Reports of Stones Falling from the Sky

When Benjamin Silliman and James Kingsley from Yale reported stones falling from the sky over Weston, Connecticut, Thomas Jefferson, child of the Enlightenment that he was, is reported to have said, “It is easier to believe that two Yankee professors would lie than that stones would fall from heaven.” It is therefore ironic that nearly 200 years later, convincing evidence has been amassed in Chesapeake Invader by C. Wylie Poag, a paleontologist from the U.S. Geological Survey in Woods Hole, Massachusetts, that a vastly larger meteor impacted south of the Mason-Dixon line, creating a 100-kilometer-wide crater that lies deeply buried under Chesapeake Bay and the shores of Jefferson’s Virginia.

Jefferson’s Humean skepticism about meteorites was not directed just at New England and it was not expressed out of scientific ignorance. He had published several geologic contributions, and he was aware of some details of the then-current scientific controversy over stones falling from the sky. For many skeptics the first convincing investigation had come a few years earlier, in 1803, when Jean-Baptiste Biot, a brilliant young professor of the College de France and friend of Laplace, was dispatched by the French Academy of Sciences to investigate a fall of stones near the town of L’Aigle in Normandy. He collected detailed testimony from the surrounding villages that confirmed the time and place of the fall, that about 3000 stones fell in a 4-by-10-kilometer area, and that the stones were found lying on top of the ground and were of a type unlike rocks native to the area.

Biot’s evidence was overwhelmingly convincing—but not to Jefferson, who wrote to a friend saying that the report was a result of “the exuberant imagination of a Frenchman … run away with his judgment. The evidence of nature, derived from experience, must be put into one scale, and in the other the testimony of man, his ignorance, the deception of his senses, his lying disposition.”

Nevertheless, evidence has gradually accumulated in support of stones falling from the sky, with more than a thousand such events documented and over 10,000 meteorites catalogued in collections. Irons are of course the epitome of meteorites and obviously come from somewhere else. But they are the exception, only a few percent of all meteorites. The vast majority are stony and don’t look that different from Earth rocks to an inexperienced eye. But stony meteorites are in fact utterly unearthly. They are Rosetta stones recording the primeval history of the Solar System and before.

Most stony meteorites are like agglutinations of tiny hailstones—not composed of ice, but of dense silicate spherules—with an overall chemical composition nearly identical to that of the Sun, minus its more gaseous components such as hydrogen and helium. Evidence has gradually accumulated that these meteorites are the original condensations of the solar nebula from which our sun and planets formed. They are all primeval, consistently showing evidence that nearly nothing has happened to them in the last 4.5 billion years. They are the odd fragments left over from the assembly of the Solar System and the formation of its planets.

Claude Allegre, a brilliant geochemist at the University of Paris and the politically controversial Minister of Science and Education in the current French government, tells this fascinating story of meteorites and the origin and early history of the Solar System in his outstanding classic, From Stone to Star. A major gap exists in the historical record between the primeval meteorites and the oldest remaining rocks of the geologic record on Earth. However, the tectonically dead planets preserve a record of this early era and show that it was a time of intense bombardment of asteroids and their smaller fragments, the meteorites, saturating the surfaces of the Moon, Mercury, and parts of Mars with a no man’s land of craters on top of craters.

This era of late, heavy bombardment ended about 3.5 to 4 billion years ago. The primary rock samples that calibrate this history were collected by the Apollo astronauts 30 years ago when they landed in the dark lava plains that fill giant impact craters. These dark plains, visible to the naked eye at full moon, are not dark because they are lava but because they are less cratered than the surrounding highlands. Thus the Moon and the Apollo samples provide a record of the progressive waning of impacts. In the last 3.5 billion years, since the giant impacts ceased, little has happened to the Moon except for successively fewer and smaller impacts. Since then only the occasional asteroid wandering into our part of the Solar System has been large enough to excavate a crater like Chesapeake.

Indeed, only a handful of craters this big have been found on Earth and Venus, planets whose crusts are still in motion. Nevertheless, a few ancient-impact craters are discovered each year on Earth. They generally aren’t obvious unless they are very recent—like Meteor Crater near Flagstaff, Arizona—because of later erosion, burial, and tectonic deformation.

C. Wylie Poag tells the tale of one of the larger serendipitous discoveries, starting from mundane observations of water wells drilled near the shores of the Chesapeake Bay. The subsurface arrangement of aquifers was chaotically anomalous. Poag guides us along the unplanned path that led to the realization that the anomalies might be the fingerprint of a buried ancient impact, and then along the still circuitous path of gathering stronger and stronger evidence of not just the Chesapeake impact, but also a smaller 20-kilometer impact buried offshore of New Jersey. In the process, Poag allows us to look over the shoulders of geologists as they make these fascinating discoveries in the course of their more mundane and practical work for the U.S. Geological Survey.

Poag concludes by assessing the prospect of getting smacked by an impact like Chesapeake. After all, if the Chesapeake impact happened in the recent geological past—about 35 million years ago—then such impacts could happen in the future. Here Poag repeats widely quoted estimates that the probability of our dying of meteorite impact is about the same as from commercial airline crashes (1 in 20,000), higher than from tornadoes (1 in 60,000) but less than motor-vehicle accidents (1 in 100) or murders (1 in 300).

This may raise some eyebrows, as it well should. I now think I can be a little more sympathetic with Thomas Jefferson. I have a hard time believing that we should be worrying about death by meteorites. The crux of the matter is that probabilities have widely different status. It is misleading to compare probabilities of death in airline crashes, which are known directly from statistical tabulations, with estimated probabilities of rare large catastrophic events based on uncertain data and speculative models. However, if we had a complete astronomical catalog of the orbits of asteroids and comets that cross the Earth’s orbit, then any impact could be computed and would be known with certainty centuries in advance. Several groups are working to produce just such a catalog. So far about 200 out of an estimated 1700 near-Earth asteroids larger than one kilometer have been tracked, and the closest approaches for the next century have been computed. Stay tuned. Meanwhile, be happy; don’t worry.

John Suppe is Blair Professor of Geosciences at Princeton University.