The space rock that carved Meteor Crater in Arizona hit the planet much more slowly than astronomers once figured, but still 10 times faster than a rifle bullet.
The new analysis, announced today, explains why there’s a lot less melted rock in the crater than expected. The mystery has dogged researchers for years.
The big hole in the ground—570 feet deep and 4,100 feet (1.25 kilometers) across—was blown into existence 50,000 years ago by an asteroid roughly 130 feet (40 meters) wide.
Previous calculations had the rock slamming into the ground at no less than 34,000 mph (15 km/sec), based in part on the expected speeds of large meteors in relation to Earth. Such an impact ought to have generated more melted rock in and around the crater than what’s been found.
A new computer model, reported in the March 10 issue of the journal Nature, shows the incoming object would have slowed considerably during its plunge through the atmosphere, part of it breaking into a pancake-shaped cloud of iron fragments prior to impact.
About half the original 300,000-ton bulk remained intact, smacking the planet at about 26,800 mph (12 km/sec), said the study’s lead researcher, Jay Melosh of the University of Arizona.
Meteor Crater, a popular tourist destination, was the first scar on Earth confirmed to have been gouged by a rocky visitor from beyond.
“It’s probably the most studied impact crater on Earth,” Melosh said. “We were astonished to discover something entirely unexpected about how it formed.”
The modeling is based in part on investigations decades ago by Daniel Barringer, whose name is officially associated with the crater. Barringer and others found chunks of the iron space rock weighing from a pound up to 1,000 pounds in a 6-mile-diameter circle around the crater. The new work also draws from an improved understanding of how Earth’s atmosphere cushions extraterrestrial blows.
In 1908, a good-sized asteroid—more stony in nature—exploded above the surface of Siberia, flatting hundreds of miles of forest but leaving almost no extraterrestrial trace. During the satellite era, scientists have monitored car-sized space rocks routinely exploding in the air.
“Earth’s atmosphere is an effective but selective screen that prevents smaller meteoroids from hitting Earth’s surface,” Melosh explained.
The effect of screaming through the air, even for an iron-heavy meteorite like the one that struck Arizona, is a lot like hitting a wall, Melosh said. And many space rocks are already cracked before they arrive, scientists believe.
“Even though iron is very strong, the meteorite had probably been cracked from collisions in space,” Melosh said. “The weakened pieces began to come apart and shower down from about 8.5 miles (14 kilometers) high. And as they came apart, atmospheric drag slowed them down, increasing the forces that crushed them so that they crumbled and slowed more.”
The results evolved out of a project in which Melosh and colleagues developed a “Catastrophe Calculator” that predicts the effects of asteroids of varying size and composition striking any given location on the planet.
Original article: http://goo.gl/aVwRZx
Forty-nine thousand years ago, a large 30 to 50 meter diameter iron asteroid impacted the Colorado Plateau in northern Arizona. The resulting massive explosion excavated 175 million tons of rock, forming a crater nearly a mile wide and 570 feet deep. While the Meteor Crater* impact event was too small to cause global environmental effects, its regional damage would have been significant.
Reconstructing the environment at the time of the impact is important to understanding the context of the devastation. Palynological (fossil pollen) and paleontolgical studies have provided a partial record of the paleoenvironment including the flora (vegetation) and fauna (animals) that lived in the region ~50,000 years ago.
The current data suggests that a juniper-pinyon woodland or forest covered the gently rolling countryside. Large mammals such as mammoths, large ground sloths, bison, camels, tapirs and horses may have lived in the vicinity and been victims of the 20 to 40 megaton blast.
The damage inflicted by the impact was similar to a nuclear bomb blast, but without ionizing radiation damage. The asteroid, bedrock, and any fauna or flora at ground zero would have been vaporized. Bedrock was ejected and overturned out to a distance of 1-2 km (see graphic). The explosive shockwave would have produced winds in excess of 1000 km/hr within 3 to 5 km of the impact (see effects map). These winds would have stripped away grass near the crater and flattened juniper and pinyon trees out to a radial distance of ~14 to 19 km.
The impact shock wave would also have had severe effects on animals in the region. Dramatic differences in the internal vs. external pressures exerted on animal bodies within the shock wave would have caused hemorrhaging (internal bleeding) and edema. Animals would also have been injured by displacement, their bodies propelled a short distance by the shock wave.
Branches, rocks, and other debris was also accelerated by the blast, causing shrapnel-type wounds out to a distance of 10 to 13 km. Vegetation and animals may also have been subject to the thermal emission from the blast, causing burn damage out to a maximum range of ~10km. This intense heat may have started forest fires or range fires, although no fire evidence has been reported.
The probable sum of these effects is the destruction of vegetation over an area 800 to 1500 km2. Damage to vegetation would have extended over an additional 200 to 600km2. Animals within 3 to 4 km of the impact site would probably been killed, with maiming injuries extending out to distances of ~16 to 24 km. While these effects are severe, they are confined to the immediate region and did not cause extinctions. In fact, the newly formed bowl shaped depression soon filled with water providing a lake habitat (see graphic) for aquatic plants and animals. Recolonization of the area was probably accomplished in a few to ~100 years.
These types of events, however, are large enough to destroy a modern city. They occur at an average rate of about once in 6000 years.
The crater has been known by several names. Before its impact origin was appreciated, the crater was called Coon Mountain or Coon Butte. Later it was called Meteor Crater, which is the popular or common name used today. However, the name recognized by the Meteoritical Society, composed in part of professional geologists who study impact craters, is the Barringer Meteorite Crater, in recognition of the work of Daniel Moreau Barringer who championed an impact origin for the crater.
Original article: http://goo.gl/VqA39m
On my drive out to Los Angeles, a childhood friend and I drove out to see this remarkable crater.
Makes one think.