Chapter 2-3

The Gas Giants

photo of red and white spiral clouds that cover Jupiter
Great storms of Jupiter.
An exercise in cosmic modern art, huge whirling storms and sawtoothed, turbulent flows spread out in Jupiter's atmosphere as pictured by Voyager 2 from 6 million kilometers (3.7 million miles). The Red Spot (right center) is a huge storm system, big enough to hold three Earths, that has persisted for at least three centuries. It whirls counterclockwise, producing highly contorted patterns at its left, where cloud banks moving left to right are blocked and forced to squeeze past it. Smaller white oval storms, about the size of Earth, create similar turbulent effects below the Red Spot. Most patterns in Jupiter's atmosphere are constantly changing; the structures shown here have changed significantly since Voyager 1 photographed them four months previously. Jupiter's atmosphere is composed almost entirely of colorless hydrogen and helium; the colors come from small amounts of unknown substances, perhaps compounds of sulfur and phosphorus.


Jupiter is where the action is. The planet is big enough to hold 1400 Earths and is almost 2.5 times more massive than all of the other planets put together. It is a huge, rapidly spinning blob of cold gases - hydrogen, helium, a little methane, water, and ammonia - all colored by traces of more complex but largely unknown chemicals. The outer part of the atmosphere is all that we see of Jupiter. There, clouds form in belts and stripes, in consequence of the fast rotation. This planet, 11 times the diameter of Earth, spins on its axis in less than half an Earth day.

Deep inside Jupiter's atmosphere, the pressure becomes enormous, and hydrogen, a gas at the higher altitudes, is condensed to a liquid. At about 25,000 kilometers (16,000 miles) below the cloud tops, the pressure attains a value 3 million times that of the Earth's atmosphere at sea level. At this pressure, the liquid hydrogen transforms to a metallic state.

Currents circulating through the me tallic hydrogen fluid generate a mag netic field that is about 14 times stronger at Jupiter's cloud tops than the field at the surface of the Earth. Disturbances in the magnetic field around Jupiter produce powerful bursts of radio waves, making Jupiter the noisiest radio transmitter in the solar system, other than the Sun. Jupiter's magnetic field is much stronger than the Earth's, and it is much less compressed by the solar wind, the stream of charged atoms that pour out from the Sun. Jupiter's field forms a huge magnetosphere around the planet, and the fleld sweeps out on the side opposite the Sun to give Jupiter a fat "magnetic tail" at least 150 times wider than the diameter of the planet itself.

A menagerie of moons.
Jupiter's four largest moons,first seen as tiny dots of light in Galileo's telescope, are revealed as strange new worlds by the cameras of Voyagers 1 and 2. Seen here in their relative proportions, they show a bewildering variety. Each is different from our own Moon and different from the others. Io is a red-orange world, pitted by the craters of active volcanoes that constantly renew its surface with sulfur and sodium compounds.
images of Jupiter's orange colored moon Io and blue-green Ganymede

image of the moons Europa and Callisto
Europa is a yellowish, smooth globe, crisscrossed with dark lines that may be the fractures of an icy crust. Ganymede, larger than our Moon, has light and dark regions dotted by bright impact craters that may have exposed a subsurface ice layer. Callisto, brownish and heavily cratered, has perhaps the oldest planetary surface yet discovered, its landscape sculptured by an intense meteorite bombardment during the formative stages of the solar system.

Jupiter's composition of hydrogen and helium is the same as the Sun's. Recent theoretical studies have carried the similarity even further, suggesting that if Jupiter had been only a little larger, it might have be come another Sun, the gas at its center compressed so much that nuclear reactions would have begun. Thus, Jupiter is a sort of "star that failed", although it still radiates, by compressing its own core, twice the amount of energy that it receives from the Sun.

The most striking resemblance, however, is that Jupiter is the center of a miniature solar system, surrounded by an array of at least 16 moons, just as the Sun is surrounded by planets, comets, and a belt of asteroids.

Jupiter, remarkable even as observed from Earth, revealed even more wonders as our spacecraft sped past it in recent years. Cameras gave us a close view of its banded atmosphere, and we learned that the seemingly smooth belts of color were actually separated by zones of violent turbulence. Earth-sized blobs of colored gas travel along huge "jet streams", spin, and collide. Great Plumes rise from deep in the atmosphere and leave trails 10,000 kilometers (over 6200 miles) long across the planet. The great Red Spot, a long-lived oval as big as three Earths (it was first seen at least 300 years ago), is a complex, swirling hurricane. Even the night side of Jupiter is alive with the flash of lightning superbolts and the glowing bands of huge aurorae.

The spacecraft passing Jupiter discovered three new moons, adding to the 13 already observed from Earth. Inside the orbit of the innermost moon they found a ring, a thinner version of the rings of Saturn, never seen from Earth. The ring, made of tiny particles, apparently reaches right down to the cloud tops of Jupiter, and it has stimulated a major debate about how it formed and how it continues to exist so close to Jupiter.

The moons of Jupiter are equally spectacular, remarkably different from our own Moon and from each other. The two Voyager spacecraft discovered that they are very unlike what astronomers expected. Amalthea, once considered Jupiter's innermost moon, is an irregular rock about 270 kilometers (170 miles) long, but only about 155 kilometers (96 miles) wide along one axis. Beyond it are the four Galilean satellites (so-called because Galileo discovered them).

Three of them are larger than our own Moon, including Ganymede, which is even bigger than the planet Mercury.

Volcanoes of Io.
Jupiter's moon Io displays the only active volcanoes found outside the Earth. Driven by tidal heating as Io circles mighty Jupiter, the volcanic eruptions are still shaping the moon's surface. They spray sodium and sulfur atoms, making a cloud that surrounds Io's orbit. In this computer-enhanced picture from Voyager 1, blue plume on the horizon consists of material hurled upward from volcano to more than 150 kilometers (about 90 miles) above Io's blotchy red-orange landscape.
close up photo of Io's volcanic surface

Io, the innermost Galilean satellite, was one of the biggest surprises of the Space Age, a brilliant red orange world with more than 100 volcanoes.

The two Voyagers saw seven of the volcanoes in continual eruption, shooting umbrella-shaped plumes of sulfur particles high above Io's sur face. These unique volcanoes, the only active ones known outside the Earth, seem to be ejecting molten sulfur at a temperature of a few hundred degrees, not molten rock at more than 1000 C. The heat comes, not from radioactivity as in the Earth, but from tidal forces and perhaps also huge electric currents that act on Io as it swings around Jupiter.

Europa, the next moon out, resembles a smooth, yellowish billiard ball and comprises perhaps the flat test real estate in the solar system. It is crisscrossed with thin lines, some several thousand kilometers long, that remind some observers of a kind of modern art. The lines may be the traces of a shifting icy crust, but we have no idea of what forces have acted, and continue to act, to keep this world so flat.

Ganymede, the largest of Jupiter's moons, looks a little more familiar. Like our own Moon, its surface is divided into light and dark regions, and we can see craters. There the similarity ends. Ganymede is a light weight moon, a "snowball" of mixed ice and rock. Bright rays extend from the craters, perhaps consisting of fresh ice. The light areas of Ganymede display long bands of parallel grooves, unique and puzzling structures which indicate that internal forces have shaped the surface.

Callisto, the outermost Galilean moon, is a heavily cratered, brownish world. It seems to be a snowball moon like Ganymede, but its surface is uniformly cratered and preserves a long history without any disruption.

Callisto may have the oldest surface yet observed in the solar system, its craters perhaps dating back to an ancient era of bombardment, which ended about 4 billion years ago. Callisto also bears what may be the largest crater anywhere in the solar system, a huge, multi-ringed basin more than 2500 kilometers (1600 miles) across.

Even the space around Jupiter turns out to be exciting. Jupiter's huge magnetic field is in a constant struggle with the streams of charged particles that speed outward from the Sun. As a result, the Jovian magnetosphere contains regions of highly charged trapped radiation particles, like the Earth's Van Allen belts, but much larger. The four large moons lie with in this belt of radiation, and it affects them. Sulfur and sodium atoms blasted out of Io by volcanoes form a glowing, doughnut-shaped band in Io's orbit around Jupiter. An intense 3-million ampere electric current links Io to the top of Jupiter's atmosphere, continuously flowing from moon to planet and back again. Further out, beyond the moons, there is a region of space where the atomic particles are so energetic that their temperature has risen to about 3 million degrees, the hottest place in the solar system except for the Sun.

The Jovian system of Jupiter and its moons is a place where all the forces of the solar system - atmospheres, volcanoes, cratering, magnetism, charged particles, radiation - are present on scales so vast that they inspire excitement and awe. We have seen only partially and briefly the wonders that exist there, and we are only beginning to understand it all.

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