Jupiter's Major Moons Help

By — McGraw-Hill Professional
Updated on Sep 16, 2011

Introduction to the Major Moons of the Outer Planets—Jupiter's Moons

The so-called outer planets are Jupiter, Saturn, Uranus, Neptune, and Pluto (although some people debate whether or not Pluto qualifies as a planet). All these planets have moons, some of which are bigger than Earth’s moon. In this chapter, we’ll look at planetary moons larger than 1000 km (620 mi) in diameter.

Most of the moons of the outer planets always keep the same side facing their parent planets. This is the result of long-term tidal effects and is the same phenomenon as that which has happened to Earth’s moon. Most of the moons of the outer planets orbit near the equators of their parent planets and in the same direction as the planets’ rotations. In the cases of Jupiter, Saturn, and Uranus, the planet-moon systems are thought to have evolved like star systems in miniature.

Now that astronomers have had a chance to closely examine (by means of space probes) the major moons of the outer planets along with the planets themselves, how can anyone not be amazed at the variety of worlds our Solar System has produced? The more we learn about these worlds, the more mysterious they become.

Jupiter’s Major Moons

Jupiter, the largest of the planets, has a system of moons that resembles a miniature “Solar System” with Jupiter as the “Sun.” Four of these moons can be seen through a good pair of binoculars or a small telescope. Galileo Galilei observed them and carefully recorded their behavior in the early 1600s. Their images were starlike points of light that appeared along a straight line passing through Jupiter’s disk. Galileo deduced that these points of light were natural satellites, or moons, because their relative positions changed from night to night in a way consistent with bodies traveling in more or less circular paths around Jupiter. These four little worlds, and only these four— Ganymede, Callisto, Io , and Europa —are called the Galilean moons in memory of Galileo.


This moon, Jupiter’s largest, is 5,270 km (3,270 mi) across, a little less than half the diameter of Earth but larger than Earth’s moon. Compared with Jupiter, Ganymede is tiny (Fig. 10-1). The satellite orbits Jupiter in a nearly perfect circle at a distance of about 1.1 million km (660,000 mi). Like virtually all planetary moons, Ganymede orbits near the equator of its parent planet and revolves in the same direction that the planet rotates. Also like most planetary moons, Ganymede keeps the same face toward Jupiter at all times. At the surface of Ganymede, the gravitational field is only about 15 percent as strong as it is on Earth’s surface. Thus, if you weigh 140 lb on Earth, you would weigh only 21 lb on Ganymede.

Major Moons of the Outer Planets Ganymede

Figure 10-1. The relative sizes of Jupiter’s largest moon Ganymede, the planet Earth, and the planet Jupiter.

Ganymede is believed to consist of a metallic core surrounded by rocky material, in turn surrounded by a crust of rock mixed with ice. The moon’s density is approximately twice that of water. The surface was seen close up by the Voyager probe, and a number of features were observed, including many craters, evidence that Ganymede has been bombarded by meteorites. The shallow nature of the craters suggests that the surface of Ganymede is largely made of water ice. At the extreme cold temperatures at Jupiter’s distance from the Sun, ice behaves something like rock on Earth’s surface. However, over time the ice flows and settles so that the mountains and rims of the craters flatten out gradually. Nevertheless, this ice maintains crater imprints for millions of years.

Ganymede, like most of the other moons of the outer planets, has little or no atmosphere, although Ganymede has enough of a gravitational pull to hold down trace amounts of oxygen and other gases. The presence of water (in the form of ice) and oxygen in the environment of Ganymede does not imply that this moon bears life; temperatures are too cold and conditions far too tranquil for anything biological to evolve the way it has on our planet Earth. One of the ingredients for the development of life as we know it, interestingly enough, is an environment subject to change.

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