Cosmic
sprinklers that spurt misty jets from cracks along Saturn's sixth largest moon
could hint at a vast watery lake hidden beneath the icy shell of Enceladus.
In 2005,
NASA's Cassini spacecraft revealed giant
geysers of ice grains and water vapor shooting from the south pole of
Enceladus. But how the geysers formed and the source of the ice crystals had
remained a mystery until now. New research, detailed in the Feb. 7 issue of the
journal Nature, provides a clear view of the processes beneath the
moon's crust that yield the handful of geysers.
The results
reveal there must be water beneath the moon's surface and also support the idea
that Enceladus'
geysers are the source of Saturn's E-ring, a faint circle of tiny ice and
dust particles.
"Since
Cassini discovered the water vapor geysers, we've all wondered where this water
vapor and ice are coming from," said researcher Juergen Schmidt of the
University of Potsdam, Germany, who is a team member on Cassini's Cosmic Dust
Analyzer. "Now, after looking at data from multiple instruments, we can
say there probably is water beneath the surface of Enceladus."
The
researchers are uncertain how large the water reservoir is. "It might be a
global ocean. It might be just a small lake," Schmidt said.
The finding makes Enceladus
one of only four moons in our solar system thought to harbor liquid water.
The other watery worlds are Jupiter's moons Europa, Ganymede and Callisto.
While Saturn is home to 60 identified moons, Enceladus is the first to show
signs of liquid water.
Beneath
the ice
Schmidt and
his colleagues relied on Cassini data on the ice grains along with computer
models to arrive at their conclusion about the water.
Here's what
they think is happening:
Hidden
beneath Enceladus' icy exterior is a lake with a temperature of about 32
degrees Fahrenheit (0 degrees Celsius). At these relatively warm temperatures
(for the frigid outer solar system) liquid water, ice and water vapor mingle.
The vapor moves upward through channels in the ice toward openings at the
moon's surface. Upon reaching the vacuum conditions of space found within the
channels and cracks, the vapor expands and cools leading to the formation of
ice crystals.
Both the
model and the Cassini observations suggest the vapor in the plumes moves at
roughly the same speed as a supersonic
jet, about 650 to 1,100 mph (300 to 500 meters per second). That's nearly
double the speed needed to escape Enceladus' gravity.
The ice
grains, however, trek along at a much slower rate. The researchers say as the
ice particles zigzag through crooked cracks in the ice, they ricochet off the
walls and lose speed. The water vapor moves unimpeded through the crevasses and
boosts the frozen particles to carry them upward.
Even with a
push from the vapor stream, only about 10 percent of the ice particles have
enough energy to break through Enceladus' gravity. The remaining slowpokes fall
back to the moon's surface.
Saturn's
ring
The escaped
ice crystals' liberty is short-lived, however. Scientists think the crystals
are recaptured by Saturn's gravity and coalesce to form the planet's E-ring.
"These
particles in the E-ring hit other satellites in the system or the main rings of
Saturn or they hit Enceladus itself," Schmidt told SPACE.com.
"So they are born at Enceladus, but they also have sinks so they die
somewhere, and that gives them balance which is more or less steady
today."
The heat
source that drives the interior melting of the ice is still unknown, but now researchers
think they know the conditions needed to drive Enceladus' plumes.
"If
vapor temperature is too low, then the gas density is too small to push the
grains out and we would not see such large amounts of particles," Schmidt
said. "Therefore, we believe that at the site of evaporation, we must have
temperatures near the melting point of water."
The next
Enceladus flyby is set for March, when the Cassini spacecraft will reach its
closest approach of a mere 30 miles (50 kilometers) from the surface. As the
spacecraft moves farther away to an altitude of about 124 miles (200
kilometers), it will pass through and sample Enceladus' plumes.
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