Geology Reference
In-Depth Information
Figure 8.50. A comparison of
the paucity of small craters in
the Valhalla central plains (left
image) and terrain (right image)
not associated with a multi-ring
structure. The paucity of small
craters in some areas of Callisto
is generally attributed to some
process of crater degradation
(left image, NASA Galileo frame
s0368317401; right image,
NASA Galileo frame
s0506142927).
others. Prior to these images, the expectation had been to
see increasing numbers of small craters down to the limit
of resolution, as on Earth ' s Moon. However, some surface
process appears to erase small craters and other landforms.
Jeff Moore of NASA ' s Ames Research Center concluded
that sublimation erosion could be an effective agent of
gradation in which volatile components sublimate, leav-
ing non-ice components to mass-waste downslope. The
overall effect is to reduce topography and soften the
terrain, with smaller craters being degraded or removed.
Given the near lack of small craters in some areas of
Callisto, the rate of degradation exceeds the rate of impact,
but this process (if it occurs) does not take place uniformly
over Callisto.
Degradation of Callisto ' s surface also includes mass
wasting on the floors of several craters (Fig. 8.51) .These
slides could result from collapse of the crater walls due to
oversteepening of the slopes by sublimation erosion.
Figure 8.51. A landslide deposit inside a central-peak crater (~13 km
in diameter), showing a lobate landslide extending onto the crater
8.7.4 Callisto summary
floor (NASA Galileo frame s0413382840).
Ganymede and Callisto are about the same size and den-
sity. Prior to the Voyager flybys, it was assumed that both
would show surfaces re ecting similar geologic histories.
However, images revealed strikingly different worlds
(Fig. 8.52) and generated debate on the comparative evo-
lution of the icy satellites. Nonetheless, some agreement
has been reached on the general events in their histories, if
not on the detailed evolution. In the early history of the
Jupiter system, both objects formed from materials lead-
ing to water - silicate bodies. Heating from various sources
(impact cratering, radioactive decay, tidal stresses) led to
differentiation and the formation of a core. However,
differences that would result in less heat on Callisto
include its smaller size (i.e., less radioactive decay) and
lack of significant tidal stresses.
8.8 Small moons and rings
In addition to the four Galilean satellites, at least 59 more
moons orbit Jupiter. All are small, with Amalthea, meas-
uring 270 km by 166 km, being the largest. Four orbit
Jupiter within the path traced by Io. The innermost two,
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