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Space Topics: UranusFacts and PicturesUranus’ AtmosphereThe atmosphere of Uranus is 84% molecular hydrogen gas, 14% helium gas, and 2% methane, with trace constituents of acetylene, hydrogen cyanide and carbon monoxide. It’s the methane that gives Uranus its serene blue color; methane absorbs red and longer wavelengths more than blue wavelengths. But there are a few infrared “windows” through the methane absorptions. Views of Uranus through these windows peer deeper into the atmosphere than views in visible light and yield pictures of bright storm systems.
Uranus’ storms tend to show up in “alleys” of storm activity within one latitude band. Some of the storms seem to persist for a long period of time; one storm, dubbed “the Great Spot at 37S,” or GS-37S, may have persisted from the time of the Voyager 2 flyby in 1986 to recent Hubble and Keck II observations. Other clouds seem to bloom and disappear over periods of only days. The changes in Uranus’ circulation patterns being seen today are not explained by present theories for Uranus’ atmospheric structure and circulation. Uranus’ InteriorTraditionally, Uranus and Neptune have been lumped with Jupiter and Saturn as the solar system’s “gas giants,” because from the outside they all look like balls of hydrogen and helium gas. But there are differences. Uranus and Neptune are smaller and much less massive than Jupiter and Saturn. Given this size difference, Jupiter should be by far the densest of the outer planets (because its great gravity squeezes its core at very high pressures into a very dense state), followed by Saturn, then Neptune, then Uranus. In fact, Neptune is the densest of them all, followed by Uranus and Jupiter, followed by Saturn. So Uranus and Neptune must be made of denser materials to begin with than the much more massive Saturn and Jupiter. The similar outsides are hiding very different insides. There must be a much greater proportion of ice and rock in Uranus and Neptune than there is in Jupiter and Saturn. In fact, Uranus and Neptune are mostly made of what planetary scientists refer to as “icy” materials -- water, methane, and ammonia -- topped by an atmosphere of gaseous hydrogen. They appear to lack the tremendously thick envelopes of metallic hydrogen that make up the bulk of both Jupiter and Saturn. In this way, scientists say, Uranus and Neptune probably resemble the cores of their bigger neighbors. Because they are composed of these so-called “icy” materials, Uranus and Neptune are now being called “ice giants.” However, the icy materials are likely not solid; they probably exist in a liquid, soupy state in the interiors of the two planets. There might or might not be a solid, rocky core underneath it all. Unfortunately, there is no way to find out for sure what the interiors of these planets are like without an orbiting spacecraft that can measure the planets’ gravitational fields highly accurately. Uranus’ Magnetic Field
Uranus possesses one of the oddest magnetic fields in the solar system. Like all the giant planets, it generates its own powerful magnetic field. However, the north-south axis of Uranus’ magnetic field is tilted 60 degrees to the rotation axis. Even weirder, the center of the magnetic field is not at the center of the planet; it is offset from the center by about a third of the planet’s radius. That may mean that it is not generated within Uranus’ core, but rather at a shallower depth. Neptune’s magnetic field has similar characteristics. Voyager 2 found that, because of the extreme tilt of Uranus, the planet’s magnetotail is wound into a corkscrew shape stretching 10 million kilometers behind the planet. Uranus’ TiltUranus is definitely tipped on its side, with a rotational axis that lies almost in the plane of the ecliptic (the plane in which all the major planets travel around the Sun, or more formally, the plane of Earth’s orbit). The sideways tip gives Uranus the most extreme seasons in the solar system. The southern hemisphere has baked in a decades-long summer while the northern hemisphere has been staring into black space, sunless, for an equally long winter. The situation is changing, though. Uranus is now entering northern hemisphere spring, and on December 7, 2007, the north pole of Uranus will see the Sun for the first time in 42 years.
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