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Hot Start Might Explain Geysers on Enceladus
03.12.07
A hot start billions of years ago might have set into
motion the forces that power geysers on Saturn's moon Enceladus.
The
ice jets of Enceladus send particles streaming into space
hundreds of kilometers above the south pole of this
spectacularly active moon.
Image
credit: NASA/JPL/Space Science Institute
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"Deep inside
Enceladus, our model indicates we've got an organic brew, a heat
source and liquid water, all key ingredients for life," said
Dr. Dennis Matson, Cassini project scientist at NASA's Jet
Propulsion Laboratory, Pasadena, Calif. "And while no one is
claiming that we have found life by any means, we probably have
evidence for a place that might be hospitable to life."
Since NASA's Voyager spacecraft first returned images of
the moon's snowy white surface, scientists have suspected
Enceladus had to have something unusual happening within that
shell. Cameras on NASA's Cassini orbiter seemed to confirm that
suspicion in 2005 when they spotted geysers on Enceladus ejecting
water vapor and ice crystals from its south polar region. The
challenge for researchers has been to figure out how this small
ice ball could produce the levels of heat needed to fuel such
eruptions.
A new model suggests the rapid decay of
radioactive elements within Enceladus shortly after it formed may
have jump-started the long-term heating of the moon's interior
that continues today. The model provides support for another
recent, related finding, which indicates that Enceladus' icy
plumes contain molecules that require elevated temperatures to
form.
"Enceladus is a very small body, and it's made
almost entirely of ice and rock. The puzzle is how the moon
developed a warm core," said Dr. Julie Castillo, the lead
scientist developing the new model at JPL. "The only way to
achieve such high temperatures at Enceladus is through the very
rapid decay of some radioactive species."
The hot
start model suggests Enceladus began as a mixed-up ball of ice
and rock that contained rapidly decaying radioactive isotopes of
aluminum and iron. The decomposition of those isotopes - over a
period of about 7 million years - would produce enormous amounts
of heat. This would result in the consolidation of rocky material
at the core surrounded by a shell of ice. According to the
theory, the remaining, more slowly decaying radioactivity in the
core could continue to warm and melt the moon's interior for
billions of years, along with tidal forces from Saturn's
gravitational tug.
Enceladus.
Image
credit: NASA/JPL/Space Science Institute
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Scientists have also found
the model helpful in explaining how Enceladus might have produced
the chemicals in the plume, as measured by Cassini's ion and
neutral mass spectrometer. Matson is lead author of a new study
of the plume's composition, which appears in the April issue of
the journal Icarus. Although the plume is predominantly made up
of water vapor, the spectrometer also detected within the plume
minor amounts of gaseous nitrogen, methane, carbon dioxide,
propane and acetylene.
Scientists were particularly
surprised by the nitrogen because they don't think it could have
been part of Enceladus' original makeup. Instead, Matson's team
suggests it is the product of the decomposition of ammonia deep
within the moon, where the warm core and surrounding liquid water
meet.
The thermal decomposition of ammonia would require
temperatures as high as 577 degrees Celsius (1070 degrees
Fahrenheit), depending on whether catalysts such as clay minerals
are present. And while the long-term decay of radioactive species
and current tidal forces alone cannot account for such high
temperatures, with the help of the hot start model, they can.
The scalding conditions are also favorable for the
formation of simple hydrocarbon chains, basic building blocks of
life, which Cassini's spectrometer detected in small amounts
within Enceladus' plume. The team concludes that so far, all the
findings and the hot start model indicate that a warm,
organic-rich mixture was produced below the surface of Enceladus
and might still be present today, making the moon a promising
kitchen for the cooking of primordial soup.
To gather
more information about the chemistry within Enceladus, the team
plans to directly measure the gas emanating from the plume during
a flyby scheduled for March 2008.
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-Huygens mission for NASA's Science Mission Directorate,
Washington. The Cassini orbiter was designed, developed and
assembled at JPL.
Source:
NASA / JPL
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