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.
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
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.
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.