Chapter 11

Which planet has the strongest magnetic field, and hence the largest magnetosphere? What is its source?

Jupiter has the largest magnetosphere. The large, rapidly spinning, liquid-metallic hydrogen above the core is the source of its magnetic field.

In the context of the giant planets and the conditions in their interiors, what is meant by "rock" and "ice"?

Scientists describe materials in these environments that are composed primarily of iron, silicon, and oxygen as "rock." Similarly, those composed of carbon, nitrogen, and oxygen in combination with hydrogen are described as "ice." The layers described in this way don't necessarily resemble rocks and ice sheets on Earth.

Why do the upper levels of Neptune's atmosphere appear blue?

The gaseous molecules in Neptune's atmosphere scatter blue light, giving Neptune its color. The same process makes Earth's sky appear blue.

Why is it difficult to drop a probe like Galileo? How did engineers solve this problem?

Aside from having to engineer the craft to survive the magnetosphere of Jupiter, the high speeds involved would guarantee that the probe would burn up on entering the atmosphere. The problem was solved using a heat shield in front of the spacecraft to absorb the heat. After the probe had slowed down, the heat shield was jettisoned and the parachute was deployed, further slowing the probe.

What is the primary source of Jupiter's internal heat?

Most of the internal energy of Jupiter is primordial heat, left over from the formation of the planet 4.5 billon years ago, when it gathered together from smaller "planetesimals" and accreted gas; the gravitational energy lost as these "building blocks" fell together turned into heat.

Which of the gas giants has the largest icy/rocky core compared to its overall size?

From Table 11.3 Basic Properties of the Jovian Planets, we see that Neptune has the largest core, extending out to about 20,000 km from the center of the planet.

What are the main atmospheric heat sources of each of the giant planets?

For Jupiter, Saturn, and Neptune, both sunlight and internal sources provide energy to the atmosphere. Uranus has no or very little internal heat, so it gets its energy from the Sun. Jupiter has the largest internal energy source, about the same as the total solar energy absorbed by Jupiter. Most of the internal energy of Jupiter is primordial heat, left over from the formation of the planet 4.5 billon years ago. Saturn has an internal energy source about half as large as Jupiter's; since its mass is only about one quarter as great, this means that it is producing twice as much energy per unit mass of material as does Jupiter. The source of this energy is the separation of helium from hydrogen in Saturn's interior. In the liquid hydrogen mantle, the heavier helium forms droplets that sink toward the core, releasing gravitational energy. In effect, Saturn is still differentiating—letting lighter material rise and heavier material fall. Uranus and Neptune are different. Neptune has a small internal energy source, while Uranus does not emit a measurable amount of internal heat.

Compare the atmospheric circulation (weather) of the four giant planets.

For both Jupiter and Saturn, convection from their internal heat sources mixes the atmosphere and promotes cloud formation. Their rapid rotation spreads out these cloud features into parallel bands that circle the planets at all latitudes. For Uranus, the circulation is also equatorial and the wind speeds are high. Since there is no convection to mix the gases and create many clouds, Uranus' atmosphere is smeared out and rather featureless. Neptune's wind speeds are much higher than its rotational speed, so the atmosphere smears out into parallel bands like Jupiter and Saturn.

Describe the seasons on the planet Uranus.

From Exercise 5 we know the planet is "on its side." Each pole experiences a 21-year sunlit "summer" and later a 21-year dark "winter." For the other half of the "Uranus year," the equator receives the light of the Sun and each hemisphere experiences what we would call a normal "day." In all, each pole experiences 42 years of light and 42 years of dark during its 84-year-long orbit.

What are the seasons like on Jupiter?

Since Jupiter's spin axis is only tilted about 3° from the perpendicular, it does not experience seasons at all.

What are the visible clouds on the four giant planets composed of, and why are they different from each other?

The clouds of Jupiter and Saturn are primarily crystals of frozen ammonia. On Uranus and Neptune, the clouds are composed of methane. The temperatures of these worlds dictate the cloud composition. For Jupiter and Saturn, the temperatures keep methane in a gaseous state, while on Uranus and Neptune, the colder temperatures allow the methane to freeze and condense into clouds.

How do storms on Jupiter differ from storm systems on Earth?

The cyclonic storm features on Jupiter are regions of high pressure, whereas storms on Earth, such as hurricanes, are low-pressure areas.

Describe the interior heat source of Saturn.

The mantle of Saturn is still differentiating: The heavier helium is sinking, displacing the lighter hydrogen, which then rises. The "falling" of the helium releases gravitational energy, which heats the interior.

At the pressures in Jupiter's interior, describe the physical state of the hydrogen found there.

The pressure is so high that the hydrogen, normally a gas, has been compressed into a liquid-metallic form not seen on Earth.

What are the main challenges involved in sending probes to the giant planets?

There are many challenges, mainly the huge distances that require many years of flight time. The spacecraft must be reliable and robust to survive the journey. The low temperatures of space require onboard heating so the components don't freeze. The light levels are too low for solar panels to provide enough energy, requiring onboard power systems. Transmitters must be powerful enough to communicate with Earth over the vast distances of space. A somewhat more advanced answer might also include: at low light levels, photographs require longer exposures; however, fast-moving spacecraft will change position during these times and smear the image unless the arm with the camera moves backward at the same rate that the spacecraft moves forward.

What is the consequence of Uranus' spin axis being 98° away from perpendicular to its orbital plane?

This essentially means that Uranus is on its side, with the poles alternately facing toward and away from the Sun. Each pole experiences long periods of light and darkness. The seasons alternate between half the planet being in the light and half in darkness for one season, to the planet's axis being sideways to the Sun and its rotation causing regular alternation of light and dark for the next season.

Explain why visual observation of the gas giants is not sufficient to determine their rotation periods, and what evidence was used to deduce the correct periods.

What is seen visually on each of these worlds is the upper atmosphere, with winds and storms that do not necessarily move at the same rate as the planet as a whole. Radio observations revealed the existence of magnetic fields originating in the cores of these planets. The rotation periods are determined from the fact that the magnetic fields rotate at the same velocity as the interiors.