Astr 301 Ch16 Homework Solutions

3. WINDS

The winds on Jupiter are prograde in a broad equatorial zone at about 350 km/h, retrograde in the north and south equatorial zones either side of the equatorial zone, and continue to reverse between prograde and retrograde with reducing speed toward both poles.

The winds on Saturn are also prograde in the equatorial zone, but at the much higher speed of 1700 km/h. Either side of the equatorial zone the winds diminish to almost zero or even to a slight retrograde flow in a narrow region. Farther toward the poles the winds again become prograde at about 400 km/h. The difference between the winds on Saturn and Jupiter is not understood.

The winds on Uranus are retrograde near the equator at about 350 km/h, which is a surprising reversal of the wind pattern seen near the equator on both Jupiter and Saturn. The winds are prograde between 50 and 60 degrees S latitude at about 600 km/h

The winds on Neptune are similar to the winds on Uranus, but reach the high speed of 1700 km/h near the equator. The difference between the winds on Uranus and Neptune and the winds on Jupiter and Saturn are not understood.

MAGNETIC FIELDS

Jupiter's magnetic field is about 14 times stronger than Earth's, but the orientations of the magnetic axes are both about 10 degrees from the rotation axis.

Saturn's magnetic field is 71% of the strength of Earth's, but the orientation is unusual in that Saturn's magnetic axis is exactly lined up with its rotation axis.

Uranus's magnetic field is 74% of the strength of Earth's, and the magnetic axis is tilted 59 degrees from the rotation axis. Similarly, Neptune's magnetic field is 43% of the strength of Earth's, and the magnetic axis is tilted 47 degrees from the rotation axis. In addition, the magnetic dipole of Uranus is offset 30% of the way from the center to the surface, and the magnetic dipole of Neptune is offset 55% of the way from the center to the surface. The large tilts and offsets are not understood. The dipoles of Jupiter and Saturn are not offset.

4. Uranus and Neptune have significant amounts of methane in their atmospheres, and their atmospheres are so cold that some of the methane condenses into droplets and forms methand hazes. These methane droplets absorb red light strongly, so Uranus and Neptune appear blue in color. Jupiter has a somewhat reddish color due to ammonia clouds in its upper atmosphere. Saturn is more yellowish, probbly because the ammonia clouds are deeper in its atmosphere and partly obscured by haze.

6. Pluto and Triton are very similar in size and mass (and thus in density), and have similar atmospheres of nitrogen, carbon monoxide, and methane (though the atmosphere of Pluto contains carbon dioxide, which Triton's atmosphere does not). Pluto's atmosphere probably all freezes out near aphelion. Both have polar caps. Triton was probably originally a free body in an orbit much like the orbit of Pluto before being captured by Neptune. Pluto has a satellite, and Triton does not.

7. The abundance of He (really the ratio of He to H) in Uranus's atmosphere is about the same as in the Sun, and the He abundance in Neptune's atmosphere is somewhat higher. The He abundance in Jupiter's atmosphere is only about half as great, and the He abundance in Saturn's atmosphere is only one sixth as great. One reason for this difference is that the weaker gravity of Uranus and Neptune has caused them to lose more H from their atmospheres than Jupiter and Saturn have lost. Another reason is that He has been condensing into droplets and raining into the interior of Saturn, and this process is apparently beginning within Jupiter too.

8. Uranus has no detectable source of internal heat. Neptune radiates 2.6 times more energy than it receives from the Sun, which is comparable to the heat radiated by Jupiter and by Saturn.

(9. not assigned, but interesting) Since the rotation axis of Uranus is tilted by 98 degrees to the plane of the ecliptic, the tropics would be at latitudes 82 degrees N and S, and the arctic and antarctic circles would be at latitudes 8 degrees N and S.

10. 1985 + (249 years)/2 = 2109

11. 1989 + (84 years)/2 = 2030

12. Jupiter and Saturn probably have cores made mostly of rock with some ice, and they were able to gather large amounts of H and He gas from the solar nebula and grow very massive. Uranus and Neptune are mostly rock with some ice, but were not able to gather large amounts of gas from the more tenuous outer reaches of the solar nebula where they formed. Uranus and Neptune may be similar to the cores of Jupiter and Saturn.

13. Collisions during the late stages of the planet forming process probably drastically altered the rotation rates of the relatively small terrestrial planets. In the case of the Earth, tidal drag from the Moon has slowed the rotation rate. The Jovian planets are so massive that they are were much less affectd by such effects and retain much of their original rotation.

16. The strange surface structures indicate that Miranda was probably broken into pieces by a large impact, and then gravitationally reassembled itself.

17. Triton has a retrograde orbit, which means it is probably a captured body. It was once thought that perhaps Pluto had been a satellite of Neptune, but the orbital inclinations of Neptune and Pluto are very different so there is really no reason to believe that Pluto every belonged to Neptune.