Borderlands of Science

by Charles Sheffield

  It has been known since the 1950s that Jupiter is an intense emitter of radio noise, but the mechanism for its production was vague. It was known that somehow it seemed to correlate with the position of Io.

  As for satellites, in 1960 a round dozen of them were known. These included the four major ones discovered by Galileo in that marvelous year of 1610 when he first applied his telescope to astronomy. Now termed the Galilean satellites, they are, in increasing distance from the planet, Io, Europa, Ganymede, and Callisto. In 1892 a fifth satellite was found, inside the orbit of Io. It was named by its discoverer, E.E. Barnard, simply "V," the Roman number for five. Later it became known as Amalthea. The other satellites, all more distant than Callisto, were numbered in the order of their discovery. Other than size estimates and orbit parameters, not much was known about any of the moons of Jupiter in 1960. The larger ones showed a few light and dark spots, and none seemed to have an atmosphere. The four outermost moons are much farther from Jupiter. They are in retrograde orbits, i.e. they are moving around Jupiter in the opposite direction from the planet's spin, and they were generally thought to be captured asteroids.

  Today's picture of the Jovian system, thanks largely to observations by the Pioneer, Voyager, and Galileo spacecraft, is vastly different from that of even thirty-five years ago. The satellites that were then little more than points of light are now well-mapped worlds, each moon with its own unique features and composition. The atmosphere of Jupiter itself has been looked at in great detail, and it is known to contain complex churning cloud patterns, with infinitely detailed vortices. The Great Red Spot has given up its secrets: it is a vast semipermanent storm system, a hurricane fueled by Jupiter's rapid rotation and lasting for hundreds of years.

  We still know less than we would like to about Jupiter's interior. The escape velocity from the planet is about 60 kms/sec, and once you go there it is hard to get away. The present picture of the planet's interior is of a deep, slushy ocean of hydrogen under fabulous pressure. At three million Earth atmospheres, seventeen thousand kilometers deep in Jupiter's atmosphere, hydrogen is believed to change to a metallic form. Deep below that is perhaps a small central core of rock and iron about the size of the Earth.

  We now have confirmation that Jupiter is composed largely of hydrogen and helium, with an observed 19 percent helium in the upper atmosphere. And we have confirmation that Jupiter gives off more energy than it receives, a result that was still tentative twenty-five years ago. Since the planet is a net emitter of energy, that energy must be produced somewhere in the deep interior. And there must be adequate convection mechanisms to bring the heat to the outer layers. In fact, Jupiter is almost a star; a bit bigger, and it could support its own fusion reactions.

  Jupiter has electric and magnetic fields in keeping with its size. The powerful magnetic field captures and accelerates the "solar wind," the stream of energetic charged particles emitted by the Sun. As the nearest large moon, Io, moves through that swarm of particles it generates and sustains a "flux tube," a tube of current, five million amperes strong, that connects Io and the atmosphere of Jupiter. This in turn stimulates intense electrical activity in the Jovian cloud systems. The cloud tops seethe with super-bolts of lightning, and they generate powerful radio emissions from the planet. The night side shimmers with auroras, also observed by the electronic eyes of the Voyager spacecraft in their 1979 inspection of the planet.

  The Voyager and Galileo spacecraft sent back quite extraordinary images of the major moons of Jupiter. Amalthea, the smallest and nearest-in of the previously known Jupiter satellites, proved to be a lumpy, irregular ellipsoid, about 265x170x155 kms. The longest axis always points towards Jupiter. Amalthea is tidally locked to face the parent planet.

  Io, the next one out, is tidally locked also. Io is a spectacular sight. It looks like a smoking hot pizza, all oranges and reds and yellows. As it sweeps its way through that high-energy particle field surrounding Jupiter, tidal forces from the parent planet and its companion satellites generate powerful seismic forces within it. Io is a moon of volcanoes. Many active ones have been observed, spewing out sulfur from the deep interior.

  Europa is my own favorite of the Galilean satellites, and much of my novel Cold as Ice is set there. Europa is the smallest of the four, with a mass about 2/3 of our own Moon. And it seems to be an ice world. There is a smooth, flat surface of water-ice, fractured by long linear cracks, ridges and fissures. Underneath those there is probably liquid water, kept from freezing by the tidal heating forces from Jupiter and the other Galilean satellites. Europa has an estimated radius of 1,565 kms, and an estimated density of 3 grams/cc. It is believed to possess a rocky silicon core, with an outer ice/water layer maybe 100 kilometers thick. There has been speculation, some of it mine, that the ice-locked waters of Europa could support anaerobic life-forms. These would derive their energy from hydrothermal ocean-floor vents, much like similar life-forms in Earth's deep oceans.

  Ganymede is the biggest moon in the solar system, with an estimated radius of 2,650 kms. It has a low density, about 1.9 grams/cc, and is thought to be about 50 percent water. The brightness of Ganymede's surface suggests that it may be largely water-ice. The surface is a mixture of plains, craters, and mountains, not unlike the Moon.

  Callisto, the outermost of the Galilean satellites, is all craters—the most heavily cratered body in the Jovian system. It has a radius of about 2,200 kms, slightly smaller than Ganymede and Saturn's biggest moon, Titan. It has the lowest density of any of Jupiter's moons, again suggesting that we will find lots of water-ice there. The surface of Callisto seems very stable. It has probably not changed much in four billion years, in contrast to Io's fuming surface, which changes daily.

  As for the other satellites of Jupiter, we still know little about them. However, the Voyager mission did add one to their number—a small one, less than 40 kms across. That moonlet orbits at the outer edge of Jupiter's ring system.

  All this, and rings too? Yes. Twenty years ago, Saturn was thought to be the only ringed planet. Now we know that Jupiter, Uranus, and Neptune are all ringed worlds. Jupiter has a thin ring, well inside the orbit of Amalthea. It has a sharply defined outer edge, and it sits about 120,000 kms out from the center of Jupiter.

  TABLE 7.1 (p. 185) shows a "score card" of the moons of Jupiter.

  7.9 Saturn. Saturn is about twice as far as Jupiter from the Sun (and hence from us—as most solar system distances go, we sit very close to the Sun). Saturn is a little smaller (58,000 kms radius, to Jupiter's 70,000); and since it is farther from the Sun it is less strongly illuminated. For all these reasons, Saturn is more difficult to observe from ground-based telescopes, and our knowledge of a generation ago reflected that fact. The most famous feature of Saturn is the ring system. Those rings were first observed, like so much else in the solar system, by Galileo in 1610, but he was baffled by them and had no idea what they might be. Huygens, working forty-five years later with a better telescope, was the first person to deduce the nature of the rings. Nearly two hundred years after that, in 1857, Maxwell showed on mathematical grounds that the rings could not be solid. They have to be a swarm of some kind of particles. However, the size and composition of those particles were unknown even as recently as twenty-five years ago, although the popular theory was that they were small chunks of ice. The rings of Saturn were imagined as snowballs, of varying sizes.

  It was known that there was not one ring, but several. In 1675 Cassini observed at least two rings, separated by what we now call the Cassini division. A third ring, the Crape ring, was observed in 1838, and again in 1850.

  As for the planet itself, Saturn seemed a smaller, lighter version of Jupiter. Its radius was close to Jupiter's, but its density was only 0.7 grams/cc (it is the least-dense large body in the solar system; Saturn would float in water, if you could find a big enough bathtub. Presumably it would leave a ring).

  Saturn weighs in at 95 Earth masses, versus 320 for Jupiter. The surface shows the
same banding as Jupiter's, but with less visible detail. The equatorial bulge is even more pronounced, with a polar radius of 54,000 kms and an equatorial radius of 60,000 kms. The planet's volume is about 750 times that of Earth, and the rotation period is 10 hours and 15 minutes (although that period is not the same at all latitudes; Saturn rotates faster at the equator than near the poles). Saturn's axis is inclined at 26.75 degrees to its orbit, so that unlike Jupiter it has substantial "seasons."

  By 1960, nine satellites of Saturn had been discovered. In order, moving outward from the planet, these are Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe. Percival Lowell thought he had seen a tenth one in 1905, and he named it Themis, but he had no more luck here than he did with the canals of Mars. No one else has ever seen it.

  Today, thanks again mainly to the Voyager spacecraft, we know that the atmosphere of Saturn is mostly hydrogen, with rather less helium than Jupiter (11 percent above the clouds, versus 19 percent for the larger planet). Methane, ammonia, ethane, and acetylene have also been observed in the atmosphere; and like Jupiter, Saturn gives off more energy than it receives from the Sun, so there must be internal sources of heat. The clouds of Saturn show a number of long-lived features, including atmospheric cyclonic patterns like the Great Red Spot on Jupiter. Saturn at the time of the 1981 Voyager 2 encounter had nothing of that size, though it did have one red spot about 6,000 kms long in its southern hemisphere. However, in September, 1990, a new "Great White Spot" was found on Saturn by the ground-based observations of amateur astronomers. Images taken by the Hubble Space Telescope revealed that this feature is a huge cloud system, extending a third of the way around Saturn's equator. Its cause and its degree of permanence are unknown.

  The rings of Saturn are known to be infinitely more complex than anyone dreamed of twenty-five years ago. There are not two or three rings but thousands of them, each one very narrow. And they are not just simple rings. Sometimes there are radial gaps in them, "spokes" that come and go within a period of a few hours. Some of the rings are interwoven, plaited together in ways that seem to defy the laws of classical celestial mechanics. (They don't, but they do call for nontraditional techniques of orbital analysis.) Other rings are "herded" along in their orbits by small shepherding satellites that serve to control the location of ring boundaries. The composition of the rings has been confirmed. They are indeed mostly water-ice—bands of snowballs, hundreds of thousands of kilometers across.

  The count of satellites for Saturn, not including the rings which are themselves composed of innumerable small satellites, has gone up substantially. Eighteen have been named. Not surprisingly, the new satellites do tend to be on the small side, although one of them, Janus, circling Saturn at about 150,000 kms distance, is comparable in size with Phoebe.

  Of all these moons, Titan has received the most attention. We know that it has a substantial atmosphere, with a surface pressure of 1.6 Earth atmospheres. It is composed mainly of nitrogen, with a good fraction of methane (as much as 10 percent down at the surface, and less higher up). The dark-red color of Titan is due to a photochemical smog of organic (i.e., carbon-containing) compounds, and ethane, acetylene, hydrogen cyanide, and ethylene have all been detected. The surface temperature has been measured as about -180deg.C. One plausible current conjecture is that Titan has an ocean—but an ocean of ethane and methane, rather like liquefied natural gas. All water on Titan will be well-frozen, but water-ice may lie below that frigid sea. Just as the old canals of Mars seem to have appeared as linear features on Europa, the petroleum oceans of Venus may be here, on Titan.

  The rest of the satellites are much smaller, devoid of all signs of atmosphere, and their low densities suggest that they contain a good deal of water-ice. All the known moons are cratered, and Mimas has one gigantic crater on it, nearly 130 kms across. Iapetus shows dark-red material on its leading face, suggesting that water-ice may have been eroded from that hemisphere by meteor impact as the moon moves in its orbit around Saturn. Another possible explanation is that water-ice has been preferentially deposited on the trailing hemisphere.

  The "score card" for Saturnian satellites is given in TABLE 7.2 (p. 186). The surface radius of Titan, 2,575 kms, makes it a little bit smaller than Ganymede. It is still bigger than Callisto or any other moon in the solar system.

  7.10 Uranus. Until 1781, the solar system ended at Saturn. William Herschel's discovery of Uranus changed that forever; now no one is sure where the "edge" of the solar system should be placed.

  Uranus, smaller than Saturn and almost twice as far from the Sun, revealed few of its secrets to ground-based telescopes. The "day" on Uranus was poorly determined even thirty years ago, estimated as anything from 10.5 to 18 hours. The large uncertainty in that number stemmed from an inability to see any features on the Uranus surface by ground telescope observation.

  Soon after the planet was discovered, it was learned (by observing the moons of Uranus) that the rotation axis is highly tilted relative to the orbital plane. The planet progresses around the Sun "on its side" like a rolling ball. Other than the size (about 25,000 kilometers estimated radius) and color (greenish, suggesting an atmosphere of hydrogen and helium plus methane and ammonia) not much more was known about the planet. The images of Uranus obtained by Voyager 2 in 1986 were something of a disappointment. The planet resembled a hazy billiard ball, with scattered high-lying clouds, probably of methane. The rotation of those clouds, plus direct observation of a rotating magnetic field (a source of observations previously quite unavailable) yields a Uranus day of 15.6 hours.

  That rotating magnetic field is one of the most interesting facts about the planet. It is sizable (0.25 gauss at the planet's surface, compared with 0.31 gauss for Earth) and it is markedly off-axis compared to the planet's rotation. For Earth, Jupiter, and Saturn, the magnetic field axis and the rotation axis point in almost the same direction. For Uranus, they are inclined at 55 degrees to each other.

  Analysis of atmospheric composition shows Uranus to be between 10 and 15 percent helium, much the same as Jupiter. Heat balance calculations confirm that Uranus lacks any internal source of heat.

  Let us move from the planet itself, to the objects that orbit around it.

  Before 1977, Saturn was believed to be the only ringed planet. In that year rings around Uranus were discovered by ground-based observation (stars disappeared and reappeared when the rings of Uranus were passing in front of them). Voyager 2 showed that all the rings are narrow and extremely dark in color; thus they cannot be water-ice like Saturn's rings. The pattern of scattered light from the rings suggests that there is little fine dust in them, which makes them quite unlike the rings of Saturn. That might be due to the off-axis magnetic field. Small particles with a high charge-to-mass ratio could be cleared out of the rings by the regular magnetic field variation, so only the larger particles would be left. Six of the rings appear elliptical, which was unexpected and suggests that they may have been created recently (speaking in astronomical terms; i.e. no more than a few million years ago).

  The search for moons around Uranus began as soon as the planet was discovered. The biggest two, Titania and Oberon, were discovered by Herschel himself in 1787. And from 1851-52, William Lassell found two more, Ariel and Umbriel. No one else saw those two for over twenty years, and many must have wondered if they really existed; but Lassell was at last proved right. The fifth and final one of the "old" set of moons (those known before the Voyager flyby) was discovered in 1948 by Gerald Kuiper. It was named Miranda.

  Today, 15 moons of Uranus are known and named. The new ones are between 13 and 77 kilometers in radius. We know little of their surface detail or composition. However, high-resolution images are available of Miranda, Ariel, Umbriel, Titania, and Oberon.

  The score card for the moons of Uranus is given in TABLE 7.3 (p. 187). Note that all the newly discovered small moons are closer to Uranus than the five previously known. The bigger moons show more evidence of internal
activity than anyone expected, though at -210deg.C they are even colder than the pre-Voyager estimate of -190deg.C. They reveal what appear to be old impact craters, fault structures, and newer extruded material in crater floors. The exception is Umbriel, which displays a bland, dark, featureless disk.

  Voyager 2 came within 29,000 kms of Miranda's surface, the spacecraft's closest approach to anything in the Uranus system. The images of that moon show an object with unexpectedly complex and inexplicable surface geology. For a first-rate science fiction story set on Miranda, try G. David Nordley's "Into the Miranda Rift" (Nordley, 1993).

  7.11 Neptune. Unlike the other planets of the Solar System, which first appeared to humans as bright points of light in the night sky, Neptune was not discovered by observation. It appeared as an abstract deduction of the human mind.

  The planet showed its presence in the first half of the nineteenth century as a small anomaly, a difference between the calculated and observed position of Uranus in its orbit. An Englishman, John Couch Adams, and a Frenchman, Urbain Le Verrier, took that small discrepancy, solved (independently) a difficult celestial mechanics problem of "inverse perturbations," and correctly predicted the existence and location of Neptune. When the planet was observed in 1846, to many people of the time it must have seemed like a magic trick. A paper-and-pencil calculation, unrelated to the real world, had somehow told of the existence of a new planet. This was mysterious, even mystical. When Gustav Holst composed his orchestral suite, The Planets, he labeled Neptune as "The Mystic" and wrote music to match.

  Neptune has a mean distance from the Sun of 4.5 billion kilometers and a period (the Neptunian year) of almost 165 years. The great distance makes Earth-based observations extremely difficult. Light takes four hours to travel from Neptune to Earth. Out at Neptune, the Sun subtends only one minute of arc in the sky, and the intensity of sunlight is one nine-hundredth of what we experience here.