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The Planet Earth Today:
Meteorite vs. Meteor, Meteoroid and Micrometeoroid
Meteorite Craters
People usually call a bright streak of light in the sky a meteor, a shooting star or a falling star. Very bright lights are called fireballs. The sky show is produced by the entry of a small meteoroid into Earth's atmosphere. A skywatcher under a dark clear sky might see a few per hour on an average night. On the other hand, during one of the annual meteor showers, a skywatcher might see up to 100 per hour.
Barringer Meteor Crater
click to enlarge
Meteor. Meteors are the streaks of light associated with the burning of small chunks of rock or interplanetary debris as they arrive in Earth's atmosphere from space.
Meteoroid. The rocks and debris are known as meteoroids and often are the size of a grain of sand or smaller.
Micrometeoroid. Very fine grains of space dust are called micrometeoroids.
Shooting Star or Fireball. Friction heats the rock plunging through Earth's atmosphere and makes the meteoroid glow in the air, causing the streak of light. The streak also is known as a shooting star or fireball.
Meteor Shower. On a dark, moonless night, a dozen meteors per hour may be seen all over the sky. During meteor showers, seen regularly at certain times of the year, as many as 30 to 100 meteors per hour may be seen.
Meteorite. A meteorite is a meteoroid large enough to survive the fall to Earth's surface. Many meteorites have been recovered by persons who happen to be near where they fall to Earth.
Types of meteorites:Iron. primarily iron and nickel, similar to type M asteroids
Stony Iron. mixtures of iron and stony material like type S asteroids
Chondrite. by far the largest number of meteorites fall into this class, similar in composition to the mantles and crusts of the terrestrial planets
Carbonaceous Chondrite. very similar in composition to the Sun less volatiles, similar to type C asteroids
Achondrite. similar to terrestrial basalts, the meteorites believed to have originated on the Moon and Mars are achondritesEarth is pocked with more than 120 impact craters. An example of what happens when a small rock hits is the Barringer Meteor Crater near Winslow, Arizona, which was gouged out of Earth's surface 50,000 years ago by an iron meteorite 90-150 feet in diameter. The crater is 3,600 feet in diameter and 600 feet deep.
Meteorite Craters
Tunguska forest. A more recent impact occurred in 1908 at Tunguska, a remote uninhabited region of western Siberia known. The incoming rock probably was about 180 feet in diameter consisting of loosely bound pieces. Unlike Barringer, Tunguska disintegrated before hitting the ground and no crater was formed. However, all trees were flattened within 30 miles. The sound of the explosion was heard half-way around the world in London.
Missouri rock balls. One of the largest meteorite craters in the United States is in the state of Missouri where a rock 1,200 feet in diameter plunged into Earth's atmosphere sometime around 310 to 340 million years ago. It crashed into territory where we find the modern towns of Weaubleau and Osceola. The impact crater encircles the modern community of Vista, making it one of only two U.S. communities, along with Middlesboro, Kentucky, totally encircled by a meteorite crater or crater remnants, according to geologist George H. Davis, a member of the team that used shallow core drilling to investigate the Weaubleau-Osceola meteorite impact in July 2003. Osceola and Weaubleau are not within the impact crater.
Geologists say such a quarter-mile-wide meteorite could explain how some rocks they found on the ground had come to be folded over and other rocks containing shattered quartz had ended up in the Ozark Mountains.
Residents of the area around Osceola have found a lot of strange, perfectly-round rocks. Geologists theorize the round stone balls are chert concretions. They suggest the impact blasted up gravel-sized pieces of shale that fell back to the ground. Silica-rich solutions seeped in around the small shale pieces and hardened in place. Those chert concretions are evidence that the impact actually occurred, according to Davis.
The folded rocks are visible in a local quarry and Davis describes them as "fascinating." The team of geologists theorize that the folding happened in "mere milliseconds." Their drilling into the structure turned up one large piece of granite that had been lifted nearly 1200 feet vertically through rock and sediment by the force of the impact.
They estimate the force of the impact must have been equal to, or greater in magnitude than, 68 of the largest U.S. military nuclear weapons, Davis says. The structure of the ground around the site first was first identified as an impact crater by Dr. Kevin Evans of Southwest Missouri State University. Davis is geotechnical liaison for the Missouri Department of Transportation (MoDOT) Materials Division.
Cambodian meteorite. Farmers were angered by the fires when a 10-lb. meteorite torched more than 1,000 acres of Cambodian rice paddies January 25, 2005. Villagers in Banteay Meanchey province, 200 miles northwest of the national capital, Phnom Penh, thought the large black lump was a divine omen. They wanted to place the rock in a peace shrine on the spot of impact, but police took it away for scientific analysis.
Kebira Crater. Boston University scientists sorting through satellite photos have found the largest crater seen to date in the Great Sahara Desert of North Africa. It's 19 miles wide, which is more than twice the next largest crater known in the Sahara. It's considerably bigger than the Barringer Meteor Crater in Arizona, which is three-quarters of a mile in diameter. On the other hand, it's way smaller than the 100-150 mile-wide Chicxulub Crater in the Gulf of Mexico.
Landsat image of the Kebira Crater in the Great Sahara Desert of Egypt at the border with Libya.
Click to enlarge this Boston University Center for Remote Sensing image.
The crater, known as Kebira, which means "large" in Arabic, is on the northern tip of the Gilf Kebir region of southwestern Egypt near Libya. The meteorite that fell to Earth and gouged out Kebira probably was three-quarters of a mile wide. The terrain around the crater is 100 million year-old sandstone. Kebira Crater has two rings eroded over the eons by wind and water. Two ancient rivers run through the crater site from the east and west.
The shock of such a large object crashing into Earth tens of millions of years ago undoubtedly distorted the surrounding terrain and destroyed plant and animal life for hundreds of miles in all directions. It may have left behind the field of yellow-green silica chips – desert glass – seen today on the surface among the giant dunes of the Great Sand Sea in southwestern Egypt. By comparison, the larger body that dug out the Chicxulub Crater may have killed off all the dinosaurs on Earth.
To learn more:
Meteors, Meteorites and Impacts SEDS Nine Planets Asteroids STO Kebira Crater Boston University
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