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The moon Callisto

CallistoCallisto. © based on NASA images

Callisto is one of the Jovian moons discovered by Galileo. Callisto is one of the biggest moons in the solar system; only Ganymede and Titan are larger. It’s also the second largest moon in orbit around Jupiter. Callisto is roughly the same size as Mercury. Callisto orbits Jupiter every 17 earth days. This moon is tidally locked. This means that it rotates once per orbit, similar to Earth’s moon. Thus one “day” or rotation on Callisto is equal to its orbital period of 16.69 Earth days.

Callisto is the farthest of the Galilean moons discovered by Galileo. Unlike the other Galilean moons, Callisto is not caught in the orbital resonance like Io, Europa and Ganymede. This means that it is not subject to the tidal forces of the other Galilean moons. Its core is not heated by the gravitational tug of war between Jupiter and the other Jovian moons.

History of Callisto

Callisto was discovered in 1610. The moon is named for a nymph who attracted the attention of Zeus. A jealous Hera turned her into a bear. Zeus, also called Jupiter to put her in the heavens where she became the Great Bear. Her son by Zeus became the Ursa Minor or Little Bear constellation.

Callisto probably formed over several million years from a rocky ring in orbit around Jupiter, in a miniature replay of the coalescence of rocky debris around the Sun forming the inner planets. This theory is supported by the number of craters and age of its craters, some of which approach four billion years old.

Geography of Callisto

Callisto is estimated to be almost as old as the solar system, roughly four billion years old. Callisto is considered geologically dead. Unlike Io, it has no volcanic activity and likely never has. Callisto lacks the underground oceans of Ganymede and Europa. It has no significant atmosphere like Titan to weather craters. Its atmosphere is comprised of a thin layer of carbon dioxide.

The largest impact crater on the moon is called Valhalla (2,500 miles).
Its largest impact crater is called Valhalla (2,500 miles). © NASA images

The constant bombardment of asteroids caught by Jupiter’s gravity over four billions years without any weathering has made Callisto the most heavily cratered moon in the solar system.

The largest impact crater on the moon is called Valhalla. It is about 2,500 miles or 4,000 kilometers across. Asgard is roughly 1,000 miles or 1,600 kilometers across. Both craters are named for characters from Norse mythology. Gipul Catena is a series of craters that form a straight line. These craters probably form when a single large object was broken up by Jupiter’s gravity and the pull of the other Jovian moons before the pieces hit Callisto.

Callisto lacks mountains, even those that normally created by impacts. It is thought that the craters have been smoothed by the flow of ice, though the ice has not filled in the craters as it has on Ganymede or Europa. It is thought that the layers of ice on Callisto melt, flow and then refreeze after an impact. This creates concentric waves in the rock and ice from the meteor’s impact without generating massive mountains of ejected debris as has occurred on Earth’s moon.

Asgard is roughly 1,000 miles across.
Asgard is roughly 1,000 miles across. © NASA images

Callisto is similar in size to Mercury but has one third of its density. This may be due to ice mixed in with its rock. Callisto is a frigid place. In the daytime, it reaches -108°C. The night side drops to -193°C. There are white deposits on Callisto’s surface that are probably the result of sublimation, the direct conversion of exposed ice to vapor upon exposure to the vacuum of space.

Callisto’s surface is a dark gray due to the predominance of silicate rock on its surface. There is probably a layer of mostly ice under the rocky surface. Callisto’s escape velocity is slightly higher than Earth’s moon. And Callisto has almost no orbital eccentricity relative to Jupiter. Its orbital eccentricity is 0.0074, very close to Jupiter’s equator. Callisto orbits just outside of Jupiter’s massive magnetosphere. This gives it the lowest radiation levels of the Galilean moons.

Inside Callisto

Callisto has a faint ionosphere. Callisto’s ionosphere is more powerful than would be explained as an interaction between its thin carbon dioxide atmosphere and Jupiter’s magnetic field. Magnetic resonance imaging shows that there is a layer below the surface that conducts Jupiter’s magnetic field. If it were an ocean, it would be very salty and lie about 100 to 200 kilometers below the crust. If the layer is comprised entirely of ice, it would be up to 300 kilometers thick. Due to the lack of plate tectonics and volcanic activity, Callisto has probably never had liquid water under its surface and has been frozen to its core since its creation. There are astronomers who theorize that Callisto has an ocean at least 150 kilometers below its surface, but there is less evidence for this than the other Jovian moons.

Internal structure of Io, Europa, Ganymede and CallistoInternal structure of Io, Europa, Ganymede and Callisto. © based on NASA images

Its core is mostly comprised of silicate rocks. There is evidence of iron-oxide rocks, frozen ammonia and carbon compounds. The carbon compounds on Callisto’s surface are probably the debris left by impacts of carbon-based asteroids. There are signs of water ice mixed with the rocky debris of the moon.

Exploration of Europa

Voyager 1 sent back the first finely detailed photos
Voyager 1 sent back the first finely detailed photos. © NASA images

Pioneer 10 and Pioneer 11 flew past Callisto, providing data that confirmed its true size. Voyager 1sent back the first finely detailed photos of Callisto’s marred surface in 1979.

Further images were sent back by the Galileo mission to Jupiter, though that mission’s true purpose was to drop a probe into Jupiter’s atmosphere. The New Horizons probe on its way to Pluto passed by Callisto and sent back new pictures of the moon. Callisto will be further studied by the Jupiter Icy Moon Explorer, or JUICE probe. JUICE is slated for launch by the European Space Agency or ESA in 2022. If sent in 2022, it would reach Jupiter in 2030. NASA’s comparable mission, Jupiter Icy Moons Orbiter or JIMO, has been cancelled due to lack of funding. If humans are sent to the Jovian moons, Callisto is considered the best site for a base due to its low radiation levels. This decision was reached during NASA’s Human Outer Planets Exploration project. However, all plans for human missions are literally on the drawing board, with no space agency actively planning a manned trip to Jupiter or its moons.