Some facts about meteorites

• Meteoric debris occurs in three flavors:
  • Meteoroid--a small particle from a comet or asteroid orbiting the Sun.
  • Meteor--the light streak which results when a meteoroid enters the Earth's atmosphere and vaporizes; also called a shooting star and, when really bright, a fireball.
  • Meteorite--a meteoroid that survives its passage through the Earth's atmosphere and impact with the Earth's surface.
• Although most meteorites fall into the oceans, the meteorites found in museums are usually discovered in Antarctica.
  • Antarctica is considered the best place to look for meteorites because meteorites are relatively easy to spot on an ice sheet and are usually well preserved there. In addition, the motion of the ice sheet tends to concentrate the meteorites at blocking locations such as mountain ranges, with wind erosion bringing them to the surface after centuries beneath accumulated snowfall.
  • Many meteorites have also been discovered in the deserts (e.g., Sahara Desert), in flat, featureless areas covered only by small pebbles and minor amounts of sand. Dark-colored meteorites can be easily spotted in these places, where they have been well-preserved due to the arid climate.
• Most meteorites are pieces of asteroids or comets. Some discovered meteorites originate from the Moon and Mars.
• Mostly made of stony material. In rare cases, they contain nickel and iron.
• Meteorites weighing a quarter of a pound or more hit the Earth thousands of times a year.
• Famous meteorites include:
  • The Willamette meteorite weighs about 15 tons and is the largest meteorite ever found in the U.S.
  • The Hoba meteorite weighs about 60 tons and is the largest meteorite in the world. It fell to Earth about 80,000 years ago and was discovered in Namibia by a farmer.
  • ALH 84001 (Allan Hills 84001) was discovered in Antarctica in December 1984. It made its way into headlines worldwide when scientists announced that it contained evidence for microscopic fossils of Martian bacteria.
  • The Peekskill meteorite broke up over the United States on October 9, 1992 and landed on a parked car in Peekskill. The meteor was witnessed by thousands across the East Coast, was captured on 16 different videos, and remains one of the most famous meteor sightings.
  • In February of 2013, a 10-ton meteor exploded in the Russsian sky and produced a shockwave which shattered windows and injured hundreds of people.
  • In 1954, Ann Hodges of Sylacauga, Alabama became the first (and so far only) person of record to be struck by a meteorite. While she was napping in her living room, the grapefruit-sized meteoroid (now called Sylacauga meteorite or, sometimes, the "Hodges meteorite") crashed through the roof of her house, bounced off her large wooden console radio, and struck her on the arm and hip. She was badly bruised but able to walk.
    
     

Some large impacts

• For reference, the world's nuclear arsenal consists of about 100,000 nuclear devices, each capable, on average, of releasing about 1 megaton of TNT (an explosive agent). By comparison, the Hiroshima and Nagasaki bombs of World War II released about 20 kilotons of TNT each.
• In 1994, the Shoemaker-Levy 9 comet smashed into Jupiter.
  • the first observed collision in modern history.
  • 50 million megatons of energy.
  • 5 million Hiroshima-sized atomic bombs.
  • 5 thousand times world’s nuclear arsenal
• A mass extinction about 250 million years ago was probably caused by 10-km bolide.
  • Layers deposited more than 250 million years ago contain fossils of many species that are not found in layers deposited more recently. The dividing line is rather sharp and is called the Permian-Triassic Boundary, marking the transition between those two periods in the Earth’s history.
  • This extinction wiped out most life on land and in the oceans.
  • A large crater (about 300 miles wide) may have been found in Antarctica.
  • Exotic molecules, called Fullerenes, that form under extremely high temperatures and pressure (as would be found in a massive impact) have been found at the boundary.
• Chicxulub impact, about 65 million years ago
  • Impactor about 10 km in diameter, roughly the size of Mount Everest.
  • The impact was 2 million times more powerful than the Emperor Bomb (which is one of the most powerful nuclear devices ever detonated on Earth). About 1000 times more powerful than the world's nuclear arsenal.
  • 99% of all living things (75% of all species) on Earth, including the dinosaurs, were wiped out.
  • The impact gouged a 125-mile-wide (200 km) crater in Earth surface resulted in a cascade of extreme weather events (tsunamis, forest fires, etc.) and lethal changes in the ecosystem.
    • Plumes of vaporized rock and debris high in the air globally blocked out views of the Sun ('impact winter') for years or decades.
    • The reduction of sunlight halted photosynthesis and caused Earth’s surface temperature to plummet by as much as 50 degrees Fahrenheit (28 degrees Celsius).
    •   All this killed off plants, which killed off herbivores, which killed off carnivores.
• Evidence for this impact includes:
  • Crater in Mexico's Yucatan Peninsula, with the right size and age.
  • Impact residue (e.g., melted bedrock, rock spherules, tektites, shocked quartz, etc.) similar to that around nuclear test sites.
  • 65 million-year-old layer of iridium, with lots of dinosaur fossils below and none above. (Iridium is rare on the Earth's surface but common in space debris.)
• Note: While the impact theory is the prevalent explanation for the extinction of the dinosaurs, some scientists have hypothesized that extreme volcanic activity and climate change may have contributing factors.
• Barringer Crater was created about 50,000 years ago
  • Impactor was about 50 m in diameter.
  • First recognized Earth crater.
  • 20-40 megatons of TNT (x1000 Hiroshima).
  • Gigantic shock waves and hurricanes.
  • Local devastation, but no mass extinction.
  • Damage to about 100 km.
• The mysterious Tunguska explosion of 1908 may have been our most recent encounter with a relative large impactor.
  • Impactor was about 30 meters in diameter; may have been comet or asteroid (issue not resolved)
  • Left no crater--exploded in the air.
  • Equivalent to about 1000 Hiroshimas.
  • Estimated to have measured 5.0 on the Richter scale.
  • Destroyed Siberian forest, but produced no human casualties.
• A stony meteoroid of about 10 meters (30 ft) in diameter can produce an explosion of around 20 kilotons, comparable to the energy released in Hiroshima and Nagasaki.
• One of closest near-collisions in recent decades took place in 2002, when asteroid 2002MN 73 meters in diameter passed Earth at a distance of around 120,000 km (75,000 miles), about one-third of the distance to the Moon. Had it hit the Earth, the damage would have been comparable to that of Tunguska.
• An even closer call occurred on Feb 15, 2013, when an asteroid 50 m in diameter (roughly half the size of a football field) came within 30,000 km of the Earth, well within range of many Earth-orbiting satellites.
• Weathering and tectonic activity has erased most craters on Earth, but there are still about 200 impact craters.

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