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A moon of saturn has a sea, scientists say
New York Times
Published 5:36 pm, Thursday, April 3, 2014
Inside a moon of Saturn, beneath its icy veneer and above its rocky core, is a sea of water the size of Lake Superior, scientists announced Thursday.
The findings, published in the journal Science, confirm what planetary scientists have suspected about the moon, Enceladus, ever since they were astonished in 2005 by photographs showing geysers of ice crystals shooting out of its south pole.
For many researchers, this tiny, shiny cue ball of a moon, just over 300 miles wide, is now the most promising place to look for life elsewhere in the solar system, even more than Mars.
"Definitely Enceladus," said Larry Esposito, a professor of astrophysical and planetary sciences at the University of Colorado, who was not involved in the research. "Because there's warm water right there now."
Enceladus is caught in a gravitational tug of war between Saturn and another moon, Dione, which bends its icy outer layer, creating friction and heat. In the years since discovering the geysers, NASA's Cassini spacecraft has made repeated flybys of Enceladus, photographing the fissures where the geysers originate, measuring temperatures and identifying carbon-based organic molecules that could serve as building blocks for life.
Cassini has no instruments that can directly detect water beneath the surface, but three flybys in the years 2010-12 were devoted to producing a map of the gravity field, noting where the pull was stronger or weaker. During the flybys, lasting just a few minutes, radio telescopes that are part of NASA's Deep Space Network broadcast a signal to the spacecraft, which echoed it back to Earth. As the pull of Enceladus' gravity sped and then slowed the spacecraft, the frequency of the radio signal shifted, just as the pitch of a train whistle rises and falls as it passes by a listener.
Using atomic clocks on Earth, the scientists measured the radio frequency with enough precision that they could discern changes in the velocity of Cassini, hundreds of millions of miles away, as minuscule as 14 inches an hour.
They found that the moon's gravity was weaker at the south pole. At first glance, that is not so surprising; there is a depression at the pole, and lower mass means less gravity. But the depression is so large that the gravity should actually have been much weaker.
"Then you say, 'Aha, there must be compensation,' " Stevenson said. "Something more dense under the ice. The natural candidate is water."
"It's a lot more water than Lake Superior," Stevenson said. "It may even be bigger."