Callisto

Image Credit:

NASA/JPL/DLR

The farthest of the Galilean moons, and the second largest of all Jovian moons, Callisto has the smallest density of the four major moons of Jupiter of 1.81 g/cm³. From the density, we know that Callisto is mostly water and ice. It has a radius of 2410 km, giving it a total mass of 0.018 Earth masses. Orbiting at 1.883 million km from Jupiter, it takes 16.7 Earth days or 40 Jupiter days. Like Ganymede, Io, and Europa, it is tidally locked to Jupiter, so it rotates on its axis once ever 40 Jupiter days, as well. It does not share a resonance with Io, Europa, and Ganymede, which, if you recall, is 1:2:4. Callisto's resonance would be 9.4 if you would want to know that.

Since it is the farthest Galilean moon, it does not go as intense tidal friction as do the others. Because of this, it is not as active as the other three, and is not as well differentiated even though it is larger than both Io and Europa. It may have a liquid ocean below the ice crust, but it is less likely to harbor life than Europa.

The Four Galilean Moons of Jupiter. Clockwise from top left:

Io, Europa, Callisto, and Ganymede. Note how Callisto is the only one that has a uniform interior.

Image Credit:

NASA/JPL

 

Since it is not as strongly affected by tidal forces, Callisto's surface is slightly older than the surfaces of Io, Europa, and Ganymede. It is dark, dusty, and pockmarked with multiple craters. The dark and dusty material is from millennia of asteroid and comets impacts, disintegrating and covering the surface.

Callisto, however, is the best bet for human presence in the Jupiter system. Since it is farther from Jupiter than the rest, the magnetic field of Jupiter is less intense at the Callisto than the other Galilean moons. Also, the radioactivity from the moon itself is not as strong since the moon is geologically inactive. The thin atmosphere contains both carbon dioxide and molecular oxygen which can be used by the colonists or scientists on the surface, and with the liquid ocean below the crust, water is in abundance, as well. Though it is farther from Jupiter, it is still well within Jupiter's gravity well, so any asteroids or comets coming nearby will most likely be capture by Jupiter and crash into Jupiter (see Comet Shoemaker-Levy 9).

Callisto, like the other Galilean moons, formed at roughly the same time as Jupiter, but they were far enough away to be prevented from being accreted by the newly formed planet. They, obviously, are still under the gravitational influence of Jupiter itself.