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Jupiter: Moons

Jupiter's moons hold a special place in the history of human knowledge. Galileo Galilei's discovery in 1610 of a "solar system" with Jupiter at its hub helped prove Earth is not the center of the universe.

Jupiter family portrait
Family Portrait with 4 of Jupiter's satellites: Io, Callisto, Ganymede, and Europa.
Galileo observed Jupiter's four largest moons - Ganymede, Io, Europa, and Callisto - making them the first celestial objects discovered with a telescope. The twin Voyager 1 and Voyager 2 spacecraft and the one named for Galileo gave us close-up looks at these "Galilean" moons and discovered many more. There are currently 63 known moons orbiting Jupiter.

Ganymede, Callisto, and Europa are all suspected of having oceans of liquid water beneath their icy crusts. If true, Ganymede and Callisto can probably thank the natural radioactivity of their rocky interiors for keeping their slushy water from freezing solid.

As the closest of the three to giant Jupiter, Europa has an additional heat source. Jupiter and the other large moons subject Europa to a gravitational tug-of-war that stretches and squeezes the satellite in much the same way that Earth's moon raises tides in our oceans. This generates enormous heat inside the moon, which may keep its ocean (if any) in a liquid state. That and the ocean's suspected location only a few kilometers beneath the smooth, icy surface may make Europa a prime target in the search for extraterrestrial life.

Io, innermost of the Galilean satellites and slightly larger that Earth's moon, goes through even greater gravitational flexing, with "tides" of as much as 100 meters (328 feet) in its solid rock surface. The resulting frictional heat keeps Io's subsurface rock layer molten and pressurized, constantly ready to feed the moon's many volcanoes.

Covered in sulfurous lava that makes it look a little like a giant pizza, Io is the most volcanically active body in the solar system. Voyagers 1 and 2 saw nine eruptions and the Galileo spacecraft has since spotted hundreds of smaller eruptions. Plumes of red and yellow sulfur dust shoot 300 km (186 miles) into the sky. Some escapes Io and paints a bright red coat on a small, neighboring moon called Amalthea.

Io contributes to fireworks on Jupiter, too, giving rise to powerful radio transmissions and contributing to auroras as the rapidly spinning planet sweeps its powerful magnetic field across the natural satellite.

Ganymede is the biggest moon in the solar system and, in fact, is larger than the planets Mercury and Pluto. It has two distinct surface types - one dark and rugged, the other bright and smooth, with parallel grooves hundreds of meters deep and thousands of kilometers long. The bright areas show fewer impact craters, indicating a much newer terrain, possibly resurfaced by ice volcanoes.

Diagram of Jupiter's Moons
Click on the image for a diagram of Jupiter's moons.
Callisto, about the size of Mercury, is the third largest moon in the solar system (Saturn's Titan is #2) and is the outermost of the Galilean satellites. It is completely covered in craters, more heavily cratered than any other moon or planet in the solar system. Unlike the other large bodies, which have recoated at least parts of their surfaces, Callisto remains as it was when it formed four billion years ago, during the period of intense meteoroid bombardment that the entire solar system experienced.

Four "ring moons" circle Jupiter inside Io's orbit: Metis, Adrastea, Thebe, and the red-coated Amalthea. As meteoroids strike them and explode, they blow off the dust and debris that form Jupiter's rings.

Many more small moons circle Jupiter outside the orbits of the four Galilean satellites.
Jupiter's Moons
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Last Updated: 04.04.05