A Hot Start Might Explain
Geysers on Enceladus
(Additional comments below)
A hot start billions of years ago might
have set into motion the forces that power geysers on Saturn's moon
"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.
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
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.
For a response,
see our TPOD
Enceladus Plumes Explained?
Also see our TPODs
Jets of Enceladus,
The Hot Poles of Enceladus