The Planets (13 page)

Countervailing winds that tear east and west across Jupiter arrange its clouds in a canopy of horizontal stripes. The east-flowing jet streams alternate with the westward trade winds to form some dozen dark belts and bright zones, each one confined to its own latitude band, where it remains fixed over time. Generations of Jupiter
watchers have marveled at the persistence of these neat divisions.

Every band of wind hosts a meteorological drama within its bounds. In the South Equatorial Belt, for example, a stable, oval-shaped storm known as “the Great Red Spot” has been studied continuously since 1879. The Spot has faded from its once vivid vermilion to pale orange, and shrunk to half its former width (though it still exceeds the diameter of Earth) without ever changing lanes. When the Great Red Spot meets other clouds traveling faster or slower in the same direction in the same belt, it sweeps them up and keeps them circling its perimeter for weeks, until they either merge with it or whirl past. Small oval storms that form in the dangerous furrows between east- and west-rushing flows, however, quickly fall victim to shear forces and shred apart in a day or two, like downgraded hurricanes.

Jupiter’s clouds take their red, white, brown, and blue colors from sulfur, phosphorous, and other impurities in the atmosphere. Winds marble the cloud colors, as though with an eye to beauty, and eddies feather the edges of designs. All the colors might well have blended and muddied by now, after
eons of swirling, had not each pigment generally held fast to its own layer at a designated altitude in the atmosphere. The low-down cloud base of warm blues can be glimpsed only through breaks in the overlying browns and whites, which give way, a few hundred miles up, to the high-flying cold reds.

A faint but detectable glow of infrared radiation leaks through gaps in the cloud cover. This is the lingering heat of the planet’s original accretion, rising slowly by convection from the core as Jupiter continues to cool and contract. Half a billion miles from the Sun, Jupiter releases more warmth than it receives. Most of the energy to drive the Jovian winds thus derives from within, augmented only slightly by faint Sunlight falling from afar. Jupiter’s radiance has earned it a reputation as a “failed star,” but its internal temperature, estimated at 17,000 degrees, falls very far short indeed of the fifteen-million-degree inferno that makes the Sun shine.

The vast, variegated clouds, which are all anyone ever sees of Jupiter, constitute only a thin veneer surrounding the planet; they comprise less than 1 percent of its forty-five-thousand-mile radius. Underneath the clouds the atmosphere grows denser and hotter because of mounting pressure,
and the weather stranger. Here the carbon content of methane and other trapped gases may be crushed to tiny diamonds in the sky. Gradually the gases cease to behave as gas, as they dissolve into a sea of liquid hydrogen.

Some five thousand miles down into this milieu, where the pressure reaches at least a million times Earth’s norm, the liquid hydrogen turns opaque, metallic, molten, and electric. By far the greatest part of Jupiter consists of hydrogen compressed to this exotic phase.

According to astrological lore, each planet corresponds to a specific metal, so that silver, for example, pairs with the Moon, gold with the Sun, and mercury with Mercury. Jupiter’s assigned metal has been tin, not hydrogen. But then, no medieval alchemist knew of hydrogen’s existence, let alone the bizarre concoction of liquid metallic hydrogen produced inside Jupiter.

Modern scientists have fabricated only the minutest quantities of liquid metallic hydrogen, by means of reverberating shock waves inside laboratory apparatus, and each such painstakingly made sample lasts just one-millionth of a second. Nevertheless, theorists have gleaned the essence of the substance, and, by extrapolation, explained many
aspects of Jupiter’s nature. Its magnetic field, for example, which is twenty thousand times the strength of Earth’s field, and extends all the way to the orbit of Saturn, arises from the liquid metallic hydrogen interior. A genuine Jovian dynamo is created deep inside the planet, where warm currents of escaping heat stir a susceptible fluid shot through with electric currents generated by Jupiter’s rapid rotation.

The whole mammoth bulk of Jupiter rotates in just under ten hours, faster than any other planet. Its massive body honors the memory of the Solar System’s earliest beginnings as a spinning disk, and none of Jupiter’s attendant moons can slow it down. As to the giant’s rate of revolution in orbit, however, its far remove from the Sun relaxes its pace and adds many miles to its annual travels.

At five times the Earth-Sun distance, Jupiter takes a long year, the equivalent of twelve Earth-years (11 years and 315 days), to orbit the Sun. En route it spends about one Earth-year passing through each of the twelve zodiac constellations. In traditional Chinese astrology, Jupiter’s slow gait earned it the title of “Year Star” (
Sui xing
)—the determiner of the Chinese years of the rat, ox, tiger, rabbit, dragon, snake, horse, sheep, monkey,
rooster, dog, and pig. The Chinese cycle of animals, however, bears only scant relation to the twelve signs of the western zodiac, which include a bull, a lion, and a crab, as well as half-human twins, a virgin, and a water-bearer.

In western astrology, one or another planet “rules” the sign with which it shares a natural affinity. Jupiter, long regarded as the most fortunate planet, rules Sagittarius, the archer, the sign of people born in mid-November through mid-December, who are said to express themselves with open-minded vision and honesty. For many centuries Jupiter also ruled the sign of Pisces, the fish, whose February-March natives (including Galileo) are masters of memory and introspection. But then, after the discovery and naming of Neptune in 1846, the new planet became astrologically associated with water, and so took Pisces away from Jupiter.

Unlike dim, distant Neptune, Jupiter makes such a naked-eye spectacle of golden light in the night sky that its presence has been known since antiquity, and therefore its discovery cannot be dated. Although the time of Jupiter’s birth has been deduced, the place of birth may lie far beyond the region where the planet now resides.

Planetary astronomers say that Jupiter formed 4.5 billion years ago from a seed of rock at a fortuitous location that predisposed it to gigantism. Far from the proto-Sun, the proto-planet rolled through the cold reaches of the primordial nebula, gathering icy tufts of hydrogen-rich compounds such as methane, ammonia, and water. Upon quickly attaining ten or twenty times Earth’s mass, young Jupiter drew in the nebula’s still plentiful light gas, and grew fat on hydrogen and helium.

No small world could have retained such a large envelope of gas, but Jupiter succeeded because of its superior mass, and consequently stronger gravity. Jupiter’s attractive power, the strongest of all the planets, also detoured passing comets from their elongated paths around the Sun, and forced them into Jovian orbit instead. It was most likely by consuming some number of these comets that Jupiter augmented its stores of carbon, nitrogen, and sulfur.

The whole world witnessed one such comet capture when Periodic Comet Shoemaker-Levy 9 crashed through the Jovian cloud banks. In 1992, this comet brushed so close to Jupiter that the planet tore it into twenty-one chunks as big as icebergs, plus many more as small as snowballs. The
pieces then circled Jupiter for two years in single file, like a flying string of pearls, before diving to their destruction, one by one, over a week’s time in mid-July of 1994. As they fell through Jupiter’s atmosphere, they exploded in fireballs and thousand-miles-high plumes of debris.

Each detonation left a huge bruise on the clouds, until a whole necklace of black pearls hung around Jupiter, just south of the Great Red Spot. Although every comet fragment had struck the planet’s far side, out of telescope range, rapid rotation soon carried each new impact into view. The dark stains then spread thin on shock waves and winds, dispersed from day to day, and all but disappeared by late August, before scientists could distinguish between the cometary material and the inventory of elements dredged up from the planet.

Following the comet’s natural, unintentional probing of the Jovian atmosphere, the
Galileo
spacecraft reached Jupiter seventeen months later, in December 1995, and dropped a robot probe carrying seven scientific instruments through the clouds.

In the single hour it operated before being wrecked by heat and pressure, the
Galileo
probe radioed back eyewitness reports. It found that the high winds seen at high altitudes blew far more forcefully lower down, reinforcing the idea that the winds draw their energy from deep within the planet. The probe also measured relatively large quantities of the noble gases argon, krypton, and xenon on Jupiter. The abundance of these substances was what forced astronomers to consider a Jovian birth place far from the planet’s present home—out where frozen caches of noble gases could be incorporated into the infant planet. Later on, they reasoned, Jupiter drifted closer in as a result of countless gravitational interactions with other Solar System bodies.

The uniqueness of the on-site vantage point empowered the
Galileo
probe to overturn long-accepted theories by its every discovery. Likewise the things it failed to find caused consternation and conjecture throughout the planetary science community, as when water turned up missing from the returned data.

Astronomers had predicted that the probe, after piercing the visible, colorful ammonia cloud level, would fall through a thick lower layer of ice- and water-laden clouds, where it could be rained on, even struck by lightning. Classical astrologers had also characterized Jupiter as “moist,” in a
medieval medical system that claimed the various planets’ hot, cold, moist, and dry qualities influenced human health by shifting the balance among the four bodily humors—blood, phlegm, and black and yellow bile. Moist Jupiter, holding sway over the blood, also inspired the “sanguine” temperament in individuals, making Jupiterians generally cheerful, or “jovial,” as opposed to mercurial, martial, or saturnine.

Contrary to all expectations, the
Galileo
probe had by chance encountered a dry area, entering a rare hot spot—one of those breaks in the clouds where Jupiter’s heat escapes into space. In time, however, the
Galileo
orbiter, mother ship to the probe, photographed titanic lightning bolts a thousand times brighter than Earthly discharges, and confirmed the presence of atmospheric water vapor. Indeed, outside the hot-spot “deserts” that continually shift their locations around Jupiter, many parts of the atmosphere appear saturated with water.

The orbiter portion of the
Galileo
spacecraft went on to explore the Jovian system for seven years. Unlike the probe, which made only a quick diagnostic descent into Jupiter, the orbiter became
a long-lived artificial companion to the Galilean satellites.

Galileo
took commands from mission controllers at the Jet Propulsion Laboratory in southern California, who periodically fired the spacecraft’s rocket engine to adjust its orbit, sending it now close in toward Jupiter to visit Europa, now out on a wide loop to fly by distant Callisto. As
Galileo
navigated among the Galilean moons, it discerned the defining characteristic of each: Nearby Io, the reddest, most volcanic body known; Europa, host to an ice-capped salt water ocean; Ganymede, the Solar System’s largest satellite; Callisto, one of the most primitive and pummeled ones.
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Just as the planetary alignments in a horoscope limn the possibilities of a life, so the relative positions of these moons have determined their destiny. Io, the nearest, exhibits the trauma of a too-close attachment. Jupiter’s gravitational pull
has racked Io with tidal stress, keeping its interior permanently melted, so that fire fountains of lava spew unceasingly from some one hundred fifty active volcanoes.

Europa, the next nearest to Jupiter and the smallest of the Galilean satellites, also shows signs of internal heating by tidal stress. But the material melted on Europa has apparently been ice, not rock. Thanks to
Galileo,
many scientists now believe a salty sea, more voluminous than the Atlantic and Pacific together, lies sandwiched between Europa’s frozen surface and its rocky depths, and moreover that its waters might support some form of extraterrestrial life.

Ganymede, though larger than the planet Mercury and farther from Jupiter than Io or Europa, also endures tidal stress. Internal heat keeps Ganymede’s iron core partially molten, and this conductive, convective interior sustains the moon’s own magnetic field, similar to Jupiter’s field, albeit much smaller and weaker.

Only Callisto, beaten and scarred by large ancient impacts, stands aloof from tidal effects. Callisto lies so far from Jupiter that it requires more than two weeks to orbit the planet, while Io makes its way around in less than two days, Europa in
three, and Ganymede in seven. Meanwhile the mammoth invisible bubble of the Jovian magnetosphere, which extends millions of miles into space and engulfs all the planet’s many moons, spins in synch with Jupiter every ten hours.