, they should have very little salt in them.
The Cassini mission discovered Enceladus' water-vapor and ice jets in 2005. In 2009, scientists working with the cosmic dust analyzer examined some sodium salts found in ice grains of Saturn's E ring, the outermost ring that gets its material primarily from Enceladean jets. But the link to subsurface salt water was not definitive.
The new paper analyzes three Enceladus flybys in 2008 and 2009 with the same instrument, focusing on the composition of freshly ejected plume grains. The icy particles hit the detector target at speeds between 15,000 and 39,000 mph (23,000 and 63,000 kilometers per hour), vaporizing instantly. Electrical fields inside the cosmic dust analyzer separated the various constituents of the impact cloud.
The data suggest a layer of water between the moon's rocky core and its icy mantle, possibly as deep as about 50 miles (80 kilometers) beneath the surface. As this water washes against the rocks, it dissolves salt compounds and rises through fractures in the overlying ice to form reserves nearer the surface. If the outermost layer cracks open, the decrease in pressure from these reserves to space causes a plume to shoot out. Roughly 400 pounds (200 kilograms) of water vapor is lost every second in the plumes, with smaller amounts being lost as ice grains. The team calculates the water reserves must have large evaporating surfaces, or they would freeze easily and stop the plumes.
"This finding is a crucial new piece of evidence showing that environmental conditions favorable to the emergence of life can be sustained on icy bodies orbiting gas giant planets," said Nicolas Altobelli, the European Space Agency's project scientist for Cassini.
Cassini's ultraviolet imaging spectrograph also recently obtained complementary results that support the presence of a subsurface ocean. A team of Cassini researchers led by Candice Hansen of the Planetary Science Institute in Tucson, Ariz., measured gas shooting out of distinct jets originating in the moon's south polar region at five to eight times the speed of sound, several times faster than previously measured. These observations of distinct jets, from a 2010 flyby, are consistent with results showing a difference in composition of ice grains close to the moon's surface and those that made it out to the E ring. That paper was published in the June 9 issue of Geophysical Research Letters.
"Without an orbiter like Cassini to fly close to Saturn and its moons -- to taste salt and feel the bombardment of ice grains -- scientists would never have known how interesting these outer solar system worlds are," said Linda Spilker, NASA's Cassini project scientist at the Jet Propulsion Laboratory in Pasadena, Calif.
Strongest evidence yet indicates icy Saturn moon hiding saltwater ocean
University of Colorado at Boulder | 2011 June 22
Samples of icy spray shooting from Saturn's moon Enceladus collected during Cassini spacecraft flybys show the strongest evidence yet for the existence of a large-scale, subterranean saltwater ocean, says a new international study led by the University of Heidelberg and involving the University of Colorado Boulder.Water vapor jets spewing from Enceladus
The new discovery was made during the Cassini-Huygens mission to Saturn, a collaboration of NASA, the European Space Agency and the Italian Space Agency. Launched in 1997, the mission spacecraft arrived at the Saturn system in 2004 and has been touring the giant ringed planet and its vast moon system ever since.
The plumes shooting water vapor and tiny grains of ice into space were originally discovered emanating from Enceladus -- one of 19 known moons of Saturn -- by the Cassini spacecraft in 2005. The plumes were originating from the so-called "tiger stripe" surface fractures at the moon's south pole and 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," said Frank Postberg of the University of Germany, lead author of a study being published in Nature on June 23. Other co-authors include Jürgen Schmidt from the University of Potsdam, Jonathan Hillier from Open University headquartered in Milton Keynes, England, and Ralf Srama from the University of Stuttgart.
"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 CU-Boulder faculty member and study co-author Sascha Kempf of the Laboratory for Atmospheric and Space Physics.
"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," said 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."
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 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," said Kempf.
Salt in the rock dissolves into the water, which accumulates in a liquid ocean beneath the icy crust, according to the Nature authors. 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," said 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 favorable to the emergence of life may be sustainable on icy bodies orbiting gas giant planets."
Flash-frozen seawater sprays from Enceladus
Max Planck Gesellschaft | 2011 June 22
New measurements confirm a liquid ocean under the icy crust of Saturn's moon
Is there an ocean on Enceladus or not? This question had been occupying researchers since the Cassini space probe discovered fractures in the icy crust around the southern pole of Saturn’s moon which eject plumes of water vapour and ice grains. Earlier measurements of the composition of these particles, which had been undertaken with the dust detector of the Max Planck Institute for Nuclear Physics, provided indications for a liquid ocean. Other scientists rejected this explanation, however. New data from a close fly-by of Enceladus have now dispelled these doubts.
- [size=85]Icy spray in space: The different shades of blue show the calculated fraction of the salt-rich ice particles in the plumes over the southern pole of Saturns moon Enceladus (top left). The bright supersonic jets from three vents which Cassini has traversed (grey vertical line) are clearly recognisable. The red curves correspond to different average particle radii – with the largest close to Enceladus. [i](© NASA/JPL/University of Potsdam)[/i][/size]
Enceladus has a diameter of 500 kilometres or so, making it one of Saturn’s smaller moons. It has a rocky core below an icy crust some 80 kilometres thick. Around its southern pole the surface has a series of fractures and fissures. These “tiger stripes” eject plumes of water vapour and tiny ice particles. The water vapour is ejected at supersonic speed from individual vents and thus feeds the diffuse outer E-ring around the gigantic planet.
In 2009 the scientists published the analysis of the chemical composition of ice particles in Saturn’s E-ring. The researchers detected three types of ice particle in the data, which was obtained with the Cosmic Dust Analyzer (CDA) of the Max Planck Institute for Nuclear Physics aboard the Cassini space probe. One of the types of ice particle contains salts in a quantity and composition which supports the assumption of there being an ocean between icy crust and rocky core. A discussion then followed about whether the salt-rich ice particles could not also have been formed without liquid water.
When Cassini flew through the plumes at an altitude of only 21 kilometres above the south pole of Enceladus, it gave the researchers the opportunity to directly scrutinize the freshly ejected ice grains. The dust detector found the same three ice particle types as in the E-ring, although their proportions changed markedly as a function of the distance from the source: near the source the salt-rich particles dominate; further away, the pure ice particles are in the majority (just like in the E-ring).
The fraction of particles containing silicates or organic material is slightly higher in the plumes. Moreover, the salt-rich ice grains are larger and slower than those without salt. When Cassini flew through the supersonic jet of a vent, the CDA detected an increased proportion of the small, salt-free particles compared to the material ejected by the other “tiger stripes”.
For Frank Postberg, researcher at the Max Planck Institute for Nuclear Physics and at the University of Heidelberg, there is only one plausible explanation for these findings and for the sources of the plumes: large saltwater reservoirs, fed by an ocean between the icy crust and the rocky core of Enceladus. The salt-rich ice particles are flash-frozen seawater spray and comprise the lion’s share of the ejected grains, while the pure ice particles are produced from water vapour – mainly in the vents. Since they are lighter, they are more strongly entrained in the jet of water vapour, and make it to the E-ring. Most of the heavier, salt-rich particles, on the other hand, fall back to the surface of Enceladus.
A salt-water reservoir as the source of a compositionally stratified plume on Enceladus - F Postberg et al
- Nature 474(7353) 620 (30 June 2011) 10.1038/nature10175
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