--------------------- 2633 - METEORS - we call them shooting stars.
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- It sounds like something from the pages of a science fiction novel, but it’s a true story and it’s written in stone. At the time of the World’s Columbian Exposition in 1893, the collection of curiosities that would become the Field Museum included 170 meteorites. The museum collection includes more than 15,000 meteorites from over 1,500 falls.
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- “A fall,” is an event in which the arrival of a meteorite is observed and recorded. This allows fragments to be identified with that event. Meteorites not identified with a fall are known in the field as “finds.”
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- The museum has a thousand-pound remnant of the Canyon Diablo meteorite that gouged the great meteor crater in the Arizona desert. There are fragments of the Chelyabinsk meteor, which made a fiery descent over Russia in 2014. There is even a meteorite that fell on a garage in Illinois in 1938, displayed alongside the holed seat and dented muffler of the car it struck.
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- But possibly the most intriguing meteorites in the collection are four that are embedded in slabs of limestone. They are surrounded by a whitish discoloration of the rock, evidence of something leaching from them. These are “fossil meteorites“.
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- One of them rests only inches away from the mineralized shell of a cephalopod, an extinct mollusk related to the modern nautilus. Fossils of prehistoric sea creatures are common in rocks dating from the Ordovician Period, which lasted from 500 to 435 million years ago.
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- But fossil meteorites are rare. In fact, they were unknown until 1952, when the first find was made in Sweden. To date, only 115 have been found, almost all of them at a single quarry.
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- Striations in the stone there represent layers of sediment laid down over millions of years, when this place was at the bottom of an ancient sea. In that age there were no land animals or even true vertebrates; the first dinosaurs were still 230 million years in the future.
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- Life was in the ocean, teeming with creatures now long gone. Trilobites scuttled along the sea floor and crinoids waved with the movement of the water. Among them swam cephalopods of the genus Orthoceras residing within a long, conical shell, tentacles groping for whatever creature would become its next meal.
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- This was the pattern of life in the Ordovician ocean; preserved by the long, slow process of fossilization to record a chapter in the history of life on Earth. But something extraordinary happened in the middle of the Ordovician Period. Meteorites began to fall among the future fossils, lots of them.
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- As the meteorites settled into the sea floor, they were buried along with the remains of the sea creatures to become fossils themselves. And just as the fossilized animals tell a story about the history of life on Earth, the fossil meteorites relate a chapter in the history of the solar system.
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- That story begins in the asteroid belt. The vast majority of meteorites that fall to Earth come from this ring of irregular, rocky bodies that never managed to form a planet. The asteroids occasionally jostle each other and when they do, the bits and pieces broken from them are sometimes sent Earthward.
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- 470 million years ago, there was a titanic collision among the asteroids. It shattered one of them completely in the biggest breakup the solar system has seen in the last 3 billion years. Immense amounts of debris from this collision were flung toward the inner solar system, where it quickly subjected Planet Earth to an intense bombardment. On a cosmic time scale, the number of meteorites falling to Earth jumped overnight.
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- The first step in proving the story is to verify that the objects found in the Thorsberg quarry are meteorites. Identifying fossils as meteorites poses a special challenge. They have been buried and embedded in stone for hundreds of millions of years. Just as the seashells underwent a mineralogical change, turning to stone over the eons, so did the meteorites.
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- The meteorites, of course, were already stone, but they underwent a process which changes their mineral content. That’s what caused the whitish discoloration in the stone surrounding them. Researchers have to study their microscopic structure, along with their chemical composition, to be sure of their identity.
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- The presence of the mineral chromite (FeCr2O4) in the fossils was one thing that identified them as meteorites. Another important giveaway is the presence of chondrules: small, round particles found embedded inside the meteorite.
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- Although meteorites come in many varieties, there are three basic types: iron, stony iron, and stony. Stony meteorites are by far the most common, making up about 96 percent of those that fall to Earth. They are composed largely of silicate minerals, such as olivine and pyroxene.
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- Iron meteorites are mostly iron, although they often contain other elements, such as nickel and cobalt.
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- Stony irons, the most rare of the three types, combine the characteristics of the other two.
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- The type of meteorite depends on where it came from in the parent body that broke apart to produce it. Some asteroids are differentiated, they have an iron core surrounded by a rocky mantle and crust, just as Earth does. Differentiation occurs when a body is heated by collisions or radioactivity to the point where its materials can flow. The iron meteorites come from the core of such a body, stony irons from a narrow region between the core and mantle, and stony meteorites from the mantle and crust.
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- Stony meteorites also come from asteroids that are not differentiated. Some asteroids were never hot enough to form an iron core, but did generate enough heat to melt bits of material embedded in them. These melted mineral grains are called chondrules and meteorites that contain them are called chondrites.
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- Having established that the embedded stones in the quarry are meteorites, and specifically that they are chondrites, can scientists go a step further in demonstrating that they all came from the same parent body?
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- As a matter of fact, they can, because chondrites come in several varieties. The family of ordinary chondrites includes three basic types: These are the H, L and LL chondrites.
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------------------------------ The H chondrites have a relatively high metal content;
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------------------------------ the L chondrites have a low metal content;
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------------------------------ the LL chondrites have both a low metal content overall and a low iron content relative to other stony meteorites.
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- Of the meteorites that fall to Earth today, 38 percent are L chondrites. In contrast, all of the fossil meteorites found to date are all L chondrites. They were the predominant type of meteorite in the Ordovician and moreover, they all came from the same asteroid, the L Chondrite Parent Body.
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- The limestone in which they are embedded was laid down as layers sediment over millions of years. The bottom layers are the oldest and the upper layers are the youngest.
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- Geologists determine the ages of the layers by several methods. By applying sedimentation rates measured in modern environments, they can estimate how long it took to build up a layer.
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- Index fossils, which are fossils of species known to have lived at a particular point in time, can pinpoint the age of a specific layer. There were many species of the mollusk and they are so common. Finally, the decay products from natural radioactivity can date the rocks.
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- A combination of these methods shows that the L chondrites began to arrive here just under 470 million years ago. And when they arrived, they came en masse.. The search has covered all of the old sea floor that has been quarried since then. The abundance of meteorites on the mid-Ordovician sea floor is far too high to be explained by a meteorite flux similar to that of today.
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- When the debris breakup began to reach this planet, meteorites fell with a frequency 100 times the modern rate for a million years. Furthermore, data from preserved craters indicates an order-of-magnitude increase in the flux of small asteroids as well.
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- Not only did the L chondrites bombard Earth in huge numbers, they did it very quickly. The typical travel time for a meteorite is millions to tens of millions of years. This is the time from when its parent body breaks apart to when a piece of it lands on Earth as a meteorite. The fossil meteorites began to arrive mere tens of thousands of years after the breakup of the meteor.
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- How is it possible to know how long a meteorite traveled through interplanetary space? There would have to be some kind of clock built into it. There is such a clock and the chemists are able to read it by measuring the amount of certain rare isotopes of elements that are in the meteorite.
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- The clock works like this: Every atom of every element has a mass number equal to the combined number of protons and neutrons in its nucleus. For example, helium with two protons and two neutrons has a mass number of 4 and is written as 4He.
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- Likewise, an atom of neon normally has 10 protons and 10 neutrons and is denoted as 20Ne. In rare cases, a nuclear reaction can produce a nucleus with fewer or more neutrons than normal, resulting in isotopes like 3He (2 protons, 1 neutron) or 21Ne (10 protons, 11 neutrons).
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- On Earth, naturally occurring neon has only about a quarter of a percent 21Ne; naturally occurring helium has a mere ten-thousandth of a percent 3He. These rare nuclei are produced by cosmic rays, the high-energy radiation that permeates outer space
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- While an asteroid is intact, the rock in its interior is shielded from the cosmic rays and isotopes are not produced. When the asteroid breaks apart, the fragments are exposed to cosmic rays and production of isotopes begins.
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- When the fragment falls to Earth as a meteorite, it is shielded once again by Earth’s atmosphere and production of the isotopes stop. Thus, the quantity of isotopes in the meteorite provides a measure of how long it was traveling in space.
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- Isotopes of 3He and 21Ne were cross-checked against the layers of rock where they were found. Their remarkable result was that the older the rock layer, the less time the meteorites in it had spent in transit. This is basically the result you’d expect if all of the meteorites came from a single parent body, with the ones making the shortest trip sitting here longest.
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- As if being embedded next to a cephalopod didn’t make it rare enough. The probable explanation for this rapid transit is that the breakup occurred near an orbital resonance, a spot where the gravity of Jupiter pulled them out of the asteroid belt and onto an Earthbound expressway.
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- Meteoritic material is constantly falling to Earth. The planet typically gains about five tons per year from meteorites, much of it in the form of tiny grains or microscopic fragments coming off of larger bodies.
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- With the proper chemical methods, these micrometeorites can be recovered from the limestone. In a recent international study, scientists recovered micrometeorites from a Russian quarry and studied their isotopes to determine their type. They found that prior to 470 million years ago, most of the meteorites falling to Earth were stony meteorites.
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- Over the years, some scientists have conjectured that meteorites have been at least partially responsible for mass extinctions and subsequent evolutionary events. So far, only the Chicxulub Meteor, which struck at the end of the age of dinosaurs, is known to be associated with such an event. A more thorough understanding of fossilized meteorites could further test such hypotheses.
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- Just before midnight on March 26, 2003, residents of Park Forest, Illinois, were startled by what many thought was a bomb. Bright flashes in the sky, resounding booms and falling debris were reported throughout this Chicago suburb. It was, of course, a meteorite. Several homes were hit and one strike left the occupants looking up at a softball-sized hole in their ceiling. Fortunately, nobody was hurt.
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- For the Chicago Center for Cosmo-chemistry at the nearby University of Chicago and for the Field Museum, the fall was a bonanza. When the fragments were collected and studied they were found to be L chondrites from the same source as the fossils in the Thorsberg quarry.
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- Pieces of the Park Forest meteorite now reside in a display case alongside the fossil meteorites from Sweden. After 470 million years, they are reunited at last.
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------------------------------------- Other reviews about meteors:
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- 2586 METEORITE - the oldest meteorite? A meteorite that crashed into rural southeastern Australia in a fireball in 1969 contained the oldest material ever found on Earth, stardust that predated the formation of our solar system by billions of years
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- 2389 - METEORS - we call them shooting stars. Shooting Stars are meteoroids that enter Earth’s atmosphere in the night sky. On average on a dark night you can see a shooting star once an hour. These meteoroids are rocky dust and debris zipping around our solar system at 30,000 mph.
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- 2343- The Story of a Rock A meteorite rock was found in Oman, Africa in September, 2002. The rock tells us that it came from the Moon and it even tells us which crater on the Moon it was ejected from. About 30 Earth rocks have been found and identified as originating on the Moon. This is the story of two of these rocks. The first rock was found in Oman. a country southeast of Saudi, Arabia on the Arabian Peninsula.
1619 - How often do meteors hit Earth and how big are they? What were the more famous meteors that impacted Earth. What is the likelihood another big one is on its way?
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- 1611 - Meteor Impacts. How many meteors of all types hit us each year? When is the next big hit expected? Learn the equations that give the answers.
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- 1567 - The Russian Meteor - The Russian Meteor. Valentine’s Day was a real surprise for many people in Russia. February 14, 2013, a 10,000 ton meteor blasted through the atmosphere. The fireball was traveling 40,000 miles per hour. The shockwave created when the meteor hit the atmosphere blew out glass windows in over 3,000 buildings, over a 1,000 square kilometers. Over 1,000 people were injured. Mostly glass cuts, one with a broken back. People instinctively went to the windows to see what caused the giant flash of light. They were standing in front of the windows when the shockwave reached them.
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- 1557 - Tektites in Healdsburg, California - January 23, 2013, our local paper ran an article about the tektites found in Dry Creek Valley around Healdsburg, California, just 20 miles from my house. What in the world are tektites?
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- 1019 - The Sudbury Asteroid. A giant asteroid struck Earth 1,850,000,000 years ago creating the Sudbury Basin in Ontario, Canada. The impact blew a crater in the shallow sea floor that was 160 miles across. Along the seashore were dense colonies of cyan bacteria. Their stumpy masses are call stromatolites.
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- 1017 - Meteorite and Asteroids. Meteorites have been landing on the surface of Earth for millions of years. We have found many of them. You can buy them on E-Bay. In 2004 in Placid, Florida a 5.3 pound asteroid was recovered. E-bay says it came from the planet Mercury so it would yield a higher price. But, astronomers do not think it came from Mercury. They believe it came from the Asteroid Belt.
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- 719 - Geminid Meteor Shower. December 14, 2006, is the peak of the Geminid meteor shower in the night sky. A meteor is a streak of light, often called a shooting star. We see the light trailing a meteoroid. A meteoroid is an interplanetary object that is bigger than a speck of dust and smaller than an asteroid.
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- 523 - The Story of a Rock
- February 25, 2020 2633
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