1The Outer WorldsChapter Sixteen2Guiding Questions1. How did Uranus and Neptune come to be discovered?2. What gives Uranus its distinctive greenish-blue color?3. Why are the clouds on Neptune so much more visible than those on Uranus?4. Are Uranus and Neptune merely smaller versions of Jupiter and Saturn?5. What is so unusual about the magnetic fields of Uranus and Neptune?6. Why are the rings of Uranus and Neptune so difficult to see?7. Do the moons of Uranus show any signs of geologic activity?8. What makes Neptune’s moon Triton unique in the solar system?9. Are there other planets beyond Pluto?Uranus was discovered by chance, but Neptune’sexistence was predicted by applying Newtonian mechanics • Uranus recognized as a planet in 1781 by William Herschel• Neptune’s position calculated in mid-1840’s because of slight deviations in Uranus’ orbit• Credit shared by Le Verrier and Adams3Uranus is nearly featureless and has an unusuallytilted axis of rotation• Both Uranus and Neptune have atmospheres composed primarily of hydrogen, helium, and a few percent methane• Methane absorbs red light, giving Uranus and Neptune their greenish-blue colorExaggerated Seasons On Uranus• Uranus’s axis of rotation lies nearly in the plane of its orbit, producing greatly exaggerated seasonal changes on the planet• This unusual orientation may be the result of a collision with a planetlikeobject early in the history of our solar system. Such a collision could have knocked Uranus on its side4Neptune is a cold, bluish world with Jupiterlikeatmospheric features• No white ammonia clouds are seen on Uranus or Neptune• Presumably the low temperatures have caused almost all the ammonia to precipitate into the interiors of the planets• All of these planets’ clouds are composed of methane• • Much more cloud activity is seen on Neptune than on Uranus.• This is because Uranus lacks a substantial internal heat source.Neptune’s Clouds• Much more cloud activity is seen on Neptune than on Uranus• This is because Uranus lacks a substantial internal heat source5Uranus and Neptune contain a higher proportionof heavy elements than Jupiter and Saturn• Both Uranus and Neptune may have a rocky core surrounded by a mantle of water and ammonia• Electric currents in the mantles may generate the magnetic fields of the planetsThe magnetic fields of both Uranus and Neptuneare oriented at unusual angles• The magnetic axes of both Uranus and Neptune are steeply inclined from their axes of rotation• The magnetic and rotational axes of all the other planets are more nearly parallel• The magnetic fields of Uranus and Neptune are also offset from the centers of the planetsUranus and Neptune each have a system of thin,dark rings6Some of Uranus’s satellites show evidence of pasttidal heatingUranus has five satellites similar to the moderate-sized moons of Saturn, plus at least 22 more small satellites7Triton is a frigid, icy world with a young surfaceand a tenuous atmosphere• Neptune has 13 satellites, one of which (Triton) is comparable in size to our Moon or the Galilean satellites of Jupiter• Triton has a young, icy surface indicative of tectonic activity• The energy for this activity may have been provided by tidal heating that occurred when Triton was captured by Neptune’s gravity into a retrograde orbit• Triton has a tenuous nitrogen atmospherePluto and its moon, Charon, may be typical of athousand icy objects that orbit far from the Sun • Pluto was discovered after a long search • Pluto and its moon, Charon, move together in a highly elliptical orbit steeply inclined to the plane of the ecliptic• They are the only worlds in the solar system not yet visited by spacecraft8• Several hundred small, icy worlds have been discovered beyond Neptune• Pluto and Charon are part of this population9Key Words• Kuiper belt• magnetic axis• occultation• radiation
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