Sunday, February 28, 2010

The Jovian Magnetospheres

Jupiter’s magnetosphere is the most powerful in the solar system. Its extent reaches some 18,600,000 miles (30 million km) north to south. Saturn has a magnetosphere that extends about 600,000 miles (1 million km) toward the sun. The magnetospheres of Uranus and Neptune are smaller, weaker, and (strangely) offset from the gravitational center of the planets.
The rapid rate of rotation and the theorized presence of electrically conductive metallic hydrogen inside Jupiter and Saturn account for the generation of these planets’ strong magnetic fields. While Uranus and Neptune also rotate rapidly, it is less clear what internal material generates the magnetic fields surrounding these planets, since they are not thought to have metallic hydrogen in their cores. With charged particles trapped by their magnetospheres, the jovian planets experience Aurora Borealis, or “Northern Lights,” just as we do here on Earth. These “lights” occur when charged particles escape the magnetosphere and spiral along the field lines onto the planet’s poles. The Hubble Space Telescope has imaged such auroras at the poles of Jupiter and Saturn.

Inside the Jovians

How do you gather information about the interior of planets that lack a solid surface and that are so different from the earth? You combine the best observational data you have with testable, constrained speculation known as theoretical modeling. Doing just this, astronomers have concluded that the interiors of all four jovians consist largely of the elements found in their atmospheres: hydrogen and helium. As we go deeper into the planet, the gases, at increasing pressure and temperature, become liquid. In the case of Jupiter, it is believed that the hot liquid hydrogen is transformed from molecular hydrogen to metallic hydrogen and behaves much like a molten metal, in which electrons are not bound to a single nucleus, but move freely, conducting electrical charge. As we shall see in just a moment, this state of hydrogen is likely related to the creation of Jupiter’s magnetosphere—the result of its powerful magnetic field. Astronomers are less confident about the nature of the very core of Jupiter, though most believe that it is a rocky core the diameter of the earth. Of course, the incredible temperatures and pressures at this depth in Jupiter mean that the material in the core might behave very differently from materials that we have studied on Earth. Saturn’s internal composition is doubtless similar to Jupiter’s, though its layer of metallic hydrogen is probably proportionately thinner, while its core is slightly larger. Temperature and pressure at the Saturnine core are certainly less extreme than on Jupiter.
Uranus and Neptune are believed to have rocky cores of similar size to those of Jupiter
and Saturn surrounded by a slushy layer consisting of water clouds and, perhaps, the
ammonia that is largely absent from the outer atmosphere of these planets. Because
Uranus and Neptune have significant magnetospheres, some scientists speculate that the ammonia might create an electrically conducting layer, needed to generate the detected magnetic field.
Above the slushy layer is molecular hydrogen. Without the enormous internal pressures present in Jupiter and Saturn, the hydrogen does not assume a metallic form.

The Atmospheres of Uranus and Neptune

The atmospheres of Uranus and Neptune have not been probed by unmanned space vehicles, but they have been studied spectroscopically from the earth, revealing that, like Jupiter and Saturn, they are mostly hydrogen (about 84 percent) and helium (about 14 percent). Methane makes up about 3 percent of Neptune’s atmosphere, and 2 percent of Uranus’s, but ammonia is far less in abundance on either planet than on Jupiter and Saturn. Because Uranus and Neptune are colder and have much lower atmospheric pressure than the larger planets, any ammonia present is frozen. The lack of ammonia in the atmosphere and the significant presence of methane give both Uranus and Neptune a bluish appearance, since methane absorbs red light and reflects blue. Uranus, with slightly less methane than Neptune, is blue-green, while Neptune is quite blue.
Uranus reveals almost no atmospheric features. Those that are there are submerged under layers of haze. Neptune, as seen by Voyager 2, reveals more atmospheric features and even some storm systems, including a Great Dark Spot, an area of storm comparable in size to the earth. Discovered by Voyager 2 in 1989, the Great Dark Spot had vanished by the time the Hubble Space Telescope observed the planet in 1994.