Prockter and Pappalardo, a senior research associate at Brown, first noticed
the folds in high-resolution images of Europa's Astypalaea Linea fracture
region (seen in photo above), 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 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," said
Pappalardo, who next May will become an assistant professor in the University
of Colorado's Astrophysical and Planetary Sciences Department in Boulder.
The
Astypalaea Linea region of Europa's southern hemisphere, shown in this photo from NASA/JPL/Johns Hopkins University Applied Physics
Laboratory and Brown, is the smooth gray area that stretches from north to south across the image mosaic. It is thought to have formed by a combination of pulling apart and
sliding of the icy surface. The telltale fold features are within the smoother
portions of the surface between the more dominant ridges, which are attributed
to upwelling of material through surface ice.
"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," said Prockter, who graduated from Brown last
May with a doctorate in geological sciences, and who conducted a good deal of
the research of Europa in Lincoln Field Building. "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."