Rapid radioactive decay billions of years ago may be responsible for the active geysers on the otherwise icy surface of Saturn's moon Enceladus, NASA scientists said Monday.

The Cassini-Hyugens spacecraft first spotted a geyser on Enceladus ejecting what appeared to be liquid water and ice crystals in 2005, fuelling speculation the distant moon might be able to support life.

As it swooped past the south pole of Saturn's moon Enceladus on July 14, 2005, Cassini acquired high resolution views of the icy satellite.
(NASA/JPL/Space Science Institute)

But the mystery for scientists is how this tiny moon — with a diameter about one-seventh that of our own moon — could produce the levels of heat needed to power such eruptions.

The new model suggests a rapid decay of radioactive elements in the moon's core shortly after it formed may have given the moon's interior heating system a jump-start.

"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 model suggests the decomposition of radioactive isotopes of aluminum and iron over a period of seven million years would have generated enormous amounts of heat.

Cassini project scientist Dennis Matson said the greater heat was needed to explain the presence of gaseous nitrogen, which the Cassini-Hyugens spacecraft detected in trace amounts from the 2005 geyser's plume.

Nitrogen is not thought to be part of the moon's original makeup, said Matson, but may have appeared as a result of the thermal decomposition of ammonia, a process that would require temperatures as high as 577 degrees Celsius.

Matson's analysis of the plume will appear in the April issue of the journal Icarus.