|Space Chronicles on Ice #10||
by Don Pettit|
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Dec 12, 2006:
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. The reason for this is generally understood, 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 passing of time on a geologic scale (thousands to millions of years), they will forever remain theories (at least until humanity has been around for periods approaching geologic time and is able to observe first hand these events in the making).
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 that try 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 the fall rate of meteorites intrinsic to Earth's is; however, reasonable estimates based on observation can be made. It is estimated that 1,000 to 10,000 tons of meteors enter Earth's atmosphere every day. Most of these burn up and never make it to the surface as a recoverable chunk of rock which we call a meteorite.
A study 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 and could actually be recovered. 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 or one meteorite per 360,000 square miles per year. 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 there that not only preserves them but actually concentrates them well above the natural abundance.
Can we come up with an estimate for how great this concentration factor is in Antarctica? Since the inception of the ANSMET meteorite gathering expeditions to Antarctica (starting in 1976), they have searched in detail about 2,000 square kilometers and have collected 12,500 meteorites (about 2,600 kilograms or 5,720 pounds) as of 2003. This equates to about 6.3 meteorites per square kilometer (2.3 meteorites per square mile). The average time over which these Antarctic ice sheets have collected meteorites is estimated to be about 10,000 years, thus 6.3 meteorites per square kilometer represents a rate of about one meteorite per 1,000 square kilometers per year.
This is about 1,000 times greater than the estimated intrinsic fall rate, thus, it is inferred that concentration mechanisms operating in the Antarctic glacier ice somehow work to concentrate the meteorites by about a factor of 1,000 over the intrinsic fall rate to Earth. That, coupled with an environment that tends to preserve them, allows a team such as ANSMET to gather perhaps 800 meteorites in a single expedition.