Space Topics: Jupiter
Callisto
Callisto in color
Galileo captured this global view of Callisto on its 30th orbit
of Jupiter, on May 26, 2001. The filters used for this image cover
a broader range of the spectrum than human eyes can see. Galileo ranged
from 740,000 to 744,000 kilometers from Callisto when it took this
image, at a phase angle of just 3°. Image scale is 7.5 kilometers
per pixel. Credit: NASA / JPL / Ted Stryk |
Diameter: 4,820.6 kilometers -- 0.3779 Earth diameters -- 12th largest solar
system body
Orbital distance: 1,882,700 kilometers from Jupiter
Orbital period: 16.689 days
Discovery: 1610 by Galileo Galilei
Compared to the dynamic and varied surfaces of Io, Europa, and Ganymede,
Callisto’s looks boring by comparison. It is one of the most
heavily cratered bodies in the solar system, indicating that its surface
is very, very old, roughly 4 billion years in age. Over geologic
time, the originally sharp-edged craters “relaxed,” so Callisto’s
craters look more subdued than those on Earth’s Moon.
Callisto is believed to have a very primitive interior, in which its component
ice and rock are mostly jumbled together, rather than being separated into
layers as they are in nearly every other large body in the solar system. Callisto
likely never got warm enough for the contrast in density between ice and
rock to drive the differentiation of its interior. It has been a very
cold, very inactive world for a long time. Strangely, though, Galileo
did detect a thin liquid layer at a depth of about 200 kilometers (120 miles)
below Callisto’s surface.
Galileo’s close-up views revealed that there was some previously unexplained
weathering process eroding jagged, icy peaks and leaving crater floors filled
with a strange, dark dust. Scientists proposed that the ice in the bright
peaks may sublime, leaving behind silicate dust, which then drifts downward
to fill crater floors. The dark dust would be relatively warmer than
the bright icy peaks; sublimation near dust deposits would be enhanced, accelerating
the process of peak erosion.
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Callisto in color
Galileo captured this global view of Callisto on its 11th orbit of Jupiter, on November 5, 1997. The filters used for this image cover a broader range of the spectrum than human eyes can see. Galileo was 687,000 kilometers from Callisto when it took this image, at a phase angle of just 1°. Image scale is 6.9 kilometers per pixel.
The image was reconstructed from the original data by Ted Stryk. About 10% of the right side of the disk was marred by data dropouts; Stryk reconstructed missing data from images taken through other filters.
Credit: NASA / JPL / Ted Stryk
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Callisto from New Horizons
New Horizons captured this image of Callisto as it zipped by the Jupiter system on February 27, 2007. With a diameter of about 4,800 kilometers, Callisto is the second largest moon of Jupiter and the 12th largest object in the solar system.
Credit: NASA / JHUAPL / SwRI
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Crater degredation on Callisto and Ganymede
Credit: NASA / JPL / Jeff Moore
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Debris slides on Callisto
Callisto is unique among the Galilean satellites of Jupiter for having debris avalanches from oversteepened crater walls.
Credit: NASA / JPL / Jeff Moore
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A model for the degradation of Callisto's topography
In this model, Callisto starts out with a surface roughened by impacts. Volatile materials -- water ice and, probably, carbon dioxide ice -- sublime from this surface and then get redeposited. Physical models suggest that the floors of these craters will experience net sublimation, while the peaks will experience net deposition of frost, a process that accelerates as the floor accumulates a lag deposit of dark materials (which absorb more solar radiation and thus get warmer) and the peaks accumulate a deposit of bright, reflective frost. Bedrock is only exposed in the steepest part of the crater walls.
Credit: Jeff Moore
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