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Q1. What two processes allow Saturn to radiate more energy than it receives from the sun? What observation supports the existence of one of these processes? Answer
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Q1. Why does the large moon of Saturn have an atmosphere even though an equally large moon at Jupiter does not? Answer
Q2. For any one of the intermediate-size moons of the solar system, describe its unique or interesting characteristics. Answer
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Q1. Explain the concept of resonance. Answer
Q2. Why do planetary rings exist? Why do they not exist at the terrestrial planets? Answer
Q3. Why should the lifetime of planetary rings be fairly short? Why are they still present? Answer
Q4. Compare and contrast the properties of the rings at Jupiter and Uranus. Answer
Q5. Why should rings be short lived phenomena? Why do they instead survive for long periods? Answer
Q6. What is the Roche limit? What is its role in ring formation? Answer
Q7. Why do we believe that a given ring particle will last only a short time in its orbit? What two things can happen to make rings more long lived? Answer
Q8. How can a resonance affect the motion of an object? Describe a resonance in the solar system, and describe how the motion has been altered. Answer
Q9. How do we know that Saturn’s rings are composed of small particles? Give two different observations, one ground-based and one from a satellite. Answer
Q10. Suppose you discovered a giant planet like Jupiter in another solar system and found that it had no moons. Would you expect it to have rings? Explain why or why not. Answer
Q11. How can we measure the size of individual rings particles? What is different about the results found on Jupiter and Saturn? Answer
Q12. Why are rings so incredibly thin, compared to their diameter? Answer
Q13. Describe the overall appearance of Saturn’s rings, as observed by the Voyager spacecraft. Answer
Q14. Describe a typical ring particle in the Saturnian ring system. Answer
Q15. Why are most ring systems very thin? Answer
Q16. Why do terrestrial planets not have ring systems? Answer
Q17. What happens if there is a small moon just outside a planetary ring? Answer
Saturn Answers
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A1. Saturn produces some energy from the slow
release of gravitational energy as it continues to shrink at a very gradual (by human
standards) pace. Even more energy is released when droplets of liquid helium form inside
Saturn and begin to "rain" on deeper layers. These falling droplets release
gravitational energy and heat the material through which they fall. This model of energy
production inside Saturn is supported by the observation that there is less helium at the
surface of Saturn than there is at the surface of either Jupiter or the sun. The outer
layers of Saturn are slowly being depleted of their supply of helium as it falls to deeper
layers.
Saturn' s Moons Answers
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A1. Saturn’s moon Titan has virtually the same gravitational force as Jupiter’s moon Ganymede. Hence they should both be able to hold an atmosphere equally. However, Ganymede is just enough closer to the sun that any atmosphere it might have would be warmer than an atmosphere at Titan. Since the warmer atoms move faster, they are harder for a gravitational force to hold on to. Thus, an atmosphere at Ganymede is much more likely to escape because the individual atoms would be moving faster.
A2. Examples include: Mimas (with a crater almost large enough to have shattered the moon), Enceladus (most reflective body in the solar system), Iapetus (one side whiter than snow, the other side darker than coal), Hyperion (largest non-spherical object in the solar system with chaotic rotation), Phoebe (an extremely dark surface which reflects only 5 % of the light that strikes it), and Miranda (features large angular regions that may represent blocks of material that have recently reformed the moon itself).
Saturn' s Rings Answers
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A1. Resonance occurs when a small force is applied at the same point in a cycle of motion. For example, a child kicks in the air at the back of each swing on a playground swing. This small force gradually increases the amplitude of the swing to a large effect.
A2. Planetary rings exist inside the Roche limit of their planet. The Roche limit is the minimum distance a liquid body could survive in orbit around the planet without being torn apart by the tides from the planet. Ring particles represent material that was prevented from collecting together to form a moon because of their position. Terrestrial planets have such weak forces of gravity that their Roche limits are within or very near their atmospheres. Any particles inside their Roche limits will quickly fall into the planet.
A3. Planetary rings are composed of fairly small particles. Collisions among these particles will quickly begin to spread them out – after each collision one particle will move closer to the planet while the other will move further away. Ring particle should be lost both by falling into the planet and by escaping from their orbits. Rings are still present because of two factors. Some rings have shepherding moons which keep the particles from wandering off. Even with this mechanism, there must also be a source of new ring particles – boulders within the rings that are slowly being ground up as particles collide with them.
A4. The rings of Jupiter form broad sheets and are composed of very tiny particles, while the rings of Uranus are confined to very narrow bands and are composed of relatively large, dark particles. Both ring systems are very faint.
A5. Collisions between ring particles should cause them to gradually spread out, with some particles eventually falling into the planet and others escaping outwards. Ring particles are maintained both by the effects of shepherd moons which keep them confined to a particular orbit and are manufactured as larger bodies are ground up within the rings.
A6. The Roche limit is the distance from a planet where the internal gravity of a liquid body would no longer be strong enough to hold it together against the tidal forces of the planet it orbits. The rings of the giant planets all lie inside the Roche limit of their planet, indicating that material in that zone was prevented from collecting together to form satellites but instead became the rings we see today.
A7. The orbits of ring particles are not stable because collisions can so easily redirect their motion. The net effect of collisions among ring particles is to spread the rings out -- both toward the planet where the particles collide with the planet and away from the planet where they are eventually lost to space. Rings can be preserved with by the presence of shepherding moons which confine the ring particle orbits or by the presence of larger bodies in the rings which resupply the rings with small particles as they are ground up.
A8. A resonance occurs when a small force acted repeated at the same point in the cycle of motion of an object. Even though the force is very small, its effect accumulates over time to produce a noticeable change in the motion of the object. The force of Earth acting on the "heavy" side of the Moon has slowly pulled that side to always face Earth as the Moon orbits Earth. The action of the sun on Mercury has produced a similar result there, except that Mercury rotates three times for every two orbits around the sun.
A9. Ground-based observations of stars that pass behind the rings, but blink in and out of view, show that the rings are neither solid nor gaseous. Satellite observations of the rate at which the particles cool off after they enter Saturn’s shadow tells us that the ring particles are quite small, only about a centimeter across on average.
A10. Without moons it would be extremely difficult for a planet to keep a ring system. Moons act to shepherd or contain the ring particles so they do not wander away to be lost from the ring system. Without a moon to keep ring particles from being lost the rings will quickly disappear.
A11. The sizes of ring particles can be measure by how quickly they cool off as they enter the planet’s shadow (smaller particles will cool faster) or by how much light is scattered into the forward direction (that is, away from the sun) compared to the backward scattering (large particles produce more backward scattering). Jupiter’s ring particles are much smaller on average than Saturn’s ring particles.
A12. Again, collisions play a key role. If one or both of the particles which collide have some vertical motion before the collision, the collision will tend to cancel out some of the vertical motion. Over time, the motion of the particles becomes more and more uniform in the plane of the rings.
A13. Saturn’s rings contain thousands of tiny ringlets within the broad band of the rings visible from Earth. Even in the dark division between the main rings, there are dimmer ringlets. There is also a thin braided "F" ring outside the main ring system, confined by two shepherding moons. Dark spokes are also seen to rotate with the rings of Saturn.
A14. A typical particle in the rings of Saturn is the size of a small pebble and composed of water ice.
A15. If a ring particle is in a tilted orbit (compared to the average of all orbits) it is likely to collide with another particle every time it passes through the plane of the rings. These collisions will tend to cancel out the vertical motion of the orbit until everything is moving in the same plane.
A16. Planetary rings usually occur inside the Roche limit, defined as the distance at which a liquid moon would be broken up by the tidal forces of the planet. The terrestrial planets are so small that their Roche limits are within the outer reaches of their atmospheres, where ring particles quickly burn up.
A17. A small moon just outside a ring acts as a shepherd for the ring particles, keeping them confined to the ring. As a ring particle catches up with the moon, it is pulled forward by the gravity of the moon. This extra speed allows it to move to a slightly small orbit. The result is a sharp outer edge to the ring, and a longer lifetime for the ring.