Enceladus, a tiny moon orbiting Saturn, appears to be venting water into space from a series of fractures over its south pole.
The water vapour is pouring out of the body through an anomalous hotspot centered over its south pole, in only the second extra-terrestrial observation of thermal activity in the solar system.
According to the latest data from Cassini, the moon might have lost as much as five per cent of its mass since its formation, and figures suggest it is losing enough material to account for Saturn's E-ring.
The difficulty is, scientists can't explain how the South Pole region might be getting so hot.
All the instruments on board Cassini point to something very unusual happening at Enceladus' south pole, the area towards the bottom of the moon in the image here, marked by four heavy crevasses, known as the 'tiger stripes', reports the Register.
According to News Telegraph Prof Michele Dougherty, of Imperial College London, and principal investigator for Cassini's magnetic field measuring equipment, said: "It was a complete surprise to find these signals at Enceladus.
"These new results from Cassini may be the first evidence of gases originating either from the surface or possibly from the interior of Enceladus."
Dr Torrence Johnson, from Nasa's Jet Propulsion Laboratory in Pasadena, California, said at a news conference in London yesterday: "We've been here before. These are early days in understanding the data. In a sense, it's nice to be baffled at this point.
"At the moment the maths does not add up. We did not expect what we know to be the available power sources here to be able to produce this kind of heat.
"I think we can expect some of our clever colleagues with models of evolutionary history of orbits and interior structure to come up with ways to explain this." Mission scientists made their first surprising discoveries about the moon after examining data collected by Cassini during two flybys, at 725 miles and 310 miles above the moon, on Feb 17 and March 9.
The spacecraft's magnetometer showed Saturn's magnetic field was being bent around Enceladus, suggesting it had an atmosphere, or layer of gas bound to it.