1. Department of Geology, Box 871404, Arizona State University, Tempe, Arizona 85287-1404 , USA
2. National Optical Astronomy Observatory , Tucson, Arizona 85719, USA
3. US Geological Survey, Menlo Park, California 94025, USA
4. Southwest Research Institute, Boulder, Colorado 8032, USA
5. Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA
6. Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
7. Ames Research Center, Moffett Field, California 94035, USA
8. Cornell University, Ithaca , New York 14853, USA
9. Present address: 308 Space Sciences, Cornell University, Ithaca, New York 14853 , USA

Correspondence to: Robert Sullivan^1,2 Correspondence and requests for materials should be addressed to R.S. (e-mail: Email: sullivan@cuspif.tn.cornell.edu).

Abstract

Images obtained by the Voyager spacecraft revealed dark, wedge-shaped bands on Europa that were interpreted as evidence that surface plates, 50–100 km across, moved and rotated relative to each other¹. This implied that they may be mechanically decoupled from the interior by a layer of warm ice or liquid water²,³. Here we report similar features seen in higher resolution images (420 metres per pixel) obtained by the Galileo spacecraft that reveal new details of wedge-band formation. In particular, the interior of one dark band shows bilateral symmetry of parallel lineaments and pit complexes which indicates that plate separation occurred in discrete episodes from a central axis. The images also show that this style of tectonic activity involved plates < 10 km across. Although this tectonic style superficially resembles aspects of similar activity on Earth, such as sea-floor spreading and the formation of ice leads in polar seas, there are significant differences in the underlying physical mechanisms: thewedge-shaped bands on Europa most probably formed when lower material (ice or water) rose to fill the fractures that widened in response to regional surface stresses.