le> Nov 18: Uranus/Neptune
Uranus:Uranus is the seventh planet from the Sun and is the third largest in thesolar system. It was discovered by William Herschel in 1781. It has anequatorial diameter of 51,800 kilometers (1.9 Earth radii) and orbits theSun once every 84.01 Earth years. It has a mean distance from the Sun of19.1 A.U.”s. The length of a day on Uranus is 17 hours 14 minutes. Uranusmasses 14.5 Earth masses.Uranus is distinguished by the fact that it is tipped on its side with arotational axis that is inclined to the orbit by 98 degrees. Itsunusual position is thought to be the result of a collision with aplanet-sized body early in the solar system”s history (also note themoons show evidence of a violent event in the past). Note that thisunusual axial tilt leads to a peculiar seasonal and diurnal motion asviewed from the “surface””. For example, during summer in the northernhemisphere an observer would see the Sun making circles in the sky every17 hours. As summer wanes, the Sun would gradually move south.Eventually, the Sun would rise and set to the autumnal equinox of equalday and night 21 years after the summer solstice. Then the nightswould grow longer until one day the Sun would not rise and a long 21year night would begin.Uranus”s atmosphere:The atmosphere of Uranus is composed of 83% hydrogen, 15% helium, 2%methane and small amounts of acetylene and other hydrocarbons. Methane inthe upper atmosphere absorbs red light, giving Uranus its blue-greencolor. In fact, as we move outward in the Solar System, the innerJovian worlds (Jupiter and Saturn) are dominated by ammonia for theircoloration. But as the temperature drops below 70 K ammonia gas freezesinto ice crystals and drops out of the atmosphere. Methane becomes moredominate and, being a blue gas, the outer Jovian worlds (Uranus andNeptune) go from blue-green to deep blue.The atmosphere is arranged into cloudsrunning at constant latitudes, similar to the orientation of the morevivid latitudinal bands seen on Jupiter and Saturn. Winds at mid-latitudeson Uranus blow in the direction of the planet”s rotation. These winds blowat velocities of 90 to 360 miles per hour. Uranus lacks an internal energy source such as Jupiter and Saturn, andthus its atmosphere energy system is much less active, resulting in fewerfeatures (storms, eddies, etc.). Cloud patterns are only seen at thewarmer, lower levels deep below the atmospheric haze. In addition, thetilted axis of Uranus produces uneven warming in the two hemispherewhich produce long-term North-South flows across the latitude zones. Thecombination of these effects means that the atmospheric features arewashed out much like Saturn.
Neptune:Neptune is the outermost planet of the gas giants. It has an equatorialradius of 24,900 kilometers (about 1.4 Earth radii). If Neptune werehollow, it could contain nearly 60 Earths. Neptune orbits the Sun every165 years at a distance of 30 A.U.”s. It masses 17 Earth masses and hasa mean density of 1.7 gm/cc. Neptune has eight moons, six of which werefound by Voyager. A day on Neptune is16 hours and 6.7 minutes long.
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Neptune was discovered on September 23,1846 by Johann Gottfried Galle, of the Berlin Observatory, and Louisd”Arrest, an astronomy student, through mathematical predictions made byUrbain Jean Joseph Le Verrier. Note that mass estimates of Neptune since its 1846 discovery have varied by a factor of 3! Thus for many decades the mass of Neptune was unknown and therefore one could not accturately calculate its gravitational ppertubatinon of the orbit of Pluto. It is not until the 1989 Voyager flyby thata high accurate (good to 0.001 earth masses) measurement of Neptune”s mass was obtained. The official value is 17.147 earth masses. Neptune”s atmosphere:Neptune is a dynamic planet with several large, dark spots reminiscent ofJupiter”s hurricane-like storms. The largest spot, known as the Great Dark Spot, is about the size of theearth and is similar to the Great Red Spot on Jupiter. Other dark spots display cyclone-likestructure in their centers. Just like the storms on Jupiter, the dark spots on Neptune “tumble”” along the zones absorbingsmaller storms to power themselves. The most surprising thing about thesestorms is that, unlike Jupiter, they are short-lived. Recent HST images do not show the Great DarkSpot.Long bright clouds, similar to cirrusclouds on Earth, were seen high in Neptune”s atmosphere. At lownorthern latitudes, Voyager captured images of cloud streaks casting theirshadows on cloud decks below.The strongest winds on any planet were measured on Neptune. Most of thewinds there blow westward, opposite to the rotation of the planet. Nearthe Great Dark Spot, winds blow up to 1,200 miles an hour.Neptune emits 2.7 times more energy than it receives from the Sun. Thisaccess energy powers the atmosphere to produce the storms that are notseen on its twin planet Uranus. The source of internal energy should notbe leftover energy from formation (i.e. Jupiter) since Neptune is toosmall. Nor is it due to an unusual chemical change, such as the heliumrain for Saturn. Rather, it is due to the fact that methane is highlyabundant in Neptune”s atmosphere, and methane is an excellent insulatorof heat (i.e. the greenhouse effect). Neptune has a sub-zero typegreenhouse effect that is trapping formation heat that should have beenradiated billions of years ago like Uranus.Uranus/Neptune interior:The interiors of Uranus and Neptune are almost identical. Both haverocky cores like Jupiter and Saturn. But at that point the similarityends. The pressures are never sufficient to convert molecular hydrogento metallic hydrogen in the interiors of Uranus and Neptune. Instead, alarge mantle of icy water and ammonia forms about 20,000 km below thesurface.
The best theory for the origin of these magnetic fields involves thehigh concentration of ammonia in the planet”s interiors. Ammonia insolution is high electrically conductive. This is due to a high amountof free ions (atoms missing electrons so that they have net positivecharge). These free ions could form a conducting ionic layer in themantle which would then produce a magnetic field with Uranus andNeptune”s high rotation rates.