A Hot Start Might
Explain Geysers on Enceladus
March 12, 2007
(Source: Jet
Propulsion Laboratory)
A hot start billions of years ago might have set into motion the forces that
power geysers on Saturn's moon Enceladus.
"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.
|
The ice jets of
Enceladus send particles streaming into space hundreds of kilometers above
the south pole of this spectacularly active moon. |
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. [somehow this is viewed as being more plausible than a
'georeactor' ??]
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.
|
Plumes of icy material
extend above the southern polar region of Enceladus |
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 fountain-like sources of the fine spray of
material that towers over Enceladus' south polar
region. |
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.