The explanation, presented in this week's issue of the journal Science, appears to answer a riddle scientists have puzzled over since the Voyager spacecraft revealed these cracks 20 years ago.

Scientists call these cracks cycloidal features, or flexi. The arch-shaped segments are called arcuate, after the shape of an archer's bow. The arcuate lines etched across Eurpoa's surface have defied explanation by common ideas about how cracks form.

Then Gregory Hoppa and Randall Tufts, both post-doctoral researchers at the Universiy of Arizona, began playing with the idea that the cracks may be formed by the daily tidal cycle of a subsurface ocean. Hoppa, the lead author of the Science article, had developed a computer model to calculate the stresses of such tides over the course of Europa's orbit. At Tufts' suggestion, he analyzed the cycloidal cracks and found that his model of changing ocean tides could explain their formation.

"The basic assumption is that Europa has a significant water layer underneath the ice that's going to go up and down by about 30 meters (100 feet)" during the moon's 85-hour orbit around Jupiter, Hoppa said.

Scientists know that Europa has a layer of water on its surface that is some 100 miles (160 kilometers) thick -- almost 15 times as deep as deepest point in any of Earth's oceans. Whether that water is entirely frozen or just covered by ice on the top has been a topic of much debate.

The explanation suggested by Hoppa's group works so well at explaining the arcuate cracks, that it has become one of the best arguments for the existence of a global ocean hidden underneath the ice.

"If there wasn't a global large fluid layer to produce significant large tidal stress, you probably would not form these features," Hoppa said. "There has to be a significant water layer that applies significant tidal stress to the ice shell on this kind of time scale."

If Europa's water were all solid ice, the tidal changes on the surface would amount to changes of tens of centimeters per day, not tens of meters per day. With such small tides, the distorting stresses would not be great enough to cause cracks.

Hoppa and members of his team are not the only ones impressed by their work. The explanation is winning praise from other researchers, some of whom have been reluctant to conclude that liquid water may exist below Europa's ice.

Bob Pappalardo, a planetary geologist at Brown University has called himself "one of the greatest skeptics." He aknowledged that the evidence for a liquid layer, at least in the past, is slowly building.

"This is a very elegant solution that Hoppa and his coworkers have come up with for how these cycloidal features form," Pappalardo said. "I think this offers probably the best geologic evidence for a subsurface ocean."

Pappalardo doesn't agree with everything the Arizona group concludes, however.

In his view, the calculations assume the ice surface to be thinner than can be justified by what scientists know about Europa. While Hoppa and his colleagues suggest the ice to be only a few kilometers thick, Pappalardo said it is more likely at least 10 kilometers thick. He added, though, that Hoppa's model can probably work with a thicker ice shell.

The explanation raises several questions that will need to be studied further, Pappalardo said. One of the lingering questions of Hoppa's analysis is why would the cracks travel so slowly? To fit the proposed explanation, some of the cycloidal cracks on Europa would need to move as slow as 2 mph. Cracks traveling through brittle material most often move at about the speed of sound.

Another Europa expert who doesn't believe the evidence is yet in that Europa has a large liquid ocean -- but is impressed with Hoppa's solution -- is Bill Moore, a post-doctoral researcher in planetary geophysics at UCLA. Moore said that there could be many explanations for slowly-propagating cracks.

"This ice has been just broken to hell by hundreds of cracks. It's not as strong as a sort of monolithic block of ice," he said. Cracks through soft ice might simply travel slowly.

"Ice, unlike rock, can relax stresses away. It doesn't always act as a brittle solid, like rock. It can be more like a plastic gooey solid," Moore said. "There's certainly a lot we don't know about ice, especially under these (Europan) conditions."

If there is indeed an ocean of liquid water concealed below Europa's frozen exterior, the moon could be the most likely place elsewhere in the solar system where extraterrestrial life might have a chance of forming.