This image shows the surprise that startled Cassini scientists on the
composite infrared spectrometer team when they got their first look at the
infrared (heat) radiation from the south pole of Saturn's moon Enceladus.
There is a dramatic warm spot centered on the pole that is probably a sign
of internal heat leaking out of the icy moon. The data were taken during
the spacecraft's third flyby of this intriguing moon on July 14, 2005.
Based on data from previous flybys, which did not show the south pole
well, team members expected that the south pole would be very cold, as
shown in the left panel. Enceladus is one of the coldest places in the
Saturn system because its extremely bright surface reflects 80 percent of
the sunlight that hits it, so only 20 percent is available to heat the
surface. As on Earth, the poles should be even colder than the equator
because the sun shines at such an oblique angle there.
The right hand panel shows a global temperature image made from
measurements of Enceladus' heat radiation at wavelengths between 9 and
16.5 microns. Cassini made the observation from a distance of 84,000
kilometers (52,000 miles) on the approach to Enceladus, and the image
shows details as small as 25 kilometers (16 miles). Equatorial
temperatures are much as expected, topping out at about 80 degrees Kelvin
(-315 degrees Fahrenheit), but the south pole is occupied by a
well-defined warm region reaching 85 Kelvin (-305 degrees Fahrenheit).
That is 15 degrees Kelvin (27 degrees Fahrenheit) warmer than expected.
The composite infrared spectrometer data further suggest that small areas
of the pole are at even higher temperatures, well over 110 degrees Kelvin
(-261 degrees Fahrenheit). Evaporation of this relatively warm ice
probably generates the cloud of water vapor detected above Enceladus'
south pole by several other Cassini instruments.
The south polar temperatures are very difficult to explain if sunlight is
the only energy source heating the surface, though exotic
sunlight-trapping mechanisms have not yet been completely ruled out. It
therefore seems likely that portions of the polar region are warmed by
heat escaping from the interior of the moon. This would make Enceladus
only the third solid body in the solar system, after Earth and Jupiter's
volcanic moon Io, where hot spots powered by internal heat have been
detected.
The Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory,
a division of the California Institute of Technology in Pasadena, manages
the mission for NASA's Science Mission Directorate, Washington, D.C. The
Cassini orbiter was designed, developed and assembled at JPL. The
composite infrared spectrometer team is based at NASA's Goddard Space
Flight Center, Greenbelt, Md.
For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov.
The composite infrared spectrometer team homepage is http://cirs.gsfc.nasa.gov/.