August 30, 2005
Cassini Finds ‘Hot Spot’ On Enceladus
By Gideon Long
LONDON (Reuters) - There is a hot spot on one of Saturn's moons which should not be there and has yet to be explained, scientists said on Tuesday.It is located at the south pole of Enceladus, a moon with a diameter of just 500 km (310 miles) which orbits Saturn at a distance of around 238,000 km.
The hot spot is unusual because it occurs at the pole, scientists said. Usually, the hottest part of any planet or moon is around the equator, as is the case with the earth.
This suggests that the heat at Enceladus' southern pole is generated from within, said scientists from the Cassini-Huygens mission to Saturn and its moons.
But they acknowledged they had no idea how.
"It shouldn't be that warm," said John Spencer, one of the scientists working on the project.
"It's like flying past Antarctica and finding that it's warmer than the earth's equatorial regions. It's that strange."
Spencer, of the Southwest Research Institute in Colorado, the United States, is one of a group of scientists examining data sent back to earth from the Cassini spacecraft, which was launched in 1997 to examine Saturn.
The spacecraft has flown past Enceladus three times -- most recently on July 14 when it passed within just 175 km of the small, icy orb.
The scientists expected to find that the temperature was around 80 degrees Kelvin (-193 degrees Celsius, -316 degrees Fahrenheit) at its hottest point, which they assumed would be near the equator.
Instead, they found that the heat was concentrated at the south pole, where the temperature hit 91 degrees Kelvin near a series of fissures, or "tiger stripes" on the moon's surface.
"It is an extremely conspicuous hot region," Spencer told a news conference. "Something is different about that area."
The scientists have come up with two theories to explain the hot spot. The first is that the heat comes from decaying radioactive material below the moon's surface and the second is that it is caused by gravitational tides.
But they say neither theory adequately explains the heat.
"We don't have anything we could call a complete hypothesis yet," said Torrence Johnson from NASA, which is working on the project alongside the European and Italian space agencies.
The team says the hot spot suggests there might be volcanoes and geysers on Enceladus.
If this is true, it would be one of only three "active" moons known to man. The others are Io, which orbits Jupiter, and Triton, which circles Neptune.
The Cassini spacecraft has been sending spectacular images back from Saturn, its rings and its moons since last year, when it reached the planet. It also launched a probe which landed on the surface of Titan, another of Saturn's 31 known moons.