Saturn Moon Harbors Ocean, Raising Possibility of Life
Scientists have found a large body of water beneath the surface of Enceladus.
Published April 3, 2014
The
ocean lies between the moon's rocky core and a layer of thick ice, and
is estimated to be about the size of Lake Superior. That's large for a
moon that is only 310 miles (500 kilometers) in diameter and could fit
within the borders of Arizona.
In our solar system, the only other moon known to have similar contact between liquid water and rock is Jupiter's Europa.
Both the rock and the water are considered to be essential for the
chemistry that could, over eons, turn nonliving matter into living
entities.
"The main implication of our work is that
there are potentially habitable environments in our solar system that
are entirely unexpected," said Luciano Iess, an aerospace engineer at the Sapienza University of Rome and lead author on the study published Thursday in the journal Science.
The
surface temperature on Enceladus is -180 degrees Celsius (-292 degrees
Fahrenheit), but enough heat is produced by tidal forces that flex and
melt the ice to create liquid water below. That the ice is heated by the
effects of Saturn's gravity—the source of those tidal forces—rather
than by radiation from the sun, Iess said, reinforces the theory that
even distant and frigid celestial bodies might be habitable.
"This
strong indication of a regional sea under the ice on Enceladus
heightens our interest in Enceladus as a possible habitable world," said
Carl Pilcher, former director of NASA's Astrobiology Institute.
Mapping a Moon With Gravity
Scientists were surprised in 2005 when images taken by the Cassini-Huygens mission revealed substantial jets of salty water were spitting out from the south pole of Enceladus. Later, close flybys produced dramatic photos of the geysers and the additional discovery of carbon-based, organic compounds being released from nearby darkened fissures called "tiger stripes."
Those
discoveries strongly suggested there was water below the surface, but
imagery alone could not confirm the liquid's presence. The scientists
had to use the Cassini spacecraft in a different way, in effect turning
it into a divining rod. (See "Ten Best Pictures From NASA's Cassini Probe—Saturn, More.")
During
three flybys Cassini made of Enceladus between 2010 and 2012,
scientists studied the moon's gravitational field in detail by measuring
ever-so-slight shifts in the satellite's path. The gravitational field
of a moon or planet subtly varies due to changes in the density and
amount of land or liquid across its surface. The changes in that field
can affect a spacecraft orbiting above.
Cassini scientists used Earth-based radio telescopes from the Deep Space Network to measure those tiny gravity-induced changes in the satellite's path, and to then indirectly map facets of the moon below.
Comparing
those gravity measurements with the moon's topography allowed
researchers to conclude that large amounts of liquid water were present
well below the surface. Key to the research is the fact that liquid water is 7 percent more dense than ice, which means it produces a greater gravitational effect.
"To get our data, we had to detect exquisitely small changes in motion with a pretty impressive precision," said co-author David Stevenson,
a planetary scientist at the California Institute of Technology in
Pasadena. "That was new, but the rest of the story of how to understand
masses from our data is basically Newton's laws of motion."
