Meteor Crater

• 1. Introduction

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  Meteor Crater sits in the desert of northern Arizona, about 37 miles east of Flagstaff, and what you see there stops people cold. A bowl carved into the earth roughly 1,200 meters across and 170 meters deep, rimmed by walls that rise 45 meters above the surrounding plains. From the air, it looks almost geometric, its outline noticeably squared off rather than round. That squared shape is no accident of chance. It traces the ancient cracks already running through the rock when something arrived from space about 50,000 years ago.

  The object that made this hole was a nickel-iron meteorite about 50 meters across, traveling at a speed scientists have revised downward over the years to around 12.8 kilometers per second. Most of it simply vanished. The energy released has been estimated at around 10 megatons of TNT equivalent. The crater it left behind was later designated a National Natural Landmark in November 1967, and institutions including the Lunar and Planetary Institute and the American Museum of Natural History call it the best-preserved meteorite crater on Earth.

  But recognition like that did not come easily. For decades, scientists argued about what made this hole. A mining engineer spent 27 years and most of his fortune drilling for ore that was never there. A meteoriticist tried to nationalize the site and lost his fieldwork rights. An astronaut trainer found here the closest thing on Earth to the Moon. The questions that shaped this crater's history are not just geological. They are about who gets to own a scientific discovery, and who gets to own the ground it sits on.

• 2. The Impactor and the Blast

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  Fifty thousand years ago, the Colorado Plateau looked nothing like the arid landscape that surrounds the crater today. The climate was cooler and damper, and the area was open grassland dotted with woodlands. Mammoths and giant ground sloths moved across that terrain.

  The meteorite that struck here was made of nickel and iron, a composition that distinguishes it from the more common stony meteorites. At roughly 50 meters in diameter, it was not enormous, but the speed of arrival made the difference. Early modeling put the impact velocity at up to 20 kilometers per second; more recent work lowered that estimate to 12.8 kilometers per second. Even at the revised figure, the collision released energy equivalent to about 10 megatons of TNT.

  About half the meteorite's mass is thought to have burned away during its descent through the atmosphere, and virtually all of what remained was vaporized on impact. The fragments known as Canyon Diablo meteorites, scattered across the surrounding area, broke away from the main body before and during the strike. They were named after the closest post office, Canyon Diablo, Arizona. The canyon itself passes about 2.5 miles west of the crater, crossing the strewn field where those fragments came to rest.

  The impact folded the rock layers outward and upward in a process geologists call inverted stratigraphy. Climbing the crater rim from the outside, a visitor passes through rock formations in reverse chronological order. Coconino Sandstone, formed around 265 million years ago, sits near the top. Below it lie the Toroweap, Kaibab, and Moenkopi formations, each progressively younger, stacked in the opposite order from how they normally appear underground. This overturning extended outward one to two kilometers from the crater's edge.

• 3. A Crater Disputed

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  When American settlers encountered the crater in the 19th century, explanations leaned toward the volcanic. Evidence of geologically recent volcanic activity was everywhere in that part of Arizona. The southeastern edge of the San Francisco volcanic field lay only about 20 miles northwest of the site.

  In November 1891, Grove Karl Gilbert, chief geologist for the U.S. Geological Survey, visited the crater and walked away unconvinced it was anything other than a volcanic steam explosion. His reasoning was methodical. If the crater had been punched from space, he argued, the volume of the impactor should still be detectable, either as displaced rock or as a buried mass generating a magnetic anomaly. His calculations showed the crater volume and the debris on the rim were roughly equal, meaning the mass of any hypothetical impactor was unaccounted for. Magnetic measurements turned up nothing significant. Gilbert concluded that the meteorite fragments found on the rim had been placed there coincidentally, or by other means. He announced his findings in a series of public lectures.

  That same year, mineralogist Albert E. Foote had presented a different picture to the scientific community. In 1891, Foote published the first scientific paper about the meteorites of northern Arizona. He had originally received an iron rock for analysis from a railroad executive, recognized it as meteoritic, and led a retrieval expedition. His team collected samples ranging from small fragments to specimens weighing over 600 pounds. Among the minerals he identified were microscopic diamonds, a detail that pointed toward extreme formation pressures. His paper to the Association for the Advancement of Science gave the first geological description of the crater to a wider scientific audience.

  Gilbert's volcanic interpretation held sway for years, but his methods had a flaw that neither he nor his contemporaries fully appreciated. Impact physics was barely understood. The idea that a meteorite could vaporize entirely on striking the ground was not yet part of scientific thinking. And because no large buried mass existed, the conclusion seemed to validate the volcanic hypothesis. It would take a mining engineer's obsession, and eventually a geologist's chemistry, to overturn it.

• 4. Barringer's Expensive Certainty

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  Daniel M. Barringer was a mining engineer and businessman who had made money investing in the Commonwealth Mine in Pearce, Cochise County, Arizona. He was also certain, in the early years of the 20th century, that a massive deposit of meteoric iron lay buried beneath the crater floor.

  Barringer incorporated the Standard Iron Company and filed a mining claim. He received a land patent signed by Theodore Roosevelt covering 640 acres around the center of the crater in 1903. At Roosevelt's request in 1906, a post office was established unconventionally named "Meteor," located at Sunshine on the Atchison, Topeka and Santa Fe Railway, six miles north of the crater. That post office closed on the 15th of April 1912, having fallen out of use.

  Barringer estimated the buried meteorite weighed around 10 million tons. At the market price of US $125 per ton for the metals it contained, he believed he was sitting on a deposit worth more than a billion 1903 dollars. By 1928, he had spent $500,000 of his own money on the search. Drilling reached a depth of 419 meters. No significant deposit was ever found.

  The reason was simple and devastating in hindsight. Most of the impactor had vaporized on contact. Impact physics was poorly understood at the time, and no one had told Barringer that the very event he believed had buried his fortune had actually destroyed it. In 1929, astronomer F. R. Moulton was hired by the Barringer Crater Company to study the physics of the impact. Moulton calculated that the impactor had likely weighed as little as 300,000 tonnes, and that the heat generated by such an impact would have vaporized it instantly. Barringer died just ten days after Moulton's second report was published.

  The Standard Iron Company was renamed the Barringer Crater Company in 1953. The crater itself remains privately owned by the Barringer family to this day.

• 5. Nininger and the Fight for Recognition

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  Harvey Harlow Nininger was an American meteoriticist and educator who built the largest personal collection of meteorites of his era. Working in the 1930s and 1940s, he did more than almost anyone else to shift scientific and popular thinking about meteor craters.

  Nininger was based in Denver, Colorado, when he published a pamphlet titled "A Comet Strikes the Earth," describing how an asteroid impact had formed Meteor Crater. In 1942, he moved his home and operations to the Meteor Crater Observatory, near the Route 66 turnoff for the crater, and named the building the American Meteorite Museum. From there he published books and conducted fieldwork at the crater itself.

  His discoveries were substantial. He identified impactite and iron-nickel spherules connected to the vaporization of the asteroid. He also found half-melted slugs of meteoric iron mixed with melted target rock. These findings were gathered into a seminal 1956 publication, Arizona's Meteorite Crater. Features he documented at Meteor Crater were later recognized at other fresh impact craters, including Henbury and Monturaqui.

  Nininger believed the crater should be nationalized as a public monument. In 1948, he petitioned the American Astronomical Society to support that effort. The petition included a claim, later acknowledged as unauthorized and false, that the Barringer family would accept a fair purchase price. The Barringers found out. They terminated Nininger's exploration rights and barred him from further fieldwork at the crater. A private museum was constructed on the crater rim in 1953, the same year the mining company was renamed.

  The conflict between Nininger and the Barringers shaped the crater's long-term status. It remained private property, and the museum and visitor facilities that exist today were built by the family, not by the federal government Nininger had hoped would take over.

• 6. Shoemaker and Scientific Proof

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  Eugene Merle Shoemaker brought the question of the crater's origin to a definitive close by looking not at the crater's shape or its buried contents, but at what the rock itself had been through.

  The key was a pair of minerals called coesite and stishovite, rare forms of silica that form only when quartz-bearing rock is subjected to instantaneous overpressure. Volcanic action cannot produce shocked quartz. The only natural mechanisms are lightning strikes and impacts from space. Artificially, nuclear explosions can produce the same result. In 1960, Shoemaker and Edward C. T. Chao identified coesite at Meteor Crater. That find added decisive chemical evidence to the impact hypothesis.

  Shoemaker also confirmed what Moulton and Nininger had already argued: the impact vaporized nearly all of the impactor. The Canyon Diablo meteorite fragments scattered around the site were pieces that had broken away before and during the collision, not remnants of the main body. Shoemaker published his conclusions in the 1974 Guidebook to the geology of Meteor Crater, Arizona.

  Geologists working in this period also drew on an unexpected source of comparative data. Nuclear test craters from the era of atmospheric testing, including the Sedan crater, gave researchers a way to establish upper and lower bounds on the kinetic energy of natural impact events. By 1967, the evidence was strong enough that the U.S. government designated Meteor Crater a National Natural Landmark. Grove Karl Gilbert, who had concluded in 1891 that the crater was volcanic, had nevertheless gone on in 1892 to be among the first scientists to propose that the Moon's own craters were formed by impacts. The same reasoning he resisted at Meteor Crater, he had extended to the sky.

• 7. Astronauts, Aircraft, and Ongoing Science

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  During the 1960s and 1970s, NASA used Meteor Crater as a training ground for Apollo astronauts preparing to walk on the Moon. The site's geological resemblance to lunar terrain made it practical. Astronaut field training at the crater continues to the present day.

  On the 8th of August 1964, two commercial pilots flew a Cessna 150 low over the crater. After crossing the rim, the aircraft could not maintain level flight. The pilot tried to circle inside the crater to gain altitude and clear the rim. During that attempted climb, the plane stalled, crashed, and caught fire. Both occupants were severely injured but survived. The crash has often been attributed to fuel exhaustion, but that account is wrong. A portion of the wreckage that was not removed remains visible at the site.

  In 2006, a scientific project called METCRAX studied the way cold air pools form and break down in the basin each day, examining the physical processes behind the crater's temperature inversions. The crater's particular shape made it a useful natural laboratory for that kind of atmospheric research.

  Today, roughly 270,000 visitors come to Meteor Crater each year. The Visitor Center on the north rim, formerly known as the Museum of Astrogeology, holds interactive exhibits, a movie theater, a gift shop, and observation areas looking into the crater. On display is an Apollo boilerplate command module designated BP-29, a 1,406-pound meteorite found in the area, and meteorite specimens from Meteor Crater that visitors can touch. The center of the crater itself is filled with 210 to 240 meters of rubble above the bedrock, a compressed record of everything the impact left behind and everything the desert has added since.