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 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

 June 2000

News Notes
 Planetary Geology

Accordion folds on Europa

Recent study of Galileo images may present the strongest evidence to date for understanding the strange surface tectonics of Jupiter’s moon Europa. One of the most striking features of Europa is its fracture-dominated, water-ice surface. But since planetary scientists began studying the jigsawed moon, it appeared devoid of any signs of compressional features, such as folds, typical of those on Earth. Now, researchers have found evidence for large-scale folding in a region of Europa that includes the well-studied Astypalaea Linea fault.

“The apparent lack of compression on Europa has puzzled researchers since Voyager days,” says Louise Prockter of The Johns Hopkins University Applied Physics Laboratory. Explanations for the moon’s lack of compressional features have ranged from subduction mechanisms that escaped detection to a theory that Europa was expanding. Prockter and Robert Pappalardo of Brown University presented a new analysis of Galileo data suggesting that Europan folds do exist. Their March 16 announcement at the 31st Lunar and Planetary Science Conference in Houston has created “an element of excitement that compressional features have finally been identified,” Prockter says.

    Astypalaea Linea fault is similar in size to 
    the California segment of the San Andreas 
    fault. Fold structures appear in the lower 
    region of the image. NASA/JPL.

Their hypothesis, they say, supports the theory that Europa’s crust is deformed by gravitational stresses that result from the moon’s proximity to Jupiter. As Europa travels around its own axis and around Jupiter, the huge planet’s gravity exerts a continuous pull. Much like the interaction of Earth’s moon on the ebb and flow of the tides, the jovian gravitational stresses appear to result in the flexing of Europa. The axes of the fold structures in the Astypalaea Linea region are oriented perpendicular to the maximum compressional stresses predicted from the gravity deformation models. Therefore, if Jupiter continually exerts a deformational force on Europa, the newly discovered folds are oriented exactly as pre-existing deformation models would predict.

Galileo collected the images at a resolution of 43 meters per pixel, revealing to the researchers fine-scale features that are often associated with fold structures on Earth. For example, joints can form along the crests of terrestrial folds, and these are seen on Europa as sets of smaller fractures striking along the crests of the regional-scale anticlines. The researchers observed compressional ridges two to three kilometers long within the synclinal lows. The small ridges are akin to the bunching observed in troughs composed of pliable materials on Earth.

By measuring shaded textures on the Galileo images, Prockter and Pappalardo could distinguish adjacent anticlines and synclines with distances between crests of approximately 25 kilometers. Combining their measurements of fold wavelengths with their knowledge of the physical properties of Europa’s icy crust, Prockter and Pappalardo were also able to calculate a regional crustal thickness of two kilometers around Astypalaea Linea. The two-kilometer depth correlated well with the results of several pre-existing crustal models.

In the fall of 1998, Patricio Figueredo of Arizona State University suggested that undulations in another region of Europa may also have been caused by gravity-driven compressional stresses. “The wavelength of the undulations,” Figueredo says, “is about 25 kilometers — similar to that reported by Prockter and others.” Figueredo is teaming up with Prockter and Pappalardo to search for new sites that may show fold evidence.

According to Figueredo, the research may color future missions to Europa. Subsequent data may be collected while the sun is at a low angle, enhancing the shadowing that led to the researchers’ discovery of the folds.

Galileo, already operating almost three years beyond its original completion date, will continue to observe Jupiter at least through the end of this year. Europa may not get its next observation until 2003, the year NASA has proposed sending a new orbiter to the icy moon. Until then, the researchers will have to be content re-examining the images retrieved by the Voyager and Galileo missions.

Joshua A. Chamot
Geotimes contributing writer

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