by Ben Bova
THE GIANT PLANET
Jupiter is indeed the giant of the solar system, 11 times wider and nearly 318 times more massive than Earth. Yet this enormous world spins so fast that its “day” is only nine hours and fifty-five minutes long. Jupiter is spinning so rapidly that even a glance at the planet shows a noticeable flattening at the poles, like a squashed beach ball. And like a beach ball, Jupiter is colored with gaudy stripes of red, white, and brownish hues.
The atmosphere is mostly hydrogen, with about 10 percent helium. There are three layers of clouds: the highest is composed of ammonia ice crystals, the middle of ammonium hydrosulfide (a mixture of ammonia and hydrogen sulfide), and the lowest may contain water ice or even drops of liquid water. The clouds cover the planet completely; it is impossible to see what lies below them.
The clouds zip along, strung out in bands parallel to the equator, one band heading east and the adjacent band west. Driven by the planet’s rapid rotation and heat welling up from the interior, hurricane-strength wind speeds are common. The cloud bands rub against one another and stir up cyclonic eddies the size of Earthly continents.
The Great Red Spot is an enormous oval system three times the size of Earth. It is obviously some form of atmospheric disturbance, but that is like calling a killer hurricane a low-pressure weather system. It rotates counterclockwise, completing one rotation in about six Earth days. The Great Red Spot has been observed for more than 300 years. Is it a perpetual storm? How far down into Jupiter’s interior does it reach? What gives it its red coloration? Unknown, until spacecraft delve beneath those cloud tops to investigate the hidden world of Jupiter.
HEATED FROM WITHIN
Even though the temperature at the cloud tops averages -150°C, Jupiter is hotter than it should be if its only source of heat were sunlight. Something deep inside the planet is generating heat. Perhaps Jupiter is still contracting, its massive gravitational field slowly pulling the planet in on itself, and this contraction generates the heat flow. The temperature at Jupiter’s core has been estimated at nearly 20,000°C.
Astronomers believe Jupiter may be a “failed star”: that is, a body that might have become a star if it had collected more mass during its accretion phase. Could our solar system have been a double-star system? Jupiter’s mass is 0.001 times the mass of the Sun, much more than all the other planets combined, but it is still far too low to trigger hydrogen fusion reactions and become a true star.
As we will see in Chapter 17, astronomers have discovered objects in the sky that are “almost” stars: that is, bodies that are far more massive even than Jupiter, yet not quite massive enough to generate the fusion reactions in their interiors that would make them true stars. They call such bodies brown dwarfs (dwarf stars, that is). Brown dwarfs radiate in the infrared, sullenly giving off heat generated by their contraction. Jupiter is not big enough to be a brown dwarf, even though it is generating heat internally.
Jupiter’s density, only 1.3 times that of water, shows that it is composed mainly of hydrogen and helium, just as the Sun is. While the inner planets were too close to the Sun to retain light gases, which quickly boiled away, at Jupiter’s distance from the Sun (5.2 AU), the original protoplanetary core was big enough to hold on to a good deal of the hydrogen and helium it collected from the solar accretion disk. The more gas it pulled in, the more massive the protoplanet became, the stronger its gravitational field, and the more additional material it could gather.
JUPITER’S INTERIOR
Spectroscopic studies of Jupiter’s clouds confirm that they are mainly hydrogen and helium, with ammonia and methane (both hydrogen compounds) as minor constituents, plus traces of ammonium hydrosulfide and water.
What lies beneath the clouds? The fact that Jupiter’s rapid rotation has noticeably flattened the planet’s sphere shows that much of Jupiter must be either gaseous or liquid. To a physicist, gases and liquids are both fluids; Jupiter is a fluid world. However, calculations show that the planet should have a dense, presumably rocky core that is some five to twenty times the mass of Earth.
Thousands of kilometers beneath the clouds, pressure squeezes Jupiter’s hydrogen into a liquid. On Earth hydrogen liquefies only at a temperature of -252.8°C, a scant 20.35 degrees above absolute zero. Yet under the titanic pressure of Jupiter’s enormously deep atmosphere hydrogen liquefies, even though the temperature is thousands of degrees, and takes the form of a liquid metal (somewhat like mercury). Metallic hydrogen conducts electricity, and it is believed that electrical currents in the metallic hydrogen, abetted by the planet’s rapid spin rate, generate Jupiter’s intense magnetic field. Deeper still, the increasing pressure forces hydrogen to solidify.
AN OCEAN WORLD?
Jupiter’s rocky core, then, lies buried beneath thousands of kilometers of atmospheric gases that condense with depth and pressure into solid hydrogen.
Yet at some lesser depth below the clouds the gases of the atmosphere must condense into liquids, squeezed by the weight of the gases above. At some level beneath those clouds there must be a planet-wide ocean. Of water? Traces of water vapor have been detected in the cloud tops. More likely, any ocean in Jupiter will be liquid hydrogen. Yet, because of the heat welling up from the planet’s core, there might be layers of liquid ammonia, liquid methane, even liquid water.
Ammonia and methane seem noxious to us, yet they were very common ingredients in Earth’s early oceans and atmosphere nearly 4 billion years ago, when life began on our planet.
LIFE INSIDE JUPITER?
An ocean more than ten times bigger than Earth, warmed from beneath by whatever is driving Jupiter’s interior heat flow. Might there be a thermally habitable zone inside Jupiter, warmed by the planet’s inner heat flow? Might there be life beneath the Jovian clouds?
As of this writing, we simply do not know. In 1995, the Galileo spacecraft dropped its probe into Jupiter’s swirling clouds to give us our first observation of what’s going on in the planet’s interior. Although it detected the presence of organic molecules—carbon-chain molecules that are essential to life—it did not detect living organisms, because it could not get deep enough into Jupiter’s atmosphere before being destroyed. But as Carl Sagan often said, absence of proof is not proof of absence.
MAGNETOSPHERE AND RADIO EMISSIONS
Jupiter possesses the most powerful magnetic field in the solar system, some 20,000 times stronger than Earth’s.
The Jovian magnetosphere is actually wider than the Sun’s 1.37-million-kilometer diameter. If our eyes could see magnetic fields, Jupiter’s magnetosphere would look several times bigger than the full Moon in our night sky. The stretched-out tail of Jupiter’s magnetosphere, blown away from the Sun by the solar wind, extends out to the orbit of Saturn and beyond, some 650 million kilometers.
That huge magnetosphere traps charged particles from the solar wind, just as the Earth’s magnetosphere does. The particles generate spectacular auroras in Jupiter’s polar regions, the same way that charged Van Allen Belt particles cause the northern and southern lights on Earth. Jupiter has its own set of Van Allen Belts, where the levels of radiation are much higher and more dangerous than Earth’s, because the more powerful Jovian magnetosphere accelerates the particles to extremely high energies. Jupiter’s radiation belts are so broad that they encompass the orbits of Jupiter’s seven innermost moons—including the four big Galilean satellites (more on them shortly). Human explorers will need heavy radiation shielding if they hope to stand on the surfaces of Jupiter’s major moons.
Jupiter is also the noisiest planet of the solar system, in radio frequencies. Powerful bursts of radio energy flicker all around the planet, most likely caused by massive lightning bolts deep within the clouded atmosphere. There is also a form of radio emission caused by Jupiter’s moon, Io, as it orbits through the intense Jovian radiation belts.
JUPITER’S GALILEAN MOONS
In 1610, when Galileo first turned a telescope to the heavens, he saw four “stars” orbitin
g around Jupiter. These four moons are called Jupiter’s Galilean satellites. In order of their distance from Jupiter, they are named Io, Europa, Ganymede, and Callisto. To date, a total of sixty satellites have been discovered orbiting Jupiter; most of them are small chunks of rock, probably captured asteroids or bits of debris left over from the planet’s accretion phase.20
Jupiter also has a thin, dark ring of pulverized material girdling it; this was not discovered until the Voyager 1 spacecraft flew past the planet in 1979. Studies and computer simulations of the dynamics of Jupiter’s ring (as well as the rings of Saturn, Uranus, and Neptune) show that most of the rings are probably young, in geological terms, and ephemeral. That is, they were probably created no more than a few thousand years ago and will probably be drawn down into the planets’ atmospheres within an equally short time. The fact that they exist now may be evidence that the rings are renewed by the breakup of small moons orbiting these giant planets, and these moons are themselves asteroids that have been captured by the planets’ gravitational pulls.
TABLE 1.THE GALILEAN MOONS OF JUPITER
Name Distance* Diameter**
Io 421,600 3,642
Europa 670,900 3,130
Ganymede 1,070,000 5,268
Callisto 1,883,000 4,806
The Galilean satellites are worlds in their own right. Ganymede, the largest moon in the solar system, is 8 percent larger than the planet Mercury and three-quarters the size of Mars. Callisto is almost Mercury’s size. Yet these moons look minuscule next to giant Jupiter.
Io is the closest of the Galilean satellites. With a diameter of 3,642 kilometers, it is about 2 percent larger than Earth’s Moon. Jokingly called “the pizza world,” Io’s surface is a mottled red, yellow, orange, white, and black. The colors come from sulfur compounds spewed out by volcanoes that are apparently powered by the enormous tidal pull that Jupiter exerts on Io’s crust and interior. This constant flexing of the moon’s entire body keeps Io’s interior molten.
Nine active volcanoes have been seen on Io, plus some 200 volcanic calderas larger than 19 kilometers across. These volcanoes have blasted gaseous plumes of sulfur and sodium chloride (salt) 400 kilometers high above Io’s mottled surface. The volcanoes spew out about 11 billion tons of sulfur-bearing material each year, which erases any evidence of cratering from meteoroid impacts on Io. The most powerful volcanic explosion ever recorded in the solar system occurred on Io on February 22, 2001, when an eruption near the volcano Surt blasted molten rock over an area larger than London in an out-of-this-world Washington’s birthday fireworks.
Io’s volcanoes even hurl sulfur, sodium, and chlorine compounds to escape velocity (Io’s escape velocity, not Jupiter’s), so that there is a ring of thin ionized gas girdling Jupiter along Io’s orbital path. This doughnut-shaped flux tube of sulfur, sodium, and chlorine compounds interacts with Jupiter’s powerful magnetosphere, causing intense electrical currents that generate powerful radio emissions.
It is difficult to envision volcanic, sulfurous Io as a possible abode for life—until we think back to the acidophilic sulfur-loving bacteria and archaea that have been found on Earth. Io might be a paradise for organisms that can metabolize sulfur.
The other Galilean satellites are ice worlds, with densities not much above that of water. Craters pockmark their surfaces, and long cracks scratch their icy crusts. However, the ice is often covered with darker material, perhaps carbon-based dust or soot.
Europa is not as heavily cratered as the other Galilean moons. It seems logical that all the Galilean moons should have been bombarded equally. In fact, astronomers use crater counts to help estimate a planet’s age. Why does Europa appear to be “younger” in terms of cratering than the other Galilean satellites? The answer may be that Europa’s ice surface repeatedly melts and then refreezes, erasing the oldest craters.
In 1995, the Hubble Space Telescope detected a very tenuous atmosphere of molecular oxygen on Europa, barely 100-billionth of Earth’s atmospheric pressure, little better than a complete vacuum. On Earth, molecular oxygen is pumped into the atmosphere by photosynthetic plants. Considering Europa’s -133°C surface temperature, astronomers do not believe the oxygen on Europa stems from plant life. Rather, they think that the intense radiation of Jupiter’s magnetosphere “sputters” water molecules off Europa’s icy surface and then dissociates the water into hydrogen and oxygen. The hydrogen quickly escapes; the heavier oxygen stays long enough to be detected.
These gases “sputtered” off Europa’s icy surface have created a ring around Jupiter, similar to the “flux tube” torus at Io, but composed of electrically neutral gases, according to observations made in 2003 by the Cassini spacecraft as it passed Jupiter on its way to a rendezvous in early 2004 with the planet Saturn.
LIFE BENEATH THE ICE?
Europa’s ice is frozen water. The fact that the ice mantle is apparently reforming itself constantly to erase the scars of meteoroid impacts has led astrobiologists to believe that there may be an ocean of liquid water beneath the ice layer. There are long cracks in the ice, such as those that form in the Arctic Ocean as the ice pack shifts and breaks up because of the motions of the water beneath it. Spectrographic analysis of those cracks shows that they contain mineral salts composed mostly of magnesium sulfate and sodium sulfate, which presumably well up from a briny sea below the ice. Similar salt minerals have been detected on Ganymede’s surface.
Possibly, an ocean of liquid water exists beneath Europa’s ice crust, heated by the tidal flexing caused by Jupiter’s powerful gravitational pull. As the ice-mantled moon orbits around Jupiter, the giant planet’s gravity raises tides in the solid rock core of Europa, and this constant flexing of the rock generates heat from friction. Europa’s interior may be the site of a thermally habitable zone that does not depend on the Sun’s warmth.
In the frozen wastes of the Antarctic, there are bodies of water such as Lake Vostok that are eternally covered by ice. Beneath the ice—even inside the ice, in some cases—living colonies of microbial organisms have been found. Biologists were stunned in 1978 to find flourishing ecologies of stromatolites living in water that is a scant degree or two above freezing. In effect, the permanent ice caps at these lakes’ surfaces act as the glass of a hothouse; although only a meager 1 percent of the Antarctic sunlight penetrates the ice, that is enough to serve as the energy source for spongy, jelly-like mats of microbial colonies.
Could a similar biosphere exist beneath Europa’s surface of ice? The ingredients for life are apparently present on Europa: carbon to form long-chain molecules, water in abundance, and energy welling up from the interior of the moon due to the internal frictional heating generated by the tidal flexing caused by Jupiter.
The Galileo orbiter found that Callisto has its own magnetic field, which has been interpreted as evidence for an ocean beneath Callisto’s thick crust of ice. If there actually are oceans of liquid water beneath the frozen mantles of Europa, Ganymede, and Callisto, then the temperatures inside those moons are well within the region where carbon-based life can exist. Each of these moons may sustain its own internal habitable zone.
CATACLYSMIC COMET CRASH
In 1994, Jupiter was struck by twenty-one fragments of Comet Shoemaker-Levy 9, which had been broken up during a close encounter with the planet on an earlier swing through the solar system. Blasting into Jupiter’s atmosphere at an estimated 200,000 kilometers per hour, the cometary fragments hit Jupiter with the energy of a million hydrogen bombs.
Every major telescope on Earth, as well as the Hubble Space Telescope and cameras aboard the Galileo spacecraft, watched the cataclysmic event. Each fragment of the comet exploded shortly after it penetrated the Jovian cloud tops, throwing up huge plumes of hot gaseous debris almost as wide as North America that rose more than 3,000 kilometers above the cloud tops. The plumes were composed of methane, ammonia, oxygen, carbon monoxide, sulfur, water, and even metals—although which constituents came fr
om the comet and which from Jupiter itself is still uncertain.
Nine months after the impacts, several of the sites were still visible to Earth-based telescopes as bright patches against the cloud deck. The largest impact scar was almost the diameter of Earth. If there is life beneath Jupiter’s clouds, could it have survived such a disaster? Jupiter is a very large planet and must be constantly bombarded with asteroids and comets. Still, the thunderclap shock waves of explosions thousands of times greater than a full-scale nuclear war may have had a disastrous effect on any life that might have existed below Jupiter’s roiling clouds.
THE GALILEO PROBE
Launched in 1989, the Galileo spacecraft spent six years in space before it reached Jupiter in December 1995. Originally designed to study Jupiter and its major moons for two years, the spacecraft has spent more than seven years observing Jupiter and making close flybys of each of the four Galilean moons, despite the intense radiation from the Jovian magnetosphere. Doughty little Galileo (and the spacecraft’s operators at Jet Propulsion Laboratory) has coped with more than four times the radiation levels it was designed to withstand.
An atmospheric entry probe separated from the main spacecraft and entered the Jovian clouds on December 7, 1995. Slowed by a heat-shield aeroshell and then by parachute, the Galileo probe carried the first instruments to enter Jupiter’s tremendously deep atmosphere. The probe transmitted data for 58.5 minutes as it sank into Jupiter’s deep, turbulent atmosphere, about 20 minutes longer than required for the mission’s primary scientific objectives.
