Meteorite Impact: Behind Craters and Clues to life on Earth - Space Essay Example
Meteorite Impact: Behind Craters and Clues to Life on Earth
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When alien rocks fall from the sky, it seems like speeding fireball rushing in as it smashes in a distance forming a mushroom cloud. This might not be just a scene from another science fiction film but a meteorite is on its way to make a mark in the face of the Earth. Meteorites are as natural objects “from space that hits the surface of the Earth or other planetary body. Impacts by large meteorites are believed to have created most of the craters on the planets and their satellites” (Ridpath, 2007). Ridpath (2007) also informed that an estimated of 50 to 100 tonnes of cosmic debris enters the Earth’s atmosphere every day, but only about a tonne reaches the ground. Thus, the fate of a meteorite entering the Earth’s atmosphere depends mainly on its mass and velocity.
Beech (2004) analysed meteorite fall statistics have revealed that about 30,000 meteorites of mass greater than 3.5 oz (100 g) fall to Earth each year. Of these meteorites the majority weigh just a few hundred grams, only a few (about 5,000) weigh more than 2.2 lb (1 kg), and fewer still (about 700) weigh more than 22 lb (10 kg). In general, the number of meteoroids hitting the Earth’s atmosphere increases with decreasing meteoroid mass: milligram meteoroids, for example, are about a million times more common than meteoroids weighing a kilogram. However, there are meteorites that weigh more than that. Because of the sheer size, Earth’s atmosphere has no significant slowing effect on meteorites that weigh over 1000 tonnes. The average entry velocity of incoming bodies which fall as meteorites is about 20 km/s. Incoming bodies with high velocities are more likely to disintegrate in the atmosphere than those with lower velocities; those with an entry velocity in excess of 30 km/s suffer more than 99% ablation (McSween 1999, p. 5). Also, the composition of an incoming meteorite also affects whether it survives to reach the ground. Also, scientists use sophisticated chemical analysis, mass spectrography, X-rays, and other technologies to determine meteorites’ origin and composition.
Schiff (2002) unravelled that there are three basic types of meteorites: stones, stony-irons, and irons. Stones are divided into two main subcategories: chondrites and achondrites. Chondrites are the main type of stony meteorite, constituting 84 percent of all witnessed meteorite falls. Most chondrites are characterized by small spherical globules of silicate, known as chondrules. Interestingly, carbonaceous chondrites also contain organic compounds such as amino acids, which may have contributed to the origin of life on Earth. Chondrites are the most primitive of the meteorites, suffering little change since their origin. Achondrites, on the other hand, come from chondritic parent bodies that have been heated to the melting point, destroying their chondrules and separating heavy and light minerals into a core and mantle. These are known as differentiated meteorites. Early volcanism occurred on the surface of their parent bodies forming a thin crust. A subcategory of achondrites called SNC (shergottites, nakhalites and chassignites) achondrites are believed to have come from Mars. However, some meteorites have been believed to originate from asteroids. From studying asteroids, scientists measure the intensity of sunlight at different wavelengths reflecting off their surfaces. This is then compared to the light reflected off pulverized meteorites in the laboratory. Reflectance spectra from various asteroids can be matched with different types of meteorites, which further strengthen the theory that some meteorites may have come from asteroids.
Another consequence of meteorites hitting the face of the Earth is the craters formed during the impact. How would someone recognise a meteorite crater if they discovered one? When the meteorite shatters at the moment of impact, the pulverised earth and meteorite fragments are hurled out of the crater and scattered around it, but a considerable part falls back into it. This causes craters to display raised and overturned rims. But most of the time this above-surface evidence is erased as the Earth’s surface is always changing. After thousands of years of weathering and erosion by wind, rain, ice, changes in temperature, gravity and activities of animals and plants, a crater may not look like a crater (Verma 2006, p. 25).
How many meteorite craters exist in the face of the Earth? Where are they found? What is the role of the meteorite impacts on the life on Earth? These are the questions we will try to delve into whilst we uncover the scientific mysteries that shroud around meteorites.
Where are the world’s meteorite impact craters and how old are they?
Meteorites were observed and collected for thousands of years, but their extraterrestrial origin was not accepted. Not until 1803 was the possibility that pieces of rock or iron might fall to earth from space regarded as fact. Skepticism against this possibility used to be so great that two centuries ago Thomas Jefferson is alleged to have said, “I would prefer to believe that two Yankee professors would lie rather than that stones could fall from heaven.” How shocked he would have been to learn what else falls from the sky. The turning point in dispensing with that skepticism came when in 1803 near the town of L’Aigle in France bright flashes lit up the heavens and stones were seen to fall out of the sky over a large area (Verschuur 1997, p. 33).
At present, there are nearly 160 known meteorite impact craters that exist around the world in 17 locations (Wynn & Shoemaker 1998, p. 66). Recognising impact meteorite craters on Earth can be difficult, unlike the moon that is covered with craters, it has no water, no weather, no continental drift – so the craters just stay where they formed, barely changed over the years. On the earth, however, all these factors have erased what would otherwise have been an equally pockmarked surface. To confuse matters further, more familiar processes, such as volcanism and erosion, also leave circular holes. Not until early this century did geologists first confirm that some craters are caused by meteorites.
The Arizona Meteorite Crater (also called the Barringer crater) is one of the best known and the first recognized meteorite crater on the Earth’s surface. D.M. Barringer first identified it as a meteorite crater in 1905 and it is near Flagstaff, Arizona. It is outstandingly well-preserved because it is in the desert in the south western United States. The crater is just over 1.2 km in diameter and 200 metres deep; it seems to be 20,000 to 50,000 years old. Iron meteorites in abundance are found nearby (Ince 2002, p. 19). It is thought that a small asteroid might have hit the Earth to form this crater because tonnes of nickel-iron meteorite debris have been found in the surrounding area from the original 50-meter-wide impactor surface at sufficiently high velocity. The kinetic energy of the meteorite is converted to heat, which vaporizes the surrounding rock as well as much of the meteorite, producing an explosion equivalent to a large nuclear device.
Over the years aerial photography and satellite imagery have revealed many other astroblemes (the scientific term for craters or scars left on Earth’s surface by the high velocity impact of large objects from outer space). The largest astrobleme is South Africa’s Vredefort Ring, whose diameter spans 24 mi (40 km), was formed 2 billion years ago (Lerner & Lerner, 2004). The Wabar crater in Saudi Arabia was discovered in 1932 as it fell in the middle of the largest contiguous sand sea in the world. A dry, isolated place, it is perhaps the best-preserved and geologically simplest meteorite site in the world and it is estimated to be 6,400 years old (Wynn & Shoemaker 1998, p. 67).
Although most craters have been ancient in origins, there are newer meteorite craters that are formed. In 1908, there was an enormous atmospheric explosion above Tunguska, Siberia. The resulting blast leveled 2,000 square kilometers (770 square miles) of forest, and the shock wave circled the globe. Such an event is predicted to happen once every few hundred years or so. As recently as 1947, the Sikhote-Alin meteorite crashing north of Vladivostok, Russia, made an array of craters, some of which were one-fourth the size of a football field (Norton, 1998).
What is the role of meteorite impacts in the history of life on planet Earth?
A meteorite crater in Yucatan, Mexico has been studied to have triggered the Cretaceous-Tertiary (K/T) mass extinction. Alan Hildebrand and his colleagues at the University of Arizona in Tucson made a connection between the readings and some unusual 65-million-year-old rocks at the Brazos River in Texas, in which a turbulent sandstone bed appears between quiet layers of marine sediment. It was thought to be a “tsunami bed”, created from material washed along by a massive wave, and conjectured that there would be a crater nearby. This made Hildebrand suggest that a meteorite had hit Yucatan 65 million years ago, generating a tidal wave that swept across Central America. Most convincing evidence was the rock samples from deep inside the crater itself. These came from two cores that Pemex drilled into the Chicxulub crater in the 1980s. Pemex’s primary aim was to find oil, not craters, and it only sampled small parts of the rock. But what it pulled out was enough to prove that Chicxulub was an impact crater (Ravilious, 4 May 2002).
However, not all meteorite impacts can be bad or scary. Some have helped bring to the surface important minerals, such as gold in the Vredefort structure in South Africa, uranium at Carswell in Canada and in Vredefort, and iron ore at Ternovka in the Ukraine. At Ternovka, blocks of iron hundreds of meters across were fractured and tossed about by the impact and mixed with rocks that contain no iron. The Vredefort structure may originally have been 300 kilometres across and much of South Africa’s gold wealth is associated with this 2 billion-year-old structure, whose riches set the scene for the growth of a nation. In some cases the economic advantage is created because of the impact; for example, impact diamonds. They tend to be harder that normal diamonds and a variety called Carbonados has been sold for industrial uses. Impact events sometimes alter the local geology to produce an economic resource; for example, oil is brought up from the 16 kilometre-diameter Ames crater in Oklahoma, which lies buried beneath 3 kilometers of sediments. Drinking water is obtained from the Manson crater in Iowa and salt is mined in the Saltpan in South Africa. Oil is also found at the Red Wing Creek and the Newporte impact craters in the United States (Verschuur 1997, p. 52).
In November 2006, the Johnson Space Centre reported that organic matter in meteorites has become a subject of interest because this may “contain the material formed at the dawn of the Solar System and may have seeded the early Earth with the building blocks of life”. In focus was the Tagish Lake meteorite “because much of it was collected immediately after its fall over Canada in 2000 and has been maintained in a frozen state, minimizing terrestrial contamination. The collection and curation of the meteorite samples preserved its pristine state”. It was found that this meteorite contained “organic globules” that “have very unusual hydrogen and nitrogen isotopic compositions, proving that the globules did not come from Earth”.
Earlier, the fall of the Murchison meteorite in 1969 provided fresh extraterrestrial material for amino acid analysis by newly developed chromatographic and mass spectrometric methods. The majority of studies from 1969 to present have focused on various stones of Murchison since there were no additional observed falls of carbonaceous meteorites until the fall of the Tagish Lake meteorite in 2000 (Hoover 2005, p. 28). The presence of protein and organic materials in meteorites are clues that might lead us to know the origins of life on Earth itself. This is the reason why the role of meteorite impacts is very essential in knowing the real story of how life on Earth came to be.
Indeed, meteorites can be considered as the “true extraterrestrials”. The craters and the debris that they leave behind are valuable not only to science but also it can provide clues about the history of life on Earth. They are reminders that we can all be wiped out by one gigantic impact hitting us and it can be remnants of proof if some other creatures beyond the Solar System exist. Thus, it is recommended that scientists continue their quests in exploring meteorite craters and debris. They might just discover new things in their endeavours that will complete the meaning our existence in this planet.
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