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| Uranus, rings and moons. The moons from the top going clockwise are Desdemona, Belinda, Portia, Cressida, and Puck (Credit: HST) |
Uranus is distinguished by the fact that it is tipped on its side with a rotational axis that is inclined to the orbit by 98 degrees. Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history (also note the moons show evidence of a violent event in the past). Note that this unusual axial tilt leads to a peculiar seasonal and diurnal motion as viewed from the ``surface''. For example, during summer in the northern hemisphere an observer would see the Sun making circles in the sky every 17 hours. As summer wanes, the Sun would gradually move south. Eventually, the Sun would rise and set to the autumnal equinox of equal day and night 21 years after the summer solstice. Then the nights would grow longer until one day the Sun would not rise and a long 21 year 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 in the upper atmosphere absorbs red light, giving Uranus its blue-green color. In fact, as we move outward in the Solar System, the inner Jovian worlds (Jupiter and Saturn) are dominated by ammonia for their coloration. But as the temperature drops below 70 K ammonia gas freezes into ice crystals and drops out of the atmosphere. Methane becomes more dominate and, being a blue gas, the outer Jovian worlds (Uranus and Neptune) go from blue-green to deep blue.
The atmosphere is arranged into clouds running at constant latitudes, similar to the orientation of the more vivid latitudinal bands seen on Jupiter and Saturn. Winds at mid-latitudes on Uranus blow in the direction of the planet's rotation. These winds blow at velocities of 90 to 360 miles per hour.
Uranus lacks an internal energy source such as Jupiter and Saturn, and thus its atmosphere energy system is much less active, resulting in fewer features (storms, eddies, etc.). Cloud patterns are only seen at the warmer, lower levels deep below the atmospheric haze. In addition, the tilted axis of Uranus produces uneven warming in the two hemisphere which produce long-term North-South flows across the latitude zones. The combination of these effects means that the atmospheric features are washed out much like Saturn.
Neptune:
Neptune is the outermost planet of the gas giants. It has an equatorial radius about 3.9 Earth radii). If Neptune were hollow, it could contain nearly 60 Earths. Neptune orbits the Sun every 165 years at a distance of 30 A.U.'s. It masses 17 Earth masses and has a mean density of 1.7 g/cc. Neptune has eight moons, six of which were found by Voyager. A day on Neptune is 16 hours and 6.7 minutes long. Neptune was discovered on September 23, 1846 by Johann Gottfried Galle, of the Berlin Observatory, and Louis d'Arrest, an astronomy student, through mathematical predictions made by Urbain 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 accurately calculate its gravitational perturbation of the orbit of Pluto. It is not until the 1989 Voyager flyby that 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 of Jupiter's hurricane-like storms. The largest storm detected was known as the Great Dark Spot, approximately the size of the earth and similar to the Great Red Spot on Jupiter. Other dark spots display cyclone-like structure in their centers.
Just like the storms on Jupiter, the dark spots on Neptune "tumble" along the zones absorbing smaller storms to power themselves. The most surprising thing about these storms is that, unlike Jupiter, they are short-lived. The Great Dark Spot disappeared in HST images taken in 1996.
Long bright clouds, similar to cirrus clouds on Earth, were seen high in Neptune's atmosphere. At low northern latitudes, Voyager captured images of cloud streaks casting their shadows on cloud decks below.
The strongest winds on any planet were measured on Neptune near the Great Dark Spot at 1,200 miles per hour. Most of the winds there blow westward, opposite to the rotation of the planet.
Neptune emits 2.7 times more energy than it receives from the Sun. This access energy powers the atmosphere to produce the storms that are not seen on its twin planet Uranus. The source of internal energy should not be leftover energy from formation (i.e. Jupiter) since Neptune is too small. Nor is it due to an unusual chemical change, such as the helium rain for Saturn. Rather, it is due to the fact that methane is highly abundant in Neptune's atmosphere, and methane is an excellent insulator of heat (i.e. the greenhouse effect). Neptune has a sub-zero type greenhouse effect that is trapping formation heat that should have been radiated billions of years ago like Uranus.
Uranus/Neptune interior:
The interiors of Uranus and Neptune are almost identical. Both have rocky cores like Jupiter and Saturn. But at that point the similarity ends. The pressures are never sufficient to convert molecular hydrogen to metallic hydrogen in the interiors of Uranus and Neptune. Instead, a large mantle of icy water and ammonia forms about 20,000 km below the surface.
Uranus/Neptune magnetic field:
The magnetic fields for both Uranus and Neptune are unusual and are not well understood at this time. As the diagram below shows, the magnetic fields of the strongest three worlds, Jupiter, Saturn and the Earth, are all roughly aligned with the rotational axis of the planets. The generation of these magnetic fields occurs in liquid mantles around solid cores (liquid rock for the Earth, metallic hydrogen for Jupiter and Saturn). Uranus and Neptune have radically different magnetic fields. Not only are they not aligned with the rotational axis of the planet, but neither are they located at the center of the planet either. The magnetic fields are probably be generated by local events in the icy mantles of both planets and may be unstable.
The best theory for the origin of these magnetic fields involves the high concentration of ammonia in the planet's interiors. Ammonia in solution is high electrically conductive. This is due to a high amount of free ions (atoms missing electrons so that they have net positive charge). These free ions could form a conducting ionic layer in the mantle which would then produce a magnetic field with Uranus and Neptune's high rotation rates.
