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Callisto

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Jupiter spiral manoever 40% complete. Callisto closest approach in approx. 1 hour.

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Callisto completes my collection of Jupiter's four Galilean moons. It is the exact opposite of Ganymede. Because, whereas Ganymede has one of the youngest non-tectonic surfaces in our solar system, Callisto possesses one of the oldest. It is probably four billion years old and has probably retained many of its original composites. Chances are it has not changed since its formation.

Except, of course, for the inevitable meteorite impacts. All of these impacts have exposed its icy interior, hence the white spots. Its most prominent features are two huge craters, called Valhalla and Asgard, are surrounded by multi-ring structures. It is the most heavily cratered surface in our solar system. Its crater density is so great, it is awfully close to saturation (any new crater on Callisto will almost certainly erase an older one).

It is slightly smaller than Mercury, and at 4862 km, is the only moon to fall between 4000 and 5000 km.

Picture by Galileo.
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