The first person to observe lunar craters was Galileo. In 1609 he trained his telescope on the moon and saw the seas (the maria), the highlands, and some circular spots. He observed that as the termina-tor—the sharp line separating the light and dark sides of the moon— moved across, the far edges of the circles lit up before the centers. This told him that the rims of the circles were higher than their centers, which meant that they were depressions, or craters. From the length of their shadows, Galileo calculated the heights of the crater walls.
As the techniques of astronomy improved, the full extent of lunar cratering emerged and needed to be explained. Many eminent scientists and philosophers, including Robert Hooke, Immanuel Kant, and William Herschel (polymaths all), took a crack at the question. Almost to a person they concluded that the craters were the remnants of lunar volcanoes. The idea that impact created the craters was proposed from time to time but never taken seriously.
In 1892, the great American geologist G. K. Gilbert, whose interest in craters was fostered by his research on one in northern Arizona, began to study lunar craters by telescope. He observed that their shape, and their central peaks and collapsed terraces, showed them to be markedly different from terrestrial volcanic craters. For that reason, he concluded that they could not be volcanic but instead had to have been formed by impact. He conducted scale-model experiments and found that impact could indeed form craters, but that when the experimental projectile struck at an angle, the resulting crater was elliptical. Since meteorites arriving randomly on the surface of the moon surely must strike at an angle most of the time, at least some lunar craters should be elliptical, but as far as Gilbert could determine, all were circular. To reconcile experiment with observation, Gilbert proposed that a ring of solid objects in orbit around the moon, like the rings of Saturn, gradually released chunks that fell vertically onto the moon's surface. But since no evidence supported this theory, it too sparked little interest.
Gilbert conducted his tests in a hotel room, which meant that the experimental impactors fell at low velocities. He had no way of knowing that a projectile arriving at interstellar speeds is destroyed and its energy converted to an explosion that leaves a circular crater almost regardless of the angle of incidence. That knowledge had to await the early twentieth century and additional observations, many of them on the bomb craters that soon became all too available in the pockmarked fields of Europe.
Gilbert's negative conclusion essentially shut down research on the origin of terrestrial craters until, in 1961, a new era began when President John F. Kennedy, calling space the new ocean to be explored, declared that within that decade the United States would send a man to the moon and return him safely. Wisely, before astronauts were sent, unmanned vessels such as Ranger, Surveyor, and Orbiter mapped the moon. They found it densely cratered on every scale from thousands of miles to fractions of an inch. Missions from the Soviet Union and the United States showed that the lunar far-side was also heavily cratered. Craters of every size saturated many lunar terrains, leaving no room for a new one without obliterating one or more previously existing craters. The larger craters had features not displayed by volcanic craters: central peaks, terraced rims, and rays of splashed debris. As this kind of evidence accumulated, it gradually became clear that lunar craters were not volcanic but were formed by impact. There was no scientific reason to believe that the other inner planets would have had a different history.
Astronauts brought back from the lunar highlands samples of impact breccia, rocks composed of broken, angular fragments embedded in a fine-grained matrix. This is what would be expected when impact breaks apart the rocks at ground zero, which are later cemented back together. The first Apollo astronauts, however, stepped out not on the highlands but onto a plain of basaltic lava, the type extruded by the Hawaiian volcanoes, showing that although there were no volcanic cones or craters any longer visible, at some earlier time vast sheets of lava had flowed out onto the lunar surface. When the lunar samples were dated, those from the highlands gave ages of 4.5 billion to 4.6 billion years, the same as the oldest meteorites and the calculated age of the earth, thus supporting the view that all the objects in the solar system have the same original age of formation. The volcanic rocks, however, which appeared both to earthbound geologists and astronauts to be the youngest lunar material, gave ages ranging from 3.' billion to 3.7 billion years. In other words, the youngest moon rocks were almost as old as the oldest rocks on the earth, which date to 3.8 billion years. Geologists quickly realized that the moon had not had a continuous, steady geologic history like the earth—everything had been crammed into the first '.5 billion years or so, after which the only significant process was meteorite impact.
The Apollo missions and the analysis of the returned samples had the ironic effect of debunking all the existing theories of the origin of the moon. As the full extent of cratering in the solar system came to be appreciated, Donald Davis and the versatile William Hartmann— scientist, artist, and author—proposed that early in the history of the solar system, a protoplanet the size of Mars struck the earth and blasted both itself and a large chunk of the earth into near-earth orbit, where the debris gradually amalgamated into the moon.6 The mass that stuck became part of the earth's mantle. This has now become the theory of choice among planetologists. (In yet another example of a theory ahead of its time, Harvard's Reginald Daly proposed this very idea in the '940s, but no one paid it any mind.7)
The space age had hardly begun when it brought evidence of cratering on other planets. The Mariner '0 spacecraft missions to Mercury in '974 and '975 found a surface as densely packed with craters as that of the moon. On Venus, Magellan found huge active volcanoes, vast lava flows, and a surface pocked with medium- to large-size craters. (Smaller craters were not found, probably because Venus's dense atmosphere causes small meteorites to burn up before they hit.) The Voyager mission showed that Jupiter's moons—Cal-listo, nearly as big as Mercury, and Ganymede, even bigger—are cratered on a lunar scale. Mars is not only heavily cratered but contains titanic volcanoes. And thousands of pockmarked asteroids float in space, some of them in orbits that cross that of Earth. Indeed, of all the bodies observed since the space age began, only three—Earth, and Jupiter's moons Io and Europa, all of them with active surfaces—lack obvious and plentiful craters.
Thus over the course of the twentieth century, impact cratering has gone from being viewed as extremely rare to being regarded as a paramount process in the history of the solar system. To have existed as a solid body in the solar system is to have been massively bombarded since the beginning. But where, then, are the craters that must have formed on Earth?
WHERE HAVE ALL THE CRATERS GONE?
After all, Earth not only has a much bigger cross-section than the moon to present to incoming meteorites, it also has a greater mass and therefore exerts a greater gravitational pull. Calculations combining area and mass show that at least 20 times as many meteorites should have hit Earth as hit the moon. The moon has 35 impact basins larger than 300 km in diameter, most of them nearly 4 billion years old. In the same period in its early history, 700 giant basins should then have formed on Earth. Such saturation bombing would have caused the entire surfaces of the moon and Earth to melt, forming giant magma lakes that persisted for millions of years (evidence for this is more visible on the moon than on Earth, where no trace remains of the early bombardment). Subsequently, both the moon and Earth were struck countless times, though not as often as during the first few hundred million years. Why then could the Alvarezes not find ready support for their theory in an abundance of terrestrial craters?
The first footprint at Tranquillity Base will outlast the pyramids and the tallest skyscraper—the moon contains no wind or water to erode the evidence of human visitation. Eternal, this giant fossil of the early solar system awaits our return. Earth, on the other hand, has been internally active since its creation, constantly renewing and reworking its surface materials, altering them beyond recognition. The erosive action of wind, ice, and water, and the large-scale effects of volcanic, plate tectonic, and mountain-building activity, have transformed the surface of Earth, and thus would likely have obscured or obliterated most terrestrial craters. The oceans cover 70 percent of Earth's surface and any crater that formed under the sea would be hidden. To add insult to injury, plate tectonics constantly recycles the seafloors, none of which therefore is older than about 125 million years. Since the end of the Cretaceous 65 million years ago, 20 percent of the seafloor has been carried down the deep-sea trenches that abut most continents, taking any accompanying craters with it.
It might be tempting to posit that terrestrial craters are rare because Earth, unlike the moon and Mars (but like Venus), has an atmosphere that incinerates incoming meteorites. We know that shooting stars suffer such a fate, but they come from tiny meteorites that weigh only a few grams and burn up completely. Slightly larger meteorites survive the trip but are slowed by atmospheric drag so that they arrive intact at nonexplosive velocities. But any meteorite larger than 40 m or 50 m in diameter blasts its way straight through the atmosphere and reaches the surface of Earth unimpeded and at cosmic velocities, whereupon it explodes, releasing tremendous amounts of energy.
It was not until the nineteenth century that scientists were prepared to believe that meteorites of any size actually came from space. Thomas Jefferson allegedly was told of the claim of two academics that they had observed a meteorite fall in Connecticut in 1807 and responded that he "would rather believe that two Yale professors would lie rather than that stones could fall from hea-ven."8 (Even if apocryphal, this story does capture the sentiment of the day.) But actual meteor falls observed at Siena, Italy, in 1794 and at L'Aigle, France, in 1803, when more than 2,000 dropped, made it impossible to deny that stones do fall from the sky. Establishing the direct link between meteorites and impact craters was much longer in coming, however. It was first found at the end of the nineteenth century, near the Painted Desert, east of Flagstaff, Arizona.
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