Cassini Spacecraft Discovery: An Ocean Lies Under The Surface Of Saturn Moon, Enceladus
The spacecraft Cassini has confirmed the presence of a subterranean ocean on one of Saturn's moons, Enceladus, an international team of space officials said Wednesday, suggesting that presence of water on Enceladus brings with it the possibility of future habitability there. The spacecraft also detected the presence of some organic chemicals - the building blocks of life - in plumes of water that emanated from the moon's ocean through 18 to 24 miles (30 to 40 kilometers) of ice. Their research appears in Thursday's issue of the journal Science.
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Astrophysicists have suspected the presence of a subterranean ocean on Enceladus ever since the space craft Cassini detected mineral-rich jets of water vapor flowing from long, distinctive fractures in the moon's southern polar region nine years ago. A large subterranean body of water has already been detected on a moon of Jupiter, Europa, but that one is underneath an even thicker vault of ice than is the ocean on Enceladus.
Cassini made three flybys, which brought the spacecraft within 62 miles (100 kilometers) of Enceladus's surface, to train Doppler instruments on the moon in an effort to explore the disparity between the gravitation forces exerted by its northern and southern hemispheres. Italian Space Agency astrophysicists Luciano Iess and Marzia Parisi from the Sapienza Università di Roma in Italy surmised that nothing but a massive body of water would account for the southern and northern hemispheres' inconsistent gravity fields: Water emits a heavier field of gravity than a solid. The south-polar region of Enceladus doesn't have enough mass at its surface to account for the hemisphere's gravity field. Something dense beneath the moon's surface, most likely water, had to be the compensating factor.
The sub-surface ocean, concentrated in the moon's southern hemisphere, extends to about 50 degrees south latitude. Cassini spacecraft instruments detected organic chemicals from the vapor plumes, thought to be leached from a silicate bedrock in the subterranean ocean. This ocean could be leaching elements such as phosphorous, sulfur and sodium. "The configuration of the base of the ocean is probably very much like the base of our ocean in earth," Lunine said. "Liquid water would circulate through the rock become warmer and pick up nutrients."
The organic chemicals the comprise Enceladus's plumes are mostly small amounts of methane, some carbon dioxide and a few other light organic molecules-ones that just have one or two or three carbon molecules. "These types of molecules you see even in comets," Jonathan I. Lunine, Cornell University's director of the Center for Radiophysics and Space Research, said. "On Enceladus's surface, there are some deposits of organic molecules, but it's very difficult to tell what they are. Cassini's instruments were designed to analyze the atmosphere of Titan: We didn't know at the time that Enceladus had a plume. Just the presence of those organic molecules gives us a taste of what we would see if they were more sensitive."
Astrophysicists think that Jupiter's moon, Europa, may have organic chemicals as well. "We do not know yet whether Europa, or the Jupiter system has organic molecules," Dr. Lunine said. "The Saturn system may have been endowed with more organic molecules. We just don't know. Both objects are of keen interest." The plumes emanating from Europa aren't nearly as active as those on Enceladus.
Until the two Voyager spacecraft passed near it in the early 1980s, very little was known about this small moon besides the identification of water ice on its surface. "Before Cassini, the jets were unexpected," Dr. Iess from the Italian Space Agency told the International Science Times at the press conference Wednesday. "Few people really expected this level of activity on a tiny moon like Enceladus." Enceladus, the sixth-largest of 62 moons of Saturn, has a diameter of only 310 miles (500 kilometers); about a tenth of Saturn's largest moon, Titan.
The large bodies of water on these moons takes the possibility of living in outer space out of the realm of science fiction and into them realm of real possiblity, some of the astrophysicists said. "With this remarkable presence of subsurface ocean, both Europa, like Enceladus are inhabitable environments in our solar system," D. J. Stevenson, an astrophysicist at Caltech, Pasadena, Calif., said. Indeed, in 2011 NASA scientists at an Enceladus Focus Group Conference reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it." And the freezing temperatures of Enceladus would not provide an impediment for the possiblity of life already exisiting on the moon, either. "We certainly have organisms here on earth that survive in water below the freezing point," Lunine said. "In its geological past it is conceivable that it was warm enough to allow life to form, even if today the ocean is maintained by antifreeze, there are terrestrial organisms that would be perfectly comfortable in that environment."
The subterranean ocean does have tides, and how those tides behave may be tied to the resonance of another moon of Saturn's Dione, as the two moons in tandem are moving away from Saturn slowly over billions of years, said Dr. Stevenson. "We would like to disentangle that story."
More flybys are planned over the northern and southern hemispheres of the tiny moon. Cassini, the unmanned spacecraft run by the Italian Space Agency, the European Space Agency and NASA, has been exploring Saturn and its moons for about 10 years now. It is expected to fly for about three more years, with a final swansong as it dives between the largest rings of Saturn, and flies directly into Saturn's atmosphere.
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