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 Earth—and 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 plen
ty 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 true—there 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 across—nearly 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 su
mmarized 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.
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