Crescent view of Europa obtained by Voyager 2 in July 1979, shown in approximately natural color. (See image below for high-resolution version.)


Europa is the smallest of Jupiter's four planet-sized moons, yet it is only slightly smaller than Earth's Moon. Its bright surface (roughly five times as reflective as the Moon), infrared water ice absorption bands, and the near absence of impact craters (only about five have been identified to date) indicate that the surface is ice rich and very young, perhaps only 30 million years old. Europa is covered by a water-ice shell no more than 150 kilometers thick. Calculations suggest that there could be liquid water at the base of this icy layer, leading to speculation that a primitive life form could have evolved in this dark, watery world. The thickness of the surface ice and the possible presence of liquid water have intrigued planetary scientists since the late 1970s. Europa was the most poorly observed of the Galilean satellites when Voyager passed through the Jupiter system in 1979, and scientists eagerly anticipate Galileo's first images of this fractured icy world.

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Highest-resolution view of Europa obtained by Voyager 2 in July 1979. Smallest features that are resolvable are about 2 kilometers across. Most of the geologic features that characterize Europa are visible. Enigmatic triple bands (long dark linear features) occur in the top half of the mosaic. Wedge-shaped bands occur near the anti-jove region (left center), and cycloidal ridges occur at southern latitudes (bottom). Linear ridges a few hundred meters high are seen near the terminator. Irregularly shaped patches of relatively dark material, known as mottled terrain, are seen in the central (or equatorial) areas. The south pole is near the bottom of the mosaic. This mosaic shows Europa in approximately natural color.

Southern Hemisphere of Europa

This mosaic provides a better view of the variety of linear tectonic features visible in the southern hemisphere. Dark, wedge-shaped bands are visible at upper left, narrow linear dark bands are present in most areas, and curvilinear cycloidal ridges are present in the bottom half of the image. Three small impact craters are also visible, near the bottom, top right, and along the terminator near the top of the mosaic.


Wedge-shaped Bands

These dark straight-edged lineaments fit together like jigsaw puzzle pieces and show that large blocks of icy crust a few hundred kilometers across have moved with respect to each other. The inset map shows how the blocks in this region can be reconstructed. The dark bands are gaps that have formed by block rotation and separated. These gaps are up to 25 kilometers across and have been filled by a darker, brownish material. Tectonic rotation of blocks (or plates) suggests that the icy layer is mechanically decoupled from the rocky interior. A soft ice or liquid water layer may be responsible.

Triple Bands

These structures are poorly understood. They could be fractures formed by extension of the lithosphere or by outflows of relatively dark, brownish material from the interior. They extend for up to a few thousand kilometers and appear to be formed or controlled by global stress patterns, possibly by the rotation of the icy shell. This scene is about 600 kilometers across.

Cycloidal Ridges

These unusual structures are not seen on any other planet or satellite. The cycloidal loops do bear a resemblance to the shapes of island arcs on Earth, which form when one lithospheric plate slides under another. This suggests that the Europa ridges could be compressional, but their origin remains a mystery. Several generations of ridge formation are also evident.

All images by Paul M. Schenk, Lunar and Planetary Institute, Houston, TX.

©Lunar and Planetary Institute, 1997

Supplemental Reading Materials

Beatty J. K., O'Leary B., and Chaikin A., eds. (1990) The New Solar System. Sky Publishing Corporation, Cambridge, Massachusetts; Cambridge University Press, New York. 326 pp.

Moons and Rings (1991) Voyage Through the Universe series. Time-Life Books, Alexandria, Virginia. 144 pp.

Rothery D. (1992) Satellites of the Outer Planets. Clarendon Press, Oxford. 208 pp.

Morrison D., ed. (1982) Satellites of Jupiter. University of Arizona Press, Tucson. 972 pp.

Burns J. and Matthews M. (1986) Satellites. University of Arizona Press, Tucson. 1021 pp.

Schenk P. and McKinnon W. (1989) Fault offsets and lateral plate motions on Europa. Icarus, 79, 75-100.