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 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.
[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.