Chapter 8Moons, Rings, and Plutoids

Units of Chapter 8

The Galilean Moons of Jupiter

The Large Moons of Saturn and Neptune

The Medium-Sized Jovian Moons

Planetary Rings

Beyond Neptune

Plutoids and the Kuiper Belt

Summary of Chapter 8

8.1 The Galilean Moons of JupiterAll four Jovian planets have extensive moon systems, and more are continually being discovered.

The Galilean moons of Jupiter are those observed by Galileo: Io, Europa, Ganymede, and Callisto.

This image shows Jupiter with two of its Galilean moons.

The Galilean moons and their orbits

Their interiors

Io is the densest of Jupiter’s moons, and the most geologically active object in the solar system.

• It has many active volcanoes, some quite large.

• Io can change surface features in a few weeks.

• Io has no craters; they fill in too fast – Io has the youngest surface of any solar system object.

Io is very close to Jupiter, and also experiences gravitational forces from Europa. The tidal forces are huge, and provide the energy for the volcanoes.

Europa has no craters; surface is water ice, possibly with liquid water below.

Tidal forces stress and crack ice; water flows, keeping surface relatively flat.

Ganymede is the largest moon in the solar system – larger than Pluto and Mercury.

It has a history similar to Earth’s Moon, but with water ice instead of lunar rock.

Callisto is similar to Ganymede; no evidence of plate activity.

8.2 The Large Moons of Saturn and Neptune

Titan has been known for many years to have an atmosphere thicker and denser than Earth’s; mostly nitrogen and argon.

Makes surface impossible to see; the picture at right was taken from only 4000 km away.

Infrared image of Titan, showing detail, and possible icy volcano

Few craters, consistent with active surfaceComplex chemical interactions in atmosphere

The Huygens lander took these images of the surface of Titan.

Trace chemicals in Titan’s atmosphere make it chemically complex.

Triton is in a retrograde orbit; its surface has few craters, indicating an active surface.

Nitrogen geysers have been observed on Triton, contributing to the surface features.

8.3 The Medium-Sized Jovian Moons

Densities of these moons suggest that they are rock and water ice.

Moons of Saturn, in natural color

Moons of Uranus and Neptune

Miranda shows evidence of a violent past, although the origin of the surface features is unknown.

The only body in the solar system known to have active volcanos other than the Earth is:A titan.B callisto.C Io.D triton. 

The Galilean satellite with the smoothest surface is

A. IoB. EuropaC. GanymedeD. Callisto 

The largest satellite (moon) in our planetary system is

A. PhobosB. TitanC. GanymedeD. The Moon 

Io appears more active than the other Galilean satellites of Jupiter. this activity may arise from

A. stronger heating of Io due to tidal friction.

B. the radiation close to Jupiter.C. Io's smaller density.D. Jupiter's magnetic field. 

Which solar system body is believed to have deep oceans of liquid water under a surface of solid ice?

A. IoB. TitanC. TritonD. Europa 

The surfaces of some moons of Jupiter and Saturn are heavily cratered. we infer that these surface regions are

A. regions of volcanic activity.B. relatively old.C. relatively young.D. highland regionsE. .lowland regions. 

When a celestial body (like a moon) comes within the roche limit of a large body, it

A. will stop rotating.B. will break apart because of tidal

forces.C. will fall into the larger body.D. will be accelerated and ejected.

Which of the moons of the giant planets is known to have substantial atmosphere?

A. CallistoB. TitanC. EuropaD. Phobos 

8.4 Planetary Rings

The ring system of Saturn is large and complex, and easily seen from Earth. The other Jovian planets have ring systems as well.

The rings are not solid; they are composed of small rocky and icy particles.

Our view of Saturn’s rings changes as the planets move in their orbits.

The Roche limit is where the tidal forces of the planet are too strong for a moon to survive; this is where rings are formed.

All observed ring systems are within this limit.

Voyager probes showed Saturn’s rings to be much more complex than originally thought.

Earth is shown on the same scale as the rings.

“Shepherd” moons define the edges of some of the rings.

Jupiter has been found to have a small, thin ring.

The rings of Uranus are more complex.

Two shepherd moons keep the epsilon ring from diffusing.

Neptune has five rings, three narrow and two wide.

8.5 Beyond NeptunePluto was discovered in 1930. It was thought to be needed to explain irregularities in the orbits of Uranus and Neptune, but it turned out that there were no such irregularities.

Pluto’s moon, Charon, was discovered in 1978.

It is orbitally locked to Pluto, and about a sixth as large.

Pluto also has two smaller moons, Nix and Hydra, discovered in 2005.

Charon’s orbit is at a large angle to the plane of Pluto’s orbit.

8.6 Plutoids and the Kuiper Belt

The first Kuiper belt objects were observed in the 1990s, and more than 700 are now known. Some of them are comparable in size to Pluto.

These images show Eris and its moon Dysnomia.

This figure shows several of the largest known trans-Neptunian objects, now collectively called plutoids.

The major gaps in the rings of Saturn are most likely caused by

A. mutual gravitational interactions between the multitude of particles in the rings.

B. the intervention of a massive body, which moved through the rings in their early history, leaving the gaps.

C. combined gravitational forces from Saturn and its moons, which deviated the paths of particles which stray into the gaps.

D. the rings were formed in that way, with the gaps in place.

The rings of Saturn

A. do not contain enough matter to form a satellite.

B. are too close to the planet to form a satellite.

C. were not allowed to condense into a satellite because of the intense radiation.

D. were stripped from the planet because of its rapid rotation. 

Which of the following characteristics do the rings of Jupiter, Saturn, and Uranus have in common?

A. Small particles orbiting the planet.B. Solid sheets of icy materials.C. Made of dark, uncoated rocky

particles.D. Made of light-colored, metallic

particles.E. Are unstable over short periods of

time. 

The ringlets within the ring system of Saturn are thought to be maintained by

A. tidal forces from Saturn.B. solar radiation.C. shepherd satellites.D. internal collisions. 

Summary of Chapter 8

• Outer solar system has 6 large moons, 12 medium ones, and many smaller ones.

• Titan has a thick atmosphere and may have flowing rivers of methane.

• Triton has a fractured surface and a retrograde orbit.

• Medium-sized moons of Saturn and Uranus are mostly rock and water ice.

• Saturn’s rings are complex, and some are defined by shepherd moons.

Summary of Chapter 8, cont.

• The Roche limit is the closest a moon can survive near a planet; inside this limit rings form instead.

• Jupiter, Uranus, and Neptune all have faint ring systems.

• Pluto has three moons, Charon, Nix, and Hydra.

• Dwarf planets beyond Neptune (including Pluto) are now known as plutoids.