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Astronomy Worlds Uranus Neptune & Remote Astronomy Worlds A. Dayle Hancock Uranus and Neptune [email protected] Orbits and Atmospheres Small 239 Internal Structure Magnetic Fields Office hours: Rings Uranus's Large Moon MTWR 10-11am Neptune's Triton Pluto http:// physics.wm.edu/~hancock/171/ 1 Uranus th Uranus is the 7 planet from the Sun. It was discovered in 1781 by William Herschel. 2 Neptune Neptune is the 8th planet from the Sun. It was discovered in 1846 by noting the small discrepancy in Uranus's orbit and calculating its position using Newton's law of gravity. 3 Uranus When Voyage 2 pasted by Uranus in 1986, the Jovian gas planet appeared nearly featureless. Clouds were not visible even in the close range images. The atmosphere is 82.5% H, 15.2% He which is similar to Jupiter and Saturn. Unlike Jupiter and Saturn, Uranus's atmosphere has 2.3% methane (CH4). T = 55K! Clouds, storms and bands are visible in the IR. Uranus orbits 19.2 AU from the Sun. The most unusual thing about Uranus is its axis of rotation is tilted 97.9o to its orbital plane. Since Uranus's orbital period is 84 years, it is summer at the south pole for 42 years and winter for 42 years 4 Neptune Voyage 2 past by Neptune in 1989. While it has about the same diameter as Uranus, it is 18% more massive. The atmosphere is 79% H and 18% He, It has 3% CH4 and almost no H2O. It is cold at 55K. There are clouds (like the white Cirrus clouds of CH4 ice.) There are also bands and storms visible in enhanced images. The Great Dark spot has disappeared. Neptune orbits 30 AU from the Sun. Neptune's orbital period is 165 years. Uranus and Neptune receive little solar energy. Unlike Uranus, Neptune emits more energy than it receives probably due to slow contraction. 5 Densities of Uranus and Neptune The densities of Uranus (1320 kg/m3) and Neptune (1640 kg/m3) are as large or larger than Jupiter (1330 kg/m3) and Saturn (690 kg/m3). Both planets have a higher concentration of heavy elements than Jupiter or Saturn which is unexpected. Their great distances from the sun is also surprising. That far out there should have not been enough material to form these relatively large planets. 6 Magnetic Fields of Uranus and Neptune Magnetometer on Voyage 2 reveled some surprises about the magnet fields of Uranus and Neptune. The axis of the Magnetic field on Uranus is tilted at 59o to its rotation axis. On Neptune it is 47o. Both fields are offset from the center of the planet. The fields are created by the dynamo effect. The conducting rotating liquid is high compressed H20 and NH which is conducting. 3 7 Rings of Uranus Uranus has three rings. The rings are narrow ( <10 km) and dark. They were discovered in 1977 by observing the occultation of a star with ground based telescopes. They are thought to be composed of carbon compounds which resulted from radiation darkening of CH4 ice. The two faint outer rings were only discovered in 2005. 8 Rings of Neptune The rings of Neptune are dark and thin like Uranus. They are also thought to be radiation darkened CH4. The big surprise about Neptune's rings is that they are disappearing. They could be gone in a century if the decay continues. 9 Moons of Uranus Uranus has 5 large moons and 22 small moons. Of the 5 large moons Umbriel and Oberon are heavily cratered dead worlds. Ariel and Titania surface appears to have cracked and the surface flooded with ice lava. Miranda (bottom image) is cratered but has the unique 'chevron' feature with cliffs twice as high as Mt. Everest. 10 Moons of Neptune Neptune has 13 known moons. Most are small icy bodies similar to the small moons of Uranus. Triton is the one large moon. Triton is in a retrograde orbit around Neptune. The orbit is inclined by 23o. It probably formed else where in the solar system. It surface is mainly water ice and is young from tectonic activity due to tidal forces. There are signs of old ice volcanoes. Triton has a thin tenuous atmosphere. Because of the retrograde orbit, Triton is moving closer to Neptune. It will eventually reach the Roche limit and break up forming a major ring system.11 Pluto Pluto was discovered in 1930. It is no longer considered a planet because of it small size (2300 km). Its orbit is highly eccentric (e= .25) and the plane of its orbit is steeply inclined (17.2o). This is more than any planet. Its large eccentricity causes it to be closer to the Sun than Neptune during part of its orbit. It axis of rotation is tilted by 120o (causing extremely long seasons) and its rotation is retrograde. 12 Pluto Before July 2015, the best images of Pluto were from the Hubble space telescope. Because Pluto is so far way (semimajor axis = 39.5 AU), the Hubble images do not show any distinct surface features. 13 Pluto In July of 2015, the New Horizons space probe flew past Pluto and its moons. This image from the flyby shows a more complex object than expected. Pluto's surface is composed of more than 98% nitrogen ice, with traces of CH 4 and CO. Pluto's density is 1.860 kg/m3. The interior is probably differentiated with a rocky core and an ice mantle. 14 Charon Charon is the largest moon of Pluto. In fact, the diameter of Charon (1190 km) is about half the size of Pluto (2270 km), This is a recently downloaded image of Charon from the New Horizon flyby. Note the riff near the equator and the area flat dark area near the north pole. Charon orbital period is 6.4 days which is the same as the rotation period of Pluto and the rotation period of Charon. Charon and Pluto always keep the same face toward each other. 15 Trans-Neptunian Objects Eris like Pluto is a 'dwarf planet'. Eris is the largest of the trans-Neptunian objects. Eris is highly reflective. Its simimajor axis is 68 AU. Like Pluto and the other trans Neptunian objects its orbit has a large eccentricity and the orbit plane is tilted to the ecliptic. Many have satellites of their own (Dysnomia in the case of Eris). Over 35,000 trans- Neptunian object are thought to exist. 16 Trans-Neptunian Objects 17 Kuiper Belt Most of the Trans- Neptunian object lie within the Kuiper belt which extends from 30-50 AU. Eris and Sedna are actually even further out. The Kuiper belt consists of objects left over from the formation of the solar system. It does not appear to be unique to our solar system. The image shows the edge on 'Kuiper' belt of a star which is 57 light years away. 18.
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