Published Tuesday, August 17, 1999, in the San Jose Mercury News

Earth battered through history by comets

Researchers say impacts caused global crises, mass extinctions

BY ROBERT S. BOYD Mercury News Washington Bureau

``My name is Ozymandias, king of kings,

Look on my works, ye mighty, and despair.

Nothing beside remains. Round the decay

Of that colossal wreck, boundless and bare,

The lone and level sands stretch far away.''

-- Percy Bysshe Shelley

WASHINGTON -- Recent scientific discoveries are shedding new light on why great empires such as Egypt, Babylon and Rome fell apart, giving way to the periodic ``dark ages'' that punctuate human history.

At least five times during the last 6,000 years, major environmental calamities undermined civilizations around the world. Some researchers say these disasters appear to be linked to collisions with comets or fragments of comets like the one that broke apart and smashed spectacularly into Jupiter five years ago this summer.

The impacts, yielding many megatons of explosive energy, produced vast clouds of smoke and dust that circled the globe for years, dimming the sun, driving down temperatures and sowing hunger, disease and death.

Warning for future

The discoveries are changing the way scientists and historians look at the past -- and offer a warning about what might happen to our planet in centuries to come.

The last such global crisis occurred between AD 530 and 540 -- at the beginning of the Dark Ages in Europe -- when Earth was pummeled by a swarm of cosmic debris.

In a forthcoming book, ``Catastrophe, the Day the Sun Went Out,'' British historian David Keys describes a two-year-long winter that began in AD 535. Trees from California to Ireland to Siberia stopped growing. Crops failed. Plague and famine decimated Italy, China and the Middle East.

Keys quotes the writings of a sixth-century Syrian bishop, John of Ephesus: ``The sun became dark. . . . Each day it shone for about four hours and still this light was only a feeble shadow.'' A contemporary Italian historian, Flavius Cassiodorus, wrote: ``We marvel to see no shadows of our bodies at noon. We have summer without heat.'' And a contemporary Chinese chronicler reported, ``Yellow dust rained like snow.''

Researchers say similar environmental calamities occurred around 3200 B.C., 2300 B.C., 1628 B.C. and 1159 B.C. Each led to the collapse of urban societies in widely scattered portions of the globe.

Destructive as they were, the natural disasters that have plagued Earth since the dawn of human civilization are but popguns compared with the truly titanic catastrophes of prehistoric eras.

Learning from fossils

There have been at least five of these monster events, each of which wiped out most of the creatures living at the time, the fossil record shows.

The best known was a six-mile-wide meteor that smashed into what is now the Gulf of Mexico 65 million years ago. The collision wreathed the planet in clouds of dust, poisoned the atmosphere and drove the dinosaurs, then rulers of the Earth, into extinction. Traces of the enormous crater, at least 100 miles across, created by the impact were found in 1990.

Even that wasn't the biggest blow the Earth has suffered. The mother of all extinctions, which wiped out 90 percent of living species, happened about 245 million years ago. Paleontologists say other mass extinctions occurred about 214 million, 360 million and 440 million years ago.

Although the evidence is debated, a growing number of researchers contend that most, if not all, of these ecological disasters are connected to bombardments from space.

``Recent evidence is converging on the conclusion that mass extinctions coincided with comet or asteroid impacts, and that periodic comet showers, triggered by the solar system's motions through the Milky Way galaxy, may provide a general theory to explain impact-related mass extinctions,'' said Michael Rampino, a geologist at New York University.

``After an impact, the dense dust cloud that is created by the impact spreads through the atmosphere, cuts out sunlight,'' Rampino said. ``This stops photosynthesis and causes the climate to get cold and dark, leading to the mass extinction of large numbers of organisms.''

These disasters, while terrible for their victims, opened the way for the survivors to flourish, diversify and -- for humans -- take over the world.

``We mammals may owe our pre-eminent position atop the Earth's food chain to a collision some 65 million years ago that wiped out most of our competition, including the dinosaurs,'' said Donald Yeomans, a NASA astronomer who tracks comets and asteroids.

These discoveries are lending weight to a revised theory of evolution. Instead of proceeding gradually by a series of tiny changes, as Charles Darwin proposed 140 years ago, life developed in a series of starts and stops, biologists now believe. They call it ``punctuated evolution,'' periods of slow development interrupted by wholesale extinctions and recoveries.

``It may take millions of years, but as the new organisms fill all the new niches that were emptied out, a whole new biosphere is created,'' Rampino explained.

Evidence supporting this catastrophic theory of evolution is accumulating from many sources:

Studies of oak and pine tree rings in Europe and North America provide a year-by-year chronology of good times and bad dating back 5,000 years. Extremely narrow growth rings are testimony to environmental setbacks that coincide with human catastrophes.

Ice cores recently pulled out of glaciers in Greenland and Antarctica preserve a record of environmental changes over the last 400,000 years.

Deep ocean drilling and surveys on land have detected more than 150 impact craters -- like the mile-wide Meteor Crater in Arizona -- demonstrating that Earth has been the target of frequent bombardment from space. Three or four craters are discovered each year, and many more are thought to be buried underground or in the sea.

Quiet period now

NASA and the Air Force are searching for comets and asteroids that might be on a collision course with our planet. Fortunately, nothing of a dangerous size -- arbitrarily defined as more than a kilometer (0.6 miles) in diameter -- has been spotted heading our way for at least a century. But astronomers say a major impact is inevitable.

``Earth is currently enjoying a quiescent period,'' said Robert Shoch, a Boston University geologist. ``But around 2200 AD, it is likely that a new flow of comet fragments will enter Earth-crossing orbits and pose a real threat to our planet.''

Obviously, the bigger the object and the faster it travels, the more damage it causes. A direct hit is not required; simply passing through one of the streams of cosmic rubble littering the inner solar system can have unpleasant consequences.

The civilization-shattering events of the historic era ``must have been near misses, because if we had been hit by a full-blown comet in the past 10,000 years or so, we wouldn't be here today,'' said Mike Baillie, a British archaeologist who studies tree rings.