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Calamity at Meteor Crater.

An iron asteroid that fell to Earth 50,000 years ago dug out what is today known as Meteor Crater, located 55 kilometers east of Flagstaff, Arizona. The basin measures 1.2 km across; however, living organisms may have felt effects as far away as 40 km from the point of impact. Courtesy David Roddy, Karl Zeller, USGS.

Like it or not, asteroids and comets episodically endanger and sometimes extinguish life on this planet. The realization that this natural hazard will continue to threaten Earth, and our own species, is relatively new. For decades scientists focused on only the geologic processes associated with impact events. But now, as mounting evidence links a cataclysmic collision to the mass extinction that closed the Mesozoic Era 65 million years ago, scientists are increasingly considering the ecological implications of extraterrestrial bombardment. The catastrophe that dug out the Chicxulub basin on Mexico's Yucatan peninsula, among the largest craters identified in the Earth's geologic record, produced some of the most dramatic changes imaginable. However, smaller objects, which affect Earth more frequently, can have widespread consequences as well. For that reason, crater sites of various sizes interest researchers like me who hope to determine how such calamities affect life in the target region.

This question has brought me to the Colorado Plateau in northern Arizona, where heaven and earth seem to meet. Meteor Crater (also known as Barringer Crater, among other names) is one of the world's classic impact sites. It lies 55 kilometers east of Flagstaff, not far from Interstate 40, and is easily accessible to anyone visiting the Grand Canyon and Petrified Forest areas. Turning off the interstate and looking at the basin's rim rising above the southern horizon, you might wonder what the crash would have looked like (spectacular and horrifying) and whether you, at this distance, would have survived (probably not).

I am sitting on top of an ancient lava flow, amid a mile-high forest of ponderosa pine. It is dark, and the star-studded autumn sky is brilliant from my perch in the arid Southwest. While it looks incredibly peaceful now, 50,000 years ago an iron asteroid tens of meters in diameter came hurtling out of the sky. Moving in excess of 40,000 km per hour, the object slammed into the plateau sediments below, producing an explosion approximately 1,000 times more energetic than the nuclear bombs that destroyed Hiroshima and Nagasaki. Earlier in the day I had held the shattered remnants of a once-solid stone that had been in the projectile's path. It was humbling to imagine the forces that excavated 175 million tons of rock in a flash, covering the surrounding landscape with a blanket of debris. What remained were a 1-km-wide crater and the scattered remnants of the asteroid. Even though no humans dwelt here at the time, the loss of plant and animal life that accompanied the impact must have been dramatic.

A Peaceful Scene

Fifty thousand years ago glaciers blanketed much of North America. Arizona is far enough south that it was not covered with continental ice sheets, but small alpine glaciers may have appeared high in the San Francisco Peaks, roughly 75 km from Meteor Crater. The topography near the crash site was slightly more rolling than it is today, with hills as high as 15 meters. Instead of the steep-walled canyons now evident, shallow streams probably flowed northeastward to the Little Colorado River a few kilometers away. Lower temperatures and greater precipitation meant that those long-ago streams carried abundant water.

Pollen and plant fragments preserved in the geologic record suggest that the vegetation would have been familiar to us, though differently distributed due to the cooler, damper environment. Today desert scrub and a few juniper trees stand where lush grassland and either a juniper-pinon woodland or ponderosa-pine forest once thrived.

A modern observer would notice the greatest differences between the scenes then and now among the animals. Fossils indicate a strange mix of Ice Age fauna roamed the land. Mammoths, giant ground sloths, bison, and camels likely grazed on grass and sedge, while mastodons browsed on woodland trees and bushes. We cannot know if it was day or night when the iron asteroid came hurtling through the sky 30 to 40 times faster than the speed of sound, but the mammoths and mastodons surely turned with wide eyes to see the brilliant meteor as it fell to Earth.

Ground Zero

Depending on its velocity, a meteorite of this size would have a peak magnitude of -27 to -36, appearing up to 10,000 times brighter than the Sun, according to calculations by Jack G. Hills and M. Patrick Goda (Los Alamos National Laboratory). As the object streaked across the sky, it would have seared and blinded the eyes of the animals that looked too long. About 30 seconds after entering the atmosphere the iron-rich body slammed into the Colorado Plateau, producing a 20- to 40-megaton blast, as estimated by geologists David J. Roddy (United States Geological Survey) and the late Eugene Shoemaker. The energy released upon impact vaporized or melted the plants, animals, underlying bedrock, and most of the asteroid itself at ground zero. Some of the surviving fragments of the asteroid were shocked so severely that carbon minerals in the original mass transformed into high-pressure diamonds. The blow overturned and ejected some of the remaining bedrock 1 to 2 km away, hurling 30-ton blocks of limestone out of the crater and onto its rim. Both the initial jolt and the fallback of ejected rock were ground- shaking events, producing a 5.5-magnitude earthquake and smaller rattling tremors.

An incandescent fireball likely rose from the crater, potentially big and hot enough to scorch vegetation and animals as far away as 10 km. While no evidence of a blaze survives at Meteor Crater, other impact basins bear the scars of such a brief but intense hell. In the infamous Tunguska event of 1908, an asteroid exploded about 8 km above a remote region of Siberia, charring the cambium on the sides of trees facing the blast for several kilometers around the epicenter. In some places the pulse of heat appears to have ignited vegetation. Kirill P. Florenskiy (Vernadskiy Institute, Moscow) reported that the forest's canopy burned for several days after the incident. (S&T: June 1994, page 38).

Had a similar fire raged after the Meteor Crater event, its effects might have been mitigated by the gust of air that radiated from the crater in conjunction with the shock wave. These coupled blasts were the most destructive outcome of the collision for nearby plants and animals. Within a few kilometers of the crash site, the magnitude of the shock wave soared to perhaps 100 times normal atmospheric pressure. The resulting pressure gradient generated a wind exceeding 2,000 km per hour, which stripped any grass and flattened all the trees within 14 to 22 km of the impact. Roughly a third of the trees several kilometers farther out might have fallen victim to the gale as hurricane-force winds buffeted the landscape as far away as 40 km. The blast almost completely destroyed the vegetation over an area of 800 to 1,500 square kilometers around the impact site.

The ballistic shock caused by the collision wave probably produced damage at even greater distances. In the Tunguska event, the pressure wave produced a butterfly-shaped pattern of felled tress rather than the circular pattern one might expect. This indicates that the cosmic projectile arrived from the southeast. At Meteor Crater, however, the trajectory of the iron asteroid remains unclear, leaving scientists unsure of how far and in what direction this extended damage occurred.

Past studies suggest that the pressure wave from the Meteor Crater event would have killed all large mammals within 3 to 4 km of the impact and damaged the lungs of others out to distances of 6 to 12 km. As a shock wave reached an animal, the difference between internal and external pressures induced rapid pressure oscillations in air-filled organs, ravaging areas between tissues of dissimilar densities. The results included hemorrhaging and edema in the lungs, possibly causing suffocation. The pressure changes could have also created embolisms in the heart, brain, or other organs. Even farther out, animals near rocky outcrops or other reflective surfaces (which increase effective peak pressures) also suffered. Death came within a few minutes.

Moreover, the force of the explosion literally hurled animals away from the impact site, killing roughly half of them up to 14 km from the collision and many more up to 24 km away. The blast also hurled broken branches, rocks, and other objects that could impale, lacerate, or otherwise wound those not killed outright. Even fragments of the iron asteroid likely inflicted injuries as they rained down, covering the ground out to 10 km.

Although the episode clearly would have been devastating to the local population of plants and animals, the asteroid was too small to produce globe-encircling clouds of dust or significant amounts of climatically active gases. The incoming bolide would have needed to be several times larger to inject CO2, H2O, or SO2 into the stratosphere, where winds could distribute them globally. The Meteor Crater event produced less than one part in 10,000 of the amount of CO2 already in the atmosphere - not nearly enough to have had any significant effect. Unlike the famous Chicxulub impact, the collision that formed Meteor Crater did not likely result in any extinctions. In fact, the area was probably recolonized within 100 years, and the creation of a spring-fed lake inside the crater might have provided niches for species that did not live there before.

The Threat of Future Collisions

The asteroid that formed Meteor Crater struck an area that now contains several sparsely populated towns and a small city. If the object were to fall to Earth today, human casualties would likely result. In a more densely populated region, the effects would be even more catastrophic. For example, if an impactor of similar size landed in the middle of Kansas City today, little of the metropolitan area would remain. Many of us have seen slow-motion films of houses being blown apart and carried away in winds produced by nuclear-bomb tests in the Nevada desert. Similarly, the peak shock pressures produced by an asteroid of these proportions would cause massive structural damage within a region 40 km across, shattering glass, buckling corrugated steel and aluminum paneling, knocking down thick brick, concrete, or cinder-block walls, and blowing out the wood siding used in home construction. Reinforced structures would fare better, but the wreckage would still be extensive. Hundreds of thousands of people - even millions in more densely populated cities - could be killed if such an event occurred with no warning.

Since water covers most of Earth's surface, any future incoming objects are statistically destined to land in an ocean, creating a huge tsunami. However, the destruction that follows will depend on the target area's proximity to a coastal city. For example, if something roughly the size of the asteroid that produced Meteor Crater were to hit 1,000 km from New York City, the resulting tsunami would be only about 10 meters tall where it makes landfall, reaching only 1 km or so inland. However, if the object struck within 200 km of the city, it would likely produce a 50-m- high wall of water that could travel several kilometers inland - enough to destroy a coastal city. Several factors influence these estimates, including the features of the ocean floor and the topography along the shore. Nonetheless, even a relatively small asteroid can produce catastrophic coastline flooding if it strikes water too close to an urban area.

Based on the Spaceguard Survey Report of 1992 scientists are focusing on locating the estimated 1,000 to 2,000 near-Earth objects that are at least 1 km in diameter - the impactors that can cause global, civilization-threatening consequences. Yet the 1995 Near-Earth Objects Survey Workshop Report notes that there are perhaps a million objects larger than 50 meters (roughly the size of the asteroid that produced Meteor Crater) that pass within the vicinity of Earth. According to James V. Scotti of the University of Arizona's Spacewatch Program, 100 house- size objects come closer to the Earth than the Moon's orbital distance each day. In their efforts to catalog these objects, scientists are finding smaller asteroids as well - some just a few meters across - but these are too numerous and faint to search for systematically.

Because erosion and other processes erase surface features so quickly, it is difficult to use terrestrial data to estimate how often these smaller bodies hit Earth. Using the Moon's crater record, however, Gerhard Neukum (DLR, Germany) and Boris V. Ivanov (Institute of Volcanic Geology and Chemistry, Russia) estimate that an object like the one that created Meteor Crater strikes somewhere on our planet about every 1,600 years or about every 6,000 years on a continental region. Some doubt this incredibly high rate of impact since so few craters of these dimensions exist on the surface of Earth. Nonetheless, even if their estimate is off by a factor of two or three, objects of this size pose a significant hazard to the human population of Earth. Let us hope that, unlike the fauna of northern Arizona, we will know one is coming long before it arrives.

David A. Kring, an associate professor at the Lunar and Planetary Laboratory, University of Arizona, is well known for his work with the Chicxulub impact event. His research while he was a student was recognized in this magazine's November 1980 issue on page 382.
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Author:Kring, David A.
Publication:Sky & Telescope
Geographic Code:1U8AZ
Date:Nov 1, 1999
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