Planetary Science and Fundamental Physics

Chapter 5C

Planetary Science and Fundamental Physics

In the case of planetary science, as Stephen Brush reminds us, the attempt to satisfy intellectual curiosity brought about many of the most significant advances of the past few centuries. In 1746, for example, d'Alambert won a prize for making one of the first consistent uses of partial differential equations. His was an essay on winds. The work of Laplace at the end of the 18th century resulted from the calculations in celestial mechanics of Clairut, Euler, and Lagrange. Legendre and Laplace originated the use of spherical harmonics in potential theory while trying to calculate the gravitational attraction of the Earth. This use was later of great value in electricity and in quantum mechanics. And many of Poincare's major works on mathematical analysis were inspired by problems in planetary mechanics.[1]

Similar remarks may be made about Gauss, one of the world's greatest mathematicians, who did much work in geodetic surveys and terrestrial magnetism. As Brush points out, "even an advocate of pure science might concede that Gauss' geophysical work provided the stimulus for some of his contributions to geometry and potential theory, just as his early work on the computation of orbits led to a major contribution to probability theory, the `Gaussian distribution', and could thereby be justified."[2] James Clerk Maxwell, clearly a giant of physics, won the Adams Prize with an essay on the stability of the rings of Saturn. This work was the basis for his kinetic theory of gas viscosity and eventually of his theory of transport processes. He later returned to the problem of the rings of Saturn and applied to it the methods he developed in his kinetic theory of gases.

A case of particular interest to Brush in establishing the connection between pure science and planetary science is the 19th century problem of the dependence of thermal radiation on the temperature of the source. This problem was of great significance to planetary science, of course, because the sun is the greatest source of radiation in the solar system. The outcome of the attempt to determine the surface temperature of the sun "was Stefan's suggestion (1879) that the data could best be represented by assuming that the rate of emission of energy is proportional to the 4th power of the absolute temperature."[3] Further experimental investigation, and Boltzman's theoretical derivation of such a formula, led to the subsequent work on the frequency distribution of black-body radiation. The search for the law that would govern such distribution can thus be seen as the genesis of Planck's quantum theory.

These remarks on the history of physics vindicate the claim that the study of the heavens at all levels has been a driving force in the development of fundamental science. This may come as a surprise to some. But it should not be a surprise if we consider the variety of interactions between cosmology and other areas of science, and between the different levels of cosmological research themselves. The study of the cosmos leads to the discovery of fundamental principles of physics, further development of physics in turn leads to new investigative tools of the cosmos, and so on. We may speak here of a dialectical relationship between areas and levels of science which results in the dynamic growth of science. In the case of planetary science we find no exception: fundamental research on physics and cosmology leads to changes in our ideas about the solar system and its planets. On the other hand, those ideas in turn give many hints as to useful lines of "pure" inquiry. This result is not merely part of the historical record: if anything it should receive greater prominence as space exploration multiplies our means of investigating the cosmos.

Indeed we will see presently that space science provides an excellent opportunity to enhance the relationship between astrophysics and the rest of fundamental physics.

---

[1] S.G. Brush. “Planetary Science: From Underground to Underdog.” Scientia, 113: 771-787 (1979).

[2] Brush, ibid.

[3] Brush, ibid.

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.

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.

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[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.

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.

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[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.

THE EXPLORATION OF THE SOLAR SYSTEM

Chapter4J

THE EXPLORATION OF THE SOLAR SYSTEM

The scientific exploration of the solar system provides rich support for the thesis that a better understanding of other worlds allows us to understand our own world better. In investigating other worlds we find:

(1) Valuable information that serves to refine our theories of the origin and evolution of the solar system, and hence of the Earth.

(2) Unusual phenomena that stretch our views of basic terrestrial mechanisms.

(3) Opportunities to test our ideas about the Earth — the solar system serves as a natural laboratory.

1. Valuable information about the history of the Earth

The origin and evolution of the Earth are closely tied to those of the Moon. Until the advent of the space age, three main theories had been advanced to account for the origin of the Earth-Moon system. According to the Daughter theory, first proposed by George Darwin, son of Charles Darwin, the Moon was born of Earth material. Presumably some cataclysm caused a chunk from the Earth to go into orbit (Darwin speculated that the Earth tides formed by the sun coupled with the free oscillations of a rapidly rotating Earth — every five hours — created a big bulge on the equator of the Earth, and that big bulge was thrown off).[i] According to the Sister theory, the Earth and the Moon formed side by side from planetesimals.[ii] According to the third theory, the Wife theory, the Moon was simply captured by the Earth.[iii]

A fourth theory, and the most popular view at present, is that a body the size of Mars collided with the proto-Earth.[iv] In the ensuing explosion from this giant collision, materials from the two bodies were flung far and wide. The Moon accreted from materials that remained in orbit around the Earth. This explosion vaporized a greater proportion of silicates and volatiles than it did metals. The proto-Moon did not have enough mass to hold on to volatiles such as water, carbon compounds, and even some metals like lead, which means that silicates formed a large proportion of the Moon's materials. This result made the composition of the Moon very similar to that of the Earth's mantle in some important respects. The Giant Collision hypothesis thus explains not only the lower density of the Moon, but the abundance of silicates and the poverty of volatiles found by the Apollo astronauts.[v]

As we have seen in the previous section, the origin and evolution of the Earth are of crucial importance to understand the present structure of the planet and the mechanisms of the global environment. It is in this context that we should think of the Apollo expeditions: their main merit was to challenge all the standard views of the formation of the Earth-Moon system. A consequence of that challenge was an increase in the sophistication of such views, which in turn opened the way for the Giant-Collision hypothesis.

Harold Urey, who won the 1934 Nobel Prize in chemistry for his discovery of deuterium and later became one of his century's great figures in comparative planetology, helped persuade the Kennedy administration of the value of the scientific study of the Moon. Urey, who favored the Wife theory, thought that the Moon had already been formed when the Earth captured it, and that therefore it should hold valuable evidence of the early processes in the history of the solar system. But according to Urey's model, the Moon was already a cold body when the Earth captured it; the maria (the large flat areas that resemble seas) probably had formed when water splashed up from the Earth during capture; and, perhaps most important of all, the Moon's crust should have great quantities of nickel. The reason for this last prediction is that the Moon was not supposed to have an iron core. In the formation of a larger planetary body like the Earth, when the iron goes toward the center it carries the nickel along. On the Moon, the distribution of nickel should thus be more uniform than it is on the Earth.

The astronauts' findings, however, made it clear that the Moon had been warm during its early history around the Earth, that the marias were made of basalt (probably the result of volcanism), and that nickel was not near the levels required by Urey's model.[vi] A few years after men landed on the Moon, Urey gave up the Wife theory.

The clues astronauts found in the plains, craters, and crevices of the Moon about the forces that transformed it, and particularly the age and composition of the rocks they brought back with them, allowed us to challenge and replace our previous ideas of how planets form. According to a hypothesis first proposed in the early part of the 20^th Century by T.C. Chamberlin and others, the solar system formed when a star passed too close to the proto-sun. Since the Moon and the planets would have been born of the sun, they would have been very hot and consequently their iron and other heavy metals would have collapsed into central cores. But if the solar system had been formed instead by the cold condensation of gas and dust into Moon and planets, only the more massive rocky planets like the Earth would have metallic cores. The evidence we found on the Moon thus played a part in the acceptance of the theory of planetesimals: grains of dust collecting first by intermolecular forces and then accreting by the action of gravity. It is from the perspective of this theory that theorists now explain the origin of the Moon as the result of a giant collision.[vii] This theory also makes the best sense of the heavy bombardment of the solar planets by giant asteroids and other very large bodies. This bombardment should have been at its heaviest during the first half billion years of the formation of the solar system.[viii] That is precisely the record that we have found on the craters of the Moon.

Unlike the Earth, the Moon has neither atmosphere nor oceans and has not shown much geological activity for the past two billion years. The record of the history of the solar system, let alone of the history of the Earth's immediate neighborhood, has therefore been preserved much better on the Moon. The oldest rocks found there are over 4.3 billion years old, and no rocks have been found younger than 3 billion years old.[ix] On the Earth, on the other hand, the oldest rocks are 3.8 billion years old, and most of the surface (the bottom of the ocean) is only 0.2 billion years old or even younger. Thus it is clear that in some important respects the Moon can tell us more about the early Earth than the Earth itself can.

The Moon, however, cannot tell us the whole story, for its surface has not preserved intact the record of impact upon impact. First, meteors, large and small, have altered the surface of the Moon.[x] Second, the Moon must have had some internal heat, and perhaps some volcanism as a result. Although the Moon is less dense than the Earth, it presumably had its share of the same radioactive materials that exist in the Earth's core. The Moon’s accretion, then, must have generated a good deal of heat also, although, again, much less than the Earth's.

This lunar heat would have dissipated at a faster rate than the Earth’s heat, because of the Moon’s smaller size. The reason lies in the ratio of volume to surface area. A larger planet has a smaller surface area relative to its volume. An increase in diameter increases the surface area by a power of two and the volume by a power of three (a doubling of the diameter leads to four times as much area and eight times as much volume, a tripling of the diameter leads to nine times as much area but twenty seven times as much volume). If two planets have exactly the same amount of heat per unit volume, the one with the largest relative surface area will radiate away its heat sooner. The smaller planet, in this case the Moon, will lose its heat at a faster rate. Moreover, as we have seen, the Moon had much less heat per unit volume than the Earth to begin with. Still the Moon's internal heat seems to have kept it somewhat active for over a billion years. That would have renewed the lunar surface to some extent.

In several respects, thus, there are limits to what the Moon can tell us.

To find a record that goes further back, we must look at smaller bodies in which the internal heating was negligible. The asteroids are good candidates, especially those in the main belt, between Mars and Jupiter. There is evidence that many asteroids underwent some thermal and chemical alteration about 4.6 billion years ago, but little since. Thus they offer a record of some of the forces at work in the early solar system.

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[i]. To Darwin's theory, also called the fission theory, Osmond Fisher added the hypothesis that the Moon had come out of what is now the Pacific Ocean basin. In this form the theory was popularized in the first decades of this century. For an account see S.G. Brush, "Early History of Selenogony," in Hartmann, et al, eds. Origin of the Moon, Houston, 1986, pp.3-15.

[ii]. Ibid.

[iii]. Ibid. See also Brush, "Harold Urey and the Origin of the Moon: The Interaction of Science and the Apollo Program," in the Proceedings of the Twentieth Goddard Memorial Symposium, 1982, published by the American Astronautical Society; and "From Bump to Clump: Theories of the Origin of the Solar System 1900-1960," in P.A. Hanle and V.D. Chamberlain, eds. Space Science Comes of Age: Perspectives in the History of the Space Sciences, Smithsonian Institution Press, 1981, pp.78-100.

[iv]. A.P. Boss, "The Origin of the Moon," op. cit.

[v]. Ibid. Another important piece of evidence is the fact that the Moon seems to have a very small core, about 300 to 425 km in radius, holding about 4% of the Moon’s mass. Had the moon been born side by side with the Earth, or had it been captured, it should be expected to have a much more significant core. Science News, Vol. 155, March 27, 1999, p. 198.

[vi]. See S.G. Brush, "Nickel for your Thoughts: Urey and the Origin of the Moon," in Science, 3 September 1982, Vol. 217, pp. 891-898. Maria could have also been produced by magma flowing from hot zones of convection cells. See P. Cassen et al, "Convection and Lunar Thermal History," in P. Cassen, ed., Solid Convection in the Terrestrial Planets, Physics of the Earth and Planetary Interiors, 19, 1979, pp. 183-196. A radically different alternative, according to which dust carried by low electrical currents created the maria, was suggested by T. Gold. It is described by B.W. Jones in The Solar System, Pergamon Press, 1984, pp. 177-179.

[vii]. A.P. Boss, op. cit.

[viii]. See B.W. Jones, op. cit., p. 183 and pp. 203-207.

[ix]. According to Jones, the oldest rock found on the Moon is 4.6 billion years old (a silicate of a type called dunite). Ibid. p.173.

[x]. Collisions with asteroids may have also broken open lakes of molten material near the crust, and that material might have then spilled over to form the maria. The molten material could have resulted from the heat of radioactive elements or even from previous collisions with giant asteroids.

Two objections

Chapter 4H

Two objections

A critic might raise two objections at this point. The first is that to understand the global environment of the Earth we need at most to have some knowledge of the present structure of the Earth. We need to take into account only the present mass and energy distribution of the Earth, not what happened billions of years ago. The second objection is that to understand the present structure of the Earth we do not need to think of Earth as a planet. The structure of Earth does not depend on that of Mars or Neptune. Why then do we need to know how they are structured in order to know how the Earth is structured?

A simple consideration alone disposes of the first objection: the history of the Earth is important to determine its possible range of behavior in the future. Take as basic a matter as the age of the Earth. If the Earth is indeed four and a half billion years old, certain mechanisms are plausible candidates to account for the transformation of the environment. Plate tectonics needs tens of millions of years for some of the feats that we impute to it. Radical changes in the chemistry of the atmosphere (e.g., the rise in oxygen from a trace gas to a large component) might have taken bacteria tens, or perhaps hundreds, of millions of years. Imagine now for the sake of argument that all the evidence for the age of the Earth is wrong, and that the Earth is only ten thousand years old. In that case, if the Earth formed roughly as we believe, it must have dissipated energy at such a high rate that the global environment must have been run by completely different mechanisms. And since many of those mechanisms would be the same ones that operate today, or would have caused them, our understanding of today's Earth would have to be seriously mistaken. Thus to understand the present global environment, and glimpse its future, we need to have some idea of how the Earth started and of how it evolved. And without planetary science, including the evidence collected by the astronauts on the Moon, the only measure we would have of the age of the Earth would be the chain of “begots” in the Bible.

I will answer the second objection in two stages. First, the structure of the Earth may be seemingly independent from those of Mars and Neptune right now, but unless we reject the theory of planetesimals off hand, Mars and Neptune did have a lot to do with how the Earth came to have the structure it has today, to be the planet it is now. And since history is important after all, as we have just seen, it follows that studying Mars and Neptune, as well as the other members of the solar system, may be very instructive to those of us Earthbound. Second, the critic seems to ignore how the rest of the solar system affects today’s planet Earth more directly. For example, energy and materials arrive constantly from outer space. If the atmosphere did not absorb ultraviolet, X-ray, and gamma radiation, life on land would be very unlikely. And life continues to survive because the Earth is the kind of planet that it is and no other, within the context of the solar system. A smaller, less dense Earth, or an Earth far closer to the sun might have defeated life's best efforts to gain a foothold and flourish.

The complex interactions between our planet’s systems presently regulate in a fortunate manner our share of solar energy. But that energy does not remain constant. It appears that the luminosity of the sun was much less during the first stages of the formation of the Earth, before its nuclear fires were ignited. And even afterward, the sun’s luminosity, according to some hypotheses, may have been 30% lower from what it is now.^[1] The sun also seems to undergo a variety of cycles in its output of energy. To complicate matters even more, the Earth's tilt with respect to the solar plane may vary slightly (the spin axis of the Earth oscillates between 22 and 24.4 degrees every 41,000 years).^[2]

The eccentricity of the Earth's orbit also changes slightly in cycles of 100,000 years (the orbit departs from its nearly circular shape). M. Milankovitch suggested many decades ago that this cycle was the cause of the Earth's ice ages, which also have a cycle of about 100,000 years. Since the two cycles could not initially be shown to coincide, and since no one proposed a generally accepted mechanism by which the expected change in luminosity would lead to an ice age, Milankovitch's hypothesis was met with skepticism.^[3] Nevertheless, recent studies of the history of the oceans provide strong evidence that the two cycles do coincide.^[4] Of course, if variations in the energy output of the sun, or in received luminosity, influence the Earth's climate, they will also influence that of other planets. We may then look in those worlds for evidence of such influence, and for a determination of the mechanisms by which that influence is exercised.^[5] A better understanding of those mechanisms will give us a better grasp of the evolution of our global environment, and consequently a better idea of its future.

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[1]. For an account see S. Schneider, op. cit., pp. 225-229. Since presumably life could not have survived under the corresponding lower temperatures, several writers have proposed a variety of mechanisms. C. Sagan and G. Mullen first suggested a large greenhouse effect driven by ammonia and methane. Then T. Owen and others argued that large concentrations of CO₂ were more likely than ammonia (up to 1000 times today's CO₂ levels). Most hypotheses depend on a large greenhouse effect created by the large out gassing from the interior of a hot young planet.

[2]. Ibid. p. 261.

[3]. Ibid.

[4]. For a report see R.A. Kerr, "Milankovitch Climate Cycles Through the Ages," in Science, February 27, 1987, vol. 235, pp. 973-74.

[5]. O.B. Toon, J.B. Pollack, and K. Rages, "A Brief Review of the Evidence for Solar Variability on the Planets," in R.O. Peppin, J.A. Eddy, and R.B. Merrill, (eds.), Proceedings of the Conference on the Ancient Sun, 1980, pp. 523-531.

Value of space science

The Dimming of Starlight: The Philosophy of Space Exploration

Ch. 1b

The notion that science and space exploration go hand in hand may seem obvious to a casual observer, but it has been bitterly contested over the years. Many scientists, perhaps the majority of scientists, were opposed to the Apollo program, to put a man on the Moon, on the grounds that it was political showbiz and not science. And just about every important field of space science has been denigrated, at one time or another, in the most prestigious and established quarters of science. Some of those fields still are.[i] And if we pay attention we may still hear rumblings that all that money should go for truly important research. Indeed, a common complaint, particularly in the physical sciences, has been that space science is merely applied science, and thus it would follow that, if we wish to forge changes to our fundamental views of the world, we should concentrate on putting our money and effort into fundamental science, not into space science.

In my reply I will show how every main branch of space science leads to new perspectives of immense value. I will argue in Chapter 4 that several of the main problems that our planet confronts now (e.g., the depletion of the ozone layer and global warming), as well as those it will probably confront in the next few centuries, are far more likely to be solved thanks to space exploration in two ways. The first is that such problems tend to be global problems and space technology is particularly well suited to study the Earth as a global system. The second is that as we explore other worlds we gain a broader and deeper understanding of our own planet.
From comparative planetology we will move on to space physics and astronomy, two fields ripe with the promise of radical changes to our scientific points of view. Such changes will in turn yield an extraordinary new harvest of serendipitous consequences for technology and for our way of life. The reason these two fields are ripe with promise is simple. The Earth’s atmosphere limits drastically the information we receive about the universe because it blocks much of the radiation that comes in our direction. This shielding is, of course, a good thing, for otherwise life could not exist on our planet. But to make even reasonable guesses about the nature of the universe, we need that information. That is why we need telescopes in orbit and eventually on the Moon and other sectors of the solar system. Until the day when space telescopes began to operate, many physicists thought of space physical science as applied science, mere application, that is, of the very successful “standard model” that explained matter in term of its constituting particles and the forces between them.

But, as I discuss in Chapter 5, physicists had been trying to explain a limited universe – a universe based on what we could observe through a few peepholes in the walls that protected us from cosmic dangers. It had already been known for some time, though not widely, that the visible mass in galaxies did not exert enough gravitational force to keep their outer rims of stars from being flung into intergalactic space. Astronomers presumed that eventually the missing mass would be found, but when space telescopes gave us the whole electromagnetic spectrum to look for that mass, we still could not find enough of it. According to some high estimates, up to 90% of the mass needed to account for the behavior of galaxies is undetectable (“dark matter”), apparently unlike the matter explained by the “standard model.”

To make a bad situation worse, in the late 1990s space astronomers discovered that the expansion of the universe was accelerating, even though we should expect that, after the Big Bang, gravity would slow down the rate of expansion. A new form of energy (“dark energy”) is supposed to explain this bewildering state of affairs, once we determine what its properties are.

Fundamental physics, which uses the “standard model” to think about the universe, explains familiar matter and energy. But most of the universe seems to be made up of unfamiliar dark matter and energy, perhaps even upwards of 90% when you combine those two. This means that thanks to space science we found out the extraordinary extent of our ignorance, and that space science is a necessary tool for developing a new physics.

Space exploration is also ripe with promise for biology. This promise is particularly interesting in the case of the astrobiologists’ attempt to search for life in other worlds. For example, when a NASA team announced in 1996 that a Martian meteorite contained organic carbon and structures that looked like fossils of bacteria, meteorite experts adduced that inorganic processes could account for all the substances and structures found in the meteorite. Therefore, these experts claimed, by Occam’s razor, we should reject the (ancient) Martian-life hypothesis (Occam’s razor is a principle that favors the simpler hypothesis; it is named after William of Occam, a medieval philosopher). Other scientists pointed out, in addition, that the presumed fossils were about one hundred times smaller than any known bacteria, too small in fact to be able to function as living organisms. But as we will see in Chapter 6, Occam’s razor would, if anything, favor the Martian-life hypothesis; and, ironically enough, the claim about the minimum size of living things spurred a search that, according to some, yielded many species of extremely small bacteria, nanobacteria, some even smaller than the purported Martian fossils![ii] Space biology proper (doing biological experiments in space) has not yet produced such spectacular and significant discoveries, but, as we will also see in Chapter 6, the main objections against its scientific value are based on misguided distinctions between fundamental and applied science not unlike those advanced some years ago against the space physical sciences. Some of these objections are also based on mistaken assumptions about genetics, and particularly about the relationship between genotype and phenotype.

[i] These points will be discussed in detail in Chapters 3-7. Stephen G. Brush has aptly illustrated the significance of the space sciences to the development of physics, as will be seen particularly in Chapter 5.
[ii] This is a controversial matter, but I will argue in Chapter 6 that this particular controversy is beneficial for biology.

Next Posting: brief summary of the additional controversies about space exploration to be explored in this book: humans vs. machines in exploration; space colonization; terraforming; travel at relativistic speeds; travel faster than light; SETI; space and war.