Chapter 15
You should know the names of the Four Galilean Moons and what order their orbits are
in relative to Jupiter. You should know that Saturn’s one large Moon is titan.
1) What is special about the orbits of Jupiter’s Galilean Moons? The first three moons
have orbital periods that are in the exact ratio 1:2:4. So Ganymede takes twice as long to
orbit the planet as Europa and 4 times as long as Io. This contributes to the tidal heating.
2) Are all the Galilean satellites made of rocky material like Earth’s moon? They all
have rocky material, but Callisto and Ganymede are primarily made of ice.
3) What could account for the differences between the inner and outer Galilean satellites?
The inner satellites are made of mostly rocky, dense materials and the outer satellites
have more low-density materials. This is similar to the formation of the solar system in
the nebular hypothesis. Astronomers think Jupiter and its planets are a miniature version
of the formation of the solar system.
4) Why does Io have active volcanoes? Io is a small moon and should have cooled off by
now, but tidal heating from Jupiter causes it to stay very hot inside and this is why it has
volcanoes. How does Io’s volcanic activity differ from that on Earth? Io’s volcanoes are
more like Earth’s geysers. Io’s volcanoes shoot plumes of sulfur dioxide into the air
rather than lava.
6) What is the evidence that Europa has an ocean beneath its surface? Europa’s surface is
covered with water ice and it has no craters so the surface is relatively new. We also see
ice “rafts” on the surface (fig 15-13) chunks of ice that are randomly distributed and
several kilometers across. These resemble the ice rafts we see floating in the Earth’s
Arctic Ocean in the spring. Also, Europa has a magnetic field indicating some geologic
activity.
7) What is unusual about the magnetic fields of Ganymede and Callisto? They vary
throughout their orbits around Jupiter which means they are induced magnetic fields
8) How is it possible for Saturn’s moon Titan to have an atmosphere? Titan is about as
the same mass as Earth’s moon, but unlike Earth’s moon, Titan can retain an atmosphere.
This is because Titan is much colder than our moon and the slower moving molecules in
Titan’s atmosphere are unable to escape its gravity.
9) Why do some of Jupiter’s Moons orbit in the wrong direction? When an object passes
by a planet in a retrograde direction, it is easier for the planet to capture that object. This
is why Jupiter has so many small moons in retrograde orbit.
10) What kinds of geologic activity are seen on Saturn’s medium-sized satellites? (p.
330) they have a variety of surface features that indicate that had internal heating and an
active geology in the past.
Jupiter and Saturn’s Moons
1)Why can’t the Galilean satellites be seen with the naked eye? The Galilean satellites
are Jupiter’s four largest Moons (discovered by Galileo). They are bright enough to be
seen by the naked eye, but they are so close to Jupiter (as viewed from Earth) that we
can’t see them because they are overshadowed by Jupiter’s brightness.
2)During the time it takes Ganyede to complete one orbit, how any orbits do Io and
Europa complete. Io does 4 orbits and Europa does 2.
3) In what ways does the system of Galilean satellites resemble the solar system? The
most dense satellites are nearest Jupiter just like the most dense planets are nearest the
Sun. In what ways is it different? The outer worlds of Jupiter (Ganymede and Callisto)
are of relatively small mass unlike the outer worlds of the solar system (the Jovians)
which are enormous.
4)How do we know about the chemical composition of the satellites if no spacecraft has
landed on them? We can look at the light they reflect and see an absorption spectrum, we
can look at their densities, we can learn about their interiors from the presence or lack of
a magnetic field.
5)In what ways is the formation of the Jovian satellites similar to the formation of the
planets in the solar system? As with the Sun, Jupiter coalesced from materials gaseous
hydrogen and helium. As with the formation of the planets, the dense rocky materials
formed satellites closest to Jupiter, and the less dense stuff formed farther away. All of
the satellites are in prograde orbits just like the planets.
8)Io has no impact craters, while Earth’s moon is covered with them. Why? Earth’s
moon is internally cold so it has no geologic activity to erase its craters. Io is internally
very active and has volcanoes and lava flows the erase the craters.
15) How was the Galileo spacecraft used to determine the internal structure of Io and the
other satellites? One of the most important things the Galileo spacecraft did was measure
the density of the Galilean satellites. They did this by measuring how the gravity of the
planet deflected the satellite as it went past each satellite.
29) Describe Titan’s atmosphere. Titan has a very thick dense atmosphere composed of
nitrogen, methane, and other hydrocarbons.
Key Words from Chap 15: (forgot to include this in the last chapter review)
Galilean satellites, ice rafts (Europa), occultation, prograde and retrograde orbits, tidal
heating, Roche limit.