The solar system as a laboratory

Chapter 4O

The solar system as a laboratory

We often extrapolate with profit the bits and pieces we learn about the Earth to our understanding of other planets. Ideas about aspects of the Earth do serve as the basis for hypotheses about what we may find elsewhere. But this ability only strengthens our dependence on planetary science. For since those ideas about the Earth are also ideas about how a particular planet behaves, their worth is often appreciated only by seeing how they apply to other planets. Thus the solar system provides us both with an appropriate context in which to interpret our observations of the Earth and with valuable opportunities to test our ideas about the Earth.

Furthermore, in order to understand the evolution of the Earth, we need to know what factors originate and shape planetary environments. But it is difficult to determine the range of those factors when we only see them in operation on the Earth. Although, as we saw earlier, we can carry out some experiments on the Earth's weather (and perhaps on its geophysics) the range of global controlled experiments is likely to be very restricted for a long time to come. That is a problem, for in science when we have an idea we want to see how it works, we want to look at it from different angles. Fortunately for us, the solar system can take the place of the laboratory. If we want to know how the mass of a planet influences its tectonics, we look at several planets with a variety of masses and examine their tectonics. We cannot vary the global conditions of our planet at will. But we can look at other worlds in which those variations occur naturally and see how other factors are correlated with them.

On Earth, the oceans provide a buffer for heat and supply most of the water vapor in the atmosphere. What other influences are there on planetary weather? The study of planets without oceans begins to answer that question. How much of a factor is the planet's rotation? Jupiter, with its gigantic, three-layered atmosphere, rotates every ten hours. In Venus, by contrast, the day is the equivalent of 243 Earth days (the atmosphere, however, rotates 60 times faster than the planet). Computer models of weather systems that control for this and other factors have been tested by the actual performance of the atmospheres of the other planets. This gives us to some degree the analog of manipulating our own atmosphere to test our ideas about the Earth's weather.

Atmospheric density is another important influence on a planet's weather. On Venus, which has a very dense atmosphere (about ninety times that of Earth), the temperature variations are minimal, 10 to 20 degrees Celsius, between poles and equator. This characteristic surely contributes to the absence of some of the weather patterns familiar on Earth. On the other hand on Mars the atmosphere is very thin, less than one-hundredth that of Earth. Apart from making practically impossible the presence of liquid water on the surface, the thin atmosphere does not provide a barrier to break the atmospheric waves. Whereas on the Earth atmospheric waves encounter resistance and form eddies, on Mars they can run their course, so to speak. The result is that on Mars the weather patterns are far more regular, repeatable, and periodic than on the Earth. Since the structures that organize the Earth's weather are not periodic, our weather is very difficult to predict. Nevertheless we can hope that the comparative study of planetary atmospheres will continue to identify the factors that contribute to the behavior of our own weather.

The point is not that comparative planetology guarantees better weather forecasting, though it might, but that in giving us a much wider experience with weather systems it may transform our views about weather. It is the transformation of those views that may provide a great impetus, some day, to much improved weather forecasting. And similar remarks may apply in the case of improvements in our knowledge of tectonics with respect to earthquake prediction. It may of course turn out that all the knowledge in the universe would not suffice to improve the prediction of weather and earthquakes past a certain level of precision. But the improvements may be great before we reach that level. A strategy that aims to give us the deepest understanding of these phenomena and the limits of their predictability is surely the best one to follow. That is what comparative planetology offers.

The transformation and fine-tuning of our views about the Earth is by no means the only benefit likely to accrue from the exploration of the solar system. We will also increase the precision with which we can observe and predict a variety of environmental states. This increase will come, partly because we will be better able to specify relevant factors and parameters of the global environment, as a result of our comparative study of the gravitation, magnetism, atmospheres, morphology, topography, geology and chemistry of the Earth and other planets; and partly because of the advances in the technology necessary to carry out that comparative study. At the present time, for example, lasers that bounce off satellites can be used to measure the movement of the tectonic plates and the vibrations of the ground around volcanoes. In this and many other new ways, the scientific exploration of the solar system will make it easier for us to ask more fruitful questions and secure more precise answers in our quest to understand and monitor the world to which we were born.[1]

Moreover, we have also seen that when trying to understand our own planet we often are unaware of many of the crucial factors that affect it. That is so because those factors often are the result of trends and forces that have developed sometimes for billions of years, or because they are hidden from our view or caused by interactions with the rest of the solar system. So we make up theories to guide our thinking. But those theories have little contact with the results that might shape them in fruitful directions if we limit ourselves to direct observations of the Earth. A generation ago we had observed up close only one planet: the Earth. Today we have examined over forty planetary bodies in the solar system. Those other bodies give us the opportunity to test the mettle of our ideas about our own world. The inevitable transformation of those ideas is therefore a transformation of our understanding of the Earth.

It is perhaps sensible to adopt a posture of skepticism toward the sensational predictions of catastrophe. The world has never suffered a scarcity of doomsayers. But it may well be that humans do have an increasingly greater impact upon the environment. And it also seems that the present rate of change is higher than what would have been brought about by other natural processes. The evidence is inconclusive but enough to make it at least prudent to look into these matters. The Earth may have seen greater changes in its past than we are liable to inflict upon it; but we should not rest assured that we will endure whatever we carelessly bring about.

We are rather in the position of a blind man in a china shop. If he moves he loses; if he does not move he loses too. Neither recklessness nor paralysis is to be recommended. To avoid them both he needs to know what the shop is like and how he can move about in it. To give him sight would be the greatest gift. The Earth is our china shop, and the satisfaction of our curiosity through space science can help us see where we are going. Wisdom requires that we accept that gift.

[1] One interesting illustration of this point can be found, once again, in the environmental problem of CO₂. Most of the dire scenarios include the melting of the polar caps. But determining how the caps would melt needs at least two kinds of investigations: (1) a way of measuring changes in the ice cap that can be correlated with increases in global temperature, and (2) a general theory about the formation and evolution of ice caps that will allow us to infer trends from such measurements. Fortunately space exploration has given us the means to take accurate measurements that would be practically impossible otherwise: polar satellites that make the precise comparisons needed for a fine determination of changes in the ice caps.

Leslie's Probability

Leslie’s intuitions about probability seem to be of a kind with the case of the double lottery winner. Suppose that you buy lottery tickets for Tuesday and Friday. There are on both occasions ten million possible numbers, so your chances of winning on Tuesday or Friday are each one in ten million. And now suppose that you win on Tuesday. What are your chances that you will win on Friday also? Most people will feel that the chances of winning on Friday after having won on Tuesday must be far less than your chances when you won on Tuesday: one in ten million times ten million (1/10¹⁴). But this is a mistake. It is one thing to ask, what are your chances that you will win in both Tuesday and Friday? The answer is: 1/10¹⁴. It is another thing to ask, what are the chances that you will win on Friday after winning on Tuesday? Well, you have one ticket and there are 10 million possible numbers. So your chances should be one in ten million (10⁷). But people like Leslie find this hard to accept. They think that there is something valid about the intuition that tells us that after winning the lottery once, the chances of winning it again have to be far smaller than they were the first time. The difficulty comes from thinking, in this case, that if you won on Tuesday, to win again on Friday really means that you will have won on both Tuesday and Friday and that should have an extremely small probability (even compared to the small probability of winning one lottery to begin with).

To dispel this intuition, let us look at one example in which the closer we get to an unlikely combined result, the greater its probability. Suppose you flip a fair coin a hundred times. What are the chances that you will get a hundred heads? 1/2¹⁰⁰. That is an extremely small number. Suppose now that on the first flip of the coin you get heads. What are the chances now that you will get 100 heads? There are 99 flips left. That means that you will have to get heads on all 99. Your chances then will be 1/2⁹⁹. That is a very small number, too, but it is twice as large as the number you had before (because the denominator, 2^99, is half of 2¹⁰⁰: 2x2⁹⁹ is 2¹⁰⁰). After getting 2 heads, your chances of getting 100 heads will be 1/2⁹⁸, which is a very small number, but twice as large as that of your second flip and 4 times as large as your first’s. The more heads you get, the closer you get to getting 100 heads, the more your chances improve of getting 100 heads. Suppose you have flipped 98 heads, so you have only two flips left. Your chances will be 1/2²=1/4. You have gotten 99 heads? Your chances of getting 100 heads depend on your getting heads on your last try. That is 1/2.

Of course, the chances of getting 99 heads and one tail will at that point also be 1/2. At the start, there were 2¹⁰⁰ possible combinations; getting 100 heads was just one of them (1/2¹⁰⁰). But once you got heads on your first try, you eliminated half the possible combinations, that is 2⁹⁹ hitherto possible combinations. Check it: 2⁹⁹ + 2⁹⁹ = 2¹⁰⁰. For an easier example: 2² + 2² = 2³ (4 + 4 = 8) and 2³ + 2³ = 2⁴ (8 + 8 = 16). If your base were 3, then you would have to add three numbers: 3^{2 +} 3² + 3² = 3³ (9 + 9 + 9 = 27). If your base were 4 you would have to add four numbers, and so on. Now, when you get your second head in a row, you eliminate half of the existing possible combinations left up to that point, that is, you eliminate 2⁹⁸ additional possible combinations. And so on: every time you get heads, you eliminate half of the possible combinations still in existence up to that flipping of the coin. Until, finally, when you only have one flip to go, there are only two possible combinations left. The first will give you 99 heads and then tails. The second will give you100 heads as the result of getting heads on your last flip. Both combinations are equally probable. Therefore, your chances of getting 100 heads when you have already gotten 99 are far greater than when you first got a head, while the chances of getting a head on your last flip are exactly the same as the chances of getting a head on your first flip: 1/2.

Let us remember this reasoning when we begin to think about the probability that life could evolve from inorganic materials.

Venus and the End of the World

Chapter 4M

Venus and the End of the World

Before concluding the discussion on these aspects of comparative planetology on too happy a note, I should mention that the hypothesis that Venus once had oceans, as appealing and reasonable as it may be, is by no means universally accepted. For it depends crucially on the plausible reading of the ratio of deuterium to common hydrogen as evidence that Venus has lost a substantial amount of water. But this is not the only possible reading.

Some scientists have argued that the same ratio of deuterium to hydrogen could be caused by a continuous re-supply of water to the atmosphere of Venus. They find this hypothesis more plausible because, at the present rate of escape, water would disappear from Venus altogether in a few hundred million years. This would mean that right now we are witnessing the very end of a long process. Most of us would not be bothered by this prospect, but some people feel that we should treat with suspicion all lucky coincidences in science. Imagine, Grinspoon tells us, that

You show up at a house where no one seems to have been home for a long time, but you hear water running. You go upstairs and find a vigorously draining bathtub which has only an inch of water in it. Now, it is possible that you showed up just as the last bit of water was running out -- but isn’t it more likely that the tap has been left running?[1]

The intuition behind this reasoning is a peculiar view of probability that sounds quite reasonable at first. Suppose that an urn contains two white balls and 98 black balls, while another contains two white balls and 9998 black balls. Without knowing which urn is which, you reach into one of them and come up with a white ball. You should conclude that in all probability you have reached into the smaller urn, for the chances of getting a white ball are two in a hundred, which are much higher than the two in ten thousand that you would find in the larger urn. It is not reasonable to believe that you have reached into the larger urn, since getting a white ball out of it is so much more unlikely.

These intuitions about probability come with their own problems, however. The first problem is that it turns scientific reasoning on its head. In the case of Venus, it leads us to expect that the amount of water we find is normal (that it has been like that for a long time). It leads us to assume that things are pretty much as we find them (a steady-state) because otherwise the situation would be very unusual and thus unlikely.

Notice how differently we reason in science. When we first observe any phenomenon, we take pains to determine whether our sample is representative. The history of science is littered with ideas that seemed promising but went nowhere because they were based on the assumption that we were looking at the normal state of things (this is the fallacy of induction). That is, normally we have to demonstrate that our sample of observations indeed represents the usual state of affairs. We are not allowed to take that for granted. Otherwise we may conclude, say, that we have discovered a new branch of the homo family based on one fossil with a peculiar skeleton, only to find out much later that it was the skeleton of an individual with a bone disease (a true case). We worry about whether the Viking and Venera landers ended up in representative locations in Mars and Venus, whether the Galileo probe went into a section of Jupiter that is like the rest of the atmosphere (it didn’t). Nature is rich and what we come to observe may actually be as unusual as flowers, birds, and bees are in the solar system: they exist in only one world – Earth – out of the many that orbit the sun.

The kind of reasoning that leads to a steady-state view of water in Venus also leads to some very strange views when applied elsewhere. John Leslie, for example, has concluded in The End of the World that the human species is likely to become extinct very soon.[2] He reasons that if the human species were to live for a long time, let’s say millions of years, then the amount of people alive today would be an insignificant percentage of the total amount of human beings that will ever be alive. Thus, he thinks, belonging to such an unusual group of humans (those of today) would be extremely unlikely. On the other hand, if the world were to end within fifty years or so, we would be part of the largest group of humans who will ever be alive (the six billion or so alive today are far more than all the rest of the humans who have ever lived put together). And it is more likely, then, that if you were to pick a human at random he would belong to the overwhelming majority than to a very small minority. Therefore the human species is far more likely than not to become extinct very soon.

Many sensible people would consider Leslie’s argument a reductio ad absurdum of the kind of probabilistic reasoning under examination here. Nevertheless, he, like others, sticks to his probabilistic intuitions, despite counterarguments like the following. Suppose the devil places ten people in a room and tells them that he will kill them all if he gets double sixes in a roll of the dice. If they survive, he will then place 100 people in the room, and then 1,000, and so on, always multiplying by ten. And every time the devil will roll the dice in hopes of getting a double six. Now, most of us will think that the chances of any one group getting killed will be 1 in 36, but according to Leslie, the ill-fated chances of the group of ten thousand have to be far greater than those of the group of ten. Leslie concludes not that there is something very wrong with his reasoning, but that he has encountered a paradox of probability. And he remains worried about the end of humanity, just in case the paradox should resolve itself in the direction of his reasoning.

There is no paradox, however. The probability estimate that takes into account a causal mechanism has priority (in this case, that the devil will kill all the people in the room if he rolls double sixes), as do all probability estimates that clearly have more relevance. For example, suppose that an ordinary man goes to the hospital to have an appendectomy. He is informed by his doctors that the chances that the operation will go well are 98%, for their survey shows that that 98 out of every 100 patients who undergo the operation do well. But suppose now that the patient is 89 years old and suffering from cancer and its debilitating effects. Surely the previous estimate would not apply to him. Let’s say, for the sake of argument, that statistical records have been kept for people in his condition and that people in his condition survive less than 5% of the time. This latter estimate is the one that should guide his decision to undergo the operation, for it is based on the factors most relevant to him. It would be preposterous of him to say, “But my chances might still be 98%.”

It may turn out that the amount of water of Venus in a steady state, but to have confidence in that idea we need independent scientific reasons in its favor, not peculiar intuitions about probability. A candidate to keep the Venusian atmosphere re-supplied with water is volcanism. A good possible outside source of water is the combination of comets and their fragments, for they are basically a mixture of ice water and other compounds. As Grinspoon points out, however, comets collide with planets often but not continuously and, thus, we would not know whether that hypothesis is consistent with our present readings of Venusian water (comets would supply water in spurts). Furthermore, this approach needs to assume that the exceptionally large ratio of deuterium to hydrogen would exist in Venus for any given short period of time. Otherwise we would have to conclude again that Venus has been losing water for a long time. Now, some scientists say that in first coming up with the “oceans in Venus” hypothesis we assume the original Venusian deuterium-to-hydrogen ratio should be close to Earth’s, but that this assumption may be incorrect: after all, they are different planets. This point is fine as far as it goes. But the ratio is 120 times greater on Venus than on Earth! It is difficult to imagine what could account for such a phenomenal difference in natural ratios between the two planets. The original ocean hypothesis must continue to be considered most reasonable until further notice.

This proliferation of ideas is by no means restricted to planetary geologies. But perhaps we can see this point better by discussing several illustrations in connection with another point, namely that the variety we find in the solar system permits us to test our ideas of the Earth.

---

[1]. D.H. Grinspoon, op. cit., p. 109.

[2]. J. Leslie, The End of the World, Routledge, 1996. Leslie believes that this statistical reasoning should make us more fearful of possible cosmic cataclysms, such as giant asteroid impacts and space-time-gobbling new universes growing inside our universe, as well as man-made catastrophes such as nuclear war.

Martian Systems

Chapter 4L

Martian Systems

The global understanding we may thus gain is fine-tuned by our exploration of still more planets. Let us consider Mars now. Since Mars was smaller and less dense than the Earth, it did not have available as much internal energy as the Earth. And since Mars is further from the sun, it receives less sunlight. To compensate, if Mars were to have as comfortable a climate for life, it would have to have a much greater greenhouse effect than the Earth. This would require truly large amounts of CO₂ in the atmosphere. At the present time the CO₂ in Mars is 50 times per unit volume that of the Earth's atmosphere. That sounds like much, but it produces only a puny greenhouse effect. The reason is that since the atmosphere is already so cold and thin, the Martian water is frozen at the poles or spread as permafrost under large areas of the surface. This means that the initial boost that CO₂ gives to the greenhouse effect is not multiplied by the action of water vapor in trapping even larger amounts of infrared radiation.

It seems, however, that earlier in its history, when Mars' internal energy was much higher, Martian volcanoes might have filled the atmosphere with as much as 100 times as much CO₂ as today.[1] This factor would have raised the density and temperature enough to permit liquid water and large amounts of water vapor, and hence much more of a greenhouse effect. As we look at Mars now, that appears to have been the case. For years, spacecraft photographs showed what seemed to be riverbeds and suggested other indications of significant amounts of liquid water in the past, perhaps even an ocean. After the recent exploration of the Martian surface by robots, the case for water in Mars is very strong.

This view of Mars is strengthened by the recent discovery that Mars at one time did have plate tectonics as well. It seems, then, that life could have existed on Mars. If so, why did not Martian life control the climate the way Earth's presumably did? Mars apparently did not have enough energy to run the cycles that have made a sustainable biosphere on Earth possible, just as it did not have enough heat to support the motion of tectonic plates for billion of years. Life, if it ever existed on Mars, was thus powerless to stop the ultimate collapse of its global environment.

The history of Mars should prove most instructive. If Martian life did exist, its perils might tell a tale as dramatic as it would be fascinating. For in the old layers of Martian rock, human geologists may one day find the mark of life just as they do in very old rocks in our home planet. And those rocks would provide a record of a series of interactions between life and the environment in which the mechanisms of the "thermostat" finally collapsed. Just as we learn much about the human brain by studying those brains that break down through injury or disease, we can learn about a terrestrial planet's global environment by studying terrestrial planets in which the global environment broke down.

As we will see in Chapter 6, the consequences of Martian life for our understanding of Earth’s biology would be truly extraordinary, even if we can find only fossils. The geological exploration of Mars is made even more tantalizing by the suggestion that at least one Martian meteorite, ALH84001, contains evidence of past life on our sister planet.

---

[1]. Some researchers have suggested that the Earth's early atmosphere, following the initial heavy bombardment by asteroids, also had a very high percentage of CO₂. The ensuing greenhouse would then compensate for the dimmer sun. Life eventually removed much of the CO₂, thus preventing a runaway temperature when the sun's luminosity increased. This hypothesis runs contrary to other ideas on the composition of the early atmosphere, according to which a primitive atmosphere would exhibit either a highly reducing mixture of methane, ammonia, water and molecular hydrogen (similar to that of Jupiter, Saturn and the other planetary gas giants) or else a mildly reducing mixture of carbon monoxide, carbon dioxide, nitrogen, and water, with not much molecular hydrogen. (In this context a mixture is reducing to the extent that it contains hydrogen). To decide between these and perhaps other alternatives it will be helpful to study not only the histories of Mars and Venus, but the largely methane atmospheres of Titan and Triton, the large moons of Saturn and Neptune respectively. The reason these matters are so worth looking into is that knowing more about the composition of the primitive atmosphere can tell us much about the origin and evolution of the global environment of a planet; in this case, of our planet.

Consider also one of the most interesting aspects of Jupiter's atmosphere: the famous Red Spot. In a dense atmosphere with winds of hundreds of miles per hour, how could a storm, which is what the Red Spot is, remain stable for centuries, perhaps for many thousands of years? The answer seems to be that the fast spin of Jupiter (once every ten hours) produces very strong Coriolis forces, which in turn produce the turbulent winds that drive the gigantic eddy of gas otherwise known as the Red Spot. As the planet spins, many smaller eddies develop, but these eddies eventually feed the Red Spot. In the midst of turbulence the Red Spot has achieved stability within the Jovian atmosphere. Thus stability arises from chaos. But interesting as this may be, what significance does it have for people on Earth? The significance is that space scientists see many parallels between the dynamics of the Red Spot and some weather patterns in the atmosphere of the Earth. In particular, these scientists see parallels to systems of high pressure that sit still for weeks or even months. Understanding this phenomenon, known as "blocking", would be a great help in forecasting the weather here on Earth. Of course, it may still turn out that what we learn about the stability of the Red Spot does not apply to stationary high-pressure systems on the Earth.

Planetary Mechanisms

Chapter 4K

Planetary Mechanisms

For a look at what might have been the original material in the solar system, comets are a good bet. Many of them have been under the influence of significant solar radiation for a relatively short time (sometimes only in the millions of years, whereas the planets and asteroids have been under it for billions of years now). The spacecraft missions to Comet Halley, and the future missions in which we will try to land on a comet and penetrate its core, are bound to enhance our ideas about these messengers of times long past.

We can also find clues about the many factors that affect the evolution of a planet – internal structure, tectonics, or atmosphere – on most of the bodies of the solar system. Since they were formed under different circumstances, because of their position in the solar system and the distribution of materials in the sweep of their orbits, and since their interactions with the rest of the solar system are somewhat different from ours, they offer a wide range of instances of those evolutionary factors at work. It is not surprising that under these different circumstances, unusual mechanisms have come into existence. In trying to understand such mechanisms, we modify our ideas about our own planet, as we will see in the next section.

2. Stretching our views of planetary mechanisms

Let us consider anew the standard account of the forces that lead to the geologic evolution of a rocky planet. The denser the planet, the more heat will be available from radioactive elements; and the larger the planet, the more retarded the loss of heat. By this account no planets much less massive than the Earth could still have active volcanoes or relatively young surfaces. Learning to draw the line has not been easy. Mars, for example, shows evidence of recent volcanism (within the last two million years); but even if Mars is not a dead planet, its surface is testimony to a prolonged coma. The Moon, which is definitely much smaller than the Earth, does no longer seem active at all. But matters soon become far more complex than. Harold Urey argued a long time ago for a greater variety of mechanisms that could produce internal heat in a planetary body. Some theorists, following in Urey's footsteps, went as far as speculating about volcanoes on Io, a Jovian moon about the size of ours – an idea that seemed much too fanciful to most researchers until, to their astonishment, they looked through Voyager’s camera and clearly saw the gigantic plume of a volcano rising over Io's horizon.

Another surprise greeted them when Voyager discovered that Enceladus, a small icy moon of Saturn about 1/100,000th the mass of the Earth, might be geologically active. If Enceladus were inert, as a small moon is supposed to be, it could not renew its surface, and thus it would show evidence of the large bombardment that took place during the early stages of the solar system. But the surface of Enceladus looks quite new. A similar argument can be made about Europa, a beautifully smooth satellite of Jupiter that apparently has large water oceans under a frozen surface.

Evidence of early geological activity can be found in many other moons, including Uranus' moons Oberon, Titania, and Ariel. One of the mechanisms that may explain these findings is that each of these moons is caught between two or more masses that exert significant gravitational attraction upon it. As a result of Io's specific position, for example, its mass expands and contracts in tides created by Jupiter on one side and Europa on the other. But we should not suppose that this form of tidal heating is the only additional mechanism able to produce an active geology. The bizarre geological formations in Uranus' moon Miranda (see figure) can perhaps best be explained by supposing that Miranda has broken up one or more times and is now in the process of differentiation, with very large ice formations still side by side with big chunks of dark carbon compounds.

In Ganymede, the largest moon of Jupiter and the solar system, we can see what looks like signs of the beginnings of plate tectonics, now conveniently frozen for our inspection.[1] In other worlds we can see other stages of the generation and dissipation of internal heat.

All these considerations remind me of my own experience concerning the geology of Venus. As the reader may imagine, since Venus is practically a twin of the Earth, comparative planetologists have been itching to take a good look at its atmosphere and geology. Unfortunately exploring Venus has been extremely difficult, for the dense clouds keep the surface hidden from our view. Part of the reason is that in Venus' atmosphere there is 300,000 times more CO₂ than in Earth's. The resulting greenhouse effect has helped produce a temperature of almost 900 degrees Fahrenheit, about the melting point of lead. In that oven, volatile substances are kept at a large height from the surface, where they form dense clouds that keep radiating heat downwards (most sunlight is actually reflected by those clouds into space, which explains why Venus is so bright). The density is one hundred times that of the Earth's atmosphere, which turns a wind of 10 miles an hour into a hurricane (albeit without rain). And whereas on Earth rain cleanses the atmosphere and changes the land, on Venus the rain is made of sulfuric acid and the heat evaporates it long before it can touch the ground.

In this inhospitable world, our landers perish in a matter of hours, unable to give us more than the vaguest of glimpses. This was the situation until the arrival of the spacecraft Magellan in the 1990s. Magellan’s radar gave us maps of Venus better in many respects than those we then had of Earth.

Once upon a time Venus might have been very different. When the sun was dimmer oceans, rivers, and perhaps even life may have existed there. According to a plausible scenario, as the sun became more luminous, life could not keep up with the increase in energy and a runaway greenhouse effect began to vaporize Venus' oceans. The increase in water vapor made the atmospheric temperature rise even more, which then vaporized more of the oceans. Eventually the oceans ended up high in the atmosphere, where ultraviolet radiation disassociated the H₂O to form atomic hydrogen (H) and the radical OH. Most of the atomic hydrogen was lost to space while the OH entered into a variety of reactions with other substances in the atmosphere.

If anything like this scenario took place one would expect a rather high ratio of deuterium to standard hydrogen. Normally only so many hydrogen atoms should be expected to be in the form of the isotope deuterium (deuterium has a neutron in the nucleus). But since deuterium is heavier, it is not as likely to be blown away from the planet; and thus as time went by, it should have become a larger percentage of the hydrogen still found in the atmosphere of Venus. This is exactly the case.

In conversations with planetary scientists in the early 1980s I floated the suggestion that the absence of water on Venus would change the viscosity of the rocks (viscosity is the resistance to flow) and, thus, plate tectonics was unlikely. The change in viscosity would make subduction and other plate motions very difficult. That is, Venus was unlikely to show much on the way of plate tectonics. Not to worry, I was told: high temperature can make the mantle behave like melting hot butter. By 1984, however, M. Carr and others had shown that lack of water would indeed make Venusian plate tectonics rather unlikely[2] (I am sure they had been thinking along these lines longer than I and, moreover, had the expertise and imagination to come up with convincing explanations). Most observers now agree with Carr.[3] It seems, then, that a runaway greenhouse effect can deprive a planet of plate tectonics. And it occurred to me, using the arguments about the role of life given earlier in this chapter, that we should consider an astonishing corollary: without the climatic regulation by life, plate tectonics might have disappeared from the Earth as well.

This hypothesis, which seemed so fanciful twenty some years ago, is apparently considered quite reasonable nowadays. Indeed the reasoning that takes us to the biological modulation of plate tectonics goes further. As the planetary scientist D.H. Grinspoon puts it,

If you agree to that, you must agree that Earth’s interior thermal evolution has been affected by its changing atmosphere and biosphere, because plate tectonics is the main way that Earth cools its interior. Even such remote quarters as the molten iron outer core, which produces Earth’s singular magnetic field, may not have been immune to the modifying effects of Earth’s quirky air, its unique, biologically touched, gaseous envelope.[4]

This line of thought begins to give us a sense of the extraordinary complexity involved in the global environment of a planet, particularly in the case of a still dynamic planet such as Earth, or Venus. This complexity in turn raises the suspicion that global environments are mathematically complex, and therefore practically unpredictable (“classical” complexity, by contrast, is often a measure of our lack of understanding more than of the possibility of understanding). And even if some features of a planet remain only partially predictable, such as next week’s weather, much can be learned from comparative planetology about the key factors and the trends that can be reasonably expected, just as someone raised on the equator learns that there is a most drastic difference in the mean temperatures of winter and summer in the Northern hemisphere. It is precisely in determining what environmental factors are relevant to what, and how they are relevant, that the study of Venus becomes most useful, as I trust we have seen in the preceding discussion.

---

[1]. A significant difference, however, may be that Ganymede is about 50% water, and so its crustal movements are closer to ice- tectonics than those on Earth.

[2]. The Geology of the Terrestrial Planets, op. cit., p. 77.

[3]. There are some who still manage to see something resembling the Earth’s mid-ocean ridges, but even if we grant that, it still seems a far shot from the full-blown terrestrial plate tectonics.

[4]. D.H. Grinspoon, Venus Revealed, Addison-Wesley Publishing Co., Inc., 1997, p. 179.