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In a turbulent universe, filled with supernovae, stellar and galactic collisions, meteor and cometary impacts, black holes, and burning radiation, our solar system is located in a relatively peaceful corner. The Sun's light is steady, and the possibility of a nearby supernova or stellar collision remote. There are no black holes near our solar system. The Earth's protective magnetic field and atmosphere protect life from the harshest radiation of our life-giving Sun. Our Milky Way Galaxy has been forecast to collide with the Andromeda Galaxy, but there's no need to alter life plans. The Andromeda Galaxy is nearly 2.2 million light years away, and even at its velocity of a remarkable 68 km/s will not reach us for another few billion years.

However, from July 16-22, 1994, our solar system was rocked by an event that had never been seen before: fragments of the Shoemaker-Levy 9 comet collided with the planet Jupiter. That spectacular event gave new vigor to the idea that the giant outer planets, particularly Jupiter, function as "protectors," which prevent smaller objects, such as comets originating from outside the solar system, from reaching the inner planets by drawing them into their gigantic gravitational maws. The escape velocity of Jupiter, for example, is an impressive 59.5 km/s, while the Earth's is a paltry 11 km/s. The tremendous gravitation field created by this massive gas giant can either "slingshot" incoming celestial objects back out of the solar system, or even draw these objects directly towards its surface, as in the case of Shoemaker-Levy 9.

Our Guardian Uncle?

In their book Rare Earth: Why Complex Life Is Uncommon in the Universe, geologist Peter Ward and astronomer Donald Brownlee looked at the conditions necessary for life to evolve to an intelligent level. One key condition they identified is a stable planetary environment. The protection provided by a gigantic gravitational field in the outer reaches of the system that limits the possibility of catastrophic collisions in the "habitable" inner system is a stabilizing feature that allows life to evolve to its fullest potential. So it seems that Jupiter, chief god of the Roman pantheon, may have been a key factor in the evolution of our species, acting as a distant protective uncle over the eons. However, if circumstances had been different, it might even have been a parent.

Until exploration of Jupiter began (with the spacecraft Pioneer 10 and 11, which flew by in 1973 and 1974, respectively, and the two Voyagers, both of which flew by in 1979), much of our knowledge of the planet was speculation and extrapolation. It was only with the launch of Galileo on October 18, 1989, that we began to learn much more about this mysterious giant. Instead of merely taking pictures of the planet on a flyby, as did her predecessors, Galileo was comprised of an orbiter and an atmospheric probe. It even was at the right place at the right time when Shoemaker-Levy 9 entered the Jovian atmosphere, producing some spectacular footage.

If Jupiter were about fifty to one hundred times more massive than it is—no small increase, as Jupiter is already a very massive object on the scale of our solar system—it would have had the mass necessary to become a star; in effect, we might have been a part of a binary star system. Imagine what the world would be like if that were the case. (Brian Aldiss' Helliconia series explores the possible effects of such a system on a life-bearing planet.) In Arthur C. Clarke's 2010, a mysterious monolith ignites Jupiter to become the sun, known as Lucifer, for the Jovian system of moons: a solar system within the Solar System. Jupiter's moons are fascinating objects in their own right, and deserve separate mention before we continue with Jupiter itself.

The Galilean Moons

(left to right) Io, Europa, Ganymede, Callisto
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The Jovian moons range in size from large asteroids to small planets in their own right. The largest moons are known as the Galilean moons, after their discoverer Galileo Galilei, who first saw them through his primitive telescope. The first Galilean moon is Io, known for its very youthful surface. The reason for this is that Io is the most volcanically active body in the Solar System, expelling about 100 times as much lava as the Earth, in spite of being about one fourth of the Earth's size. The next, Europa, is suspected of being another world in the solar system that may harbor life beneath its icy exterior.

Ganymede is Jupiter's largest moon, and has its own magnetic field, the only satellite in the solar system that has one. The last Galilean moon is Callisto, which is heavily pitted with impact craters. Although Callisto, unlike its Galilean brethren, appears to be geologically dead, data gathered by Galileo suggests an active world that appears to create a disturbance in the Jovian magnetic field. So far, scientists can only speculate on the cause of this; one theory is that a global layer of seawater some tens of kilometers thick might be creating this disturbance. All of these satellites are significantly affected by the presence of their enormous parent: tidal forces created by Jupiter's gravitational field are considered to be the source of Io's vulcanism, as well as the possible subsurface oceans of Europa and Callisto.

The Rings of Jupiter

In addition to its many moons, Jupiter has a ring system. It is not as complex as the ring system around the planet Saturn, but it is impressive nonetheless. Jupiter's rings were first discovered by Voyager 1 in 1979. The ring system is comprised of an inner halo, the main ring, and the Gossamer rings. The rings themselves are the result of dust from interplanetary meteoroids that smash into the four innermost moons of Jupiter: Metis, Adrastea, Thebe, and Amalthea.

The innermost halo ring is toroid in shape and extends radially from about 92 to 122,500 km from Jupiter's upper cloud deck. The main and brightest ring extends from the halo boundary to about 128,940 km, just inside the orbit of Adrastea. The Gossamer rings are uniform, with the innermost Gossamer ring extending from the orbit of Adrastea to the orbit of Amalthea at 181,000 km. The fainter Thebean Gossamer ring extends from Amalthea to the orbit of Thebe at 221,000 km. Voyager 1 showed the Gossamer ring only as a single entity, but Galileo found that the Gossamer ring is actually two rings, one ring embedded in the other. It's not mere chance that these rings' boundaries coincide with the orbital paths of the inner moons: in fact, it's the gravitational influences of these moons that keep the rings' particles in place, and hence these moons are sometimes referred to as the "shepherd moons."

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The rings and the moons of Jupiter exist in an intensive radiation belt of electrons and ions trapped by the enormous magnetic fields of Jupiter. The size of Jupiter's field is larger than that of any other planetary body in the solar system; only four other planets have magnetic fields that extend beyond their internal bounds: Mercury, Earth, Jupiter, and Saturn. Even given its size, Jupiter's field is proportionately larger than what would be expected. The solar wind shapes the field into a teardrop shape, with the charged particles continually being ejected into interplanetary space. On the sunward side of the planet, the magnetic field extends from 5 to 12 million km into space, while on the far side of the field stretches outward about 1 billion kilometers, almost to the orbit of Saturn.

The Voyager and Pioneer spacecraft detected a large donut-shaped collection of charged particles in the area of Io. It is thought that this is material from the volcanoes of Io, expelled at a rate of 1000 kg/s into the magnetic field of Jupiter to form the doughnut-shaped Torus of Io. The Torus is, in effect, the largest electrical circuit that we know of, with a generating capacity of about 2 trillion watts. This enormous field is generated by a combination of Jupiter's size and rapid orbital rate (0.41 Earth days). What lies beneath holds more clues to the source of Jupiter's enormous magnetic field.

Atmosphere, Winds, and Storms

When we look at Jupiter through a telescope, we see a banded planet made up of dark belts and lighter zones. This is only the upper cloud of the planet's atmosphere. When the Galileo probe penetrated into the upper icy ammonia clouds seen at the surface of the cloud deck on December 7, 1995, it discovered a range of chemicals, from the most common, hydrogen (comprising about 81% of the atmosphere), to helium, methane, ammonia, water, hydrogen sulphide, deuterium, neon, argon, krypton, and xenon.

At the bottom of the cloud deck, the weather was expected to turn windy, cloudy, hot, and humid, and the atmosphere to be a water cloud at 5-10 atmospheres of pressure. This turned out not to be the case at all. Evidence of clouds was non-existent; instead, the light was hazy, and the pressure was only 1.6 atmospheres. The lightning detector on the probe, which was effectively an AM receiver, detected only faint discharges. In summary, the weather on Jupiter was found to be a lot drier and clearer than had been expected. What had happened? The probe had entered one of Jupiter's hot spots, which act as shunts through which infrared radiation from lower levels leaks out. Jupiter has a number of these spots, and they move around constantly, so there was no way the probe could have avoid the hole.

Though the probe's lightning detector only found distant radio noise, the orbiter observed enormous flashes in the clouds, indicators of massive lightning activity, larger than anything seen on Earth. Like its Voyager predecessors, Galileo found that the lightning was concentrated in a few zones of latitude. The depth of the lightning was estimated from the size of the illumination on the cloud tops, with the theory that the bigger the spot, the deeper the discharge. From these observations, the lightning appeared to be originating in the area where water clouds were thought to be formed, similar to the process on Earth.

The lack of lightning activity in the vicinity of the probe, coupled with the enormous flashes of light visible to the orbiter, indicated that there are wet and dry areas on Jupiter. In hot spots such as the Galileo probe entry site, cold air descends from the upper atmosphere and is as dry as the Sahara. In other areas, there are wet spots that create the giant lightning flashes. In further support of this model, the orbiter found that the abundance of water and ammonia in the Jovian atmosphere can vary greatly.

One thing that the scientists did correctly predict is that Jupiter's atmosphere is windy—Jovian winds achieve velocities of several hundred miles per hour. This is similar to Earth's jet stream, but the velocity of terrestrial winds dies down as the winds approach the Earth's surface, while Jupiter's surface is much further below its atmosphere. Prior to Galileo, scientists had long speculated on the causes of the winds of Jupiter. On Earth, the winds are driven by temperature differentials between the poles and the equator, whereas the temperature of Jupiter's cloud tops is relatively uniform, with an average of -108 C. There were two main explanations for Jupiter's winds, depending on the energy source driving the winds. If the energy source were internal, for example caused by gravitational contraction, winds would be expected to increase in speed, or at least remain the same, with increasing depth. If the source of energy were external, perhaps sunlight, the winds would die down with depth. The Galileo probe discovered that the winds actually increased in speed with depth before leveling off and becoming constant, and thus the energy source driving the enormous winds of Jupiter is internal.

With an active atmosphere such as we find on Jupiter, we can also safely assume that it is a planet of intense storm activity, as indeed it is. Next to Saturn's rings, the Great Red Spot of Jupiter is one of the most well-known and recognizable features of our solar system. The Great Red Spot is, in effect, a giant storm, larger than the Earth itself, and has been raging for about three hundred years. The spot, a high pressure system (unlike Earth cyclones, which are low pressure zones), rotates in a counterclockwise pattern every six days. Its reddish color is somewhat of a mystery, but there is speculation that phosphorous may be the cause. In addition to the Great Red Spot, Galileo found two cold storms, called white ovals, that merged to form the strongest storm system known in the solar system other than the Great Red Spot.

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What about rain? Does it rain on Jupiter? Given all the lightning activity and storms, the idea seems plausible, but then again we are talking about Jupiter, which is far different from Earth. Recent speculation, based on the findings of the Galileo probe, suggests the following line of reasoning. Helium is present on Jupiter, but in lesser concentrations than it is on the Sun. Scientists believe that something must be draining helium from the atmosphere. So there is a possibility, based on these findings, that Jupiter is deluged with a helium rain. And if that isn't bizarre enough, the relative rarity of neon on Jupiter indicates that neon, too, is being drained away, and the hypothesis that neon is actually dissolving in helium rain has been suggested.

What Lies Beneath?

There has been speculation that below the cloud cover lies a metallic hydrogen sea about 40,000 km deep. Metallic hydrogen does not occur naturally on Earth, but under the extreme pressures that exist on Jupiter (up to three million times that of Earth), hydrogen molecules can become so tightly compacted that they break up and become electrically conductive. This electrical conductivity is speculated to be one of the sources of Jupiter's unusually large magnetic field. Underneath it all is a core at an estimated temperature of 30,000 C. This heat makes its way up to create the hot spots such as the one the Galileo probe entered. The core may be solid and is estimated to be about 1.5 times that of the Earth's diameter, and 10-30 times as massive.

This is not likely to be an ocean teeming with life, but then one has only to read Robert Forward's Dragon's Egg, which speculates about life on a neutron star, another astronomical entity with (even more) enormous gravitational and magnetic fields, to find an imaginative depiction of life existing under extreme conditions. A few science fiction writers have speculated on the possibility of life existing in Jupiter's turbulent atmosphere; Arthur C. Clarke's short story "A Meeting With Medusa" and Ben Bova's novel Jupiter both envision creatures that "swim" in the currents that flow through the Jovian atmosphere. Most writers have avoided Jupiter itself as a locus of life, concentrating instead on the less climactically extreme moons.

Birth of a Giant

According to modern theories of planetary formation, Jupiter emerged from the primordial nebula in two stages. First, icy planetisemals condensed out of the cloud of dust and gas, and as the protoplanet grew so did its ability to gather more material. At a certain point in the process of agglomeration, a critical mass was reached that promoted a rapid growth spurt as the cumulative gravitational effect of the conglomerated protoplanet became sufficient to trap all the nearby hydrogen and helium. Jupiter is somewhat similar to the primordial Sun in chemical content; however, krypton, xenon, and argon are found at higher levels on Jupiter than in the Sun. This suggests that Jupiter's material of formation may have come from the outer reaches of space, since trapping these gases at the levels found on Jupiter would require the gases to be "frozen out," which isn't possible in Jupiter's present location. Thus, it may have formed further out in the solar system and drifted to its present distance of 778 million km from the Sun.

Our guardian uncle doesn't really bear any resemblance to the rest of the family in our solar system. It can be difficult to grasp the scale of Jupiter: at over 300 times the mass and 1300 times the volume of Earth, its immensity renders our human yardsticks barely adequate for the task. Its uniqueness is what makes it worth the visit, and hopefully there will be plenty more visits to follow Galileo. Particularly given the possibility that we have Jupiter to partially thank for our existence as an intelligent species, Jupiter is really a family member worth knowing.

Further Reading

Bagenal, Fran, Timothy Dowling, and William McKinnon. Jupiter: The Planets, Satellites and Magnetosphere.

Cole, Michael D. Jupiter: The Fifth Planet.

Fischer, Daniel. Mission Jupiter: The Spectacular Journey of the Galileo Spacecraft.

Harland, David M. Jupiter Odyssey: The Story of NASA's Galileo Mission.

Hanlon, Michael and Arthur C. Clarke. The Worlds of Galileo: The Inside Story of NASA's Mission to Jupiter.

Leutwyler, Kristin and John R. Casani. The Moons of Jupiter.

Rogers, John H. The Giant Planet Jupiter.

Seymour, Simon. Destination: Jupiter.

World Spaceflight News. 21st Century Complete Guide to Jupiter and the Galileo Mission. (CD-ROM)

Peter Jekel runs the Infectious Diseases Program for one of the largest Health Department Districts in Ontario, Canada. He lives with his family, made up of wife, daughter, dog, 3 cats, 20 fish, and a rabbit. His previous publications for Strange Horizons can be found in our Archive.
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