Observing Project 10B The Ballet of the Galilean Satellites

The discovery of Jupiter's four bright satellites was one of the most important in the history of science because it proved to be the deathblow to the idea that Earth was the center of the universe and that everything else circled it. The finding of four bodies circling Jupiter proved the Copernican theory that the Sun was as the center of the solar system. For daring to oppose the Church in his beliefs, Galileo was humiliated, disgraced, imprisoned and broken.

Though the moons are bright and will stand up to high power, there really is in fact very little to be viewed in amateur telescopes. The surface features of the moons are far too tiny to be viewed with the human eye from Earth at any magnification. Use a low to medium power to study the moons, perhaps a little more magnification to split apart two moons that are very close together.

The innermost moon is Io and it shines at about magnitude +6. Io is slightly larger than Earth's moon at 3,630 kilometers in diameter and zips around Jupiter in just under two days at a distance of 420,000 kilometers. Europa is slightly smaller than Earth's moon at 3,138 kilometers and circles Jupiter each four days at a distance of 670,900 kilometers. Europa also shines at magnitude +6. The two outer moons are larger and slightly brighter. Ganymede is the third moon and circles





Figure 10.3. Jupiter's moons on Sept. 25, 2005. Graphic created by author with Redshift 4.

Jupiter each eight days at a distance of 1,070,000 kilometers. Ganymede shines at magnitude +5 and at 5,262 kilometers is the largest satellite body in the solar system. Callisto is the fourth moon and circles Jupiter every fourteen days at a distance of 1,883,000 kilometers from Jupiter. Callisto also shines at magnitude +5 and measures slightly smaller than Ganymede at 4,800 kilometers.

As the moons zip around the planet, you might note that the satellites do not move in random patterns from day to day. Watch the movement of Io and Europa relative to each other over one revolution of Europa. Where is Io relative to Europa when Europa is at greatest elongation? Now watch Europa make a full revolution around the planet. At the instant Europa returns to greatest elongation, where is Io? You will find Io in about the same place it was four days ago, except that Io has made two revolutions around the planet while Europa made one. This is not a coincidence. Europa and Io orbit in resonance with each other. The organizing effects of the gravity of both Jupiter and the satellites themselves cause the resonance. Then try including Ganymede in your experiment over one revolution of Ganymede. Where are Europa and Io compared to where they were when you first marked Ganymede at greatest elongation from Jupiter? They are both in the same place as they were eight days prior, except that Europa has gone around the planet twice and Io four times while Ganymede has made one revolution. All three moons

Figure 10.4. Jupiter's moons on Oct. 2, 2005. Graphic created by author with Redshift 4.

are part of this resonance. Europa's orbital period is almost exactly twice as long as that of Io and Ganymede's is twice that of Europa. The gravitational and tidal interaction you are watching has important effects on the moons as well. Io's interior is heated as a result of tidal interactions with Europa and Ganymede as well as Jupiter. These tides stretch Io by as much as 100 meters during an orbit. Can you imagine tides 330 feet high? The flexing of the moon is responsible for the volcanoes that wrack Io's surface. Europa may have an enormous subsurface ocean of liquid water, kept from freezing by tidal stresses. Ganymede shows signs of tectonic activity, folded mountains and dynamic surface activity.

Now let's try Callisto. Mark the positions of the three inner moons when Callisto is at greatest elongation and see what happens when Callisto returns to greatest elongation fourteen days later. The moons are in different positions. Callisto is not part of this resonance among the Galilean moons and Callisto is a very different moon. No tectonics, no warmth, no volcanoes. Callisto's surface is the oldest in the solar system, virtually unchanged by any force except for the occasional impact for some four billion years. Callisto does not even exhibit any evidence of internal structure at all with no clearly definable mantle or core. But Jupiter's gravitational organizing power is relentless and eventually Callisto will drift out away from Jupiter far enough that it will fall into resonance with the other four moons.

The eternal dance of Jupiter's moons gives us joy to watch their beautiful changing patterns from night to night. But beauty is not always random accident. The resonance of the inner three Galilean satellites is a large part of what makes Io, Europa and Ganymede as dynamic as they are while Callisto is old, dull and monolithic.

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