This find makes Enceladus only the third place in the solar system, after Earth and Jupiter's volcanic moon Io, where "hot spots" associated with ongoing geological activity have been detected.

The discovery was made with Cassini's Composite Infrared Spectrometer (CIRS), which was built by and is operated from the NASA Goddard Space Flight Center in Greenbelt, Md., under the direction of Principal Investigator Michael Flasar. NASA announced the findings in a July 29 press release.

Enceladus has been a puzzle since the first pictures of it were taken by the Voyager spacecraft in the early 1980s. It is only 500 kilometers (300 miles)in diameter - so small that it ought to be cold and dead - yet its surface is torn by innumerable fractures, indicating great geological violence in the relatively recent past.

It is also the brightest moon in the solar system, with a surface that is almost pure white, and seems to be the source of a huge ring of dust, called the "E ring," that surrounds Saturn.

On July 14, 2005, Cassini flew a mere 175 km above the surface of Enceladus, returning unprecedented information about this mysterious place. Following plans devised by NASA Goddard scientist John Pearl and SwRI scientist John Spencer, the CIRS instrument scanned the south polar region, measuring the thermal (heat) radiation from the surface at wavelengths between 9 and 16 microns.

Though the pole, like Earth's poles, should have been one of the coldest places on Enceladus, at about 75¿ Kelvin (-324¿ Fahrenheit), Spencer's analysis of the data found that the south pole was instead the warmest place on the moon, with temperatures in small areas reaching well over 110¿ K (-261¿ F) - so "warm" that these temperatures are difficult to explain by solar heating alone. More likely, heat is being generated inside Enceladus and is being released to the surface through fractures near the pole.

"Looking for signs of internal heat on Enceladus was always one of the goals of the CIRS instrument, but it always seemed like a long shot because we couldn't imagine that an object so tiny could produce enough heat to detect," said Spencer, a staff scientist in the SwRI Space Studies Department. "We are amazed that we were actually successful."

Other Cassini instruments discovered a huge cloud of water vapor over the warm south pole, as described in the NASA press release. Cassini's cameras also showed that the south polar region is one of the youngest parts of Enceladus' surface. The water vapor cloud is probably continually supplied by evaporation of the warm ice discovered by CIRS.

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