A web exclusive story from Astronomy magazine

METEOR CRATER
INSIDE AND OUT

Standing on the rim of Meteor Crater gives one a spectacular view of the best-preserved impact crater on Earth.

David J. Eicher

Take a trip down to the bottom of the best-preserved impact crater on Earth.

by David J. Eicher

With the start of the 50th anniversary of the legendary Apollo missions, you might be dreaming about what it would have been like to explore lunar craters up close. Well, there’s a pretty good way to live that dream, right here on Earth. Meteor Crater, the world’s best impact scar, lies on the Arizona plain about 18 miles west of Winslow — you know, that town from the Eagles song.

This summer I had the chance to travel to Meteor Crater along with its owner, Drew Barringer, whose family has owned Meteor Crater since 1903 — that’s right, the crater is privately owned — and a small group led by planetary scientist David Kring of the Lunar and Planetary Institute in Houston. One of the natural wonders of the world, the crater attracts some 270,000 visitors per year. The site contains a beautiful, expansive visitor center and educational museum with spectacular scientific displays, and normally the visit includes walking around a portion of the 2.4-mile circumference around the crater’s rim. Rarely do visitors get the chance to climb down to the crater’s floor and explore the whole thing. But this year, that’s exactly what we did, and it afforded a unique view of what a small asteroid can do to our planet.

The best crater on the planet

Long ago the inner solar system was bombarded repeatedly with asteroids, planetesimals, and comets. All you need to do to see the evidence of the so-called Late Heavy Bombardment, when much of this frenzy of impacts occurred, is to look at the Moon or Mercury. These worlds have a nearly complete record of ancient scars dating to the early days of the solar system, more than 4 billion years ago. Earth has not been exempted from this aerial attack, but our planet has a widespread system of resurfacing. It comes from plate tectonics, mountain building, volcanism, wind, water, continental drift, and the slumping of craters, mountains, and volcanoes. The record of impacts on our planet gets covered up pretty quickly.

And yet we know many impacts have taken place on Earth in its past. Famous among them is the K-Pg Impact in the Yucatán Peninsula, which among other things wiped out the dinosaurs 66 million years ago. More recently, impacts such as the Tunguska event, an airburst explosion of an asteroid over Siberia in 1908, and the Chelyabinsk impact in 2013, come to mind.

And then there’s Meteor Crater, the first proven and best preserved of all Earth impact craters, gouged into the Arizona desert. Because we know of past impacts and what caused them doesn’t mean they will stop. At some point in the future, humans will have to do something to prevent another such impact, and so Meteor Crater has a great deal to teach us.

An overhead view of Meteor Crater shows the roughly 1.2-km diameter and its somewhat square appearance, caused by corners that are associated with tear faults. Remnants of mining operations are visible in the center of the crater floor. The Visitor Center and Museum are visible at top, above the rim. David Kring

Meteor Crater is unlike any other spot on Earth. Perched at an elevation of 5,710 feet (1,740 m) above sea level, the impact scar stretches 3,900 feet (1,200 m) across — three-quarters of a mile — and the floor is 560 feet (170 m) deep. The crater’s rim rises 148 feet (45 m) above the surrounding desert plain.

The story of understanding Meteor Crater begins in 1891, with the Philadelphia physician, mineralogist, and mineral dealer A. E. Foote (1846–1895), who heard of the crater from a railroad executive who sent him a sample of iron from the crater. Foote analyzed the iron and deduced the sample came from a meteorite. He immediately traveled to the location with a team of assistants, to a point “185 miles due north of Tucson,” and collected masses and fragments of meteoritic iron. Foote found that the meteorites contained signature minerals and elements such as troilite, daubréelite, carbon, and diamonds, of extraterrestrial origin.

Foote wrote a scientific paper about his find and presented it at a meeting of the American Association for the Advancement of Science in Washington, D.C. In it he described the meteorites, wrote about “Crater Mountain,” but he did not connect the crater with a meteorite impact, despite writing that he could not locate lava, obsidian, or “other volcanic products.” He believed that a large iron meteorite of 500 to 600 pounds impacted near the site, but as unconnected with the crater. The pieces of meteorite were linked with the surrounding plain, and at one point the crater was called Coon Mountain. Meteorites found near the site were shipped from a small town nearby called Canyon Diablo, and so the meteorite fragments found were called Canyon Diablo meteorites.

In the audience at Foote’s Washington lecture was none other than Grove Karl Gilbert (1843–1918), chief geologist of the U.S. Geological Survey. He was immediately entranced by the story of the crater and the meteoritic iron. He believed that perhaps the crater was the result of the impact of a large iron mass, or that the crater was produced by a large steam explosion and had nothing to do with the meteorites. He conducted measurements in late 1891 to detect meteoritic material underneath the base of the crater. He carefully measured the crater’s volume and the volume of material ejected into the crater’s rim. If the two were equal, he would reason that no mass lay beneath the crater floor. He also measured the crater’s magnetic field. He found the same volume and also detected no magnetic anomaly. So Gilbert figured the crater had formed from a steam explosion, unrelated to the meteorites.

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Unaware of this primitive geological work, geologist Daniel Moreau Barringer (1860–1929, and pronounced Bear-in-GER, with a hard G) learned of the crater while talking to a friend on the veranda of a Tucson hotel. The son of Daniel Moreau Barringer, U.S. Congressman from North Carolina, and nephew of Confederate Brig. Gen. Rufus Barringer, Daniel was astonished by the site and dreamed of a lucrative find of iron and nickel from the meteorite mass. He was already an experienced silver miner and was well financed, and so in 1903 he acquired the crater through a series of mining claims. The site is more commonly known as Meteor Crater, but its proper scientific name is the Barringer Meteorite Crater. After making 10 trips to study the crater, Barringer produced an analytical paper of his own, aided by his business partner, Benjamin Chew Tilghman.

Unlike Gilbert, Barringer carefully studied the distribution of the meteoritic iron and found that pieces were scattered concentrically around the crater. He described strata in the crater walls, and suggested they were uplifted by a forceful impact. He examined and correctly described the inverted layers of material upturned by the impact. He found that the largest ejected blocks were oriented in an east-west line, suggested the trajectory of an impacting body. He found that silicate minerals were crushed so finely that “grittiness” could not be detected. Barringer also noted the absence of any volcanic material to a depth of 1,400 feet below the surrounding desert plain. A steam explosion seemed out of the question. Barringer concluded correctly that the crater must have been the result of an impact from space.

Daniel Moreau Barringer’s map of the distribution of Canyon Diablo meteorite specimens, made in 1908 and published in 1910, shows where major meteorite finds were made around the crater. David Kring

The earlier work of Foote and especially Gilbert, with his reputation, continued to color the interpretation of the site for many years. In an age when catastrophic events seemed too unusual, and geologists adhered to belief in slow, continuous, uniform processes, Barringer’s conclusions — correct as they were — were largely ignored well beyond his lifetime.

And then came a renaissance for the ideas of Daniel Barringer. The mountain of additional confirming evidence came from the work of Gene Shoemaker (1928–1997), geologist and one of the founders of planetary science. Reexamining the problem and studying Meteor Crater for his PhD. work at Princeton, in 1960, Shoemaker incorporated new studies of nuclear blast explosions and developed a new model for understanding high-speed, energetic impacts. He conducted immensely detailed studies of the geology of the crater and correlated them with features found in the nuclear-bomb crater Teapot Ess in Nevada.

Along with geologist Edward Chao (1919–2008), Shoemaker discovered telltale shocked quartz from impact energy in the form of the minerals coesite and stishovite. Shoemaker‘s studies confirmed what Barringer believed: the crater was definitely an impact from a meteorite. Shoemaker went on to train Apollo astronauts at Meteor Crater (and nearby Sunset Crater, a volcanic cinder cone), and pioneered the field of astrogeology with the founding of the U.S. Geological Survey Astrogeology Branch in Flagstaff.

This aerial view of Meteor Crater shows some of the features labeled. David Kring/Courtesty of Meteor Crater Enterprises

A trip down into the crater

Gene Shoemaker was very much on my mind when I arrived at Meteor Crater in June 2018 for our journey down to the crater floor. Gene died tragically in an auto accident in Australia, far before his time, and I had gotten to know he and his wife Carolyn, the world’s leading discoverer of comets, when I was hanging around Flagstaff in the 1980s. Also on my mind was David Kring, the leading expert on Meteor Crater, and my new friend Drew Barringer, Chairman and CEO of The Barringer Crater Company and my fellow Lowell Observatory Advisory Board member.

Our group began our hike into the crater on a sweltering day. Indeed the humidity was low, but the temperature tipped the scales at 95° F (35° C), so we brought plentiful supplies of water with us. We expected the adventure to last at least three to four hours. Members of our group were all well acclimated; I was not, having come from an elevation of 800 feet in Wisconsin two days beforehand. The difference in altitude and heat would come back to haunt me later in the journey.

After getting our stuff together and assembling our group, we traversed through the beautiful visitor center and headed west along the crater rim trail. This would take us several hundred feet to a century-old trail leading downward into the crater. As we walked along, we had to be extraordinarily careful, led and at times warned by David Kring, who is certainly the world’s leading expert on Meteor Crater as a planetary scientist. Great care must be taken about where to step so as not to disturb the placement of rocks, damage water-needy plants, or leave impressions and prints here and there. The crater is very much an active research laboratory in which many factors are studied, the disturbing the positions of artifacts can disrupt the knowledge to be gained.

As our group descended the rough walls of the crater, planetary scientist David Kring lectured on the geologic history of the crater. David J. Eicher

We traversed down the antique path carefully, pausing occasionally to listen to David’s scientific explanations of various key points, positions, or vistas that would help to explain the crater’s history. One of the first things we noticed in climbing down was to turn our heads back up to see the raised rim, and the enormous blocks of uplifted rock above us that were thrown out of the crater. One could easily imagine what it would be like to climb down into a crater on the Moon and see such similar features; large boulders that had been blasted up and landed atop the rim, in an upheaval that turned the rock’s natural layering upside-down.

David then took us to a point and showed us close-up views of some white Coconino Sandstone, the plentiful rock that is made almost entirely of finely ground quartz, pulverized into small particles. This is sedimentary rock, the lowest layer of the upper portion of the native landscape, and preserves a cross-layered structure that formed from fossilized sand dunes, left from a time when Northern Arizona was dune field similar to the modern Sahara Desert.

Other types of rock are exposed in the crater walls, and include — rising above the Coconino — the Toroweap Formation, a thin layer of sandstone and dolomite, and above it, the Kaibab Formation, a thicker layer of dolomite, dolomitic limestone, and thin sandstone. The represents a time when the area was covered in a sea, and abundant fossils exist in these layers. They include trilobites, brachiopods, cephalopods, gastropods, and pelecypods, and date from the Permian, more than 250 million years ago. Lastly, there is the reddish siltstone Moenkopi Formation, closest to the surface.

Walking down into Meteor Crater gives you an appreciation of what it must be like to climb down into a crater on the Moon. Looking upward at the crater walls, you see blocky uplifts like this one where the explosive force of the event threw huge chunks of rock outward. David J. Eicher

As we kept winding our way downward, slowly, along the trail, the heat was astonishing. We stopped here and there to look at a wall of Coconino Sandstone, appreciating the finely pulverized quartz that makes it up, and to look at the reddish Moenkopi, and to gaze at fossils in the Kaibab. The march downward lasted much longer than an hour, and as we rested and regrouped, carefully climbing over rocks, sometimes scaling down small boulders, and making sure we didn’t cause any rockslides or smash native plants with our feet.

Meteor Crater’s geology

As we stopped to rest, David continued his narrative of the structure of the crater for us. The crater has a simple bowl-shape, typical for impact craters that are less than roughly 2 km in diameter in sedimentary rock and smaller than 4 km in crystalline targets. The crater has no features of large craters visible on the Moon such as central peaks, rings, modification zones of collapsed walls, or deformational rings surrounding the impact. Meteor Crater does have so-called tear faults that cross-cut the terrain in four areas, giving the crater a squarish appearance. Joints in the landscape are thought to have existed before the impact, and the creation of the crater fractured these faults.

In 1960, Gene Shoemaker produced the definitive geological map of Meteor Crater. His work demonstrated that the upper crater walls and uplifted crater rim are composed of Coconino, Toroweap, Kaibab, and Moenkopi formation rocks. Debris is scattered around the crater’s perimeter and within it, too, of course: Shoemaker mapped the impact debris within the crater in a careful way.

This geologic map of Meteor Crater was produced by the planetary scientist Gene Shoemaker in 1960. In addition to identifying different varieties of bedrock, Shoemaker mapped the interior deposits of breccia, exterior areas of debris, and faults that cross-cut the crater walls. David Kring

Geologists have a special term for rock that has been smashed and mixed up and fused back together — breccia. Shoemaker mapped several types of brecciated rock from the impact within the crater. Some of them contain fragments of rock from more than one of the geological formations. Some of them contain shock-melted rock along with debris from the meteorite. The crater has changed dramatically since it formed. Erosion and the accumulation of sediments have played major roles in remaking Meteor Crater. Debris has collected at the base of the crater’s walls. This has reduced the steepness of the walls. The apparent size of the crater has changed due to erosion of the rim’s height, shallowing of the wall slope, and large quantities of sediment filling in the crater’s floor. As the rim shrank in height and the upper walls eroded, the apparent diameter grew.

Atop the debris from the impact on the crater’s floor, the brecciated rock, lie ancient lake sediments to a level of about 100 feet (30m). The ancient lake sediments have comingled with material shedding downward from the crater walls. Following the impact, the environment in this area became far more arid, and the lake evaporated. So standing on the floor of the crater is now standing well over 100 feet higher than one would have stood just after the impact.

The environment of what is now northern Arizona was far different tens of thousands of years ago. Ground water and precipitation partially filled the crater during the Late Pinedale of the Late Pleistocene, producing a crater lake as recently as about 11,000 years ago. Sediments deposited on the crater floor, and along with material that fell onto the floor from slumping walls, raised the height of the floor more than 100 feet from its original surface. David Kring

As we continued to wind our way down, it was stunning to look up and see how large the walls are as viewed from within. It‘s clear why astronauts were routinely brought here for training, first by Shoemaker and now by Kring. The landscape certainly offers a lunar-like world to climb into, and exposes explorers to a variety of mesmerizing geological features. It’s also a challenging environment, well suited for testing astronaut durability. The 95° F heat on our day was starting to wear on me, the easterner. After the better part of an hour, we reached a point where the downward climb tapered off a bit and we could see we were approaching the horizontal trek across the crater’s floor. It strikes you as almost incredible that you start by thinking, “sure, we can walk right down there!” And then the amazingly large scale of the place hits you. It takes a long time to make your way down toward the floor.

As I mentioned, Daniel Barringer, Drew’s grandfather, was a geologist who was interested in the potential for finding iron that could be mined. With a pair of binoculars atop the rim, you can spy down to the center of the crater’s floor to see the remnants of mining operations from the past. You can also spy a life-sized cutout figure of an astronaut placed there for scale. That brings home the enormous distances involved with the crater. As we made our way down to the floor, starting to carefully walk across it, Kring described some of the history of the elder Barringer’s mining activities.

Early in the 20th century, Daniel Barringer attempted to mine in the crater’s center, hoping to recover the main mass of the asteroid, consisting of valuable iron and nickel. The main mass was vaporized, and never found, but the pumps, boilers, and hoists from this era remain on the crater floor. David J. Eicher

In 1892 Barringer had purchased a gold and silver mine near Cochise, Arizona, east of Tucson. A few years later he discovered a silver mine near Pearce, Arizona, a short distance south of Cochise. Funded by these ventures, excited over the crater near Winslow, he founded the Standard Iron Company and commenced drilling operations on the crater’s floor in 1903. He believed of course that the main mass of the meteoritic iron must be buried below the surface of the crater’s center. Over the next five years, Barringer and his colleagues drilled 28 holes into the crater’s floor, searching for the iron. The deepest of these reached 1,085 feet (323m). The team excavated seven shafts on the floor, the deepest of which reached 222 feet (68m), and sank several other shafts in other locations, as well as digging trenches.

The mining team extracted core samples along with chips and sand. None of the core sample material survives in the present day. The drilling suggested that the broken rock from the impact bottoms out at about 700 feet (210m), and amazingly, drilling detected a huge (5-acre) chunk of Coconino sandstone that was reported to have slumped off and landed in the middle of the zone of broken rock, some 200 feet (60m) deep.

Following the first mining explorations, which failed to find meteoritic iron, the United States Refining, Smelting, and Mining Company drilled more explorative holes between 1920 and 1922, on the south rim. This hole reached a depth of 1,376 feet (419m), more than 800 feet (240m) below the crater floor. That effort, too, failed to find significant amounts of iron.

A coherent picture of what happened, exactly, when a small asteroid slammed into the desert, began to emerge after Gene Shoemaker’s work in 1960. Geologists still don’t know such details as the asteroid’s trajectory, the exact energy unleashed, and so on, but over the last few years they’ve assembled a good general picture of what happened. The impactor, an iron-nickel object, slammed into what‘s now the Arizona desert and penetrated the Moenkopi formation to a depth of about its own diameter, exploding and releasing tremendous energy, generating a downward and radial shock wave. The flow of rock moved downward and outward before moving upward and outward. This made the crater and ejected debris into the surrounding landscape. Material along the walls slumped inward, forming a breccia lens. The total timeframe in making the crater was only a few seconds. The iron-nickel material, by the way, did not come from the core of an asteroid, as is often stated. Rather, type IAB irons appear to be metal from the differentiated base of an impact melt sheet on an asteroid — the result of an ancient collision.

As we approached the crater’s floor, we said to ourselves, “Ahh, we can just walk right over there to the center of the crater.” And yet it is a long, long way! David J. Eicher

Marching toward the crater’s center

Now our group was close to the floor of the crater. We climbed down a small vertical distance further, and made our objective the relics of the mining equipment we could see at the floor’s center. Remnants of the mining attempts of the early 20th century, surrounding the main central shaft, include a boiler and relics of a hoist, among other large iron pieces. Again, the scale of the crater fools you. You say to yourself, “Ah, I’ll just walk right over there.” And you find out that it is a very long walk.

As we traversed the crater floor, Kring continued his geological explanation of the crater. One of the key pieces of evidence to prove the crater was an impact came from analysis of what geologists call shock metamorphism. By finding shocked crystals of quartz, for example, they can tell a walloping force was unleashed there. Back in the first decade of the 20th century, Barringer and Tilghman discovered “rock flour,” pulverized Coconino sandstone. Barringer also found that much of the Coconino had been shocked. Later, geologists identified a mineral, coesite, in the crater, which is a high-pressure form of quartz. Soon after that find, they identified stishovite, another high-stress form of quartz, clearly indicated the great pressure that was unleashed with the impact. Other associated rocks showed indications of melting and resolidification.

We walked along, being careful not to squash plants, mindful of possible critters on the ground such as snakes and insects, and with an eye toward the center, as Kring continued his talk. He described how the impact uplifted the crater rim, and how uplift is visible in the layers of the crater walls. Fracturing within the crater walls bulked up the rock, preserving the uplifted rim. Broken rock fragments and ejecta filled parts of the walls, helping to preserve them. And the walls consist of blocks of rock that are in their normal strata, pushed up to vertical orientation, and even overturned as one looks up to the top of the rim.

But, Kring explained, the ejecta forming the crater walls was hardly the entirety of the material cast out from the crater. Rubble from the impact lies over a radial reach of more than a kilometer away from the crater’s center. The largest pieces of course lie near the crater rim, where blocks of limestone up to 60 feet across and sandstone to 100 feet in diameter were thrown. Some of these pieces weigh in at up to 5,000 tons. One of the largest blocks near the crater’s rim is called Monument Rock, or House Rock. It probably landed after about 2 seconds in the air and was traveling at about 30 mph (50 kph). Blocks of rock that landed half a kilometer beyond the rim, however, probably traveled at velocities of 225 mph (370 kph).

As we reached the central point of the crater floor, we stopped, rested, regrouped, and took some pictures of ourselves and of the mining equipment. So what, as we stood right where the colliding object landed, do we know of the asteroid itself?

Kring told us about the current thinking on the impactor. The current thinking is that it had a diameter of about 160 feet (50m). Of course what we know now, and what Barringer and his team of hopeful miners didn’t, is that the main mass was obliterated by the incredible energy released in the impact. Fragments of the iron-nickel meteorite, known as Canyon Diablo, have been picked up since prehistoric times. The estimated total mass recovered is something like 30 tons, but that’s a crude guess. A meteorites go, it is classified as a Group IAB iron. The composition of the meteorite is mostly iron, but with about 7 percent nickel and a small percentage of other elements. Many meteorites fall into Earth’s surface at relatively low velocities and survive in great numbers. The asteroid that produced Meteor Crater struck Earth when it was still traveling at a very high speed, which produced the explosive cratering event.

A diagram shows the crater in cross-section to reveal the current crater form (top) and the size of the impacting asteroid (about 50 meters in diameter, below). David Kring

We gathered ourselves after the souvenir photo shoots at the floor’s center and got ready for the hike back up to the rim. The temperature was still blazingly hot, and we were tired, but imbibing plenty of water.

It’s an amazing thing to stand in the middle of the crater and look all around at the walls towering on all sides. I shot a couple of panoramas. Aside from the blue sky and desert terrain, you certainly got a taste of the psychology of what it would be like to be standing in the middle of a small crater on the Moon.

Looking upward, you could begin to imagine how the asteroid came in. It all would have happened in a flash, of course. Standing in the blast zone, you wouldn’t have known what hit you. The Hollywood business of seeing a blazing object coming in, trailing fire, with warning before the “bomb” goes off, is of course silly. That notwithstanding, the trajectory of the incoming asteroid is a tricky problem to solve that is still under debate. Early on, Barringer and others realized that an object coming in at a 45° angle would produce a round crater. Shoemaker and others since, most notably Kring, have noted that faults in the crater walls suggest a rough south-to-north direction of the incoming asteroid. But the opposite direction of motion could also be true. This is a question that can’t yet be answered with certainty.

From near the center of Meteor Crater, the rim, although it is eroded since 60,000 years ago, makes an imposing visual wall that surrounds you. David J. Eicher

We do know something of the immense energy released by the impact, however. Assuming a roughly 50m object, the object’s mass would be somewhere in the range of half a million metric tons. The incoming velocity of this asteroid? Somewhere in the range of 7 to 12 miles per second (11 to 20 km/s). That is really moving. As a rough estimate, physicists garner the impact energy released at approximately 10 megatons, or 700 times the energy of the bomb that exploded over Hiroshima.

The incredible force unleashed by this impact shook the immediate area and the region. The impact ejected debris from the site, produced a fireball, a radiating shock wave, and a related air blast. Plants and animals at the impact site would have been vaporized, as were portions of the asteroid and some of the underlying bedrock. The shock wave would have radiated across the surrounding landscape. The winds would have severely damaged any life forms to a diameter of some 20 miles (32 km). The destruction over this region would have killed or injured most animals and plants, and the air blast would have caused bleeding and fluid buildup in the lungs of creatures, suffocating them, and obstructing blood flow to the heart and brain. The blast wave would also have struck animals and plants over this region. On a scale far greater than a hurricane, winds would send rocks, tree branches, and other debris flying outward like missiles. The ballistic shock wave would have created injuries, perhaps many fatal, over a much larger area yet.

Some 60,000 or more years ago, when the asteroid struck what is now Arizona, the surrounding sagebrush, woodland, and forest terrains were populated with mammoths, mastodons, tapirs, bison, camels, and other large mammals. Shock pressures, wind velocities, and heating were enormous within a few kilometers of the blast. The fireball scorched plants and animals out to a distance of 10 km (red circle). Large animals were killed or wounded by the pressure pulse and air blast out to a distance of some 16 km (yellow circle). The air blast produced hurricane-force winds out to a distance of some 30 km (blue circle). David Kring

The climb back up

As he said several times when we began our long hike, Kring told us to take it easy and hydrate substantially. “There are two ways back out,” he told us. “You can climb, and so you need to take care of your body. Or you can be taken out by helicopter. That costs about $10,000, and I’m not going to pay for it.” 

We started the climb back up thinking about the incredible blast, and how reality is far harsher than the movies make such things out to be. So when, when did all of this happen? That seemed to be one missing part of the story.

As we approached the edge of the floor and started our upward journey along the antique trail, over boulders, careful not to step on plants or displace rocks, Kring continued his lecture. It turns out that the thinking about the age of the impact has been changing relatively recently.

The earliest experts thought the crater so well preserved that it couldn’t be fantastically old. Barringer, in 1905, believed the crater to be between 2,000 and 3,000 years old. Finding the crater’s age is a tough process. The volumes of impact melt that could be analyzed via isotopic analysis are small, and the crater is young enough that some radiometric techniques for dating rocks are useless due to the half-lives of radioactive elements being too short in the crater samples.

In the 1930s, geologist Eliot Blackwelder of Stanford University examined the thickness of lake sediments on the floor, debris on the crater slopes, pitting of ejected limestone, and other factors, and estimated an age of 40,000 to 75,000 years. That turns out to be remarkably accurate in the more modern era.

For a time, scientists thought that Meteor Crater and the much smaller and more eroded Odessa Crater in West Texas might be linked. I asked Dave Kring about that as we climbed upward. The idea is that the ages of each were estimated to be in the range of 50,000 years for some time, and it seems strange that two craters in more or less the same region from more or less the same time period would be unrelated. But the trajectories for Meteor Crater and Odessa don’t seem to match up well. Any relationship seems hard to match up from an orbital point of view.

The business of directly measuring the age of the crater got underway in the 1980s. Using a specialized technique called thermoluminescence, which measures accumulated radiation, Stephen R. Sutton of the University of Chicago found an age of 49,000 years, plus or minus a few thousand. More recent studies have agreed with that figure. Now, however, Kring says studies in press may push the age of the crater — and of Odessa, by the way — to be somewhat older, possibly to 60,000 years or even somewhat older than that. Stay tuned.

A we climbed up, I felt more and more winded. And then a bit lightheaded. Over rocks, up the old trail we went, and I found myself having to pause more frequently. Yes, the guy from the Midwest had the heat getting to him. I didn’t really know it at the time, but I had heatstroke. I soldiered on, but as we neared the bottom of the rim, about to make our way out, I got sick. Yes, threw up in front of our group. It wasn’t my finest hour. I had learned a great deal updating my knowledge of the crater, but in one sense, the heat won the day.

As our group started the ascent, we paused to gaze one more time across the magnificent crater’s floor. David J. Eicher

Lessons for the future

As mentioned, the impactor that made Meteor Crater was a small asteroid in the range of 50 m diameter. It was material from an ancient planetesimal produced from an impact melt. Other impacts have frequently taken place on Earth and their scars have faded away. In the Arizona desert, with an impact just 60,000 or so years old, the scar is fresh.

Planetary scientists are frantically working on expanding their inventory of Near Earth Objects (NEOs), asteroids that could impact our planet in the future. Such impacts will happen. Fortunately, there are no very large objects in near Earth space, similar to the K-Pg impactor that struck the Yucatán 66 million years ago and killed a high percentage of species, including the dinosaurs. But what about small asteroids that could trike us and be city killers, or regionally devastating, like the asteroid that made Meteor Crater? Astronomers now know of about 18,500 NEOs, but the inventory of objects down to 50 meters in size is completely only to an estimated 1 percent. Yes, that’s 1 percent.

The Asteroid Day project aims to raise awareness for NEO research and discovery, and you can find out more about it here: asteroidday.org

For further details on Meteor Crater, its geology, and current science, you must read David Kring’s Guidebook to the Geology of Barringer Meteor Crater, Arizona, a.k.a. Meteor Crater, 2d ed., 270 pp., paper, Lunar and Planetary Institute, Houston, 2017. The complete publication is available online here: https://www.lpi.usra.edu/publications/books/barringer_crater_guidebook/.

For information on visiting Meteor Crater, see the website at http://meteorcrater.com/.

For information about The Barringer Crater Company, visit www.barringercrater.com.

I encourage you to visit Meteor Crater. It is an incredible spectacle to see this amazing product of solar system interaction. The visitor center is fantastic, and a walk around a portion of the crater’s rim will show you an amazing overview of the site.

Thanks are due to Drew Barringer and to David Kring for their gracious hospitality and hosting during our visit, and for their expertise in assisting the production of this story.

Watch out for space rocks!