Crescent Callisto as seen from a distance of 410,000 kilometers by
Voyager 1, with a resolution of 4 kilometers.
The north pole is located about midway along the terminator
(inside edge of crescent). Color images from
a previous image sequence were used to produce this
approximately "natural" color view.
Callisto is the
outermost of Jupiter's four planet-sized moons and is dominated by
impact craters. Despite this, a few more interesting
features are also visible, including gigantic impact basins,
suggestions of ancient volcanism, and a few ancient
tectonic features. Among the most interesting features on Callisto
are impact scars from tidally disrupted comets.
Callisto is nearly as large as the planet Mercury.
Its bulk density, however, is only ~1.8 grams per cubic centimeter,
or only twice that of water ice. This indicates that the
interior is approximately half water ice as well. Callisto is very
similar in bulk properties to Ganymede,
but apparently had a much simpler
geologic history. The different geologic histories of these two
bodies has been an important problem for planetary scientists, which
may be related to the orbital and tidal evolution of Ganymede.
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This global view of Callisto was obtained by Voyager 1 in 1979.
At this distance, Callisto is dominated by bright spots. At high resolution,
these spots are revealed to be fresh, young impact craters and their ejecta patterns.
Callisto is relatively brownish in color, possibly due to
the contamination of the icy surface by meteoritic material.
Callisto & Ganymede Compared
Both Callisto and Ganymede are covered by huge tracts of
relatively dark, heavily cratered terrains, shown here at
similar image resolution and solar illumination. Ancient dark terrain on
Ganymede is complex. Numerous parallel
troughs (furrows) are probably part of a vast impact structure, similar to
the rings observed at Valhalla (see below).
Numerous scarps, smooth dark features, and thick crater-floor domes
occur throughout dark terrain on Ganymede and indicate a complex geologic
history. Ancient dark terrain on Callisto appears to have had a simpler
geologic history. Scarps, furrows, and domes are rarely seen.
Valhalla Multiring Basin
This 4000-kilometer-wide impact structure consists of at least 25 concentric
rings or ring arcs, and is one of the most unusual impact structures in
the solar system. The unusual number of rings may be due to impact
into a thin, icy lithosphere, which fractured very easily when the
IMPACT OF TIDALLY DISRUPTED COMETS ON CALLISTO
There are eight prominent crater chains on Callisto. They
probably formed by the impact of comets that were
tidally disrupted by the planet Jupiter, like the split
comet Shoemaker-Levy 9 in 1992-1994. The observed
number of chains suggests that comets are disrupted by Jupiter an
average of once per 275 years or so. Gomul Catena is ~350 kilometers
long, with craters up to 25 kilometers wide (scale bar is 100 kilometers long).
The arcuate scarps are part of the vast
Valhalla multiring impact basin.
This and other unusual crater chains on
Callisto formed by the impact of tidally disrupted comets. This chain
is ~620 kilometers long, with craters up to 40 kilometers across
(scale bar is 100 km long). Gipul Catena is located near the north
pole of Callisto. Several similar crater chains have also been identified on
Comet Shoemaker-Levy 9
This comet was shattered into ~21 fragments by tidal forces of
Jupiter in July 1992. This Hubble Space Telescope image shows the
comet in early 1994, prior to their plunge into Jupiter's atmosphere.
The planet Earth has been added to show the scale of this string of comet
fragments, although each individual fragment was probably no
bigger than a kilometer or so. The bright areas surrounding
each fragment are large halos of reflective dust. Compare this
"string of pearls" with the prominent crater chains on Callisto.
(Image courtesy of STScI and LPI.)
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.
McKinnon W. and Parmentier E. (1986) Ganymede and Callisto. In Satellites (J. Burns and M. Matthews, eds.), p. 437, University of Arizona Press, Tucson.
Morrison D., ed. (1982) Satellites of Jupiter. University of Arizona Press, Tucson. 972 pp.
Schenk P. (1995) The geology of Callisto. Journal of Geophysical Research, 100, 19023.
Schenk P. (1993) Central pit and dome craters: Exposing the interiors of Ganymede and Callisto. Journal of Geophysical Research, 98, 7475.
Schenk P. et al. (1996) Cometary nuclei and tidal disruption: The geologic record
of crater chains on Callisto and Ganymede. Icarus, 121, 249-274.