Gravity measurements made with the Saturn-orbiting Cassini spacecraft indicate the small moon Enceladus has an ocean sandwiched between its rocky core and icy shell, a finding that raises the prospects of a niche for life beyond Earth.
The Cassini data shows the body of water, which is in the moon's southern hemisphere, must be as large or larger than Lake Superior and sitting on top of the moon's rocky core at a depth of about 31 miles.
"The ocean may extend halfway or more toward the equator in every direction," said planetary scientist David Stevenson, with the California Institute of Technology in Pasadena.
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Scientists infer the ocean is salty because water plumes shooting out of cracks in Enceladus' southern pole and sampled by Cassini contain salts, as well as organic molecules. That would happen if minerals from underlying rock were leaching into water, a chemistry that bodes well for the development and evolution of life.
"It was not a surprise to find a water reservoir certainly, because we knew that there are plumes, there is liquid water," said planetary scientist Luciano Iess with the Sapienza University of Rome.
"There have been clues all along," added Candice Hansen-Koharcheck, senior scientist with the Tucson, Ariz.-based Planetary Science Institute.
"But until you actually get this gravity data, it's still kind of a circumstantial evidence-story. This is proof of the pudding," Hansen-Koharcheck told Discovery News.
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The measurements were painstakingly taken as Cassini flew close to Enceladus three times between 2010 and 2012. Two flybys were over the moon's south pole, at distances of 65 miles and 44 miles above the surface. One flyby was 31 miles above the North Pole.
During the passes, radio signals were transmitted 850 million miles back to Earth so scientists could ferret out minute shifts in frequency caused by gravitationally tugging on Cassini by the moon, which is about 300 miles in diameter. The slight signals were buried in mountains of other frequency-shifting phenomena, including Earth's rotation and even the density of the plumes Cassini passed through.
"Tracking the spacecraft to a fraction of a millimeter per second, when you think about it, is truly extraordinary. If only we could do that with Malaysian aircraft," Stevenson told Discovery News. "
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In the end, scientists discovered a notable asymmetry between the moon's northern and southern hemispheres.
Taking into account what planet- and moon-building materials were available in the outer solar system, scientists agree the gravity measurements point to one thing: a subsurface ocean.
"It's always that with this kind of analysis that if you insist on working only with the data and ignoring everything else in the universe that you know, that you could have all sorts of weird things. So what you do in a situation like this is that you don't just look at the data. You say, ‘OK what does this satellite have?' When you take that into account, water is the only thing that works," Stevenson said.
With the discovery, Enceladus may come to rival Jupiter's ocean-bearing moon Europa as candidates for follow-on missions to look for signs of life.
The research appears in this week's Science.