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Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

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Jupiter and Saturn

Chapter 23

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As we begin this chapter, we leave behind the psychological security of planetary surfaces. We can imagine standing on the moon, on Venus, or on Mars, but Jupiter and Saturn have no surfaces. Thus, we face a new challenge—to use comparative planetology to study worlds so unearthly we cannot imagine being there.

One reason we find the moon and Mars of interest is that we might go there someday. Humans may become the first Martians. But the outer solar system seems much less useful, and that gives us a chance to think about the cultural value of science.

This chapter begins our journey into the outer solar system. In the next chapter, we will visit worlds out in the twilight at the edge of the sun’s family.

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I. Jupiter

A. Surveying Jupiter

B. Jupiter's Magnetic Fields

C. Jupiter's Atmosphere

D. Jupiter's Ring

E. Comet Impact on Jupiter

F. The History of Jupiter

II. Jupiter's Family of Moons

A. Callisto: The Ancient Face

B. Ganymede: A Hidden Past

C. Europa: A Hidden Ocean

D. Io: Bursting Energy

E. The History of the Galilean Moons

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Outline (continued)

III. Saturn

A. Planet Saturn

B. Saturn's Rings

C. The History of Saturn

IV. Saturn's Moons

A. Titan

B. The Smaller Moons

C. The Origin of Saturn's Satellites

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Largest and most massive planet in the solar system:

Contains almost 3/4 of all planetary matter in the solar system.

Most striking features visible from Earth: Multi-colored cloud belts

Explored in detail by several space probes:

Visual image

Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo

Infrared false-color image

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The Mass of Jupiter

Mass can be inferred from the orbit of Io, the innermost of the 4 Galilean Moons:





Relative sizes, distances, and times to scale

1 s corresponds to 10 hr in real time.

Using Kepler’s third law MJupiter = 318 MEarth

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Jupiter’s Interior

From radius and mass Average density of Jupiter ≈ 1.34 g/cm3

=> Jupiter can not be made mostly of rock, like earthlike planets.

Jupiter consists mostly of hydrogen and helium.

T ~ 30,000 K

Due to the high pressure, hydrogen is compressed into a liquid, and even metallic state.

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The Chemical Composition of Jupiter and Saturn

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Jupiter’s Rotation

Jupiter is the most rapidly rotating planet in the solar system:

Rotation period slightly less than 10 hr.

Centrifugal forces stretch Jupiter into a markedly oblate shape.

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Jupiter’s Magnetic Field

Discovered through observations of decimeter (radio) radiation

Magnetic field at least 10 times stronger than Earth’s magnetic field.

Magnetosphere over 100 times larger than Earth’s.

Extremely intense radiation belts:

Very high energy particles can be trapped; radiation doses corresponding to ~ 100 times lethal doses for humans!

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Aurorae on Jupiter

Just like on Earth, Jupiter’s magnetosphere produces aurorae concentrated in rings around the magnetic poles.

~ 1000 times more powerful than aurorae on Earth.

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Explorable Jupiter


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The Io Plasma Torus

Some of the heavier ions originate from Jupiter’s moon Io.

Io flux tube

Io flux tube



Inclination of Jupiter’s magnetic field against rotation axis leads to wobbling field structure passing over Io Acceleration of particles to high energies.

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Jupiter’s Atmosphere

Jupiter’s liquid hydrogen ocean has no surface:

Gradual transition from gaseous to liquid phases as temperature and pressure combine to exceed the critical point.

Jupiter shows limb darkening hydrogen atmosphere above cloud layers.

Only very thin atmosphere above cloud layers;

transition to liquid hydrogen zone ~ 1000 km below clouds.

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Jupiter’s Atmosphere (2): Clouds

Three layers of clouds:

1. Ammonia (NH3) crystals

2. Ammonia hydrosulfide

3. Water crystals

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Planetary Atmospheres


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The Cloud Belts on Jupiter

Dark belts and bright zones.

Zones higher and cooler than belts; high-pressure regions of rising gas.

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The Cloud Belts on Jupiter (2)

Just like on Earth, high-and low-pressure zones are bounded by high-pressure winds.

Jupiter’s Cloud belt structure has remained unchanged since humans began mapping them.

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The Great Red Spot

Several bright and dark spots mixed in with cloud structure.

Largest and most prominent: The Great Red Spot.

Has been visible for over 330 years.

Formed by rising gas carrying heat from below the clouds, creating a vast, rotating storm.

~ 2 DEarth

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The Great Red Spot (2)

Structure of Great Red Spot may be determined by circulation patterns in the liquid interior

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Jupiter’s Ring

Not only Saturn, but all four gas giants have rings.

Galileo spacecraft image of Jupiter’s ring, illuminated from behind

Jupiter’s ring: dark and reddish; only discovered by Voyager 1 spacecraft.

Composed of microscopic particles of rocky material

Location: Inside Roche limit, where larger bodies (moons) would be destroyed by tidal forces.

Ring material can’t be old because radiation pressure and Jupiter’s magnetic field force dust particles to spiral down into the planet.

Rings must be constantly re-supplied with new dust.

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Roche Limit


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Comet Impact on Jupiter

Impact of 21 fragments of comet SL-9 in 1994

Impacts occurred just behind the horizon as seen from Earth, but came into view about 15 min. later.

Impact sites appeared very bright in the infrared.

Visual: Impacts seen for many days as dark spots

Impacts released energies equivalent to a few megatons of TNT (Hiroshima bomb: ~ 0.15 megaton)!

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The History of Jupiter

  • Formed from cold gas in the outer solar nebula, where ices were able to condense.

  • In the interior, hydrogen becomes metallic (very good electrical conductor)

  • Rapid rotation strong magnetic field

  • Rapid growth

  • Soon able to trap gas directly through gravity

  • Rapid rotation and large size belt-zone cloud pattern

  • Heavy materials sink to the center

  • Dust from meteorite impacts onto inner moons trapped to form ring

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Jupiter’s Family of Moons

Over two dozen moons known now; new ones are still being discovered.

Four largest moons already discovered by Galileo: The Galilean moons





Interesting and diverse individual geologies.

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Callisto: The Ancient Face

Tidally locked to Jupiter, like all of Jupiter’s moons.

Av. density: 1.79 g/cm3

composition: mixture of ice and rocks

Dark surface, heavily pocked with craters.

No metallic core: Callisto never differentiated to form core and mantle.

No magnetic field.

Layer of liquid water, ~ 10 km thick, ~ 100 km below surface, probably heated by radioactive decay.

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Ganymede: A Hidden Past

Largest of the 4 Galilean moons.

  • Av. density = 1.9 g/cm3

  • Rocky core

  • Ice-rich mantle

  • Crust of ice

1/3 of surface old, dark, cratered;

rest: bright, young, grooved terrain

Bright terrain probably formed through flooding when surface broke

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Jupiter’s Influence on its Moons

Presence of Jupiter has at least two effects on geology of its moons:

2. Focusing of meteoroids, exposing nearby satellites to more impacts than those further out.

1. Tidal effects: possible source of heat for interior of Gany-mede

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Europa: A Hidden Ocean

Av. density: 3 g/cm3

composition: mostly rock and metal; icy surface.

Close to Jupiter should be hit by many meteoroid impacts; but few craters visible.

Active surface; impact craters rapidly erased.

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The Surface of Europa

Cracked surface and high albedo (reflectivity) provide further evidence for geological activity.

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The Interior of Europa

Europa is too small to retain its internal heat Heating mostly from tidal interaction with Jupiter.

Core not molten No magnetic field.

Europa has a liquid water ocean ~ 15 km below the icy surface.

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Io: Bursting Energy

Most active of all Galilean moons; no impact craters visible at all.

Over 100 active volcanoes!

Activity powered by tidal interactions with Jupiter.

Av. density = 3.55 g/cm3Interior is mostly rock.

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Interaction with Jupiter’s Magnetosphere

Io’s volcanoes blow out sulfur-rich gasses

tenuous atmosphere, but gasses can not be retained by Io’s gravity

gasses escape from Io and form an ion torus in Jupiter’s magnetosphere

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The History of the Galilean Moons

  • Minor moons are probably captured asteroids

  • Galilean moons probably formed together with Jupiter.

  • Densities decreasing outward Probably formed in a disk around Jupiter, similar to planets around the sun.

Earliest generation of moons around Jupiter may have been lost and spiraled into Jupiter;

Galilean moons are probably a second generation of moons.

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Mass: ~ 1/3 of mass of Jupiter

Radius: ~ 16 % smaller than Jupiter

Av. density: 0.69 g/cm3Would float in water!

Rotates about as fast as Jupiter, but is twice as oblate No large core of heavy elements.

Mostly hydrogen and helium; liquid hydrogen core.

Saturn radiates ~ 1.8 times the energy received from the sun.

Probably heated by liquid helium droplets falling towards center.

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Saturn’s Magnetosphere

Magnetic field ~ 20 times weaker than Jupiter’s

weaker radiation belts

Magnetic field not inclined against rotation axis

Aurorae centered around poles of rotation

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Saturn’s Atmosphere

Cloud-belt structure, formed through the same processes as on Jupiter,

but not as distinct as on Jupiter; colder than on Jupiter.

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Saturn’s Atmosphere (2)

Three-layered cloud structure, just like on Jupiter

Main difference to Jupiter:

Fewer wind zones, but much stronger winds than on Jupiter:

Winds up to ~ 500 m/s near the equator!

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Planetary Atmospheres


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Saturn’s Rings

A Ring

Ring consists of 3 main segments: A, B, and C Ring

B Ring

C Ring

separated by empty regions: divisions

Cassini Division

Rings can’t have been formed together with Saturn because material would have been blown away by particle stream from hot Saturn at time of formation.

Rings must be replenished by fragments of passing comets & meteoroids.

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Composition of Saturn’s Rings

Rings are composed of ice particles

moving at large velocities around Saturn, but small relative velocities (all moving in the same direction).

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Shepherd Moons

Some moons on orbits close to the rings focus the ring material, keeping the rings confined.

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Divisions and Resonances

Moons do not only serve as “Shepherds”.

Where the orbital period of a moon is a small-number fractional multiple (e.g., 2:3) of the orbital period of material in the disk (“resonance”), the material is cleared out


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Electromagnetic Phenomena in Saturn’s Rings

Radial spokes in the rings rotate with the rotation period of Saturn:

Magnetized ring particles lifted out of the ring plane and rotating along with the magnetic-field structure

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  • About the size of Jupiter’s moon Ganymede.

  • Rocky core, but also large amount of ice.

  • Thick atmosphere, hiding the surface from direct view.

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Titan’s Atmosphere

Because of the thick, hazy atmosphere, surface features are only visible in infrared images.

Many of the organic compounds in Titan’s atmosphere may have been precursors of life on Earth.

Surface pressure: 50% greater than air pressure on Earth

Surface temperature: 94 K (-290 oF)

methane and ethane are liquid!

Methane is gradually converted to ethane in the Atmosphere

Methane must be constantly replenished, probably through breakdown of ammonia (NH3).

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Saturn’s Smaller Moons

Saturn’s smaller moons formed of rock and ice; heavily cratered and appear geologically dead.


Heavily cratered; marked by 3 km deep, 1500 km long crack.


Leading (upper right) side darker than rest of surface because of dark deposits.


Possibly active; regions with fewer craters, containing parallel grooves, possibly filled with frozen water.

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Saturn’s Smaller Moons (2)

Hyperion: Too small to pull itself into spherical shape.

All other known moons are large enough to attain a spherical shape.

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The Origin of Saturn’s Satellites

  • No evidence of common origin, as for Jupiter’s moons.

  • Probably captured icy planetesimals.

  • Moons interact gravitationally, mutually affecting each other’s orbits.

  • Co-orbital moons (orbits separated by only 100 km) periodically exchange orbits!

  • Small moons are also trapped in Lagrange points of larger moons Dione and Tethys.

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Coorbital Moons


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New Terms


liquid metallic hydrogen

decameter radiation

decimeter radiation

current sheet

Io plasma torus

Io flux tube

critical point



forward scattering

Roche limit

gossamer rings

grooved terrain

tidal heating

shepherd satellite


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Discussion Questions

1. Some astronomers argue that Jupiter and Saturn are unusual, while other astronomers argue that all solar systems should contain one or two such giant planets. What do you think? Support your argument with evidence.

2. Why don’t the terrestrial planets have rings?

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Quiz Questions

1. Jupiter’s mass is approximately 0.001 solar masses. How is the mass of Jupiter determined?

a. We use Newton’s form of Kepler’s third law.

b. We use the period and semimajor axis of Jupiter’s orbit around the Sun.

c. We can use the period and semimajor axis of Callisto’s orbit around Jupiter.

d. Both a and b above.

e. Both a and c above.

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Quiz Questions

2. What evidence do we have that Jupiter is primarily composed of hydrogen and helium rather than rock?

a. Jupiter has hydrogen and helium lines in its spectrum.

b. The density of Jupiter is 1.3 grams per cubic centimeter.

c. Jupiter’s equatorial diameter is about 6% larger than its polar diameter.

d. Both a and b above.

e. All of the above

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Quiz Questions

3. What energy source drives the weather that we see on Jupiter?

a. Thermal energy escaping from Jupiter’s interior, still hot from formation.

b. Thermal energy escaping from Jupiter’s interior, created by nuclear fusion.

c. Sunlight striking the cloud tops warms Jupiter’s atmosphere from above.

d. Sunlight heats the surface of Jupiter, then the surface radiates at infrared wavelengths, which warms the atmosphere.

e. Thermal energy escaping from Jupiter’s interior due to the condensation of helium droplets that sink beneath the less dense hydrogen.

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Quiz Questions

4. What evidence do we have that Jupiter has a very hot interior?

a. It reflects 170% of the visible light than it receives from the Sun.

b. It reflects 170% of the infrared light than it receives from the Sun.

c. It emits 70% more energy at visible wavelengths than it receives from the Sun.

d. It emits 70% more energy at infrared wavelengths than it receives from the Sun.

e. Both a and b above.

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Quiz Questions

5. Which method of heat transfer is responsible for Jupiter’s belts & zones, and the Great Red Spot?

a. Radiation.

b. Convection

c. Conduction.

d. All of the above.

e. None of the above.

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Quiz Questions

6. In the 1950s, radio telescopes first detected synchrotron radiation from Jupiter. What did this discovery tell us about Jupiter?

a. Jupiter has three distinct cloud layers in its atmosphere.

b. Jupiter began emitting radio waves in the 1950s.

c. Io orbits around Jupiter once every 1.8 days.

d. Jupiter has a strong magnetic field.

e. Jupiter rotates rapidly.

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Quiz Questions

7. The two requirements for a strong planetary magnetic field are rapid rotation, and a convective interior zone composed of an electrically conductive material. Jupiter’s rotational period is slightly less than 10 hours. What type of matter fulfills the second requirement?

a. Liquid molecular hydrogen.

b. Molten copper-aluminum.

c. Liquid metallic hydrogen.

d. A salty subsurface ocean.

e. Molten iron-nickel.

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Quiz Questions

8. What is the Roche Limit?

a. The maximum distance from a planet at which planetary rings can exist.

b. The maximum distance that a moon can travel from its planet.

c. The maximum mass of a blattella germanica.

d. The maximum mass of a Terrestrial planet.

e. The minimum mass of a Jovian planet.

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Quiz Questions

9. Jupiter’s ring appears dark in back-scattered light, yet appears bright in forward-scattered light. What does this tell us about the particles that make up Jupiter’s ring?

a. The average diameter of a particle is a few micrometers.

b. The particles are most likely dust.

c. The particles are most likely ice.

d. Both a and b above.

e. Both a and c above.

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Quiz Questions

10. How does Ganymede differ from Callisto?

a. Ganymede has more impact craters than Callisto.

b. Ganymede is differentiated, and Callisto is not.

c. Ganymede has a lower density than Callisto.

d. Ganymede is smaller than Callisto.

e. All of the above.

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Quiz Questions

11. The density of Callisto is 1.8 grams per cubic centimeter, and that of Ganymede is 1.9 grams per cubic centimeter. What does this suggest about these outer two of Jupiter’s four big moons?

a. These two moons must be made of roughly equal volumes of rock and iron.

b. These two moons must be made of roughly equal volumes of ice and rock.

c. Both moons must be larger than Earth’s moon.

d. Both moons must be smaller than Earth’s moon.

e. Both b and c above.

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Quiz Questions

12. What evidence do we have that the surface of Europa is young and active?

a. Europa has very few impact craters.

b. The icy crust of Europa is highly reflective.

c. Europa is the smallest of Jupiter’s four large moons.

d. Both a and b above.

e. All of the above.

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Quiz Questions

13. What evidence do we have that Io’s crust and lava is mostly silicate rock rather than sulfur compounds?

a. Some mountains on Io are much higher than any mountains on Earth.

b. Much of the lava flowing from Io’s volcanoes is hotter than Earth lava.

c. The various hues of yellow, orange, and red cannot be explained by sulfur.

d. Both a and b above.

e. All of the above.

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Quiz Questions

14. Why are Europa, Ganymede, and Callisto necessary for the continued heating of Io?

a. The tidal forces that these moons exert on Io are greater than the tidal force on Io due to Jupiter.

b. These moons periodically tug on Io and keep its orbit elliptical.

c. These moons send incoming comet bodies toward Io.

d. These outer moons disrupt Jupiter’s magnetic field lines, causing them to twist back and forth across Io.

e. Io is the smallest of these moons and subject to their influence.

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Quiz Questions

15. What evidence supports the model of Jupiter’s Galilean satellites forming in a mini accretion disk around Jupiter?

a. The density trend of these four moons is highest close to Jupiter and decreases with distance.

b. The two inner moons are much smaller than the two outer moons of this group.

c. Moons closer to a large planet have more impact craters on their surface.

d. Both a and b above.

e. All of the above.

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Quiz Questions

16. In which way does Saturn differ from Jupiter?

a. Saturn is less oblate.

b. Saturn has more mass.

c. Saturn has a stronger magnetic field.

d. Saturn has more distinct belts and zones

e. Saturn has a smaller zone of liquid metallic hydrogen.

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Quiz Questions

17. How do Saturn’s three layers of clouds differ from Jupiter’s three layers of clouds?

a. Saturn’s three cloud layers have a very different chemical composition than Jupiter’s.

b. Saturn’s three cloud layers are at much lower temperatures than Jupiter’s.

c. Saturn’s three cloud layers are located higher up in the atmosphere than Jupiter’s.

d. All of the above.

e. None of the above.

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Quiz Questions

18. What gives Saturn’s rings their beautiful structure?

a. The gravitational interaction between ring particles.

b. The gravitational influence of Saturn’s moons on the ring particles.

c. The gravitational interactions between Saturn and the ring particles.

d. Both and b above.

e. All of the above.

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Quiz Questions

19. How can Titan have a nitrogen-methane atmosphere with a surface pressure 1.5 times that of Earth’s atmosphere, whereas the larger and more massive Ganymede has no atmosphere at all?

a. Titan does not have the strong gravitational field of Jupiter nearby to break apart atmospheric gas molecules.

b. Titan has a stronger surface gravity and can hold onto gas molecules more easily than Ganymede.

c. Titan does not have to compete for gases with other large moons that are located nearby.

d. Titan has a sticky surface that holds onto large gas molecules.

e. Titan is farther from the Sun, and thus colder than Ganymede.

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Quiz Questions

20. What causes the leading side of Saturn’s small moon Iapetus to differ from its trailing side?

a. The leading side is darker, because it collides with and captures dark dust.

b. The leading side is brighter, since it collides with and captures fresh ices.

c. The leading side is darker, as ices melt and condense on the trailing side.

d. The leading side is darker, eroded as it is by impacts with solar wind particles.

e. The leading side is brighter, eroded as it is by impacts with solar wind particles.

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