Originally published 23 May 1983
The meteor was traveling through space at tens of thousands of miles per hour when it collided with the earth. It was as big as a house, weighed a hundred thousand tons, and blasted a hole in the ground the size of downtown Boston.
While the meteorite crashed to earth 25,000 years ago near Flagstaff, Arizona, its tourist-attraction crater remains the most recent scar of its kind on the face of the earth — as well as a vivid symbol of the danger that can come from the sky. If a similar object slammed into Arizona today, the human toll would be staggering; if it fell into Massachusetts Bay, it would raise a tidal wave that would devastate coastal New England.
It is a sobering thought to realize that such an event is not only possible, but — given enough time — certain.
In recent years, geologists have made a close census of ancient impact craters on Earth, on the Moon, and on other planets. Astronomers have studied the asteroids that swing through the solar system on Earth-intersecting trajectories. Together, they conclude that several objects capable of excavating a crater at least 6 miles wide will crash into the Earth every million years.
The enormous ecological consequences of such an impact on land and sea raise the question of whether asteroid impacts have punctuated the evolution of life on Earth.
In a theory that has gained favor in the past three years or so, many scientists say yes. But, in recent weeks, even that relatively new theory is being disputed on the basis of new field work.
The death of the dinosaurs
The asteroid theory, initially offered in late 1979, holds that an Earth-colliding asteroid — far greater that the trifling one that hit Arizona 25,000 years ago — was the cause of the wave of plant and animal extinctions that swept the earth 63 million years ago, extinctions marked by a sharp discontinuity of life forms preserved as fossils in the stratified rocks. The event defines the boundary between the Cretaceous and Tertiary periods of geologic time, and between the Age of Reptiles and the Age of Mammals. The most famous victims of the Cretaceous-Tertiary calamity were the dinosaurs.
One place where the boundary between the two geologic eras is exposed for inspection is in the hills near Gubbio, Italy. A thin layer of clay separates two formations of marine limestone. The limestone below the clay contains fossil marine organisms typical of Cretaceous times. There are no fossils in the clay. Tertiary fossils characterize the limestone above the clay.
In 1979, a group of scientists at the Lawrence Berkeley Laboratory in California announced the discovery of an abnormally large concentration of the rare element iridium in the clay layer at Gubbio. The iridium level was 30 times greater than in the rocks above or below the clay.
Iridium is rare in rocks of the earth’s crust, but it is found in much richer abundance in meteorites. This led the Berkeley group to suggest that the clay layer at Gubbio was deposited on the sea floor following the collision of a massive asteroid with the earth. The clay presumably was the fallout of a mixture of meteoric material and pulverized Earth rock, blasted into the atmosphere by the impact.
A bullet through tissue paper
Supporters of the asteroid hypothesis estimate that the colliding object had a diameter of three to 10 miles and released the energy equivalent of 100 million hydrogen bombs. The consequences of such an event are difficult to assess, but a likely scenario has the space invader passing through earth’s atmosphere and ocean like a bullet through tissue paper and excavating a crater the size of Rhode Island.
On impact, a huge mass of material, rich in extraterrestrial elements, was lofted into the atmosphere. Winds carried the debris worldwide, wrapping the earth in a gray dusty shroud. For a year or more the earth cooled, the oceans by a few degrees, the land areas by more. For several months light levels at the surface of the earth were below the limit of dark-adapted human vision. The entire planet was plunged into inky darkness. Animals, particularly large ones, had difficulty finding food. We know, in fact, that all land animals weighing more than 50 pounds became extinct at the time in question. The dinosaurs were the most conspicuous victims.
For an even longer time photosynthesis was disrupted. It is thought that the temporary cessation of photosynthesis had particularly dangerous consequences in the oceans. The destruction of planktonic organisms in surface waters could have led to a collapse of food chains.
On land, small burrowing nocturnal mammals fared better in the crisis than sun-loving reptiles. When the danger passed, the mammals inherited the earth.
This violent scenario is not a “worlds-in-collision” fantasy. The story finds ample, if not uniform support in the record of the rocks.
Did a meteor really do it?
But not all scientists were willing to accept a cosmic culprit as the agent of extinction. It was not altogether clear, for example, that the various “Cretaceous-Tertiary” iridium anomalies were in fact exactly coincident in geologic time. Some paleontologists argued that the fossil record supports a gradual and staggered wave of extinctions, rather than a single sudden catastrophe.
Recently, resistance to the asteroid theory has stiffened. A potentially fatal objection to the hypothesis came in a February paper in the journal “Science,” by Michael Rampino of the Goddard Space Flight Center and Robert Reynolds of Dartmouth College. The two scientists reported results of a careful mineralogical study of clay samples from the Cretaceous-Tertiary boundary at four widely separated sites that had shown iridium anomalies, including Gubbio.
If the clays were part of a worldwide fallout from an asteroid collision, said Rampino and Reynolds, they should be mineralogically homogeneous from site to site. Second, the clays should include a terrestrial component that is different from locally derived clays. Third, the clay should contain exotic minerals not normally found in marine sedimentary rocks.
The two scientists found that the clays differed considerably from place to place. The clays were not widely different from local sediments above and below the boundary layer. And there was no evidence of the kinds of exotic constituents that might be expected from an extraterrestrial source.
Rampino and Reynolds conclude that the boundary-layer clays more likely came from earthly volcanoes than a space intruder.
In a March issue of “Science,” Charles Officer and Charles Drake of Dartmouth College reported a survey of several other aspects of the Cretaceous-Tertiary boundary strata. They argued that the fossil sequences across the boundary are not consistent with a single catastrophic event, but show a range of transition times and transition rates depending on fossil type and locality.
It would not be fair to say that the asteroid theory has yet been mortally wounded. But the theory has suffered several damaging blows. Perhaps in the months or years ahead the asteroid hypothesis will follow the dinosaurs into extinction.
Since this essay was first written, over 35 years ago, the asteroid hypothesis has gained widespread acceptance among scientists. — Ed.
