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.

Views from the Voyagers and Galileo

During the 1970s and 1980s, two Voyager space probes gave us unprecedented images of the jovian planets. Voyager 1 visited Jupiter and Saturn, and Voyager 2 added Uranus and Neptune to the list.
The Voyager missions also revealed volcanic activity on Io, one of Jupiter’s moons. As
for Saturn, a new, previously unknown system of rings emerged: several thousand
ringlets. Ten additional moons were discovered orbiting Uranus, which also revealed
the presence of a stronger magnetic field than had been predicted. And the Neptune
flyby led to the discovery of three planetary rings as well as six previously unknown moons. The hitherto featureless blue face of the planet was now resolved into atmospheric bands, as well as giant cloud streaks. As a result of the Voyager 2 flyby, the magnetospheres of Neptune and Uranus were detected. As with the Van Allen belts around the earth, the magnetospheres of these planets trap charged particles (protons and electrons) from the solar wind.
If only its namesake could have lived to see it. Launched in 1989, Galileo reached Jupiter in 1995 and began a complex 23-month orbital tour of the planet and its almost 400 years after the Italian astronomer first gazed on its colored bands and moons. Among the most extraordinary of
Galileo’s discoveries is a new ring of dust that has a retrograde (backward) orbit around Jupiter. About 700,000 miles (1,120,000 km) in diameter, this doughnut-shaped ring moves in the opposite direction of the rotating planet and its moons. Why does it move in this fashion? No one yet knows.
The Cassini space probe passed Jupiter in early 2001 and sent back images from its many cameras.

Earthbound Views: Jupiter and Saturn

In contrast to Uranus and Neptune, Jupiter and Saturn make for easy viewing. On a good, dark night, even a quite modest telescope will reveal the planets’ belts. The use of colored filters can enhance bands in Jupiter’s atmosphere. Moreover, Jupiter rotates so fast (its day consumes a mere ten hours) that any details you see will perceptibly move across the planet’s face if you observe long enough. Its rapid rotation also makes the planet appear noticeably oblate (elongated). It is even possible to observe the near moons (like Io) emerging from behind Jupiter as they orbit. Although smaller and nearly twice as distant as Jupiter—and therefore appearing much smaller and dimmer than the larger planet—the sight of Saturn through a refractor of at least a 4-inch aperture or a reflector with at least a 6-inch aperture is thrilling. Expect to see the planetary disk and its belts and zones, as well as its celebrated rings (discussed later in this chapter). You may even catch a glimpse of the moons, including Titan, brightest and biggest of Saturn’s nine moons (which we will discuss in the next chapter). Titan’s atmospheric pressure is similar to Earth’s, although its composition and temperature are different. Titan is slightly larger in diameter than the planet Mercury.

Uranus and Neptune from Earth

It is possible for the amateur astronomer to see both Uranus and Neptune. In fact, if you know where to look, Uranus is visible, albeit very faintly, even to the naked eye, provided that the night is very dark, very clear, and you are far from sources of light pollution. To view Neptune, which is much fainter than Uranus, requires an advanced amateur telescope. You don’t have to spend years sweeping the skies to find these dim and distant worlds.
But what can you expect to see? Uranus will appear as a greenish disk, probably featureless—though it is not impossible, given a very good telescope and superb atmospheric conditions, to see atmospheric features and bright spots. It is even possible to see Titania and Oberon, the largest of the planet’s five moons. Even many advanced amateur astronomers have not seen Neptune. Blue in color, it is aptly named for the Roman god of the sea. If you locate the planet at all, it will be a featureless disk.