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.

Jupiter's Layers of Gas

On July 13, 1995, Galileo released an atmospheric probe, which plunged into Jupiter’s atmosphere and transmitted data for almost an hour before it was destroyed by intense atmospheric heat and pressure. After analysis of this data (and earlier data from Voyager), astronomers concluded that Jupiter’s atmosphere is arranged in distinct layers. Since there is no solid surface to call sea level, the troposphere (the region containing the clouds we see) is considered zero altitude, and the atmosphere is mapped in positive and negative distances from this. Just above the troposphere is a haze layer, and just below it are white clouds of ammonia ice. Temperatures in this region are 125–150 K. Starting at about –40 miles (60 km) below the ammonia ice level is a cloud layer of ammonium hydrosulfide ice, in which temperatures climb to 200 K. Below this level are clouds of water ice and water vapor, down to about –60 miles (100 km). Further down are the substances that make up the interior of the planet: hydrogen, helium, methane, ammonia, and water, with temperatures steadily rising the deeper we go.

Bands of Atmosphere

The atmospheric bands that are Jupiter’s most striking feature are the result of convective motion and zonal wind patterns. Warm gases rise, while cooler gases sink. The location of particular bands appear to be associated with the wind speed on Jupiter at various latitudes.
Anyone who watches an earthly television weather forecast is familiar with high-pressure and lowpressure areas. Air masses move from high pressure regions to low pressure regions. But we never see these regions on the earth as regular zones or bands that circle the planet. That’s because the earth doesn’t rotate nearly as fast as Jupiter. The rapid rotation of the giant planet spreads the regions of high and low pressure out over the entire planet.

The Great Red Spot

The Great Red Spot was first reported by the British scientist Robert Hooke (1635–1703). It is a storm, a swirling hurricane or whirlpool, of gigantic dimensions (twice the size of the earth), at least 300 years old. It rotates once every six days and is accompanied by other smaller storms. Neptune has a similar storm called the Great Dark Spot.
How could a storm last for three centuries or more?
We know from our experience on the earth that hurricanes form over the ocean and may remain active there for days. Once they move over land, however, they are soon spent (albeit often destructively); the land mass disrupts the flow pattern and removes the source of energy. On Jupiter, however, there is no land. Once a storm starts, it continues indefinitely, until a larger storm disrupts it. The Great Red Spot is the biggest storm on the planet.

Rotation: A New Twist

With all the bands and surface features of the biggest jovian planets, you’d think it would be relatively easy to calculate rotation rates “by eye.” Just look for a prominent surface feature and time how long it takes that feature to make one trip around.
Well, it’s not so easy. Because these planets lack solid surfaces, different features on the surface actually rotate at differing rates! This differential rotation is not dramatic in the case of Jupiter whose equatorial region rotates only slightly faster than regions at higher latitudes. East-west winds move at about 190 miles per hour (300 km/h) in Jupiter’s equatorial regions, and at a zippy 800 miles per hour (1,300 km/h) in the equatorial regions of Saturn. It turns out that the best way to clock the rotation rates of these planets is not to look at their atmospheres, but to measure something tied to the planets’ cores. The periods of fluctuation in the radio emission (which arise from the planets’ magnetic fields) are taken to be the “true” rotation rate.
While Neptune and Saturn are slightly tipped on their axes similar to the earth (30, 27, and 24 degrees, respectively), Jupiter’s axis is nearly perpendicular to the plane of its orbit; the planet tilts from the perpendicular a mere 3 degrees. The true oddball in this respect is Uranus, which tilts 98 degrees, in effect lying on its side. The result of this peculiarity is that Uranus has the most extreme seasons in the solar system. While one pole experiences continuous daylight for 42 Earth years at a stretch, the other is plunged into an equal period of darkness.
It’s interesting to note that if the earth were tipped on its axis like Uranus, a city like Atlanta would experience 70 days when the sun never rose, and 70 days when the sun never set. The North Pole would have 6 months of darkness, and 6 months of sunlight.
On the vernal and autumnal equinoxes, day and night in Atlanta would still each last 12 hours.