Cassini CIRS instrument,
showing the warm south pole.
Temperatures are in degrees Kelvin.
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Temperature map of Enceladus from the Cassini CIRS instrument, showing the warm south pole. Temperatures are in degrees Kelvin. |
Enceladus has been known to be an oddball for over 25 years. In 1980 astronomers found that Saturn's tenuous E-ring, which is made of tiny ice grains, was densest at the orbit of Enceladus, suggesting that perhaps the moon was supplying the ice grains that created the ring. Shortly afterwards, the Voyager 1 and 2 spacecraft flew past Saturn and revealed that Enceladus was a dazzlingly white world, the brightest in the solar system. Also, unlike most of Saturn's moons, Enceladus was scarred by intense faulting and other signs of recent geological activity. Based on this evidence, there was speculation through the 1980s and 1990s that Enceladus might be actively venting ice grains into the Saturn system. So scientists eagerly awaited the close-up reports from the Cassini mission when it entered Saturn orbit in July 2004. The first close Enceladus flybys, in February and March 2005, returned images showing even more intense faulting than Voyager's more distant images had revealed. Cassini's magnetometer also found that some barrier bigger than Enceladus itself was perturbing Saturn's magnetic field. Based on the magnetometer findings, the project decided to move the spacecraft's trajectory closer to Enceladus on the next flyby, scheduled for July 14th 2005.
The July 2005 flyby provided the first good view of Enceladus' south pole, allowing mapping of infrared heat radiation from the polar region by the CIRS instrument, which was built and is operated by NASA's Goddard Spaceflight Center under the direction of PI Mike Flasar. The infrared mapping effort was led by John Pearl at Goddard and John Spencer at SwRI. Surface temperature maps derived by Spencer from Enceladus' 11 - 16 micron radiation showed that the south pole was radiating several gigawatts of heat into space, and that the heat was concentrated along prominent fractures, dubbed "tiger stripes", that were seen by Cassini's cameras: the fractures seemed to be at least 130 degrees Fahrenheit warmer than their surroundings. These temperatures were too high to be explained by solar heating of the surface: an internal heat source was needed.
Other instruments reported equally remarkable results from the July 2005 flyby. Cassini's ultraviolet spectrometer detected a cloud of water vapor over the south pole, and the onboard mass spectrometer was able to analyze the gas composition directly as the spacecraft flew through the cloud. This gas cloud explained the magnetometer results found on the previous flybys. Cassini's dust detector showed that Cassini also flew through a strong concentration of ice grains over the south pole, confirming the decades of speculation that Enceladus really was supplying ice grains to the E-ring. This plume of dust was later imaged by Cassini's cameras in November 2005: the images showed numerous jets, apparently emanating from those warm "tiger stripe" fractures, spraying icy grains hundreds of miles above the surface of Enceladus. Ice grains falling back on Enceladus' surface are presumably responsible for its extremely white surface: the moon is literally snow-covered.
The energy source for this geological activity is almost certainly tidal heating- Enceladus is heated by the continual distortion of its shape by Saturn's gravity as the moon orbits Saturn in its slightly eccentric orbit. But many questions remain, and Cassini is scheduled to return to Enceladus to gather more close-up data in March 2008, with additional flybys likely in the following two years.
Cassini's Enceladus results were published in the March 10th 2006 issue of Science, including the results from the CIRS instrument (Spencer et al. 2006, Science 311, 1401-1405).