Icy cracks may betray Europan ocean.

Circumstantial evidence that Jupiter's moon Europa harbors a subterranean ocean--a possible haven for extraterrestrial life--continues to mount.

The latest findings come from an analysis of features first seen by the Voyager spacecraft in 1979. Near Europa's south pole, the craft recorded scalloped lines that continue for hundreds of kilometers. The Galileo craft, now touring Jupiter, has recorded the same scalloped pattern over Europa's entire surface.

According to a model developed by Gregory V. Hoppa, Paul E. Geissler, and their colleagues at the University of Arizona in Tucson, the arcs are fractures generated when Europa's icy surface flexes in response to the rise and fall of tides in the underground ocean. As it follows an elliptical, 3.5-day orbit around Jupiter, Europa changes its distance from the planet and thus its tidal pull. One new arc in the scalloped pattern appears to have formed during each 3.5-day interval, the team notes in the Sept. 17 SCIENCE.

Although the model only suggests that Europa once had an ocean, the moon's nearly crater-free, youthful facade indicates that the arcs seen by Galileo were generated no earlier than 100 million years ago. "It stretches the imagination to assume that the subsurface had been liquid [then] and froze up so recently," says Geissler.

"This adds significantly to the evidence for an ocean under the present, relatively thin ice crust," says William K. Hartmann of the Planetary Science Institute in Tucson. Confirmation could come with the 2003 launch of Europa Orbiter, which will use radar to measure the thickness of the moon's icy crust and may determine whether liquid water exists below.