In July, NASA's Cassini spacecraft made its latest flyby of Saturn's moon Enceladus, revealing an unexpected hot spot on the moon's south pole.
The finding flipped everything scientists knew about Enceladus on its head, because what should have been a dead moon appeared to be geologically active and what was supposed to be the moon's coldest region turned out to be its warmest.
"This is as astonishing as if we'd flown past Earth and found that Antarctica was warmer than the Sahara," said John Spencer, an astronomer from the Southwest Research Institute in Colorado and a co-investigator of the Cassini mission.
The finding could explain an old mystery concerning Enceladus, but it also presents a new puzzle of its own.
Discovered in 1789 by a British astronomer, Enceladus is named after a mythological Greek giant. Despite its namesake, it is a tiny moon, only about 300 miles in diameter, and small enough to fit snugly inside the state of Arizona.
The surface of Enceladus is coated in a thin layer of ice that reflects back nearly all of the sunlight striking it, making it the brightest object in the solar system apart from the Sun.
Cassini's July flyby of Enceladus had it dipping within 109 miles of the moon's icy surface, its closest approach yet. In addition to the south pole hot spot, Cassini also revealed that the "icy veins" were actually a series of fractures on the moon's surface. Even more surprising, the fractures appeared to be active, violently spewing a slushy jet of warm water and ice into space.
Together, the venting fractures and the hot spot provide strong evidence for geologic activity on Enceladus. If true, the findings could explain the moon's connection with one of Saturn's rings, a relationship that has puzzled scientists for years.
Saturn's E ring stretches nearly 200,000 miles from its inside edge to its outer bound and is made up of microscopic particles of ice and dust. The ring is so faint that scientists didn't discover it until about 30 years ago, but when they did, they noticed a curious thing: the ring was brightest around Enceladus, which, along with some of Saturn's other moons, wades through the E ring's plane of debris while circling the planet.
This observation caused some scientists to suspect that Enceladus was somehow supplying material for the ring. It was a strange idea, but the fact that the E ring existed at all was evidence for it.
"The particles are so small they should not last very long," Spencer told Space.com. "You couldn't have the E ring just sit there for the age of the solar system. It had to be regenerated somehow."
There were two possibilities for how Enceladus could supply material for the ring. According to one scenario, impacts from cosmic particles were blasting tiny bits of Enceladus off into space, providing Saturn with fodder for a ring.
In the second scenario, geysers or water volcanoes on the surface of Enceladus spewed out clouds of ice and dust into the moon's atmosphere, and because the moon is so small and its gravity so weak, the ice and dust soon float off into space. Like drifting steam from a tugboat, Enceladus would deposit a trail of microscopic debris in its wake as it orbited Saturn — debris that Saturn's gravity would then rope in to make a ring.
Based on what scientists knew about Enceladus, the first scenario seemed more likely. Enceladus was thought to be geologically dead, and without geologic activity, there could be no spouting fountains of water and ice on the moon's surface.
That's why the discovery of the hot spot on the moon's south pole is so important, because it provides the moon with an engine to drive the geysers and volcanoes.
Why the south pole is so active is still a mystery. One theory is that radioactive material left over from the moon's formation billions of years ago is acting as a heat source, said Linda Spilker, Cassini's deputy project scientist.
Another theory is that a change in the moon's spin rate caused fractures to form on the moon's surface.
Like many moons, Enceladus takes as long to rotate on its axis as it does to make one orbit around Saturn, thus only one of its hemispheres faces Saturn.
"If Enceladus moved in closer, then it would have to try to spin more quickly to keep one side facing Saturn," Spilker said. "And maybe that change in spin might have caused the cracking seen at the south pole."
How or when this might have occurred is still unknown.
Cassini is scheduled to make an even closer pass of Enceladus in 2008, during which scientists hope to gather new pieces for the puzzle.