The Moons of Jupiter

Earth has one moon, as we earth dwellers know. Jupiter, that mysterious “gas giant” that is the fifth planet in order from the sun and the largest planet in the solar system, has—count them—63 moons. Many of these moons, however, are not moons as we think of moons. Only eight are “regular satellites,” with prograde (a direction of rotation counterclockwise as viewed from the north pole of the sky or a planet) and nearly circular orbits not greatly inclined with respect to Jupiter’s equatorial plane. The other 55 moons are “irregular satellites” whose prograde and retrograde (moving in a direction contrary to that of similar objects) orbits are much farther from Jupiter and have high inclinations and eccentricities.

Of the eight “regular satellite” moons, four are called the Galilean moons because they were discovered by the astronomer Galileo in 1610. They are among the largest objects in the solar system and would be considered dwarf planets if they revolved around the sun instead of around another planet. They are Io, Europa, Callisto, and Ganymede and were the first objects discovered to orbit a body other than the earth or the sun. (The other four “regular satellite” moons are much smaller and closer to Jupiter, serving as sources of the dust that makes up Jupiter’s rings.)

Ganymede, the largest of the four Galilean moons, is the largest natural satellite in the solar system (larger than the planet Mercury) and the only satellite known to possess a magnetosphere (a region of space surrounding a planet that is dominated by the planet’s magnetic field so that charged particles are trapped in it). Ganymede is made up mostly of silicate rock and ice. A salt-water ocean is believed to lie underneath its surface, sandwiched between layers of ice. The surface has two types of terrain—highly cratered dark regions and younger (though still ancient) regions with many grooves and ridges. There is a thin oxygen atmosphere that includes ozone and some atomic hydrogen.

The second largest of the four Galilean moons is Callisto, ranking as the third largest moon in the solar system. Callisto is made up of approximately equal amounts of rock and ice, which makes it the least dense of the Gallilean moons. It has a very thin atmosphere of carbon dioxide and molecular oxygen. The likely presence of a subsurface ocean of liquid water suggests that Callisto can or could harbor life. Callisto is the most likely place for a human base for future exploration since it is farthest from the intense radiation of Jupiter.

Of the four Galilean moons, Io is the one closest to Jupiter and is the fourth largest moon in the solar system. With over 400 active volcanoes, it is the most geologically active object in the solar system. The surface of Io is dotted with more than 100 mountains, some of which are higher than Mount Everest on earth. Most moons (or satellites) in the outer solar system have a thick coating of ice, but Io is made up primarily of silicate rock surrounding a molten iron or iron sulfide core. Io has a thin atmosphere and is bombarded with radiation and magnetic fields from Jupiter.

Europa is the second closet to Jupiter of the four Galilean moons and is the smallest of the four—slightly smaller than earth’s moon. It is one of the smoothest objects in the solar system, with a layer of water surrounding the mantle of the planet. The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water. The smooth appearance of the surface of Europa has led scientists to believe that a water ocean exists beneath it, conceivably serving as an abode for life. The prominent reddish-brown markings that crisscross Europa indicate low topography, meaning that few craters exist because its surface is tectonically active and young. Europa is made up primarily of silicate rock and likely has an iron core. Its tenuous atmosphere is composed primarily of oxygen.

All four of the Galilean moons are bright enough that they could potentially be seen from earth with the naked eye, but the brightness of Jupiter obscures them. They are, however, visible with even low-powered binoculars if the person looking through the binoculars knows where Jupiter is and knows exactly what he or she is looking for.

Copyright © 2012 by Allen Kopp