Episodic plate separation and fracture infill on the surface of Europa

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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.

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Figure 1: Combined Voyager and Galileo image data reprojected from above 40° S, 205° W.
Figure 2: Part of Galileo image S0368639400 of Europa, with north up.
Figure 3: Bilateral symmetry of internal details within the dark, wedge-shaped band shown in Fig. 2.


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We thank N. Sleep for reviews, and K. Klaasen, H. Breneman, T. Jones, J. Kaufman, K. Magee and D. Senske at JPL for their efforts. Cooperation from personnel responsible for other instruments after a playback anomaly on Galileo's third orbit allowed recovery of the high-resolution image shown in Figs 2 and 3. This work was supported by NASA's Galileo Project.

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Correspondence to Robert Sullivan.

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