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Uranus and Neptune Uranus and Neptune The outermost planets were unknown to the ancients—they were discovered by telescopic observations: Uranus in 1781 and Neptune in 1846 Uranus and Neptune have very similar bulk properties, so it is natural to consider them together Discovery of Uranus The planet Uranus was discovered by British astronomer William Herschel in 1781 Herschel was engaged in charting the faint stars in the sky when he came across an odd-looking object that he described as “a curious either nebulous star or perhaps a comet” The object appeared as a disk in Herschel’s 6-inch telescope and moved relative to the stars, but it traveled too slowly to be a comet Herschel soon realized that he had found the seventh planet in the solar system Since this was the first new planet discovered in well over 2000 years, the event caused quite a stir at the time The story goes that Herschel’s first instinct was to name the new planet “Sidus Georgium” (Latin for “George’s star”) after his king, George III of England Uranus From Earth Uranus is in fact just barely visible to the naked eye, if you know exactly where to look It looks like a faint, undistinguished star Even today, few astronomers have seen it without a telescope Through a large Earth-based optical telescope, Uranus appears hardly more than a tiny pale greenish disk With the flyby of Voyager 2 in 1986, our knowledge of Uranus increased dramatically, although close-up images of the planet still showed virtually no surface detail Discovery of Neptune Once Uranus was discovered, astronomers set about charting its orbit, and quickly discovered a small discrepancy between the planet’s predicted position and where they actually observed it The logical conclusion was that an unknown body must be exerting a gravitational force on Uranus Astronomers realized that there had to be another planet in the solar system perturbing Uranus’s motion Discovery of Neptune In the 1840s two mathematicians independently solved the difficult problem of determining this new planet’s mass and orbit British astronomer, John Adams, reached the solution in September 1845 in June of the following year, the French mathematician Urbain Leverrier came up with essentially the same answer In September a German astronomer named Johann Galle began his own search from the Berlin Observatory, using a newly completed set of more accurate sky charts He found the new planet within one or two degrees of the predicted position—on his first attempt After some wrangling over names and credits, the new planet was named Neptune, and Adams and Leverrier (but not Galle!) are now jointly credited with its discovery With an orbital period of 163.7 years, Neptune has not yet completed one revolution since its discovery Neptune from Earth Distant Neptune cannot be seen with the naked eye, although it can be seen with a small telescope Neptune is so distant that surface features are virtually impossible to discern With Voyager 2’s arrival, much more detail emerged Physical Properties The radius of Uranus is 4.0 times that of Earth The radius of Neptune is 3.9 times that of Earth Their masses are 14.5 Earth masses for Uranus and 17.1 Earth masses for Neptune Large rocky cores constitute a greater fraction of the planets’ masses than do the cores of either Jupiter or Saturn Rotation of Uranus Uranus has a short rotation period Earth-based observations of the Doppler shifts in spectral lines first indicated that Uranus’s “day” was between 10 and 20 hours long The precise value of the planet’s rotation is now known to be 17.2 hours The planet’s atmosphere rotates differentially However, Uranus’s atmosphere actually rotates faster at the poles (where the period is 14.2 hours) than near the equator (where the period is 16.5 hours) The rotation of Uranus, like that of Venus, is classified as retrograde Rotation of Uranus Uranus’s rotation axis lies 98° from the perpendicular Relative to the other planets, we might say that Uranus lies tipped over on its side As a result, the “north” (spin) pole of Uranus, at some time in its orbit, points almost directly toward the Sun Half a “year” later, its “south” pole faces the Sun The planet experiences the most extreme seasons known in the solar system The poles are alternately plunged into darkness for 42 years at a time Rotation of Neptune The average rotation period of Neptune’s atmosphere is 17.3 hours Measurements of Neptune’s radio emission by Voyager 2 showed that the magnetic field of the planet, and presumably also its interior, rotates once every 16.1 hours Neptune is unique among the jovian worlds in that its atmosphere rotates more slowly than its interior Neptune’s rotation axis is inclined 29.6° Atmosphere Spectroscopic studies of sunlight reflected from Uranus’s and Neptune’s dense clouds indicate that the two planets’ outer atmospheres are quite similar to the atmospheres of Jupiter and Saturn The most abundant element is molecular hydrogen (84 percent), followed by helium (about 14 percent) and methane, which is more abundant on Neptune (about 3 percent) than on Uranus (2 percent) Ammonia, which plays such an important role in the Jupiter and Saturn systems, is not present in any significant quantity in the outermost jovian worlds Jupiter has much more gaseous ammonia than methane, but moving outward from the Sun, we find that the more distant planets have steadily decreasing amounts of ammonia and relatively greater amounts of methane The increasing amounts of methane are largely responsible for the outer jovian planets’ blue coloration Methane absorbs long-wavelength red light quite efficiently, so sunlight reflected from the planets’ atmospheres is deficient in red and yellow photons and appears blue-green or blue As the concentration of methane increases, the reflected light appears bluer Weather on Uranus Uranus apparently lacks any significant internal heat source Because of the planet’s low surface temperature, its clouds are found only at low-lying, warmer levels in the atmosphere The absence of high-level clouds means that we must look deep into the planet’s atmosphere to see any structure The bands and spots that characterize flow patterns on the other jovian worlds are largely “washed out” on Uranus Astronomers have learned that Uranus’s atmospheric clouds and flow patterns move around the planet in the same sense as the planet’s rotation, with wind speeds ranging from 200 to 500 km/h Weather on Neptune Neptune’s clouds and band structure are much more easily seen Neptune’s upper atmosphere is actually slightly warmer than that of Uranus Neptune has an internal energy source—it radiates 2.7 times more heat than it receives from the Sun The cause of this heating is still uncertain Neptune sports several storm systems similar in appearance to those seen on Jupiter The largest such storm is comparable to the size of Earth and is known as the Great Dark Spot Magnetospheres Voyager 2 found that both Uranus and Neptune have fairly strong internal magnetic fields—about 100 times stronger than Earth’s field and 1/10 as strong as Saturn’s Uranus and Neptune each have substantial magnetospheres, populated largely by electrons and protons either captured from the solar wind or created from ionized hydrogen gas escaping from the planets themselves Uranus’ magnetic field is tilted at about 60° to the axis of rotation On Earth, such a tilt would put the north magnetic pole somewhere in the Caribbean The magnetic field lines are not centered on the planet The internal structures of Uranus and Neptune are different from those of Jupiter and Saturn, and this difference changes how their magnetic fields are generated Internal Structure Theoretical models indicate that Uranus and Neptune have rocky cores similar to those found in Jupiter and Saturn—about the size of Earth and perhaps 10 times more massive The pressure outside the cores of Uranus and Neptune (unlike the pressure within Jupiter and Saturn) is too low to force hydrogen into the metallic state, so hydrogen stays in its molecular form all the way in to the planets’ cores Astronomers theorize that deep below the cloud layers, Uranus and Neptune may have high-density, “slushy” interiors containing thick layers of water clouds Moons of Uranus Moons of Uranus Uranus' moons are unique in being named for Shakespearean characters, along with a couple from the works of Alexander Pope Uranus now has 27 known moons Moons of Uranus Oberon and Titania (king and queen of the fairies) are the largest Uranian moons All of Uranus's inner moons appear to be roughly half water ice and half rock The composition of the moons outside the orbit of Oberon remains unknown, but they are likely captured asteroids The darkest of the moons of Uranus is Umbriel and it displays little evidence of any past surface activity Miranda has a surface unlike any other moon that's been seen It has giant fault canyons as much as 12 times as deep as the Grand Canyon, terraced layers, surfaces that appear very old, and others that look much younger Ariel has the brightest and possibly the youngest surface among all the moons of Uranus Cordelia and Ophelia are shepherd moons that keep Uranus' thin, outermost "epsilon" ring well defined There is a swarm of eight small satellites unlike any other system of planetary moons This region is so crowded that astronomers don't yet understand how the little moons have managed to avoid crashing into each other Moons of Neptune Moons of Neptune Neptune has a total of 13 known moons The largest moon, Triton, is the only large moon in our solar system to have a retrograde orbit Part of its surface resembles the rind of a cantaloupe Ice volcanoes spout what is probably a mixture of liquid nitrogen, methane, and dust, which instantly freezes and then snows back down to the surface Triton's icy surface reflects so much of what little sunlight reaches it that the moon is one of the coldest objects in the solar system, about -240 C (-400 F) Millions of years from now, Triton will come close enough for gravitational forces to break it apart - possibly forming a ring around Neptune Proteus, the second-largest, is thought to be right at the limit of how massive an object can be before its gravity pulls it into a sphere Rings of Uranus Ground-based observations revealed the presence of a total of nine thin rings The main rings, in order of increasing radius, are named Alpha, Beta, Gamma, Delta, and Epsilon A fainter ring, known as the Eta ring, lies between the Beta and Gamma rings, and three other faint rings, known as 4, 5, and 6, lie between the Alpha ring and the planet itself Voyager 2 discovered two more even fainter rings The rings of Uranus are dark, narrow, and widely spaced Rings of Neptune Neptune is surrounded by five dark rings Three are quite narrow, like the rings of Uranus The other two are broad and diffuse, more like Jupiter’s rings Moons of Uranus and Neptune Quiz to follow Quiz #1 By receiving a gravity assist from _____, Voyager II shortened its travel time to Uranus and Neptune. A. B. C. D. Mars Pluto Venus Jupiter #2 How many years did it take for Voyager II to reach Neptune? A. B. C. D. six nine twelve fifteen #3 The Hubble telescope showed that Uranus has pink clouds composed of _____. A. B. C. D. water carbon oxygen methane #4 The magnetic field of Uranus runs through one side of the planet. true false #5 Scientists think that the rings of Uranus are made of _____. A. B. C. D. water vapor carbon dioxide liquid hydrogen coal-like particles #6 The largest moon of Uranus is named _____. A. B. C. D. Ariel Titania Umbriel Miranda #7 The great dark spot on Neptune is a _____. A. B. C. D. sea crater storm volcano #8 Scientists think that Neptune’s core is _____. A. B. C. D. cold and icy hot and rocky cool and metallic warm and gaseous #9 Astronomers have identified _____ rings around Neptune. A. B. C. D. three five seven nine # 10 Triton has a dull surface, reflecting almost no light. true false Answers: #1 By receiving a gravity assist from _____, Voyager II shortened its travel time to Uranus and Neptune. A. B. C. D. Mars Pluto Venus Jupiter #2 How many years did it take for Voyager II to reach Neptune? A. B. C. D. six nine twelve fifteen #3 The Hubble telescope showed that Uranus has pink clouds composed of _____. A. B. C. D. water carbon oxygen methane #4 The magnetic field of Uranus runs through one side of the planet. true false #5 Scientists think that the rings of Uranus are made of _____. A. B. C. D. water vapor carbon dioxide liquid hydrogen coal-like particles #6 The largest moon of Uranus is named _____. A. B. C. D. Ariel Titania Umbriel Miranda #7 The great dark spot on Neptune is a _____. A. B. C. D. sea crater storm volcano #8 Scientists think that Neptune’s core is _____. A. B. C. D. cold and icy hot and rocky cool and metallic warm and gaseous #9 Astronomers have identified _____ rings around Neptune. A. B. C. D. three five seven nine # 10 Triton has a dull surface, reflecting almost no light. true false