13-1 Jupiter’s Galilean satellites are easily seen with Earth-based telescopes

When Galileo trained his telescope on Jupiter in January 1610, he became the first person to observe satellites, or moons, orbiting another planet (see Section 4-5). He called them the Medicean stars, in order to curry the favor of a wealthy Florentine patron of the arts and sciences. We now call them the Galilean satellites. Individually, these moons are named Io, Europa, Ganymede, and Callisto, after four mythical lovers and companions of the god whom the Greeks called Zeus and the Romans called Jupiter.

When viewed through an Earth-based telescope, the Galilean satellites look like pinpoints of light (see Figure 4-16). All four satellites orbit Jupiter in nearly the same plane as the planet’s equator (Figure 13-1). From Earth, we always see that plane nearly edge-on, so the satellites appear to move back and forth relative to Jupiter (see Figure 4-17). The orbital periods are fairly short, ranging from 1.8 (Earth) days for Io to 16.7 days for Callisto, so we can easily see the satellites move from one night to the next and even during a single night. These motions led Galileo to realize that he was seeing objects orbiting around Jupiter, in much the same way that Copernicus said that planets move around the Sun (see Section 4-2).

Figure 13-1: The Orbits of the Galilean Satellites This illustration shows the orbits of the four Galilean satellites as seen from Earth. All four orbits lie in nearly the same plane as Jupiter’s equator. The apparent angular size and orientation of the orbits depends on the relative positions of Earth and Jupiter.

Each of the four Galilean satellites is bright enough to be visible to the naked eye. Why, then, did Galileo need a telescope to discover them? The reason is that as seen from Earth, the angular separation between Jupiter and the satellites is quite small—never more than 10 arcminutes for Callisto and even less for the other three Galilean satellites. To the naked eye, these satellites are lost in the overwhelming glare of Jupiter. But a small telescope or even binoculars increases the apparent angular separation by enough to make the Galilean satellites visible.

Synchronous Rotation and Orbital Resonance

The brightness of each satellite varies slightly as it orbits Jupiter, because the satellites also spin on their axes as they orbit Jupiter, and dark and light areas on their surfaces are alternately exposed to and hidden from our view. Remarkably, each Galilean satellite goes through one complete cycle of brightness during one orbital period. This observation tells us that each satellite is in synchronous rotation, so that it rotates exactly once on its axis during each trip around its orbit (see Figure 3-4b). As we saw in Section 4-8, our Moon’s synchronous rotation is the result of gravitational forces exerted on the Moon by Earth. Likewise, Jupiter’s gravitational forces keep the Galilean satellites in synchronous rotation.

Much more important, however, are the effects of an orbital resonance. Recall that an orbital resonance occurs when bodies exert a regular, repeating gravitational influence on each other. As we will see in this chapter, heat generated by this effect has a profound influence on Io and Europa.

Synchronous rotation means that the rotation period and orbital period are in a 1-to-1 ratio for each Galilean satellite. Remarkably, there is also a simple ratio of the different orbital periods of the three inner Galilean satellites, Io, Europa, and Ganymede, which leads to an orbital resonance. Io orbits Jupiter in 1.77 days, while Europa has twice the orbital period of Io, and Ganymede has 4 times the orbital period of Io. Thus, the orbital periods of Io, Europa, and Ganymede are in the ratio 1:2:4, which you can verify from the data in Table 13-1. This is an example of a stabilizing orbital resonance. (In Section 12-11, we saw that a destabilizing resonance cleared out the Cassini division.)

This orbital resonance—a special relationship among the satellites’ orbits—is maintained by the gravitational forces that they exert on one another. Those forces can be quite strong, because the three inner Galilean satellites pass relatively close to one another; at their closest approach, Io and Europa are separated by only two-thirds the distance from Earth to the Moon. Such a close approach occurs once for every two of Io’s orbits, so Europa’s gravitational pull acts on Io in a rhythmic way. Indeed, Io, Europa, and Ganymede all exert rhythmic gravitational tugs on one another. Just as a drummer’s rhythm keeps musicians on the same beat, this gravitational rhythm maintains the simple ratio of orbital periods among the satellites. As we will see, this “rhythmic drumming” provides a significant source of energy that heats the interiors of Io and Europa.

By contrast, Callisto orbits at a relatively large distance from the other three large satellites. Hence, the gravitational forces on Callisto from the other satellites are relatively weak, and there is no simple relationship between Callisto’s orbital period and the period of Io, Europa, or Ganymede.

Transits, Eclipses, and Occultations

Because the orbital planes of the Galilean satellites are nearly edge-on to our line of sight, we see these satellites undergoing transits, eclipses, and occultations. In a transit, a satellite passes between us and Jupiter, and we see the satellite’s shadow as a black dot against the planet’s colorful cloudtops. (Figure 12-2a shows Europa’s shadow on Jupiter.) In an eclipse, one of the satellites disappears and then reappears as it passes into and out of Jupiter’s enormous shadow. In an occultation, a satellite passes completely behind Jupiter, and the satellite is temporarily blocked from our Earth-based view. (The word occultation comes from a Latin verb meaning “to cover.”)

Before spacecraft first ventured to Jupiter, astronomers used eclipses in a very clever way to estimate the diameters of the Galilean satellites. When a satellite emerges from Jupiter’s shadow, it does not blink on instantly. Instead, there is a brief interval during which the satellite gets progressively brighter as more of its surface is exposed to sunlight. The satellite’s diameter is calculated from the duration of this interval and the satellite’s orbital speed (which is known from Kepler’s laws).

As Table 13-1 shows, the smallest Galilean satellite (Europa) is slightly smaller than our Moon; the largest satellite (Ganymede) is larger than Mercury and more than three-quarters the size of Mars. Thus, the Galilean satellites truly are worlds in their own right.

TABLE 13-1 JUPITER’S GALILEAN SATELLITES COMPARED WITH THE MOON, MERCURY, AND MARS