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Saturday, September 25, 2010




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 20th 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.

[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.

Wednesday, September 22, 2010

Killer Asteroids

Chapter 4I

Killer Asteroids

It is also clear that the Earth is not a closed system with respect to matter. Because its position as one of the inner planets in the system, and because of its gravitation, the Earth attracts a good number of bodies, some of which collide with it. Most, if not all of these bodies, are debris left over from the formation of the solar system. They range from dust and small meteorites to comets and large asteroids. According to some hypotheses, such collisions have a most profound effect upon climate and life.

The story is as follows. Even in recent geologic times (within the last 100 million years) large meteors have collided with the Earth, altered the weather catastrophically and brought extinction to many species. One asteroid about 10 kilometers in diameter, now called the Alvarez asteroid, is held responsible for the disappearance of the dinosaurs about 65 million years ago.[1] Gravitational disturbances of the asteroid belt, the Kuiper Belt (a little beyond Pluto) or of the (possibly) billions of comets in the Oort cloud, in the outskirts of the solar system, will send several rather large bodies towards the sun.[2] Some of them collide with the planets and moons of the solar system. In 1994, for example, large fragments of Comet Shoemaker-Levy 9 hit the atmosphere of Jupiter at velocities over 200,000 kilometers per hour, exploding with a brightness as much as fifty times that of the entire planet, and ejecting searing materials thousands of kilometers above the clouds. Had Shoemaker-Levy 9 hit the Earth instead, we would have gone the way of the dinosaurs.[3]

Apart from the realization that our natural history has to make conceptual room for such catastrophes,[4] there is a most obvious practical issue of survival involved. Perhaps with a reliable tracking system in place, space technology might allow us to change the orbits of those comets or asteroids most in danger of colliding with the Earth. But how worried should we be? It depends on the probabilities of collisions, of course. According to present models, meteors large enough to create Meteor Crater in Arizona would hit an urban area every 100,000 years on average (although one could hit 10 years from now). That meteor was presumably 60 meters across; the crater is 1.2 kilometers across. A body with a diameter of 250 meters would cause a crater 5 kilometers across and destroy about 10,000 square kilometers (about the area of greater Los Angeles). These are supposed to hit the Earth once every 10,000 years on average, although most would fall in unpopulated areas. Global catastrophes would take place every 300,000 years. These would be meteors with a diameter of approximately 1.7 kilometers.[5]

At this point, however, a reader may fairly wonder why such models should be given any more credence than the catastrophic climate models questioned earlier in the chapter. Soon after impact, craters are attacked by wind, water, life, lava and a myriad of tectonic motions. In the blink of an eye, geologically speaking, all obvious traces of them disappear from the surface of our active planet. But we find a good record on the Moon. And in Venus, where most of the surface is 600 million years old, the spacecraft Magellan counted nearly one thousand impact craters at least twice the diameter of Meteor Crater. Since Venus is almost the same size as Earth, and in the Earth’s vicinity, and since the impacts are geologically recent, Magellan’s radar mappings of Venus lead me to expect on the average a truly catastrophic impact on Earth every half a million years or so.[6] Those of us living today may have little to worry about, but eventually our descendants will be thankful to us for creating a warning system and the technology to prevent disaster.[7]

This example illustrates quite well how considering the Earth as a planet has led to the sort of understanding that underlies serendipity. The idea that the extinction of the dinosaurs and so much other life was caused by an asteroid’s impact would have been unimaginable to Alvarez, or to anyone else, had it not been for his previous knowledge about planetary science, such as the bombardment of planets by asteroids, the suspicion that some large bodies had not been swept up yet, etc. With that knowledge in the background, the geological data at the boundary could give him strong hints of a catastrophic collision. Without it, he probably would not have been at all interested, and in any event it would be difficult to envision how he would have come by his hypothesis. Once his hypothesis was corroborated, we had no only an explanation of that particular extinction but also a warning about a potentially disastrous problem. Planetary science, however, also gives us hope that we may be able to solve the problem.

Apart from the sun and assorted debris, other members of the system exert some influence on the Earth. Of those others none are as significant as the Moon. Since the Moon is a very large satellite relative to its planet, sometimes people speak of the Earth-Moon system, as if the Earth were a binary planet. In any event, the Moon does have a large effect upon our global environment. In the short run the Moon affects the ocean tides; in the long run it slows down the Earth's rotation – at one time the Earth's day may have been less than ten hours long. Just as the gravitational effects of the Earth on the Moon slowed down the Moon's rotation so that now the Moon always offers the same side to the Earth, the Moon's gravitational attraction, though smaller, will eventually have a similar effect on the Earth. The night-day cycle is of course an extremely important component of our climate and presumably has played a major part in our natural history.

The Moon has also influenced the climate in a second important way: Its gravitational influence stabilizes the tilt in the Earth's axis of rotation, so that it varies only a few degrees. Mars, by contrast, may have suffered wild swings in the tilt of its axis, and this instability might have had devastating consequences for the Martian climate.[8] In other words, the Moon may have played a crucial role in ensuring that life on Earth endured and prospered while Mars became a barren world.

We have begun to see in this section that our specific theories about the Earth are inevitably tied to more general theories about the nature and behavior of the other bodies of the solar system. As we challenge our understanding of that system, we place ourselves in a position to learn new things, not only about other worlds, but also about our own. The bounty of space science will thus not be scattered by alien winds over alien lands. It will be handed down to the children of the Earth.

[1]. L.W. Alvarez, W. Alvarez, F. Asaro, and H.V. Michel, "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction, Science, 1980, vol. 208, pp. 1095-1108.

[2]. Some interesting studies by D.M. Raup and J. Sepkoski suggested that the extinction of a significant portion of terrestrial life is a periodic occurrence, with the period being about 26 million years (D.M. Raup and J.J. Sepkoski, Jr., "Mass Extinctions in the Marine Fossil Record," Science, 1982, vol.215, pp. 1501-1503. For several of the issues raised in the last few paragraphs the reader may wish to consult Chapter VI of The Evolution of Complex and Higher Organisms, D. Milne, D. Raup, J. Billingham, K. Niklaus, and K. Padian, eds., NASA SP-478, 1985. According to a hypothesis by Raup, this extinction rate depends on the orbit of a star companion to the sun, a dwarf star dubbed "Nemesis" which causes the gravitational disturbances described in the text. Nemesis was never found, though (For a very accessible account, read D.M. Raup, The Nemesis Affair, W.W. Norton & Co., New York, 1986). This idea has fallen out of favor, since Nemesis was never found.

[3]. See D. Desonie, Cosmic Collisions, a Scientific American Focus Book, Henry Holt & Co., 1996.

[4]. If they are not, we will still come away with sharpened alternative accounts of the fate of living things.

[5]. These estimates come from D. Desoinies Cosmic Collisions, op. cit., pp. 100-101.

[6]. Since the Earth is a bit more massive than Venus, its gravitational attraction is consequently larger. On the other hand, Venus is closer to the sun, and thus objects with pronounced elliptical orbits and rather small perihelions are bound to pass closer to Venus than to Earth. It is unfortunate, for the purposes of statistical prediction, that smaller craters than Meteor Crater (whose creation would be disastrous enough) do not register on the surface of Venus – such meteors burn up in the extremely dense atmosphere of that planet. The present estimates for objects around 60 meters in diameter strike me as being at least of the right order of magnitude and probably quite accurate. Incidentally, my own estimates involve some circularity, since the age of the surface of Venus has been estimated using the rate of cratering (although such rate has been calibrated to some degree with actual measurements on the Moon).

[7]. Thermonuclear weapons are the first choice, although ONeills mass drivers might also do the job. He envisioned using such drivers to transport asteroids rich in valuable minerals to a lunar orbit. The effectiveness of nuclear bombs is the subject of some controversy and the inspiration for several movies.

[8]. For an alternative account of the extinction of the dinosaurs, see R.T. Bakker, The Dinosaur Heresies, William Morrow and Co., New York, 1986. The Alvarez account has become the received view, however.

Saturday, September 11, 2010

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.

[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.

Saturday, September 4, 2010


Chapter 4G


A sketch of the view

Our global understanding of the Earth depends on what we think a planet like Earth is like. The mechanisms that regulate Earth's environment interact with each other in many loops and cycles. These loops and cycles are run by energy, and that energy comes either from the Earth itself or from extraterrestrial sources. The energy that comes from the Earth depends very much on the sort of planet the Earth is. And the useful energy that comes from the sun and the rest of the solar system depends on how the planet Earth relates to the rest of the system.

The Earth produces heat that rejuvenates its surface by creating, moving, and breaking up continents; by forming mountain ranges when the tectonic plates that carry the continents collide; by bringing new materials from the mantle into the crust, oceans, and atmosphere through volcanoes and mid-ocean ridges; and by recycling lands and gases through the spreading and subduction of the crust (the sinking of one plate under another when they collide). The rejuvenation of the Earth's surface creates a great variety in the environment, one of the crucial factors in the natural selection of living things. Moreover, the energy injected into the oceans and the atmosphere drives those systems and greatly influences how they interact with one another.

The Earth's gravitational energy keeps the atmosphere from dissipating into space, and thus it determines to a high degree the density of that atmosphere. That density in turn influences the chemistry of the environment and the climate of the planet. To see the point clearly it pays to compare our planet with others. For example, the density of Mars' atmosphere is so low (about one hundredth that of Earth's) that water cannot exist in liquid form: It goes directly from ice to vapor. One of the crucial differences between Mars and the Earth is precisely that the Earth's mass, and therefore its gravitational attraction, is much larger.

Internal heat and gravitation are both functions of the mass and structure of the Earth. Let us discuss heat. The main sources of internal heat are the release of energy from the decay of radioactive elements such as thorium, uranium, and potassium in the interior of the planet, and the energy left over from the gravitational collapse of the matter that formed the planet initially. Associated with this second source is the heat from differentiation, which occurs when denser materials move downwards and displace less dense materials towards the surface. And initially there was also, of course, the extraordinary heat that the bombardment of the Earth by asteroids generated, enough, in the bigger collisions, not only to vaporize oceans and atmosphere but to melt the entire surface.

Our ideas about these sources of heat are based partly on what we think is the structure of the Earth and partly on how we think the Earth was formed. We think that the Earth was formed by the accretion of planetesimals (chunks of the original materials of the solar system) over four and a half billion years ago. As the Earth grew, its gravitational attraction also grew and the Earth captured even more planetesimals. In a short time the Earth was colliding with many objects of diverse sizes. Many of these collisions would have generated a good deal of heat,[1] as also did the compression of the accreting materials by the increasing gravitation. In the hot terrestrial interior, the heavier elements separated towards the center, eventually creating a metallic, radioactive core. Less heavy materials concentrated first in the mantle, and then in the lithosphere (the crust and the uppermost portion of the mantle upon which the crust rests). The heat from the core stirs up the mantle and leads to the convection currents that push tectonic plates apart at the ridges. Through those long trenches, a new surface is forged from the mantle that spills forth.[2]

In trying to understand the mantle, the core, and so on, we do not observe them directly. We infer some of their properties from the measurements we make of the Earth, for example with seismic waves, using a technique called "seismic tomography,” which is based on the notion that seismic waves travel at different speeds through different materials (such as molten metal or solid rock). And then we use those measurements in the light of certain theories in order to discern other geophysical properties. Among those theories are our ideas of how a planetary body forms and how it distributes its energy. More specifically, they are theories of the evolution of a planetary body, adjusted to Earth's mass and position within the solar system (not just its distance from the sun but also its having a very large moon as well). To understand the Earth we must understand what kind of object it is. We know that Earth is a planet; thus we need to understand what a planet is, and more specifically what a planet like Earth is.

[1]. Several theories would posit a cold accretion of the Earth. If the Earth had condensed from a gas nebula, a cold accretion would make sense. But on the prevailing view that the Earth accreted from planetesimals, I do not think it makes as much sense. Recent calculations do suppose a cold accretion -- the reasoning is that when planetesimals come together at low relative velocities they can easily stick together; this result allows for the expected quick planetary accretion. At low relative velocities, however, not much heat is produced. At high relative velocities, on the other hand, the colliding planetesimals would vaporize; and thus accretion would take a long time (see Alan P. Boss, "The Origin of the Moon," Science, January 24, 1986, vol. 231, pp.341-345). Nevertheless, it seems to me, that the quick initial accretion of bodies with low relative velocities leading up to the proto-Earth would be eventually followed by collisions at high relative velocities with bodies of varying sizes, long before the completion of the Earth's accretion. As long as the proto-Earth's mass was significantly higher than those of the other objects, some of the mass of those objects would be quickly accreted into the Earth by gravitational attraction, even if those objects vaporized upon impact. This "hot" phase in the formation of the Earth is quite reasonable in view of the presently favored hypothesis of the origin of the Moon, which would have the Earth colliding with a body the size of Mars (see below). Surely if this is plausible we should favor a scenario in which the proto-Earth is constantly bombarded by bodies too small to break it up but large enough to provide for a "hot" phase during its accretion.

[2]. 70% of the heat from the interior is dissipated in the motions of the plates. M. Carr, R.S. Saunders, R.G. Strom, D. E. Wihelms, The Geology of the Terrestrial Planets, NASA SP-469, 1984, p. 76.