Researchers at The Johns Hopkins University Applied Physics Laboratory (APL) and Brown University may have solved a 20-year-old geological mystery surrounding Jupiter's icy moon Europa.

In the August 11 issue of Science, Louise Prockter of APL and Robert Pappalardo of Brown report evidence of "folds" on the moon's frozen surface. The researchers say the mountain-like features - found in three regions - are the first indication of compression on the fractured Europan crust, and provide unprecedented insight into the history and behavior of the Jovian satellite.

Europa as seen from Galileo. Photo: NASA/JPL

"We learned from Voyager images in the late 1970s that there was a lot of extension on Europa - that the surface was pulling apart and a slushy material was moving up through the gaps - but no one could find out how this new material was being accommodated," Prockter says. "Now, we have finally found folds where the icy surface material compresses, and this will help us start to understand how Europa evolved and how it resurfaces."

Prockter and Pappalardo first noticed the folds in high-resolution images of Europa's Astypalaea Linea fracture region, taken by the Galileo spacecraft. Near the large fracture zone they spotted fine-scale features that typically occur in fold structures (such as the Appalachian Mountains) on Earth: regional patterns of fractures and small ridges which mark adjacent crests and valleys.

The folds' direction and location along Astypalaea Linea coincide with models of tidal stress, the gravitational pull from Jupiter that scientists believe creates the pattern of large, canyon-like cracks on Europa's rotating surface. The size and nature of the folds - crests possibly tens to hundreds of meters high and spaced about 25 kilometers (16 miles) apart - also tell the researchers about the surface itself. They indicate warping of a thin brittle lithosphere covering a thicker region, or asthenosphere, of "warmer" and mobile glacier-like ice.

The researchers spotted similar folds in two other regions, and believe they could exist in other areas. One reason the folds have been hard to find is the planet does a good job of hiding them; over time, the researchers hypothesize, the folds "relax away" and push some material back into Europa's interior for recycling.

"There has been no solid evidence for compression folds or material cycling on any other icy satellite, though many show extensional features," Pappalardo says. "This finding potentially has applications for other icy moons as well."