JUPITER'S MOONS

``The most important thing in science is not so much to obtain new facts as to discover new ways of thinking about them.''
- Sir William Bragg

Key Concepts

The Galilean satellites, starting from the outermost -
61 moons have been discovered orbiting Jupiter. (The latest news about Jupiter's moons can be found here.)The small moons are probably captured asteroids. The 4 large moons are called the Galilean moons (after Galileo, who discovered them in 1609). The Galilean moons, starting from the outermost and moving inward, are Callisto, Ganymede, Europa, & Io (named after four of the god Jupiter's startlingly numerous lovers).

When Galileo saw the Galilean moons through his small telescope, he saw only unresolved points of light. The best images we can get without actually traveling to Jupiter are those provided by the Hubble Space Telescope. In the montage below (click on the image for a larger version), the moons, starting in the upper left corner and moving clockwise, are Io, Europa, Ganymede, and Callisto.

A sharper view of the Galilean moons was provided by the two Voyager spacecraft, which flew past Jupiter in 1979, and the Galileo spacecraft, which went into orbit around Jupiter in 1995.

The Galilean moons form a miniature analog to the solar system as a whole. They orbit Jupiter counterclockwise, in nearly the same plane, on nearly circular orbits. The orbital periods of the Galilean moons:

Note that the orbital period of Europa is twice that of Io, and the orbital period of Ganymede is twice that of Europa; these moons are in an orbital resonance. Every time Io goes twice around Jupiter, for instance, it sees Europa in the same place, and gets a little extra gravitational tug in that direction.

(4) Callisto: Jupiter's cratered moon

The surface of Callisto consists of dirty ice - that is, frozen water with interplanetary dust and bits of rock embedded in it. The surface is heavily cratered (as seen in the Voyager image below), indicating that Callisto is geologically dead, and has been for billions of years.

The craters on Callisto are paler than the surrounding ice, because they break through the superficial layer of dirty ice to the fresh, clean ice underneath.

The average density of Callisto is 1800 kg/m3, intermediate between the density of ice and the density of rock. This indicates that Callisto is probably a mix of ice and rock.

(3) Ganymede: Jupiter's largest moon

Ganymede is not only the largest moon of Jupiter, it is the largest moon in the solar system.Not only is Ganymede bigger than the puny planet Pluto, it is actually 380 kilometers larger in diameter than Mercury. If Ganymede orbited the Sun, we'd have no hesitation in calling it a planet. Although Ganymede is larger in diameter than Mercury, it is lower in mass. The average density of Ganymede is only 1900 kg/m3, much less than that of Mercury. Hence, Ganymede is less than half the mass of Mercury. The density of Ganymede is similar to that of Callisto.

The surface of Ganymede differs from that of Callisto, however. Parts of its surface, as seen in the Voyager image below, are covered with grooved terrain.

The grooves are about 10 kilometers apart and 300 meters deep. Judging from the number of impact craters lying on top of them, the grooves formed less than 2 billion years ago. They are probably `stretch marks' in the crust.

(2) Europa: Jupiter's smooth moon

Europa, the smallest Galilean moon, is indeed smooth. It has very few impact craters and very few surface features more than 100 meters high. The surface of Europa is covered with smooth, white, new ice. Europa, thanks to its pristine surface, reflects almost 70% of the light that strikes it. (Dirty Callisto reflects only 20%.) Since the ice is clean and crater-free, it must be constantly resurfaced by liquid water seeping up from below and freezing on the surface. A close look at Europa's surface, as in the Voyager image below, reveals that the icy surface is covered with cracks, through which water can flow up to the surface.

The average density of Europa is 3000 kg/m3, only slightly less than that of rock. The icy surface of Europa must be only a thin skin; Europa is mostly rock.

An even closer look at Europa reveals that the ice is fractured into numerous ice rafts and ice floes a few miles across, similar to ice rafts seen on the Arctic Ocean on Earth. The presence of these structures indicates that the outer ice layer lies atop a liquid water ocean which lies atop the central rocky core of the moon.

The image of ice rafts on Europa shown below was made by the Galileo spacecraft in February 1997. The field shown is 42 kilometers (about 26 miles) across, and the ice rafts range in size up to 13 kilometers across. (Click on the small image to get a larger, higher resolution version. In the large image, the smallest visible features are about half the size of a football field.)

Europa has liquid water; did life evolve there? No one has a clue, but it's interesting to speculate...

(1) Io: Jupiter's volcanic moon

Io has been compared to everything from a rotten orange to a bad case of acne. A color photo, such as the Voyager image below, reveals that Io is strangely colored indeed.

Io's unique appearance results from the fact that it is volcanically active; the volcanos on Io spew out molten sulfur and various sulfur compounds, which can vary in color from white to yellow to orange to brown. Because of Io's low surface gravity, the volcanic ejecta rise high above the moon's surface and spread over a large area. Any impact craters that form on Io are covered by these volcanic ejecta at a rate of roughly a meter per century. In less than 200,000 years (a short timescale, astronomically speaking), an impact crater will be buried under a mile of sulfur and sulfur compounds. Io is free of impact craters because it is constantly being resurfaced.

The images of Io shown below were made by the Galileo spacecraft five months apart, in April 1997 (left frame) and September 1997 (right frame). Note the new dark deposit of volcanic ejecta around one of the many active volcanos on Io.

The average density of Io is 3500 kg/m3, indicating that Io is all rock and no ice. The melting point of sulfur is about 400 degrees Kelvin (260 Fahrenheit), so the surface of Io, with liquid sulfur pouring across it in lava flows, must be hot - too hot for ice to exist.

What is the source of Io's heat? Melting so much sulfur and spewing it into the air requires lots of energy; much more than Io receives from the Sun. Like Jupiter, Io must have an internal heat source.

Io's internal heat ultimately comes from tides. The distance from Io to Jupiter is approximately the same as the distance from the Moon to the Earth. However, the mass of Jupiter is over 300 times the mass of the Earth. Thus, massive Jupiter creates ENORMOUS tidal bulges on Io, hundreds of meters high. Io's orbit is constantly changing shape, thanks to the repeated gravitational tugs provided by Europa (and to a lesser extent by the more distant moons Ganymede and Callisto). When Io is closer to Jupiter than usual, it has larger tidal bulges. When it is farther from Jupiter than usual, it has smaller tidal bulges. Thus, as Io moves closer to and farther from Jupiter, it is being squeezed back and forth like an accordion. The repeated flexing of Io in this way causes internal friction which heats its interior.

Like the terrestrial planets, the Galilean moons show different amounts of volcanic activity, which result from different amounts of internal heat.

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