News: Mystery of Saturn's Watery Moon Solved.
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Sun, 10 Feb 2008 10:35:13 GMT
Mystery of Saturn's Watery Moon Solved
Jeanna Bryner
Staff Writer
SPACE.com Fri Feb 8, 3:16 PM ET
http://news.yahoo.com/s/space/20080208/sc_space/mysteryofsaturnswaterymoonsolved
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.
--
Bob.
.
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