Encyclopaedia: Meteorites in Antarctica

2015-04-13 Andrew B. Collier

A contribution which I wrote for Antarctica and the Arctic Circle: A Geographic Encyclopedia of the Earth’s Polar Regions.

Interplanetary space is not a vacuum: it is teeming with objects of various sizes, from microscopic electrons and ions to large pieces of space debris. Some of these objects end up on earth. A meteoroid is a natural object with size between 10 μm and 1 m moving through space. Asteroids are bigger than meteoroids but smaller than a planet, and without their own atmosphere. Most meteoroids have a mass of less than a few grams, originating as fragments from asteroids, comets, and planets. They are often concentrated in a meteoroid stream, which is the debris left in the trail of a comet. A meteoroid or asteroid that enters the earth’s atmosphere produces a meteor (or shooting star), which is the visible trail formed as it falls through the atmosphere. When the earth passes through a meteoroid stream, a meteor shower is observed.

Meteoroids enter the earth’s atmosphere at speeds up to about 156,585 mph (251,999 km/h). Speed, size, incidence angle, and degree of fragmentation determine whether or not a meteoroid reaches the ground. Those meteoroids that penetrate the surface of the earth and survive the impact are known as meteorites. Meteorites are found everywhere on earth, but most have been discovered in Antarctica where they are clearly visible on the snowy surface or embedded in ice.

The flux of meteoritic material to the surface of the earth is between 104 and 106 kg per year. A meteorite with a size of 1 m hits the earth on average once per year. Smaller meteorites are more common, while larger ones are relatively rare. Larger meteorites can produce extensive craters like the Vredefort crater in South Africa, which is 300 km in diameter. A meteorite with a size of 3,280 ft. (1,000 m) would have a cataclysmic effect, but fortunately, such impacts are rare: roughly once every million years. Many hundred meteorites, each with a mass of a few grams, strike the earth every day. The majority of meteorites originate from tiny meteoroids that are so light that they slow down rapidly in the atmosphere. These are deposited as microscopic dust.

Meteorites are classified as stony (94% of all meteorites; composed of silicate minerals), iron (5%; alloys of nickel and iron), or stony-iron (1%; composites of stony and metallic materials). Although stony meteorites are the most common, they are more difficult to identify since they are similar to normal terrestrial rocks. Iron meteorites are readily identified since they are magnetic and denser than most terrestrial rocks. They are also the most likely to reach the ground since they seldom fragment.

Initial estimates of meteorite flux were made from eyewitness accounts. Such meteorite falls (where the meteor trail was observed) are naturally more common in densely populated areas. Unfortunately, such estimates are bedeviled by large uncertainties. Recent meteorite flux data have been recorded by camera networks. These too suffer from difficulties in calibrating the images of meteor trails to meteorite masses. However, meteorite finds (where the meteor trail was not observed, but the meteorite itself was discovered) present a robust history that extends back thousands of years. The hot deserts and Antarctica have favorable conditions for meteorite accumulation: weathering occurs slowly, and meteorites are easily recognized against a uniform background that is relatively free of other rocks. Meteorites can survive in these regions for more than 100,000 years, allowing variations in meteorite flux and mass distribution to be estimated. Some meteorites found in Antarctica had been there for more than 1 million years. Furthermore, there is evidence to suggest that some of the meteorites found in Antarctica contain material that originated either from the Moon or Mars.

More than 30,000 meteorites have been retrieved from Antarctica by various expeditions during the past 30 years. This exceeds the entire population of meteorites collected over the rest of the earth. Differences in the populations of recent meteorite falls and Antarctic meteorite finds indicate that the characteristics of meteorites landing on earth are changing with time as debris from the early solar system is progressively swept up.

The fate of a meteorite that falls onto Antarctica depends on the nature of the surface and the typical weather conditions. If it falls on ice, then it may remain exposed for a long time. Alternatively, it might become covered by snow and ultimately trapped in solid ice. There is a general flow of ice toward the periphery of the continent, so that these meteorites are eventually deposited in the Southern Ocean. The motion of the ice sheet can be slowed by mountain ranges, and meteorites embedded in the ice become concentrated in stranding zones. If the ice either evaporates via sublimation or is ablated by the wind, trapped meteorites become exposed, leading to high meteorite concentrations. The history of meteorite falls in the Antarctic can be deduced from stranding zones and ice cores.

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