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. 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. 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.
Apart from the realization that our natural history has to make conceptual room for such catastrophes, 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.
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. 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.
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. 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.
. 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.
. 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.
. See D. Desonie, Cosmic Collisions, a Scientific American Focus Book, Henry Holt & Co., 1996.
. If they are not, we will still come away with sharpened alternative accounts of the fate of living things.
. These estimates come from D. Desoinie’s Cosmic Collisions, op. cit., pp. 100-101.
. 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).
. Thermonuclear weapons are the first choice, although O’Neill’s 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.
. 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.