The solar system has provided a wonderful, fertile field for speculation since the earliest days of science fiction. Set your stories there, by all means; but unless you want those stories to be dismissed as fantasy by the critical reader, make it the new solar system, as revealed by recent observations.
Even fifty years ago, the writer had lots of freedom. Telescopic observations of the Sun, Moon, and planets had told us a fair amount, but that was overwhelmed by the things we didn't knowwhat does the other side of the Moon, never seen from Earth, look like? What is beneath the perennial clouds of Venus?
Today, those and many other mysteries have gone away. Planetary probes have had a close-up look at every world except Pluto. Space-based telescopes have given us not only images, but spectroscopic data about all the planets.
We will confine this chapter to the "edges" of the solar systemnot in terms of location, but in terms of knowledge. We will seek virgin territory for storytelling, where there is still hope for surprises.
7.1 Mercury. The planet closest to the Sun is Mercury. Before 1974, this was thought of as an airless ball, moving around the Sun in a rather elongated ellipse every 88 days. It was believed to present the same face to the Sun all the time, so that one side would be fiercely hot, and the other chillingly cold. Astronomers knew that Mercury had little or no atmosphere. A planet closer to the Sun than Earth sometimes passes between us and the Sun. Sunlight will then be refracted by any substantial atmosphere. There is no sign of that, so the surface of Mercury must be close to a perfect vacuum.
The big change in our knowledge of Mercury came with the Mariner 10 spacecraft, which in 1974-75 performed a series of flybys of the planet. It sent back pictures from three close encounters, and produced the first big surprise: the surface of Mercury looks at first sight exactly like the Moon. It is cratered, barren, and airless. Mariner also discovered a magnetic field, about one percent of Earth's. This, together with the planet's high density, suggests a substantial iron core maybe 1,500 kilometers in diameter. (Mercury itself is only 4,500 kms. in diameter.) At least part of that core should be fluid, allowing the existence of a permanent dynamo that generates the external magnetic field.
Mercury's rotation period was another surprise. The old assumption, that tidal forces would have locked it in position to present the same face to the Sun all the time, turned out to be wrong. If that were the case, the rotation period of Mercury would be the same as its year, 88 days. Mercury actually goes through one complete revolution on its axis in 58.6 Earth-days. It is no coincidence that 58.6 is two-thirds of 88. A dynamical effect known as a "resonance lock" keeps those two periods in that exact ratio. As one odd result, a day on Mercury lasts exactly two of its years (because the planet turns one and a half times on its axis in the time it takes to make one full circuit around the Sun). Since the planet does not present the same face to the Sun all the time, all sides get baked; the planet is hot all over, except possibly at the very poles, rather than just on one side as was previously thought.
Mercury has probably changed little in appearance in the past three billion years. However, it has one interesting difference from the Moon; its surface is more wrinkled, probably as a result of more cooling and contraction than the Moon has ever experienced. On the other hand, anything three billion years old has a right to be wrinkled.
The "old" Mercury allowed some fascinating science fiction stories to be written about it. The modern Mercury is rather dullor should we say, a good challenge to the writer's imagination?
7.2 Venus. If Mercury was for a long time something of a mystery to astronomers, Venus was a positive embarrassment. Galileo, back in 1610, took a look at the Planet of Love with his homemade telescope and noted that the surface seemed completely featureless. That, improvements in telescopes and observing techniques notwithstanding, was the way that Venus obstinately remained for the next three and a half centuries. Venus was known to be about the same size as the Eartha "sister planet," as people were fond of saying, coming closer to Earth than any other, and only a few hundred kilometers smaller in radius (6,050, to Earth's 6,370). But if this were our sister, we knew remarkably little about her. The length of the Venus year was determined, but not the day; and the surface was a complete and total mystery, because of the all-pervading and eternal cloud layer.
Naturally, that absence of facts did not stop people from speculating. One popular notion was of Venus as a younger and more primitive form of Earthprobably hotter, and perhaps entirely covered with oceans. The logic was simple: hotter, because nearer the Sun; and clouds meant water, so more clouds than Earth meant more water. Venus might be a steamy, swampy planet, where it rained and rained and rained.
There were competing theories. Fred Hoyle, the astronomer whom we met in Chapter 2 and will meet again in Chapter 13, speculated that Venus indeed had oceans; but according to his theory they would be oceans of hydrocarbons (the ultimate answer to a fossil fuel crisis).
Hoyle's ideas sound wild, but at least they were based on an extrapolation of known physical laws. Whereas Immanuel Velikovsky, in the early 1950s, came up with the wildest, least scientificand most populartheory of all. Venus, he said, was once part of Jupiter. By some unspecified event it was ripped out of the Jovian system and proceeded inward. There, after a complicated game of celestial billiards with Mars and the Earth, it settled down to become Venus in its present orbit. And all this took place not at the dawn of creation of the solar system, but recently, 3,500 years ago. Among other things, the multiple passages of Venus past the Earth stopped our planet in its rotation, caused a universal deluge (the Flood), parted the Red Sea, and caused numerous other annoyances.
Read Velikovsky, by all means, for wild ideasbut don't believe him. We will mention just one problem with the theory, that it violates the law of conservation of angular momentum, and leave it at that.
In the past thirty years, space probes have dramatically changed our knowledge and understanding of Venus. The present description runs as follows:
* The period for Venus to make one complete revolution about its axis is 243 Earth days. This is longer than the Venus year, of 225 Earth days. Also, since the planet rotates in the opposite sense from its direction around the Sun, its daythe time from noon to noon for a point of the planetis 117 Earth days.
Would-be world-builders please note: It is difficult to visualize the relation between the time a planet takes to rotate on its axis (the sidereal period), the length of its day (from noon to noon), and the length of its year. However, there is a simple formula that relates the three quantities. If R is the time in Earth days for the planet to rotate on its axis, D is the length of its day, and Y the length of its year, then 1/D=1/R61/Y, where the plus sign is used when the planet rotates on its axis in the opposite sense from its travel around the Sun. For Venus, Y=225 Earth days, R=243 Earth days, so D=1/(1/243+1/225)=117.
* The pale yellow clouds of Venus are not water vapor. They are sulfuric acid, the result of combining sulfur dioxides and water. These sulfuric acid clouds stop about 45 kilometers above the surface, and below that everything is very clear, with almost no dust. The whole atmosphere is about 95% carbon dioxide. The lighting level at the surface is roughly like that of a cloudy day on Earth, though there are frequent storms in the clouds, and lots of lightning.
* The pressure at the surface is about 90 Earth atmospheres. Such a pressure may seem to offer impossible problems for the existence of life, but that's not the case. A sperm whale, diving in Earth's oceans to deeper than a kilometer, comfortably endures a pressure of more than a hundred atmospheresand returns to the surface unharmed a few minutes later. We still don't know how the whale is able to do that.
* Venus is hot. In this way the modern picture of Venus is like the old one, but it is probably hotter than anyone expected. The surface temperature is somewhere between 460 and 480 degrees Celsius, and highly uniform over the whole surface. Since the axial tilt of Venus is only about 6 degrees, there are no seasons to speak of.
Venus is hot for the same reason that a greenhouse is hot. Solar radiation gets into the atmosphere easily enough, but longer wavelength (heat) radiation from the surface is then trapped by the thick carbon dioxide atmosphere (or glass, in the case of the greenhouse) and cannot escape.
* Thanks largely to the Magellan spacecraft, we have a high-quality radar map of almost the whole surface of the planet. (Note: We still lack such a complete radar map of the surface of the Earth.) Venus is a barren place of rocky uplifts and shallow, melted-down craters. It is nothing like the old stories; no swamps, no intelligent amphibious life forms, no artifacts but a few burned-out spacecraft from the Soviet Union and the United States. But there are mountain ranges, well-mapped by orbiting imaging radars, and a great rift valley, bigger than any other in the solar system.
There is an interesting difference between the general surface structure of Earth and Venus. If we plot the average altitude of surfaces on Earth (including the seabed) we find that there are two peaks in the distribution: they represent the ocean floor and the continental platforms, separated by about five kilometers. This two-story world is a consequence of plate tectonics, where moving plates lift the land surfaces. When we make the same plot for Venus, a different picture emerges. We have a single peak, at the most common average elevation. There are uplands, a vast rift valley, and shallow basins, but they all cluster around this one average value.
Why are plate tectonics not a major force on Venus? Here we are on speculative ground. Theorists argue that the high surface temperature gives rise to a thick, light crust, which is too buoyant to be subducted (forced under) even if plates collide. Others argue that Venus is like a very young Earth, where we have yet to see the effects of plate tectonics. In perhaps a billion years Venus will see the rise of continents, and conditions may perhaps change to ones more congenial to life.
* Venus possesses no appreciable magnetic field. This is strange, since the planet is so like Earth in size and composition. However, the lack of field may be related to the planet's slow rotation, which would greatly reduce the dynamo effects of a liquid iron core.
* There remains one general question: Why is our sister planet so different from Earth in so many ways? One possibility: The Earth has a large moon; Venus has none. More and more, the presence of the Moon seems important, although I have yet to see an authoritative and persuasive discussion of the reasons.
7.3 Earth. We will say little about our own planet. Not because there is nothing to say, but because there is so much. Although this is our home, we might still argue that our understanding of Earth as a planet is in its early days.
Consider just a few examples. The theory of plate tectonics, already referred to, was geological heresy fifty years ago. Alfred Wegener proposed the theory in the early part of this century, but since he was a meteorologist rather than a professional geologist, he was either ignored or laughed away. Only when the evidence of sea-floor spreading became undeniable did geologists begin to accept the ideas of plate tectonics, which today underpin almost all serious geomorphological work.
A second example is the theory of primordial methane. This proposes that methane has been present in the interior since the formation of the planet, rather than being formed recently and close to the surface by the breakdown of more complex molecules through heat, pressure, and biological processes.
A third example is the Gaia theory proposed by James Lovelock and championed by Lynn Margulis. We will discuss this in Chapter 13, and note here only that it, today, is in the same state of "scientific heresy" as Wegener's theory in the 1920s.
We know remarkably little about our own Earthand what we "know" changes with every generation.
7.4 The Moon. Other than Earth, this must be the most familiar and best-known planet or satellite in the solar system. Humans have been looking up at the Moon and studying it for all of history. Its influence on Earth, and on each of us individually, is profound. There are lunar tides running within our bodies, just as they ebb and flow in the seas of Earth. We are very familiar with our own 24-hour circadian rhythms, and how we feel at different times of day. But we are also affected by the more subtle lunar rhythm, imposing a cycle on our bodies in ways we have still to understand.
Forty years ago, our ignorance of the Moon was quite striking. For example, the Moon always presents approximately the same hemisphere to Earth (small oscillations, known as librations, allow us to see a little more than half the Moon's surface). We had no information to tell us what lay on the far side of the Moon. A good deal of wild speculation could be tolerated. It was even possible to imagine a deep depression on the back of the Moon, where there could be an atmosphere and possibly life.
That idea went away in 1959, when a Russian spacecraft, Lunik III, took and transmitted to Earth pictures of the far side of the Moon. It looked, disappointingly, rather like the side that we already knew.
However, there were still plenty of things to speculate about. For example, the craters: were they caused by volcanoes, or were they meteor impacts? Forty years ago no one had any proof one way or the other. The flat, dark "seas" on the Moon: they were certainly not water, but might they be deep dust pools, ready to swallow up any spacecraft unwise enough to attempt to land on one of them?
Today we have many of the answers. First, we know that the surface of the Moon is old. The measured ages of lunar rock samples brought back in the Apollo program are in the billions of years. Half of them are older than any rocks ever found on Earth. Even the "new" craters, like Tycho, measure their ages in hundreds of millions of years. The dust pools are not there. Astronauts who landed on the Moon reported a layer of dust, but no sign of the deep, dangerous seas of an earlier generation's speculations.
The Moon is of great interest to scientists; but it seems fair to say that to most people it is a dull place. There are no known substantial deposits of valuable minerals, no air, little water. The Clementine spacecraft, according to a widely reported Defense Department press release, in 1993 "discovered" a lake of ice in a crater near the north pole of the Moon. However, the actual scientific paper in Science concerning the radar signals was far more circumspect, and merely noted that Clementine's radar return signal was consistent with the presence of water. A 1998 observation by the Lunar Prospector spacecraft made newspaper headlines with the announcement that a hundred billion tons of water had been found on the Moon. The most impressive thing to me is how little water that is. It is a small pond, ten feet deep and seven miles across. On Earth it would hardly be noticed.
Human colonies on the Moon seem possible within a generation, but they may exist mainly to send materials back out into space, or to take advantage of the radio quiet zone on the lunar far side (we flood the near side, and most of space, with our incessant babble). The biggest advantage of the Moon may turn out to be its low escape velocity, only 2.4 kms/sec, allowing cheap shipment of materials from the Moon to Earth orbit.
I do not think that a lunar base will satisfy our urge to develop the planets. The Moon is too much an offshore island of Earth. We have already paddled our dugout canoes there a few times, and we will be going back. But it is not our new continent, our "new-found-land."
That new-found-land may be Mars.
7.5 Mars. The Red Planet has had some bad publicity over the years, in science fictional promises that were not kept.
There were the canals of Mars, which Percival Lowell thought he could see very well and believed were of artificial origin, but which other people had trouble seeing at all.
And of course there were the Martians, given very poor press by H.G. Wells in The War of the Worlds (Wells, 1898). They were sitting up there on Mars, with their "vast, cool, and unsympathetic" minds set on taking over Earth.
Regardless of whether the Martians were good or bad, at the turn of the century almost everyone agreed that there was life on Mars. Although Venus is Earth's sister planet, from many points of view Mars is a more convincing Earth look-alike. It has a day just a few minutes longer than a day on Earth (24 hrs., 37 mins.). It has an axial tilt almost the same as Earth's, so the cycle of the seasons should be similar. And it has an observable atmosphere, although one that a generation ago was of unknown composition and density. There are noticeable seasonal changes in both the planet's color and the size of the polar caps with each Martian summer and winter.
Intelligence, maybe; life, a sure thing. That seemed to be the common attitude toward Mars eighty years ago.
And the modern Mars? No canals, but a cratered sand-worn surface that looks more like the Moon than Earth. Months-long sand storms. No surface water, but lots of signs of ancient water run-off. Stupendous mountains, twice as high as any on Earth; a vast canyon (Vallis Marineris) that would easily swallow the Grand Canyon whole; and plenty of jagged surface rocks. That was the report that came back from the Mariner, Mars (Soviet) and Viking spacecraft, and also from the Viking Lander. In 1976 the Lander also looked for life with its onboard experiment package. The first results were outstandingly positive, too good to be truethere seemed to be chemical indicators of life everywhere. Then the investigators decided, yes, those results are too good to be true, and they're not true.
The most widely held view, prior to August 1996, was that Mars lacked life completely and probably never had it. That situation changed dramatically with the NASA announcement that analysis of a meteorite found in Antarctica revealed possible evidence of ancient single-celled life on Mars. The 1997 Pathfinder lander, and its roving companion Sojourner, were not designed to look for life, though they did find more evidence of long-ago surface water.
The current Mars atmosphere is not promising to support the forms of life that we know best. The pressure at the surface is only one percent of an Earth atmosphere, and it is mostly carbon dioxide and nitrogen. Surface temperatures range from the freezing point of water, at low points on the equator at high noon, to -100deg.C or colder. That is not most people's idea of a mild climate. On the other hand, there are terrestrial organisms that can stand those temperatures, and even thrive if they have access to water. And there is water on Mars. It is found in the polar caps, believed to be a mixture of water ice and solid carbon dioxide ("dry ice"). Some analyses also find evidence for deep liquid water, an idea developed in detail in Kim Stanley Robinson's monumental trilogy, Red Mars, Green Mars, Blue Mars (Robinson, 1993, 1994, 1996). Before you consider writing about Mars colonization, read Robinson's work.
In spite of everything, humans could live on Mars. The available land area is roughly equal to the land area of Earth. The atmosphere is dense enough to be useful for aerobraking spacecraft, or flying an aircraft. The low gravity, only 2/5 of Earth gravity, helps a lot. If there are no Martians now, someday there will be.
7.6 The moons of Mars. Mars has its own moons, two of them. However, if attention to objects in the solar system were to be given in proportion to their size, Phobos and Deimos would be totally ignored. They are tiny objects, each only tens of kilometers across.
In Chapter 1 we mentioned Jonathan Swift's 1726 "predictions" of the existence and major characteristics of these moons, long before there was any chance of discovering them. The little moons themselves would not be discovered for another century and a half. They were finally seen by Asaph Hall, in 1877. Later observations, between 1877 and 1882, gave estimates of their distances from Mars and their orbital periods.
Until forty years ago, distances from Mars and orbital periods were all that anyone knew of Phobos and Deimos. In 1956, Gerald Kuiper estimated their diameters, giving figures of 12 kms for Phobos and 6 kms for Deimos. But the real quantum leap in our knowledge had to wait until 1977, one hundred years exactly after Asaph Hall's discovery. In that year, the Viking 2 spacecraft took a close-up look at both moons.
Neither Phobos nor Deimos is anything like a sphere. They are ellipsoids of roughly similar shape. Phobos is 27 by 21 by 19 kilometers, and Deimos 15 by 12 by 11 kms. They are both tidally locked to Mars, so that they always have their longest axes pointed towards the planet. They have battered, cratered surfaces, and Phobos has one huge crater, Stickney (named after Asaph Hall's wife, Angeline Stickney, who encouraged him to keep looking for the moons when he was ready to give up). Stickney is about ten kilometers acrossnearly half the size of the moonlet. Both moons have a regolith, a dusty surface layer of fine-grained material, and both are thought to be captured asteroids. There is some suggestion that Phobos may have water locked within it, because some of its surface features suggest steam has escaped there after past meteor impacts. Phobos looks more and more like a tempting target for anyone interested in conducting a manned Mars expedition, perhaps in the first decades of the twenty-first century. With its low gravity and location, it is an equally good target for science fiction writers.
7.7 The asteroid belt. This is also good frontier territory for speculation. "Asteroid" means "having the form of a star" and it is a terrible name for what are, in essence, small planets. "Planetoid" would be much better. Unfortunately, we seem to be stuck with the word, and also with "asteroid belt." There is a huge number of asteroids, ranging from the biggest, Ceres, at 974 kilometers diameter, through Pallas (538 kms diameter), Vesta (526 kms), Juno (268 kms), and on down to boulders and pebbles. We still know little about most of them, beyond their shapes, rotation periods, and light-reflectance curves. We have had close-up photos of two (Gaspra and Ida, the latter a double asteroid of two bodies, Ida and Dactyl, bound to each other by gravity), and we have Hubble Telescope images of Vesta and other large asteroids.
Some asteroids have left the main belt, between Mars and Jupiter, and swing in on orbits much closer to the Sun. This class of so-called Earth-crossing asteroids includes its own subgroups: the Apollo asteroids have orbits crossing Earth's orbit; the Aten asteroids are on average closer to the Sun than is the Earth (their semimajor axis is less than Earth's); and the Amor asteroids cross the orbits of both Earth and Mars. Finding such asteroids is today an active business, because it takes less fuel to get to them from Earth than to most other places in the solar system. Many contain valuable minerals. A small, metal-rich asteroid, maybe a mile across, should provide as much nickel as all Earth's known commercial deposits, and in quite a pure form. Don Kingsbury's "To Bring in the Steel" (1978) tackled the theme of mining one.
People have proposed other uses for Earth-crossing asteroids. Moved to Earth orbit (feasible if the necessary volatile material for fuel can be found on the asteroid itself), such bodies could be used to protect other satellites and installations, or as a threat to ground-based facilities.
There is an old controversy surrounding the asteroids: Are they fragments of matter that never got together to form a planet, or were they once a planet that for some reason catastrophically disintegrated? Forty years ago, no one could offer firm evidence one way or the other. Today, most astronomers argue that the planet never formed. Jupiter's powerful gravitational field prevented the separate bodies from ever coalescing.
However, there have been other opinions. In 1972, the Canadian astronomer Ovenden examined the rate of change of planetary orbits, and concluded that they are varying too rapidly for a solar system that has supposedly been fixed in major components for hundreds of millions of years. Ovenden looked at the changes, and found they were consistent with the disappearance from the system of an object of planetary dimensions in the fairly recent past. He concluded that a body of about 90 Earth masses (the size of Saturn) had vanished from the solar system about sixteen million years ago. Three years later, Van Flandern at the U.S. Naval Observatory analyzed the orbit of long-period comets. He found many with periods of about sixteen million years, and they seemed to have left the solar system from a particular region between the orbits of Mars and Jupiter.
Where do I stand on this question? Reluctantly, I conclude that the asteroids were never a single body. They date back to the origin of the solar system, and have probably existed in their present form ever since.
On the other hand, in my novel Sight of Proteus (Sheffield, 1978), a planet between Mars and Jupiter blew itself apart and created the asteroid belt. If I could get away with it, why shouldn't you? You can do as I did, and cite Ovenden and Van Flandern.
7.8 Jupiter. It is convenient to break the discussion of the planets of the solar system into two parts: anything closer to the Sun than the asteroid belt, and anything farther out. This division is also logical. The inner system contains small, dense, rocky bodies, of which Earth is the biggest and heaviest. The outer planets are (except for Pluto, which is probably not a true planet at all) large and diffuse gaseous bodies, with little or no solid core.
Until the invention of the telescope, what we knew about the outer solar system could be summarized very simply: it was Jupiter and Saturn, seen only as specks of light in the sky.
This, even though Jupiter is by far the biggest planet of the solar system, a bully whose gravitational field grossly perturbs every other body orbiting the Sun. With a diameter eleven times Earth, and a mass 320 times as big, Jupiter contains more material than all the rest of the planets put together. Its density was estimated more than a century ago, at 1.3 grams/cc. This is a low value compared to Earth, so astronomers knew that Jupiter must contain a large fraction of light elements.
Jupiter was known to be in rapid rotation, spinning on its axis once every ten hours. This, together with its great size, means that it bulges noticeably at the equator. The equatorial radius is about 6 percent bigger than the polar radius.
The Great Red Spot on Jupiter was observed in the seventeenth century (first noted by Robert Hooke, in 1664). The feature has dimmed and brightened over the years, but it is known to have been there continuously since at least 1831. It has been observed regularly since 1878. The size varies quite a bit. At the beginning of this century it was about 45,000 kms by 25,000 kms, twice today's size. But even in its present shrunken state, the Great Red Spot could easily swallow up Earth.
Forty years ago the nature of the Great Red Spot was quite unknown. One theory, still acceptable in the 1940s, held that the Spot was a new satellite of Jupiter in the process of formation, ready to split away from its parent planet (shades of Velikovsky). Other later ideas, from the 1960s, include a floating island of a particular form of water-ice (a phase known as Ice VII), or an atmospheric cloud cap over a deeper floating island. The spot moves around on the surface of Jupiter, so it certainly has to be a floating something.
The other long-observed features of Jupiter were the striped bands that circle the planet parallel to the lines of latitude. Their appearance also suggested clouds. Given Jupiter's low density, those clouds were assumed to be very deep, but their composition was largely a matter of guesswork and something of a mystery. Speculation based on the composition of the Sun suggested that Jupiter ought to be mainly hydrogen and helium, but the direct observations of the 1960s showed only methane and ammonia.
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 cratersthe 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 numbera 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 usas 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-icebands 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 oceanbut 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.
The Voyager 2 encounter revealed Neptune's equatorial radius to be 24,700 kms (since Neptune does not have a solid surface, this is taken as the radius where the pressure equals one Earth atmosphere). Neptune has a mass 17 times that of the Earth, and an average density of 1.64 grams/cc. The Neptunian day was revised to 16.11 hours, based on the rotation of the planet's magnetic field. That magnetic field is substantial, and its axis is offset 47 degrees from the planet's axis of rotation. In addition, the center of the magnetic field does not coincide with the planet's center of mass. As a result the field at the surface ranges from less than 0.1 gauss in the northern hemisphere to more than 1 gauss in the southern.
The appearance of the planet itself is striking. Unlike bland Uranus, Neptune shows atmospheric detail more like Jupiter and Saturn. There is a Great Dark Spot of midnight blue, calling to mind the Great Red Spot of Jupiter, and around the spot are bright, cirrus-like clouds that move along lines of latitude. This atmospheric activity may be a consequence of a net heat outflow, for like Saturn and Jupiter but unlike Uranus, Neptune gives off more energy than it receives from the Sun; in this case about 2.7 times as much. The minimum observed temperature on Neptune is a frigid 50 Kelvin, up near the top of the atmosphere.
Earth-based observations of Neptune, plus theoretical arguments, had suggested that its atmosphere would be hydrogen and helium with some methane. That has been confirmed. The helium is about 15 percent of the total, and small amounts of both methane and acetylene were found.
In the mid-1980s evidence had been found of rings around Neptune based on ground observations; or rather, there seemed to be evidence of partial rings. The way to find rings is to look for a star dimming and then brightening again, just before the planet passes in front of it. If there is a ring, then the same thing should happen again when the star reappears on the other side of the planet. This stellar occultation method was used for Neptune, just as was done in the case of Uranus.
However, although applying the technique to Neptune sometimes gave a dimming of the star for a couple of seconds, and a brightening before it vanished from sight behind the planet, there was no dimming when it reappeared!
In any event, full rings were found during the Voyager 2 encounter. There are three complete rings, and an outermost ring containing three bright, dusty arcs within it. These ring arcs caused the peculiar occultation results found in the earlier ground-based measurements.
Before Voyager 2, Neptune had two known satellites. The larger, Triton, was found in 1846 by that remarkable observer and discoverer of Uranus's Ariel and Umbriel, William Lassell, just ten days after the discovery of Neptune itself. Triton is big, with a radius of 1,350 kms, and has about a third of our Moon's mass. It travels in a retrograde orbit, opposite to the direction of planetary rotation. It has a period of 5.9 days, inclined at 23 degrees to the Neptune equator.
Nereid, the second satellite, is much smaller. It was discovered by Gerald Kuiper in 1949, and it travels in a very elliptical orbit, far out from the planet, with a period of 360 days. It and Triton are almost certainly captured bodies, caught in Neptune's gravitational net.
The Voyager encounter added half a dozen to the count of Neptune's moons. I have a personal fondness for Proteus, the biggest of these moons. Proteus is shaped like a knobby apple, and it may be the largest highly asymmetrical body in the solar system. Not much is known about it. Proteus orbits close to Neptune, where its own reflected light is overpowered by the light of its primary.
As for Triton, it is bright, and it is cold. The surface temperature of 38 Kelvin is the lowest measured for any body in the solar system. Nitrogen is solid at this temperature, and so is methane. The atmosphere is very thin, surface pressure between 10 and 20 millionths of an Earth atmosphere, and it is mainly nitrogen vapor with a little methane.
Any disappointment at Triton's cold, thin atmosphere is more than made up for by the satellite's astonishing surface. It possesses active geysers, "cryovolcanoes" that blow icy plumes of particles tens of kilometers high. The surface is fantastically cracked and complex, much of it showing meteorite impact craters crisscrossed by ridges of viscous material in a pattern that the Voyager team termed "cantaloupe terrain."
The score card for Neptune's moons is given in TABLE 7.4 (p. 189).
7.12 Pluto and the limits of the solar system. This planet has never been visited by any probe, so it is still wide open for science fictional conjecture. Discovered by Clyde Tombaugh in 1930, Pluto is described in most astronomy textbooks as "the most distant planet from the Sun." Actually, from 1979 to 1999, Neptune was the most distant known planet. For part of its eccentric orbit, Pluto moves within the orbit of Neptune.
Pluto's best images have been gained by the Hubble telescope. The planet has a mean radius of 1,140 kilometers. Its average surface temperature is about 43 Kelvin. There is some evidence that the surface is partly covered with methane ice, and it is conjectured that, like Triton, which it resembles in size and distance from the Sun, Pluto may have a coat of solid nitrogen.
Pluto, smaller than some satellites of Jupiter and Saturn, surprisingly has a moon of its own. Discovered in 1978 from ground-based observations, it is named Charon. It is about 590 kms in radius. Since Pluto itself is only 1,140 kms in radius, relative to the size of its planet Charon is the largest moon in the solar system. Pluto and Charon orbit each other in 6.4 days, and are 19,400 kilometers apart. The discovery of Charon allowed a good estimate of the mass of Pluto itself. That mass turns out to be small indeed, about one five-hundredth of Earth's mass. Charon's mass is still less, only one-seventh that of Pluto.
Might there be a "tenth planet," out beyond Neptune and Pluto? The search for such an object has been proposed, because one reason for seeking Pluto was a slight discrepancy between Neptune's observed and computed positions. However, after Pluto was discovered its faintness indicated that it could not be massive enough to cause the observed differences. Hence the search for "Planet X."
No such single planet has been found, but more than thirty small bodiesplanetoids, minor planets, large comets, or whatever we choose to call themhave recently been discovered beyond the orbit of Neptune. They range in size from a hundred to four hundred kilometers in diameter, and are believed to be members of the Edgeworth-Kuiper Belt. This is often called the Kuiper Belt, but its existence was first suggested by the Irish astronomer K.E. Edgeworth in 1943. The EK Belt is believed to extend to at least twice the distance of Neptune from the Sun, but detection of its more remote members is extremely difficult because of the distance and low illumination levels there. The EK Belt is believed to be the source of many of the short-period comets that from time to time visit the inner solar system.
Even with the Edgeworth-Kuiper Belt, we are not at the "edge" of the solar system. In 1950, the Dutch astronomer Jan Oort suggested a source for the long-period comets. Oort proposed that there must be a vast "cometary reservoir," somewhere far out in space.
The roughly spherical Oort Cloud of comets drifts around the Sun, weakly bound by solar gravitational attraction. Sometimes a comet will be perturbed by another star, or perhaps by a close encounter with another Cloud member. Then its orbit will change, and it may fall in toward the Sun and become visible to us. Clearly, if comets are fairly common occurrences, there must be a lot of them in the cloud. Estimates put the number in the Oort Cloud as somewhere between a hundred billion and a trillion. Each comet is thought to be a loose aggregate of water, gravel, and other volatile substances such as ammonia and hydrocarbonsthe "dirty snowball" theory introduced by Fred Whipple in 1950. The Oort Cloud is a great setting for stories. I put my novel Proteus Unbound out there, and had lots of fun with it.
The Oort Cloud is believed to extend as far as fifteen trillion kilometers from the Sun. Fifteen trillion kilometers takes us more than a third of the way to the nearest star. Are we finally at the "edge" of the solar system?
Well, there is still Nemesis. This highly hypothetical "dark companion" to the Sun is supposed to return every 26 million years, to disturb the solar system and shower us with species-extinguishing comets that fall in from the Oort Cloud.
The existence of Nemesis is highly controversial, and I find the arguments for it unpersuasive. However, few explanations are available for periodic large-scale species extinctions. At its most distant point from the Sun, Nemesis would be almost three light-years away. At that distance, it would hardly be gravitationally bound to our Sun at all. Should it be discovered (it may be very faint, because if its mass is small enough it will not sustain its own fusion reactions), then the size of the Solar System has expanded in two hundred years from the orbit of Saturn, one and a half billion kilometers from the Sun, to the thirty trillion kilometers limit of Nemesis's orbit.
If all the natural bodies of the solar system are not enough as possible homes, there remains the possibility of making more in open space. One approach to the construction of such space colonies is discussed in Chapter 8.
7.13 Planets around other stars. Although humans can live in space and will do so in increasing numbers, planets are likely to remain our preferred home. In our own solar system, Mars is the most tempting new prospect. If Europa's water ocean exists, then that moon of Jupiter will be an equally attractive goal.
But what about more distant planets, around other stars? Do they exist? And if so, are they likely to be suitable for the development of life?
Science fiction writers have always assumed that the answer to all these questions was a definite and unambiguous Yes! In half the stories you will ever read, or movies and TV shows you will ever watch, it is assumed that planets exist around other stars, that they are suitable for life, and that they nurture intelligent life. Many of the intelligent life-forms are human-like to the point of ludicrous implausibility. Yet, up to 1996, there was no firm evidence at all that even one planet existed around any star other than Sol.
Certain properties of any such planets could be inferred, even if none had been observed. For example, no matter what shape a planet starts out at the time of its formation, gravitational forces will tend to make it spherical over time. When a planet happens to be rotating fast, like Jupiter or Saturn, centrifugal forces will give it a bulge at the equator. This oblateness, as it is called, is greater for Saturn than for any other planet in the solar system, but our eyes still see the disk of Saturn as circular. Anything big enough to be called a planet must be roughly spherical in shape.
For a spherical planet, the escape velocity at the surface (the speed of an object needed to escape from the planet completely) depends on only two things: the mass and the radius. Although internal compositionthe way matter is distributed insidewill have a small effect, the escape velocity, V, will be close to 2GM/r, where M is the mass in kilograms, r the radius in meters, and G is the universal gravitational constant, equal anywhere in the universe to 6.672x10-11. Here V is given in meters/sec. For example, in the case of Earth, M=5.979x1024, r=6,378,000 and we find V=11,180; i.e., 11.18 kms/sec.
Escape velocity is important, and not only because it tells us what speed a rocket needs to get clear of Earth's gravity. It is also one of two key variables that decide whether or not a planet can hold on to an atmosphere. The other variable is the planet's temperature. If a planet is too hot, or too small, some of the molecules of atmospheric gases will always be moving faster than escape velocity. Unless they have a scattering collision with some other, slower, molecule, they will escape the planet completely. And unless they are replaced, from the interior or in some other way, the planet will at last lose its atmosphere.
A body as cool, big, and far from a star as Jupiter (escape velocity 60 kms/sec) or Saturn (escape velocity 36 kms/sec) is from the Sun will hold onto its atmosphere indefinitely. A body as small and hot as Mercury (escape velocity 4 kms/sec) or as small as Ceres (escape velocity 0.46 kms/sec) has no chance. Any atmosphere will vanish over time.
The surface gravity of a planet, g (or gee), a quantity with which we are more personally familiar, depends on exactly the same variables. We have g=GM/r2, where M, r, and G are the same as before. For the case of Earth, we find g=9.80 m/sec2.
In the past few years, the existence of planets around other stars has changed from optimistic guess to fairly confident reality. TABLE 7.5 (p. 190) gives a list of some of them, all admittedly based on evidence that is, if not weak, at least indirect. The list is representative rather than complete, because the number is growing fast. A new planet is added every month or two. We have not yet actually seen a planet around another star, even though every planet on the list is big, Jupiter's size or more.
That should not be taken to mean that most planets in the universe are massive. It merely shows that our detection methods can find only big planets. Possibly there are other, smaller planets in every system where a Jupiter-sized giant has been discovered.
Two planets in TABLE 7.5 are more than five times the mass of Jupiter. They are so big that these worlds are candidate "brown dwarf" stars, glowing dimly with their own heat. It is also disconcerting to see massive planets orbiting so close to their primary stars. In the case of 51 Pegasi and 55 Cancri, we have planets at least half the size of Jupiter, and perhaps a good deal bigger, orbiting only seven and sixteen million kilometers out from their sun. A planet of that size and in that position in our own solar system would have profound effects on Earth and the other inner planets.
If we cannot actually see a planet, how can we possibly know that they exist? There are two methods. First, it is not accurate to say that a planet orbits a star. The two bodies orbit around their common center of mass. That means, if the planet's orbit lies at right angles to the direction of the star as seen from Earth, the star's apparent position in the sky will show a variation over the period of the planetary year. That change will be tiny, but if the planet is large, the movement of the star may be big enough to measure.
The other, and so far more successful, method of detection also relies on the fact that the star and planet orbit around their common center of gravity, but in this case we look for a periodic shift in the wavelength of the light that we receive. When the star is approaching us because the planet is moving away from us, the light will be shifted toward the blue. When the star is moving away from us because the planet is approaching us, the star's light will be shifted toward the red. The tiny difference between these two cases allows us, from the wavelength changes in the star's light, to infer the existence of a planet in orbit around it.
Since both methods of detection depend for their success on the planet's mass being an appreciable fraction of the star's mass, it is no surprise that we are able to detect only the existence of massive planets, Jupiter-sized or bigger. The size distribution of planets around other stars remains an open question. Will we ultimately find a continuum, everything from small, Mercury-sized planets on up to planets able to sustain their own fusion reactions and thus to multiple star systems? Or are there major gaps in sizes, as we find in our own solar system between the inner and outer planets?
Are all stars candidates for planets that might support life? They are not, and we can narrow the search process. First, as noted in Chapter 3, massive stars burn their nuclear fuel much faster than small ones. A star ten times the mass of the sun will consume its substance several thousand times as rapidly. As a result, instead of continuing to shine as Sol will, more or less unchanged for over five billion years, our massive star will find its fuel exhausted in just a few million years. Its end, as we saw in Chapter 4, is cataclysmic. No planet could survive the explosion of its primary as a supernova.
The chance that native life, still less intelligence, might be wiped out in such a stellar conflagration is negligible. It would not have had time to develop. We do not know how long life took to establish itself on Earth, but it was surely longer than a few million years. The solar system was a turbulent place four and a half billion years ago, and Earth did not have a surface suitable to support life for at least the first few hundred million years. A planet orbiting a massive star would be gone before its crust had solidified.
Recall our horrible example from Chapter 1. The home world of the aliens orbited Rigel. But Rigel is a super-giant star, with a mass as much as 50 solar masses. It runs through its stable phase so fast that alien intelligence would have no time to develop. Add that to the list of story problems that need fixing.
We must deal with one other obstacle to the formation of planets suitable for life. The Sun is a star, and when we speak of, for example, Sirius or Rigel, we tend to think of them as single stars also. However, double and triple star systems are very common. Alpha Centauri, the nearest star to us, is actually three stars, labeled Alpha Centauri A, Alpha Centauri B, and Proxima Centauri ("proxima," meaning "close," refers to the star's distance from us, not from its companions; it is a tenth of a light-year away from the A and B components, and has an orbital period of at least half a million years). Since Proxima is small and dim, as seen from a planet circling Alpha Centauri A or B it would not be among their top thirty bright stars.
In the same way, Sirius is two stars, Sirius A and Sirius B. The second is sometimes called the "dark companion," not because it is really dark, but because it is small and condensed. Its existence was deduced by Bessel in 1844, from observations he had made of the perturbation of the brighter Sirius A. However, no one saw the companion until Alvan Clark observed it in 1862. That only added to the mystery, because although calculations showed that Sirius B had to be about as massive as the Sun, it shone only one four-hundredth as bright. Sirius B is a white dwarf star, the first one discovered, and its average density is several tons per cubic inch. Finally, Rigel also has a companionand the companion itself seems to be a binary star.
The relevance of all this to planets suitable for life is defined by celestial mechanics. When we have a star like our Sun, planetary orbits around it tend to be stable over long periods of time. The Earth has varied little in its distance from Sol, and hence in the amount of solar heating, during its whole lifetime. The mathematical description of the motion of the Earth around the Sun is provided by the "two-body problem," solved by Isaac Newton in the late seventeenth century. Perturbation effects of other planets, particularly Jupiter, were included by later workers such as Laplace, and confirmed the stability of the Earth's orbit.
When two or more stars are in one stellar system, however, the relevant mathematical problem for the motion of a planet is termed the "N-body problem." The formal exact solution has never been found, but approximate solutions can be obtained in any particular case, using computers. When this is done, the fate of a planet in an N-body system of multiple stars is found to be very different from the stable orbits of our own solar system. Orbits are far more chaotic. Close encounters of a planet with one or other of the primary stars will take place, distances vary wildly over time, and in extreme cases a combination of gravitational forces can eject the planet totally from the stellar system.
Even if the planet does not suffer such a fate, it moves through various extreme situations, now close to a star and baking in radiation, now far away in the freezing dark. This is, so far as we know, not a promising environment for the development of life.
There are two ways for a storyteller to avoid these problems. One is to be so blissfully ignorant of basic astronomy and astrophysics that you see no problem putting life and intelligence any place that you choose, and you hope for equal ignorance on the part of the reader. If you have come this far with me, you will know that I do not approve of such an approach.
The other way is to choose a star without companions, of a stellar type close to our own Sun. Suitable candidates that are also our stellar neighbors include Epsilon Eridani, at 11 light-years, and Tau Ceti, at 12 light-years. No one knows if either star has planets, though Epsilon Eridani has a ring of dust particles which is considered a promising sign. You are free to give either of these stars a world with the size and chemistry of Earth, and explain to the reader that this is the case.
You will then not have the chore of building a plausible world, and you will be safe from criticism. But as Hal Clement, justly famous for designing and explaining exotic worlds, says, "Where's the fun in that?"
TABLE 7.1. The Moons of Jupiter.
Physical properties
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|
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Name
|
Mass
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Radius
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Density
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Albedo
|
|
|
|
(1020 kg)
|
(kms)
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Galilean Satellites
|
|
|
|
|
Io
|
893
|
1,821
|
3.530
|
0.61
|
Europa
|
480
|
1,565
|
2.990
|
0.64
|
Ganymede
|
1,482
|
2,634
|
1.940
|
0.42
|
Callisto
|
1,076
|
2,403
|
1.851
|
0.20
|
Lesser Satellites
|
|
|
|
|
Metis
|
|
20610
|
|
0.05
|
Adrastea
|
|
10610
|
|
0.05
|
Amalthea
|
|
131x73x67
|
|
0.05
|
Thebe
|
|
50610
|
|
0.05
|
Leda
|
|
5
|
|
|
Himalia
|
|
85610
|
|
|
Lysithea
|
|
12
|
|
|
Elara
|
|
40610
|
|
|
Ananke
|
|
10
|
|
|
Carme
|
|
15
|
|
|
Pasiphae
|
|
18
|
|
|
Sinope
|
|
14
|
|
|
Orbital parameters
|
|
|
|
|
Name
|
Semimajor axis
|
Period*
|
Inclination
|
Eccentricity
|
|
(1000's kms)
|
(days)
|
(degrees)
|
|
Galilean satellites
|
|
|
|
|
Io
|
422
|
1.769
|
0.040
|
0.041
|
Europa
|
671
|
3.552
|
0.470
|
0.0101
|
Ganymede
|
1,070
|
7.155
|
0.195
|
0.0015
|
Callisto
|
1,883
|
16.689
|
0.281
|
0.007
|
Lesser Satellites
|
|
|
|
|
Metis
|
128
|
0.297
|
0
|
0.041
|
Adrastea
|
129
|
0.298
|
0
|
0
|
Amalthea
|
181
|
0.498
|
0.40
|
0.003
|
Thebe
|
222
|
0.675
|
0.8
|
0.0015
|
Leda
|
11,094
|
238.72
|
27
|
0.163
|
Himalia
|
11,480
|
250.56
|
28
|
0.163
|
Lysithea
|
11,720
|
259.22
|
29
|
0.107
|
Elara
|
11,737
|
259.65
|
28
|
0.207
|
Ananke
|
21,200
|
631R
|
147
|
0.169
|
Carme
|
22,600
|
692R
|
163
|
0.207
|
Pasiphae
|
23,500
|
735R
|
148
|
0.378
|
Sinope
|
23,700
|
758R
|
153
|
0.275
|
* The symbol R after the period indicates that the moon is in retrograde motion;
i.e., it orbits in the opposite direction to Jupiter's rotation on its axis.
TABLE 7.2 The Moons of Saturn
Physical properties
|
|
|
|
|
Name
|
Mass
|
Radius
|
Density
|
Albedo
|
|
(1020 kg)
|
(km)
|
|
|
Mimas
|
0.38
|
198.8
|
1.140
|
0.5
|
Enceladus
|
0.73
|
249.1
|
1.120
|
1.0
|
Tethys
|
6.22
|
529.9
|
1.000
|
0.9
|
Dione
|
10.52
|
560
|
1.440
|
0.7
|
Rhea
|
23.10
|
764
|
1.240
|
0.7
|
Titan
|
1,345.50
|
2,575
|
1.881
|
0.21
|
Hyperion
|
|
185x140x113
|
|
0.19-0.25
|
Iapetus
|
15.9
|
718
|
1.020
|
0.05-0.5
|
Phoebe 1
|
15x110x105
|
|
|
0.06
|
Lesser Satellites
|
|
|
|
|
Pan
|
|
10
|
|
0.5
|
Atlas
|
|
18.5x17.2x13.5
|
|
0.9
|
Prometheus
|
0.0014
|
74x50x34
|
0.270
|
0.6
|
Pandora
|
0.0013
|
55x44x31
|
0.420
|
0.9
|
Epimetheus
|
0.0055
|
69x55x55
|
0.630
|
0.8
|
Janus
|
0.0198
|
99.3x95.6x75.6
|
0.650
|
0.8
|
Calypso
|
|
15x8x8
|
|
0.6
|
Telesto
|
|
15x12.5x7.5
|
|
0.5
|
Helene
|
|
16
|
|
0.7
|
Orbital parameters
|
|
|
|
|
Name
|
Semimajor axis
|
Period*
|
Inclination
|
Eccentricity
|
|
(1000's kms)
|
(days)
|
(degrees)
|
|
Mimas
|
185.5
|
0.942
|
1.53
|
0.0202
|
Enceladus
|
238.0
|
1.370
|
0.02
|
0.0045
|
Tethys
|
294.7
|
1.888
|
1.09
|
0.0000
|
Dione
|
377.4
|
2.737
|
0.02
|
0.0022
|
Rhea
|
527.0
|
4.518
|
0.35
|
0.001
|
Titan
|
1,221.9
|
15.945
|
0.33
|
0.0292
|
Hyperion
|
1,481.1
|
21.277
|
0.43
|
0.1042
|
Iapetus
|
3,561.3
|
79.330
|
7.52
|
0.0283
|
Phoebe
|
12,952
|
550.48R
|
175.3
|
0.163
|
Lesser Satellites
|
|
|
|
|
Pan
|
|
133.6
|
|
0.575
|
Atlas
|
137.6
|
0.602
|
0
|
0
|
Prometheus
|
139.4
|
0.613
|
0.0
|
0.0024
|
Pandora
|
141.7
|
0.629
|
0.0
|
0.0042
|
Epimetheus
|
151.4
|
0.695
|
0.34
|
0.009
|
Janus
|
151.5
|
0.695
|
0.14
|
0.007
|
Calypso
|
294.7
|
1.888
|
0
|
0
|
Telesto
|
294.7
|
1.888
|
0
|
0
|
Helene
|
377.4
|
2.737
|
0.2
|
0.005
|
* The symbol R after the period indicates that the moon is in retrograde motion.
TABLE 7.3 The Moons of Uranus
Physical properties
|
|
|
|
|
Name
|
Mass
|
Radius
|
Density
|
Albedo
|
|
(1020 kg)
|
(km)
|
|
|
Miranda
|
0.659
|
240x234x233
|
1.200
|
0.27
|
Ariel
|
13.53
|
581x578x578
|
1.670
|
0.34
|
Umbriel
|
11.72
|
584.7
|
1.400
|
0.18
|
Titania
|
35.27
|
788.9
|
1.710
|
0.27
|
Oberon
|
30.14
|
761.4
|
1.630
|
0.24
|
Name
|
Mass
|
Radius
|
Density
|
Albedo
|
|
(1020 kg)
|
(km)
|
|
|
Lesser Satellites
|
|
|
|
|
Cordelia
|
|
1
|
|
0.07
|
Ophelia
|
|
16
|
|
0.07
|
Bianca
|
|
22
|
|
0.07
|
Cressida
|
|
33
|
|
0.07
|
Desdemona
|
|
29
|
|
0.07
|
Juliet
|
|
42
|
|
0.07
|
Portia
|
|
55
|
|
0.07
|
Rosalind
|
|
29
|
|
0.07
|
Belinda
|
|
34
|
|
0.07
|
Puck
|
|
77
|
|
0.07
|
Orbital parameters
|
|
|
|
|
Name
|
Semimajor axis
|
Period
|
Inclination
|
Eccentricity
|
|
(1000's kms)
|
(days)
|
(degrees)
|
|
Miranda
|
129.8
|
1.413
|
4.22
|
0.0027
|
Ariel
|
191.2
|
2.520
|
0.31
|
0.0034
|
Umbriel
|
266.0
|
4.144
|
0.36
|
0.0050
|
Titania
|
435.8
|
8.706
|
0.10
|
0.0022
|
Oberon
|
582.6
|
13.463
|
0.10
|
0.0008
|
Lesser Satellites
|
|
|
|
|
Cordelia
|
49.75
|
0.335
|
0.1
|
0.000
|
Ophelia
|
53.76
|
0.376
|
0.1
|
0.010
|
Bianca
|
59.17
|
0.435
|
0.2
|
0.001
|
Cressida
|
61.78
|
0.464
|
0.0
|
0.000
|
Desdemona
|
62.66
|
0.474
|
0.2
|
0.000
|
Juliet
|
64.36
|
0.493
|
0.1
|
0.001
|
Portia
|
66.10
|
0.513
|
0.1
|
0.000
|
Rosalind
|
69.93
|
0.558
|
0.3
|
0.000
|
Belinda
|
75.26
|
0.624
|
0.0
|
0.000
|
Puck
|
86.00
|
0.762
|
0.3
|
0.000
|
TABLE 7.4 The Moons of Neptune.
Physical properties
|
|
|
|
|
Name
|
Mass
|
Radius
|
Density
|
Albedo
|
|
(1020 kg)
|
(km)
|
|
|
Naiad
|
|
29
|
|
0.06
|
Thalassa
|
|
40
|
|
0.06
|
Despina
|
|
74
|
|
0.06
|
Galatea
|
|
79
|
|
0.06
|
Larissa
|
|
104x89
|
|
0.06
|
Proteus
|
|
218x208x201
|
|
0.06
|
Triton
|
214.7
|
1,352.6
|
2.054
|
0.7
|
Nereid
|
|
170
|
|
0.2
|
Orbital parameters
|
|
|
|
|
Name
|
Semimajor axis
|
Period*
|
Inclination
|
Eccentricity
|
|
(1000's kms)
|
(days)
|
(degrees)
|
|
Naiad
|
48.23
|
0.29
|
4.74
|
0.00
|
Thalassa
|
50.08
|
0.31
|
0.21
|
0.00
|
Despina
|
52.53
|
0.33
|
0.07
|
0.00
|
Galatea
|
61.95
|
0.43
|
0.05
|
0.00
|
Larissa
|
73.55
|
0.56
|
0.20
|
0.00
|
Proteus
|
117.65
|
1.12
|
0.55
|
0.00
|
Triton
|
354.76
|
5.88R
|
156.83
|
|
Nereid
|
5,513.4
|
360.14
|
7.23
|
0.75
|
* The symbol R after the period indicates that the moon is in retrograde motion.
TABLE 7.5
Planets of other stars.
Star
|
Distance of planet
from star (Earth to Sun=1) |
Minimum
mass (Jupiter=1) |
Orbit
Period (days) |
51 Pegasi
|
0.05
|
0.5
|
4.3
|
47 Ursae Majoris
|
2.1
|
2.4
|
1,103
|
70 Virginis
|
variable
|
6.6
|
117
|
55 Cancri
|
0.11
|
0.8
|
14.76
|
HD 114762
|
variable
|
10.0
|
84
|
Tau Bootis
|
0.0047
|
3.7
|
3.3
|
Upsilon Andromedae
|
0.054
|
0.6
|
4.61
|
Lalande 21185
|
2.2
|
0.9
|
5.8 (yrs)
|
HD 210277
|
1.15
|
1.36
|
1.2 (yrs)
|