Faint Echoes, Distant Stars

by Ben Bova

  About half an hour after the last data transmission, the temperature of Jupiter’s atmosphere reached an estimated 650°C, hot enough to melt the probe’s Dacron parachute. Within six hours the probe sank to a depth of more than 950 kilometers below the cloud tops, where the 1,650°C temperature must have melted its titanium structure.

  Analysis of the data transmitted by the probe showed that the Jovian atmosphere contains neon, argon, hydrogen sulfide, ammonia, methane, and organic compounds, including hydrocarbons. This was the first time organic compounds have been found on any of the planets of our solar system. (Although organic compounds have been detected in comets and found in meteorites. As we will soon see, organics have also been detected on Saturn’s giant moon, Titan.)

  Water vapor was unexpectedly low in the Jovian atmosphere and two heavy inert gases—krypton and xenon—were unexpectedly high. Astronomers believed that the probe would find considerable water vapor because Jupiter is rich in hydrogen while oxygen should be as plentiful as it is in the Sun, since it is the most abundant element after hydrogen and helium. Also, many of Jupiter’s moons are covered with water ice.

  However, only about one-tenth of the expected amount of water vapor was detected by the Galileo probe’s mass spectrometer, a finding that puzzled planetary astronomers. Yet the discovery of organic chemicals in the Jovian atmosphere gives hope that life may exist deeper beneath Jupiter’s cloud decks.

  Helium was found to be only half as abundant as in the Sun, however. Astronomers suggest that helium condenses into droplets and rains down into the deeper levels of the Jovian atmosphere, below the altitudes the probe’s instruments sampled.

  Wind speeds at the cloud tops were more than 350 kilometers per hour and increased to more than 500 kilometers per hour before the probe’s signals ended. This indicates that Jupiter’s winds are driven by the planet’s internal heating rather than solar energy, as the winds of Earth are powered.

  On its way toward Jupiter, the Galileo spacecraft encountered several dust storms. Dust motes about the size of smoke particles—a tenth of a micrometer—hit the spacecraft’s detectors at a rate of up to 20,000 particles per day. The usual rate of dust particles between the storms was one particle every three days. The storm particles were moving at nearly 40 kilometers per second, but they were so small that they did not damage the spacecraft or its sensors. Galileo was well past the Asteroid Belt when the dust storms occurred; the dust apparently emanates from Jupiter itself, since the detectors picked up the particles only when they were pointed at the planet.

  A VIKING’S FUNERAL

  Thirteen years after its launch from Earth and more than seven years after its arrival at Jupiter, the Galileo spacecraft is blind, crippled, and ready for death.

  Despite a glitch early in its flight to the giant planet that prevented its main communications antenna from opening fully, Galileo has sent nearly 5 gigabytes of data and more than 14,000 images back to the eager scientists at the Jet Propulsion Laboratory. It has weathered extremely high radiation, nearly four times the total radiation dose it was designed to receive. It has performed faithfully for more than three times longer than its designers and NASA expected.

  Early in 2002, radiation damage forced the spacecraft to shut down temporarily, marking the beginning of its demise. Galileo’s JPL operators then shut down the spacecraft’s “eyes,” including its two cameras, its light detector, and its infrared mapping system. Only sensors that detect magnetic fields and high-energy subatomic particles remained working.

  Galileo is also running out of fuel for the small steering rockets that keep the spacecraft’s antennas pointing toward Earth. When the hydrazine fuel is finally exhausted, Galileo will become mute.

  The spacecraft has been placed in an orbit that will send it crashing into Jupiter in late 2003, so that there will be no danger of it smashing into Europa or any of the other Jovian satellites that might be sites of life. Galileo will plunge into Jupiter’s thick, turbulent atmosphere and burn up, a fitting Viking’s funeral for an intrepid explorer.

  THE BEST PLACE TO LOOK?

  Jupiter may well be the most likely place to find extraterrestrial life. The planet itself has organic compounds, water, and plenty of energy from its internal heat flow.

  If a planet-wide ocean of water really exists beneath Jupiter’s clouds, an ocean eleven times wider than the whole Earth and thousands of kilometers deep, it could be the site of great biological activity. Perhaps the organic molecules that the Galileo probe discovered in Jupiter’s clouds rain down on that world-spanning ocean like manna from heaven. Perhaps complex organisms swim in that vast sea and feed on them.

  Most astrobiologists are more interested in the Galilean moons. For one thing, they should be easier to get at with spacecraft—either robotic or eventually crewed by human scientists. Diving beneath Jupiter’s clouds is a formidable task, even for robots. The moons have solid surfaces on which to land and relatively light gravity, although they are bathed in the intense radiation of Jupiter’s Van Allen Belts.

  The proven existence of life in ice-covered Antarctic lakes gives astrobiologists high hopes that life may exist beneath the ice mantles of Europa, Ganymede, and Callisto—if those ice crusts actually do cover oceans of liquid water.

  For more than a century Mars has been the focus of attention in the search for extraterrestrial life. While there might have once been microbial organisms living beneath Mars’ surface, and they might still exist there, Jupiter and its moons seem to be a more promising site for finding alien life.

  SATURN, THE RINGED WORLD

  With its intricate system of broad circling rings, Saturn is the loveliest sight in the solar system. Like Jupiter, Saturn is a gas giant, slightly smaller, slightly colder, not quite as colorful as its bigger neighbor. But breathtakingly beautiful.

  The speculations we made about what might lie beneath Jupiter’s clouds apply to Saturn, as well, although since Saturn is nearly twice as far from the Sun as Jupiter and less than one-third as massive, there is considerably less energy available in Saturn to serve the needs of life. There might be a planet-girdling ocean beneath Saturn’s yellow and tan clouds, but it would be colder and shallower than on Jupiter. Deeper down there is likely to be metallic hydrogen and then, perhaps, a rocky core of about five to fifteen Earth masses. Perhaps the Cassini spacecraft, on its way to a rendezvous with Saturn in 2004, will tell us something more about the planet’s interior.

  Saturn’s density is so low that the planet would actually float on water if you could build a pool ten times the size of Earth. This means that it is composed more of the lightest elements, hydrogen and helium, than any of the other planets.

  SATURN’S RINGS

  Saturn’s beautiful rings are composed of particles of ice or ice-covered dust. While most of the particles are no larger than dust motes, some may be as big as houses. The rings are about 400,000 kilometers across but not much thicker than 100 meters. They have been described as “proportionally as thick as a sheet of tissue paper spread over a football field.”

  The rings’ total mass amounts to that of an icy satellite no more than 100 kilometers in diameter. They are either the remains of one or more moons that got too close to the planet and were broken up by gravitational tidal forces or leftover material from the time of the planet’s formation that never coalesced into a single body because they were too close to Saturn to do so.

  Much more substantial than the rings of Jupiter, Uranus, or Neptune, Saturn’s rings are probably self-sustaining: As particles are sucked down into the planet, new particles are chipped off Saturn’s many moons by constant collisions with ring particles.

  The dynamics of the rings are fascinating. The Voyager spacecraft detected “shepherd” satellites, small moons that circle just outside or just inside the rings and apparently keep them in place with their tiny gravitational influence. New ring particles are abraded off these tiny moonlets as they grind their way around the planet.


  The main rings are actually composed of hundreds of thinner “ringlets” that appear to be braided together. Spacecraft time-lapse photos also show mysterious spokes weaving through the largest of the rings, patterns of light and dark that remain unexplained and fascinating. Perhaps Saturn’s extensive magnetosphere electrically charges the dust particles in the rings and levitates them, which may give rise to the spokes.

  SATURN’S MOONS

  Saturn has at least twenty-nine moons. Most of them are quite small, apparently captured asteroids, although Tethys, Dione, Rhea, and Iapetus range in diameter from 1,000 to nearly 1,500 kilometers, bigger than the largest asteroid in the Asteroid Belt.

  Mimas is only 390 kilometers across, but it is scarred by a crater almost one-third its diameter. The crater, named Herschel after the astronomer who discovered Mimas in 1789, makes the moon look eerily like the “Death Star” from the movie Star Wars.

  And then there is Titan.

  TITAN AND LOS ANGELES

  The aptly named giant of Saturn’s brood, Titan has a diameter of 5,150 kilometers, larger than the planet Mercury. The only bigger satellite in the solar system is Jupiter’s Ganymede.

  Titan has something not even Ganymede has: a substantial atmosphere. Titan is the only moon in the solar system to possess a thick mantle of gases. Io has a thin haze of sulfur dioxide from its volcanoes, Europa an even thinner whiff of oxygen, and Neptune’s moon Triton has a thin atmosphere, as well. But Titan’s atmosphere is actually about 50 percent thicker than Earth’s.

  How could Titan hold on to such a thick atmosphere when Mars, which is nearly a third larger, can retain only a whiff of an atmosphere, thousands of times rarer than Titan’s? The answer lies in the temperatures of the two bodies. Although Mars is cold, with an average surface temperature of -50°C, Titan averages -178°C. Titan’s atmosphere would literally boil away from “tropical” Mars, but at nearly 10 AUs from the Sun, the frigid gas molecules of Titan’s atmosphere move too sluggishly to escape its gravitational pull.

  Titan’s atmosphere is composed mainly of nitrogen laced with methane and perhaps traces of other hydrocarbon compounds such as ethane (an ingredient in gasoline), acetylene, and propane. If there were any free oxygen in that volatile atmosphere, it could ignite the very air!

  Shine sunlight on that hydrocarbon-rich atmosphere and you get exactly the same result that you get in Los Angeles: photochemical smog. When the Voyager 2 spacecraft photographed Titan in 1981 it found a world shrouded in orange smog. However, the Hubble Space Telescope was able to peer through Titan’s cloudy atmosphere in 1995 and produce photographs of its surface.

  Titan’s surface may consist of water ice covered with “a bizarre, murky swamp” of “hydrocarbon muck,” in the words of a Voyager scientist. The methane and other hydrocarbons in the atmosphere could produce hydrocarbon rain or snow: black snowflakes or rain like the downpour of a Texas oilfield gusher. There may be lakes of methane and/or ethane atop the frozen icy surface. Imagine the titanic tides, though, pulled by massive Saturn—thousands of times stronger than the pull of the Moon on Earth’s tides. On Titan, a lake or larger sea might literally crawl all the way around the world, pulled by Saturn’s immense gravity.

  Nitrogen, hydrocarbon compounds, liquid methane, water ice: Could life exist on Titan? With a surface temperature averaging about -178°C, any chemical processes taking place on Titan’s surface or in its atmosphere must proceed with glacial sluggishness, unless some sort of catalyst were present to speed up the chemistry enough for the purposes of living organisms. The catalyst might be asteroid strikes.

  Jonathan Lunine of the University of Arizona has suggested that the impact of an asteroid striking Titan’s surface could melt some of the crust, producing a liquefied area that would “take upwards of thousands of years to refreeze.” A millennium or two is less than an eyeblink as far as geological processes are concerned, but, Lunine points out, it is a long enough time for prebiotic chemistry to take place. If there is water ice on the surface, the asteroid impact would melt it into a liquid; once it refreezes it would provide protection from ultraviolet and cosmic ray radiation for geological time periods. Lunine asks, “Could [a spacecraft] on Titan find the well-preserved remains of interesting prebiotic evolution?”

  Titan’s deep interior is probably heated by the tidal flexing of mammoth Saturn’s gravitational pull. Perhaps a deep, hot biosphere based on hydrocarbons and water exists beneath Titan’s crust.

  URANUS

  Although Uranus is less than half the size of Saturn, it is still almost four times larger than Earth and certainly qualifies to be called a gas giant.

  Uranus is weird. The axes of all the planets of the solar system tilt to some degree from the vertical. For example, Earth’s axis is tipped 23.4°, a tilt that gives us our seasons. Uranus, though, is tilted 97.9° from the vertical. Its north pole points toward the Sun for part of its year, while half a Uranian year later the south pole points sunward. What kind of climate does that produce?

  The planet’s “day” is seventeen hours, fourteen minutes long, but it spins from east to west: retrograde, in astronomical parlance.

  If the planets all formed together at the same time, why is Uranus tipped over so far and spinning backward relative to most of the other planets? We might explain Venus’ slow and retrograde rotation by theorizing that it was hit by a very large planetesimal. But Uranus is about fifteen times more massive than Venus; it would have had to be struck by an object the size of a fully grown planet to tilt like that.

  Even to the Voyager spacecraft’s close-up cameras, Uranus seems a bland, quiet world, with none of the swirling bands of clouds that mark Jupiter and Saturn. Uranus is smaller than they are and considerably colder. If there is a heat source deep within the planet, it does not seem to roil the clouds noticeably. No measurable heat flow has been detected from the planet’s interior. Uranus looks like a greenish-blue world of hydrogen and helium clouds, tinted by frigid methane.

  MAGNETOSPHERE

  Like Jupiter and Saturn, Uranus has a powerful magnetosphere, some forty-eight times stronger than Earth’s. But where all the other planetary magnetospheres are roughly aligned with the planet’s axis of rotation, Uranus’ magnetosphere is tilted almost 59° away from the planet’s rotational axis. And the center of the magnetic field does not coincide with the planet’s center. It is offset by more than 7,700 kilometers, almost one-third of Uranus’ radius. Weird indeed.

  What’s going on inside Uranus that generates such a lopsided magnetic field? Unknown, so far.

  RINGS AND MOONS—AND CARBON

  Uranus is orbited by a system of dark, narrow rings, so faint that they were only discovered in 1977 and photographed by Voyager 2 in 1986.

  Uranus has at least fifteen moons, ten of them apparently captured asteroids. The five major satellites—Miranda, Ariel, Umbriel, Titania, and Oberon—show fascinating and puzzling surfaces pitted by craters and wrinkled by grooves and cliffs. Each of these moons is different from the others; like the planets themselves, each moon is unique. Carbon-based dust seems to cover much of Uranus’ moons; we have already seen that carbon dust and carbon compounds are plentiful among the moons of Jupiter and Saturn, as well as the comets.

  Apparently, the outer solar system is rich in carbonaceous dust, which may have been driven away from the inner worlds by the Sun’s heat and the solar wind. The terrestrial planets may have absorbed a good deal of this carbon as they accreted from the original solar disk.

  Carbon is the building block of life, of course. At the frigid temperatures of the outer solar system, prebiotic chemistry may have taken place in amorphous ice particles, but life itself seems unlikely—unless there are special conditions on Uranus or its moons that have not yet been discovered.

  NEPTUNE

  Although Neptune is slightly smaller than Uranus and certainly colder, it seems to be a more active world than Uranus.

  Neptune is a blue world, much like Uranus, and most
likely due to the same reason: the presence of methane in its cloud tops. But Neptune’s atmosphere is more active than Uranus’. In its 1989 flyby, Voyager 2’s cameras showed belts of clouds and a large, dark oval storm system that immediately reminded observers of Jupiter’s Red Spot. When the Hubble Space Telescope was trained on Neptune in the mid-1990s, however, the dark spot had disappeared, although a different one showed up in 1994 near the planet’s north pole.

  Both dark spots were edged with white clouds, presumably of methane ice crystals that form at higher altitudes than the spots themselves.

  The Hubble Space Telescope has observed bright areas in the planet’s blue disc: weather patterns—giant storms, actually—that change over periods as short as a few days.

  From 1996 through 2002, astronomers using the Hubble Space Telescope and the Keck telescope in Hawaii have seen Neptune’s southern regions become brighter as the planet moves into its “springtime” weather pattern for the southern hemisphere. With an axial tilt of 29°, Neptune undergoes seasonal changes, although each season of its 165-Earth-year orbit lasts more than forty years.

  Neptune also has rings, but the surprising thing about them is that they do not seem to be complete. Rather, they are arcs of fine particles that go only partway around the planet. Or if they are complete rings, they are far thinner in some places than in others. How? Why? Perhaps we are seeing rings that have just formed out of a crumbling moon, or maybe they are ancient rings that are breaking up and dropping their particles into Neptune’s thick atmosphere.