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Scientists Study "Plumbing" in Plumes of Enceladus
Scientists on the Cassini mission have become out-of-this world "plumbers" as they try to piece
together what's happening inside the "pipes" feeding the plumes of Saturn's moon Enceladus.
Enceladus is jetting out giant geysers three times the size of the moon, and now scientists are
beginning to understand how the ice grains are created and how they might have formed.
Knowing the process of how the plume forms and the path the water-ice particles have to travel
is giving them an insight into what may be a liquid reservoir or lake lying just beneath the
surface.
"Since Cassini discovered the water vapor geysers, we've all wondered where this water vapor
and ice are coming from. Is it from an underground water reservoir or are there some other
processes at work? Now, after looking at data from multiple instruments, we can say there
probably is water beneath the surface of Enceladus," said Juergen Schmidt, team member on
Cassini's Cosmic Dust Analyzer at the University of Potsdam, Germany. This study appears in
the Feb. 7, 2008, issue of the journal Nature.
The large number of ice particles observed spewing from the geysers and the steady rate at which
these particles are produced require high temperatures, close to the melting point of ice, possibly
resulting in an internal lake. The lake would be similar to Earth's Lake Vostok, beneath
Antarctica, where liquid water exists locked in ice. The ice grains then condense in the vapor
evaporating from the water, streaming through cracks in the ice crust to the surface.
The presence of liquid water inside Enceladus would have major implications for future
astrobiology studies on the possibility of life on bodies in the outer solar system.
Scientists have studied the plume dynamics since 2005, collecting data from several Cassini
remote sensing instruments and those that sample particles directly, like the Cosmic Dust
Analyzer. They conclude that an internal lake at a temperature of about 273 Kelvin (32 degrees
Fahrenheit) is the best way to account for the material jetting out of the geysers.
At these warm temperatures, liquid water, ice and water vapor mingle. The vapor escapes to the
vacuum of space through cracks in Enceladus' ice crust. When the gas expands, it cools and the
ice grains that make up the visible part of the plumes condense from the vapor. Vapor in the
plumes is clocked at roughly the same speed as a supersonic jet, about 300 to 500 meters per
second, or about 650 to 1,100 miles per hour. However, most of the condensed ice particles fail
to reach Enceladus' escape velocity of 240 meters per second (536 miles per hour).
Pinball-like physics account for the slow speed of the particles. Shooting up through crooked
cracks in the ice, the particles ricochet off the walls, losing speed, while the water vapor moves
unimpeded up the crevasse. The vapor reboosts the frozen particles as they pinball off the walls,
carrying them upward. Reaching nozzle-like openings at the surface, the faster-moving water
vapor shoots high above Enceladus, becoming entrapped in Saturn's magnetosphere. Most of
the particles, which have lost energy through collisions in transit, fail to achieve escape velocity
and fall back to Enceladus' surface. Only about 10 percent escape Enceladus and form Saturn's
E-ring.
"Our model provides a simple concept to understand how particles form, their speed and how
they behave as they make their way out into space. 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,"
said Schmidt. "Therefore, we believe that at the site of evaporation, we must have temperatures
near the melting point of water."
Scientists say that particles seen in the plumes are too numerous to have started from processes
described in one existing model that requires low temperatures, proposing that gases may be
trapped inside ice crystals. Another model suggests that water ice, suddenly exposed to the
vacuum of space, sublimes, or boils, directly into vapor without liquefying first. But this would
mean there are short bursts of activity, rather than the steady production of particles. The new
model of grains condensing in a vent that evaporates from a liquid body is consistent with a
steady production of particles, ejected from a localized source.
This research provides fundamental knowledge about solar system bodies, in particular those
that, like our home planet, are homes to oceans – environments where life might evolve.
The next Enceladus flyby is in March 2008. The spacecraft closest approach will be at a mere 50
kilometers (30 miles) from the surface and the altitude will increase to about 200 kilometers (124
miles) as the spacecraft passes through the plumes. Cassini will sample the plumes directly and
find out more about their makeup.
More information on the Cassini-Huygens mission is available at: http://saturn.jpl.nasa.gov
and
http://www.nasa.gov/cassini .
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency
and the Italian Space Agency. JPL, a division of the California Institute of Technology in
Pasadena, manages the Cassini mission for NASA's Science Mission Directorate, Washington,
D.C. JPL designed, developed and assembled the Cassini orbiter.
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