Astronomy Chapter 11

Europa

Which moon is considered likely to have a deep, subsurface ocean of liquid water?

thought to have a deep, subsurface ocean of liquid water

Which of the above applies to Europa?

the largest moon in the solar system

Which of the above applies to Ganymede?

the most volcanically active body in the solar system

Which of the above applies to Io?

the target of the Huygens probe, which landed on the surface in 2005

Which of the above applies to Titan?

probably a captured moon

Which of the above applies to Triton?

Particles in the solar nebula were more spread out at greater distances, so that accretion took longer and there was less time to pull in gas before the solar wind cleared the nebula.

According to our theory of solar system formation, why did Uranus and Neptune end up to be much less massive than Jupiter and Saturn?

Io orbits Jupiter on an elliptical orbit, due to orbital resonances with other satellites

All the following statements are true. Which one is most important in explaining the tremendous tidal heating that occurs on Io?

TRUE

Both the existence and the location of Neptune were predicted mathematically before the planet actually was detected by telescope.

Uranus was discovered with the aid of a telescope by William Herschel. Neptune was discovered when astronomers realized that Uranus was being influenced by another body as it moved around its orbit. They were thereby able to predict the precise location in the sky in which to look for Neptune, which was subsequently found by telescope.

Briefly explain how Uranus and Neptune were discovered.

Jupiter has the strongest magnetic field by far among the planets. It is 20,000 times stronger than Earth's. Its strong field is able to deflect the solar wind about 40 Jupiter radii in front of Jupiter. The magnetosphere around Saturn is smaller because it has a much thinner layer of metallic hydrogen. Although Uranus and Neptune have no metallic hydrogen, and therefore much weaker magnetic fields generated in their cores, the solar wind is also much weaker at their great distances from the Sun. Therefore, their magnetospheres are larger than they would be if the planets were closer to the Sun. Even with the weaker solar wind, however, these planets have very small magnetospheres.

Contrast Jupiter's magnetosphere with that of Earth and of the other jovian planets.

Within two to three radii of any planet, the tidal forces tugging an object apart become comparable to the gravitational forces holding it together. This region is called the Roche zone. One scenario for the origin of the rings is that a wandering moon strayed into the Roche zone and was torn apart. Another scenario is that the material near the planet was prevented from forming a moon in the first place because of the strong tidal forces. Random orbits of the particles would cause collisions between particles that cross orbits, and this has the effect of making the rings very thin. Frequent collisions between particles or between particles and their planet's upper atmosphere cause ring particles to disintegrate within a few million years. Therefore, since the ring particles are rapidly disappearing, they must be continually replenished. Otherwise it would be very unlikely that we would be so lucky to view rings around all of the jovian planets during this short time period that they have rings.

Describe the leading scenarios for the origin of the planetary rings. What makes us think that ring systems must be continually replenished?

The white zones of Jupiter's atmosphere are white because of ammonia clouds. The red and brown colors of the belts must be produced by ingredients such as sulfur compounds or phosphorus compounds that "pollute" the ammonium-hydrosulfide crystals. Saturn's reds and tans probably come from the same compounds that produce these colors on Jupiter. However, they are more muted because these cloud layers lie deeper within Saturn's atmosphere, under a thicker layer of tan "smog." The blue colors of Uranus and Neptune are produced by methane gas, which absorbs red light and transmits blue.

Describe the possible origins of Jupiter's vibrant colors. Contrast these with the origins of the colors of the other jovian planets.

For every orbit that Ganymede completes around Jupiter, Europa completes exactly two orbits and Io completes exactly four. Therefore, all three planets line up for every orbit of Ganymede, and Io and Europa line up twice each Ganymede orbit. The gravitational tugs from their sister moons add up over time, and these tugs are always in the same direction. Therefore, the moons' orbits become slightly elliptical over time

Explain how the resonance among Io, Europa, and Ganymede makes their orbits slightly elliptical.

You are in the Great Red Spot on Jupiter. This immense storm is found at a latitude of about 35°S on Jupiter. To understand the motion of the Sun through the sky, recall that, because Jupiter has only a very slight axis tilt (3°), the celestial equator and the ecliptic are nearly coincident.

Following a series of experiments in which you are flung against walls to test the resiliency of the human body, you are ejected from the ship and find yourself in a maelstrom. Whipped by winds gusting at 10,000 km/hr, rising and falling in convective currents, you are not having a good time. On the rare occasions when you can assess your surroundings, you notice a horizon that seems to go on "forever" (or, at least, much farther than the horizon on Earth), and no solid surface anywhere. You get a glimpse of several large moons. The Sun races through your sky, requiring only about 5 hours to rise nearly due east, cross the meridian at an altitude of about 55° 8 in the north, and then set due west.

by contracting, changing gravitational potential energy into thermal energy

How do astronomers think Jupiter generates its internal heat?

by raining dense helium droplets from higher to lower altitudes, resembling the process of differentiation

How do astronomers think Saturn generates its internal heat?

All have cores of about the same mass, but differ in the amount of surrounding hydrogen and helium.

How do the jovian planet interiors differ?

They are much faster than hurricane winds on Earth.

How do typical wind speeds in Jupiter's atmosphere compare to typical wind speeds on Earth?

It is about the same size but is 10 times more massive.

How does Jupiter's core compare to Earth's?

Jupiter's magnetic field is about 20,000 times as strong as Earth's.

How does the strength of Jupiter's magnetic field compare to that of Earth's magnetic field?

100 million

How many more times is the atmospheric pressure in Jupiter's core greater than the atmospheric pressure at Earth's surface?

It emits twice as much.

How much energy does Jupiter emit compared with how much it receives from the Sun?

TRUE

Hydrogen exists as a gas, liquid, and solid within Jupiter

TRUE

If Jupiter were 10 times more massive, it would actually have a smaller radius.

FALSE

If Jupiter were 10 times more massive, it would generate nuclear fusion in its core and be a star instead of a planet.

You are on Titan, a moon of Saturn. Saturn remains stationary in your sky because Titan rotates synchronously with its revolution about Saturn. (You might want to read The Sirens of Titan by Kurt Vonnegut. Be sure to keep in mind, however, that his description of Titan is 30 years out of date.) Titan was visited by a probe from the Cassini spacecraft to Saturn in 2005, check this out!!!!!You may need it after this class is over!

It is very cold, but otherwise quite comfortable. You breathe deeply. Although there is no oxygen (not to worry-your life-support belt will take care of that problem), the air pressure is quite tolerable and seems almost Earth-like (because it is mostly nitrogen). Your vista might even include such breathtaking sights as an ocean of liquid methane before you, and snowcapped (methane snow) mountains behind (although the atmosphere is very smoggy and the visibility consequently is limited). Your solar day is about 16 Earth days long. A large, ringed object seems to hang in your sky, never rising or setting; however, it goes through phases from new to full and back to new again with the same 16-day period as the rising and setting of the Sun.

They actually contain relatively little material in a gaseous state.

Jupiter and the other jovian planets are sometimes called "gas giants." In what sense is this term misleading?

TRUE

Jupiter does not have seasons because it has no appreciable axis tilt.

TRUE

Jupiter is slowly shrinking through gravitational contraction today.

FALSE

Jupiter's Great Red Spot is a low-pressure storm like a hurricane on Earth.

clouds of sulfuric acid

Jupiter's colors come in part from its three layers of clouds. Which of the following is not the primary constituent of one of Jupiter's cloud layers?

The Sun

Overall, Jupiter's composition is most like that of...

FALSE

Pluto exerts a noticeable gravitational influence on Uranus.

TRUE

Process of Science: A prediction based on Newton's theory of gravity led to the discovery of Neptune

Io has active volcanoes, as verified through pictures of the surface, pictures of plumes, and a lack of craters. Its proximity to Jupiter means that gravitational tidal heating is likely a strong source of internal friction and heat.

Scientists have not observed Io's interior nor measured its internal temperature. What evidence do we have of Io's strong tidal heating from Jupiter?

TRUE

Some of the moons of the jovian planets have significant atmospheres.

Without ingredients besides hydrogen and helium, the jovian planets would all be gray in color, and there would be no clouds or precipitation.

Suppose the jovian planet atmospheres were composed 100 percent of hydrogen and helium rather than 98 percent of hydrogen and helium. How would the atmospheres be different in terms of color and weather?

countless icy particles, ranging in size from dust grains to large boulders

Suppose you could float in space just a few meters above Saturn's rings. What would you see as you looked down on the rings?

TRUE

Synchronous rotation is when a moon's rotation period and orbital period are the same.

features or erosion, including what appeared to be dry river valleys and lakebeds

The Huygens probe took numerous pictures as it descended to Titan's surface in 2005. What did the pictures show?

alternating bands of rising and falling air at different latitudes

The belts and zones of Jupiter are...

both the Sun and their interiors, in roughly equal proportions.

The energy in the atmospheres of most of the jovian planets comes

a natural consequence of tidal forces acting on the moons.

The fact that most moons always show the same face to their planet is

a mixture of rock and ice.

The four Galilean moons around Jupiter are

The gravitational attraction of Saturn on Mimas is less than that of Jupiter on Amalthea; therefore, Saturn's mass must be less than Jupiter's. If Saturn is less massive but almost as large as Jupiter, its density must be lower.

The satellite Amalthea orbits Jupiter at just about the same distance in kilometers at which Mimas orbits Saturn. Yet Mimas takes almost twice as long to orbit. What can you deduce from this difference qualitatively? Since Jupiter and Saturn are not very different in radius, what else can you conclude?

Temperatures on Jupiter and Saturn are too high for methane to condense.

Uranus and Neptune have methane clouds but Jupiter and Saturn do not. Which factor explains why?

FALSE

Uranus continues to generate internal heat through gravitational contraction.

Methane

What atmospheric constituent is responsible for the blue color of Uranus and Neptune?

The main evidence comes from trying to find a mechanism to explain Europa's magnetic field variation. It appears to be due to an induced field, possibly from a conducting substance, such as a salty subsurface ocean.

What evidence is there of a subsurface ocean on Europa? Do you find it compelling?

Ice geology is the formation of surface features due to ice melting and flowing, similar to lava flows on Earth. Many of the jovian moons are made of water, ammonia, and methane ices, and since these melt at far lower temperatures than rock, even small amounts of heating (from tidal effects for example) can sustain a surprising amount of geologic activity. An example is Enceladus, a small moon of Saturn, in which ice flows have filled in many craters.

What is "ice geology"? Give an example illustrating why it is important in the outer solar system.

a long-lived, high-pressure storm

What is Jupiter's Great Red Spot?

A long-lived, high-pressure storm on Jupiter

What is the Great Red Spot?

Its density would increase but its diameter would barely change.

What would happen to Jupiter if we could somehow double its mass?

Uranus

Which jovian planet should have the most extreme seasonal changes?

Triton

Which large jovian moon is thought to have been captured into its present orbit?

Titan

Which moon has a thick atmosphere made mostly of nitrogen?

core of rock, metal, and hydrogen compounds; thick layer of metallic hydrogen; layer of liquid hydrogen; layer of gaseous hydrogen; cloud layer

Which of the following best describes the internal layering of Jupiter, from the center outward?

Jovian moons are made mostly of ice that can melt or deform at lower temperatures than can the rock and metal that make up the Moon and Mercury.

Which of the following best explains why many jovian moons have been more geologically active than the Moon or Mercury?

The light stripes are regions of high clouds, and the dark stripes are regions where we can see down to deeper, darker clouds.

Which of the following best why we see horizontal "stripes" in photographs of Jupiter and Saturn?

spectroscopy of the cloud layers

Which of the following does not yield information on jovian planet interiors?

carbon dioxide

Which of the following gases is not a significant ingredient of the jovian planet atmospheres?

lakes of liquid water in the warmer equatorial regions

Which of the following is most unlikely to be found on Titan?

They are higher in average density than are the terrestrial planets.

Which of the following is not a general characteristic of the four jovian planets in our solar system?

Astronomers have detected small lakes of liquid water on Europa's surface.

Which of the following is not a piece of evidence supporting the idea that Europa may have a subsurface ocean?

Jupiter is contracting very gradually

Which of the following most likely explains why Jupiter's interior releases so much heat?

Most of the moons are large enough to be spherical in shape, but a few have the more potatolike shapes of asteroids.

Which of the following statements about the moons of the jovian planets is not true?

They all have the same exact set of internal layers, though these layers differ in size.

Which of the following statements comparing the jovian interiors is not thought to be true?

Saturn, where it generates heat as it falls downward.

Which planet may have helium rain in its interior, and what does this rain do?

It is the most volcanically active body in our solar system.

Which statement about Io is true?

Saturn's rings formed along with its moons 4.6 billion years ago.

Which statement about planetary rings is not true?

Io did have impact craters but they have all been buried in lava flows.

Why are there no impact craters on the surface of Io?

Methane does not condense into ice in the warmer atmospheric temperatures of Jupiter and Saturn.

Why do Uranus and Neptune have blue methane clouds but Jupiter and Saturn do not?

Their rapid rotation flings the mass near the equator outward.

Why do jovian planets bulge around the equator, that is, have a "squashed" appearance?

Accretion took longer further from the Sun, so the more distant planets formed their cores later and captured less gas from the solar nebula than the closer jovian planets.

Why do the jovian planet interiors differ?

That Jupiter produces more energy than it absorbs from the Sun is an observed fact. We can calculate that any heat from its formation would have been radiated away by now. Furthermore, the amount of energy is too large to explain by radioactive decay (the process that applies on Earth). We conclude that gravitational contraction is the most plausible source of the heat by a process of elimination of other possibilities, even though the rate of contraction is too small to be directly observable.

Why do we think the source of Jupiter's large internal energy come from gravitational contraction?

Different layers represent clouds made of gases that condense at different temperatures.

Why does Jupiter have several distinct cloud layers?

The three layers represent clouds made of gases that condense at different temperatures.

Why does Jupiter have three distinct layers of clouds?

The differences in the sizes of the jovian planets are due to their capturing different amounts of gas from the solar nebula, since their cores are all about the same size. Icy planetesimals took longer to accrete in the outer solar system, because they were more spread out there. Thus, more distant jovian planets didn't have as much time as Jupiter to capture gas from the solar nebula before the nebula was cleared by the solar wind.

Why does it make sense that the jovian planets farther from the Sun have less mass?

The extra mass of Jupiter compresses its interior to a greater extent than that of Saturn.

Why is Jupiter denser than Saturn?

It has a different composition than Saturn, including a higher proportion of hydrogen compounds and rocks.

Why is Neptune denser than Saturn?

Jupiter's greater mass compresses it more, thus increasing its density.

Why is Saturn almost as big as Jupiter, despite its smaller mass?

Triton's orbit around Neptune is retrograde (opposite to the planet's rotation) and highly inclined. It is a large, round satellite with an apparently active geology. Its large size and round shape suggests it formed from the accretion of large numbers of planetesimals, yet its orbit argues that it did not form in situ around Neptune as the planet condensed from the protosolar nebula. It appears to have formed in the outer regions of the solar system, perhaps in the Kuiper belt like Pluto, and was subsequently captured.

Why is Triton such an unusual satellite?

The region is full of gases that become ionized after they are released from volcanoes on Io.

Why is the radiation so intense in the region that traces Io's orbit around Jupiter (the Io torus)?

The surface temperature on Jupiter is determined more by internal heat (from gravitational contraction) than by solar radiation. Thus the poles and the equator are at nearly the same temperature.

Why is there no large temperature variation from the poles to the equator on Jupiter?