Fragile Oasis

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Concentrated Meteorites

For some reason, meteorites are found lying on the top of Antarctic ice fields in amazing numbers, much more so than what one would find in other areas of the world. The reason for this is generally understood [ref 1], however, like so many human spawned ideas into how nature works, there are a number of details that seem to escape our explanations. And of course, like any explanation that involves the passage of time on a geologic scale, they will for the most part, remain theories.

In 1969, a Japanese expedition to Antarctica discovered that meteorites are found on the ice in significantly greater abundance than what one would expect from just the normal fall rate. They proposed a number of natural mechanisms to explain this fact and scientists have been refining these theories for the last 30 years. No matter the explanation, meteorites somehow become concentrated on the glacier ice fields in Antarctica and there is great scientific utility in gathering them for study.

No one really knows what Earth’s intrinsic meteorite fall rate is. Reasonable estimates based on observation can be made. It is estimated that 1000 to 10,000 tons of meteors enter Earth's atmosphere every day. Most of those burn up and never make it to the surface as a recoverable chunk of rock which we call a meteorite.

A study [ref 1] in 2001 estimated the intrinsic fall rate of recoverable meteorites from a world wide network of cameras integrated over several decades. A recoverable meteorite is a chunk that survives the atmospheric entry. This study is perhaps the best estimate that can be made with real observables at this point in time. Their meteorite fall rate was one meteorite per million square kilometers per year (1/ 106 km2 yr) or one meteorite per 360,000 square miles per year (1/ 3.65 mi2 yr). This work also found that meteorites fall isotropically on Earth, a fancy way of saying that it does not matter where you are on Earth, the rate at which meteorites fall is the same. So meteorites fall in Antarctica at the same rate as any place else, there just happens to be special conditions here that not only preserves but concentrates them well above the natural abundance.

We can estimate this concentration factor. Since the inception of the ANSMET meteorite gathering expeditions (starting in 1976), they have searched in detail about 2000 square kilometers and have collected 12,500 meteorites (about 2600 kilograms or 5720 pounds) as of 2003 [ref 1]. This equates to about 6.3 meteorites per square kilometer (2.3 meteorites per square mile). The age of these meteorites is measured in billions of years, however their Earth-age, the time they have been on our planet, is much younger. It is estimated that their Earth-age is somewhere around 10,000 years. If we assume that once on the glacier ice these meteorites are preserved and thus accumulated over time, then one per million square kilometers per year intrinsic fall rate yields about one per hundred square kilometers after 10,000 years. Since we find them at about 10 meteorites per square kilometer, they are somehow concentrated by about another factor of 1000. It is inferred that some mechanism is operating in the Antarctic glacier ice to concentrate the meteorites by about a factor of 1000 over the intrinsic fall rate to Earth.

This concentrating mechanism, coupled with an environment that tends to preserve them, allows teams such as ANSMET to gather perhaps 800 plus meteorites in a single expedition.

1. Harvey, R.P., “The Origin and Significance of Antarctic Meteorites”, Chemie der Erde, vol. 63 (2), 93-147 (2003).