Chapter 11: Rings, Moons, and Pluto

11.1 Ring and Satellite Systems

General Properties

The terms satellite and moon are used interchangeably in astronomy. I'll try to use the word satellite instead of moon to avoid confusion with the Earth's Moon.

A satellite is an object, usually large, that orbits a planet.

All the giant planets have satellites. In fact, all the planets except Mercury and Venus have satellites. Appendix 8 has a complete listing of the known (as of 2000) satellites.

We classify the satellites as regular or irregular, depending on their orbits. Regular satellites orbit their planet in the same sense the planets orbit the Sun and roughly in the same plane. Irregular satellites are in retrograde orbits, highly elliptic orbits, or orbits far from the equatorial plane. We differentiate between the two classes because the irregular satellites likely formed elsewhere and were captured by the planet. Regular satellites most likely formed with their planet.


The Earth has one natural satellite, the Moon, and many small man-made satellites.


Mars has two satellites, Phobos and Deimos (Figures 12.7 and 12.8). Both are small, about 10 to 20 km in diameter, and are probably captured asteroids. The Moon is 3476 km in diameter for comparison.


Recall that when Galileo looked at Jupiter with his telescope, he saw four satellites circling the planet, from which he drew the shocking (at the time) conclusion that the Earth is not the center of the solar system. The four largest, Callisto, Ganymede, Europa, and Io, are known as the Galilean Satellites. In total, Jupiter has 16 (known) satellites and a faint ring.


Saturn has 19 (known) satellites and the most spectacular ring system of any planet. The largest of Saturn's satellites is Titan.


Uranus has 18 satellites and a system of rings. All the satellites and rings are tilted at 98°, just like the planet.


Neptune has eight satellites and narrow and faint rings.

11.2 The Galilean Satellites and Titan

The large gravitational force of Jupiter exerts significant tidal forces on its satellites. The closer the satellite to Jupiter, the more significant the tidal forces are.

Callisto, an Ancient Primitive World

Of the Galilean satellites, Callisto is the farthest from Jupiter, and therefore experiences the least tidal forces. It is composed mostly of ices (think water, ammonia, methane, carbon dioxide), and is heavily cratered.

Ganymede, the Largest Satellite

Ganymede is the next closest Galilean satellite, and the largest satellite in the solar system. From table 11.1, what is surprising is not the size of Ganymede, but that Earth's Moon is so large (about half the mass of Ganymede and 2/3 the diameter). Why should the Earth have such a relatively large satellite?

Ganymede is much less cratered than Callisto. It has a core of rock and metal with an icy crust above. It is believed that Ganymede is heated by tidal forces, explaining the younger more active surface.

Europa, the Satellite with an Ocean

Moving in closer we come to Europa. Now the heating from tidal forces becomes rather apparent. Europa seems to have liquid water in its interior, with ice floating above. The surface of Europa has features that look like ice floes in the arctic, believed to result from melting and refreezing of the surface ice.

Io, a Volcanic Satellite

The closest Galilean satellite is Io. Tidal forces have heated Io significantly, giving it the most active surface in the solar system. The volcanoes on Io are frequent and large. The Galileo spacecraft photographed several eruptions with clearly visible plumes rising hundreds of kilometers from the surface.

Titan, a Satellite with Atmosphere

Titan was discovered by Huygens after Galileo observed 4 satellites orbiting Jupiter. Titan orbits Saturn. Titan has a thick atmosphere with a complex chemistry. The atmosphere blocks our view of the surface. Scientists find the mixture of compounds detected in Titan's atmosphere interesting, since the chemistry here could resemble the chemistry on Earth early in its history.

11.3 Triton and Pluto

Triton and Its Volcanos

Triton is one of Neptune's satellites, not to be confused with Titan. I will skip the section on Triton.

The Discovery of Pluto

Pluto is the only planet not yet visited by spacecraft. NASA is discussing a mission to Pluto to be launched this decade, arriving at Pluto in the next decade.

Pluto was discovered in 1930 by Clyde Tombaugh. Percival Lowell (canals on Mars) searched for a planet he believed responsible for perturbations to the orbit of Uranus not accounted for by Neptune. His calculations predicted two possible locations for this ninth planet, and he searched for it until his death in 1916. in 1929, his Arizona observatory got a telescope with a photographic mount. In 1930 Clyde Tombaugh spotted Pluto by "blinking" photographs taken about a week apart. In "blinking", the two photos are alternated in the eye piece. The fixed stars don't change position in the two photos, but a nearer object, like a planet, appears to wiggle back and forth.

Pluto is too small to be Lowell's searched for planet. We now know that Lowell's calculations were incorrect, and that there is no other large planet beyond the orbit of Neptune. Yet his connection with Pluto will last forever -- the first two letters in Pluto are his initials. Some astronomers argue that Pluto is not a planet, but just a large object of the Kuiper belt (another large object was recently discovered). This story of Pluto's discovery may sway your opinion on this issue as well.

Pluto's Motion and Satellite

Pluto's orbit has the largest angle to the ecliptic (17°) and eccentricity of any planet. Pluto is actually closer to the Sun than Neptune for a good part of its orbit. Pluto takes almost 250 years to orbit the Sun. In the 72 years since its discovery, Pluto has traveled less than a third of the way around its orbit.

Pluto's satellite was discovered in 1978. Called Charon, it is more than 50% of the size of Pluto, the largest relative size of any satellite.

Pluto's axis is tilted by about 90°, like Uranus'. Charon's orbit is also tilted by the same amount. Additionally, Charon rotates with the same period as it revolves around Pluto, and both are the same as Pluto's period of rotation (day). Therefore, the same side of Charon always faces Pluto, and from the surface of Pluto, Charon is stationary. From half of Pluto Charon is always visible, fixed in the same location in the sky, and from the other half of Pluto Charon is never seen!

The Nature of Pluto


The Origin of Pluto

Skip. To briefly summarize, the leading theory is that Pluto originated in the Kuiper belt.

11.4 Planetary Rings

All of the giant planets have rings, Saturn is simply the best known and most impressive example.

What Causes Rings?

Rings are composed of small particles that, in the first approximation, each orbit the planet according to Kepler's laws. The ring does not rotate as a single object. Each of the particles moves independently, and according to Kepler's laws, the particles closer to the planet rotate faster than those further away.

The independent motions lead to more complex interactions than allowed in the first approximation (Kepler's laws). The particles in the ring exert gravitational forces on each other, and rub and bump into each other. Density waves have been seen in the rings -- similar to sound waves in air, but you might get a better picture by thinking of waves on the surface of calm water.

There are two theories for the origin of the rings. The first is that the rings are the remnants of a satellite that orbited so close to the planet that it was torn apart by tidal forces. Calculations show that all the rings exist within the region where this could occur (see Figure 11.17).

The second theory is basically the first in reverse, the rings are fragments that were unable to coalesce into a satellite.

Rings of Saturn

See pictures.

Rings of Uranus and Neptune

See pictures.

Satellite-Ring Interactions

The F ring of Saturn is interesting because it seems to be kept from spreading by interactions with two small shepherding satellites called Pandora and Prometheus. (Figure 11.22 shows the ring and the shepherd satellites.) This is an active are of study where computer modeling is used to understand how the multitude of interactions between particles works to keep the rings stable.

You are in charge of sending an unmanned space probe to a galilean satellite. Which satellite would you visit and what would be the goals of the mission?

© Robert Harr 2001