A Hot Start Might Explain Geysers on Enceladus
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."
Image right: 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+ Full image and caption
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.
Image left: Enceladus. Image credit: NASA/JPL/Space Science Institute + Full image and caption
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.
For images and information about the Cassini mission, visit:
Media contact: Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.