Sunday, March 27, 2005

Catch a comet by the tail

Micrometeorites are small particles of space dust, up to the size of grains of sand, which populate the inner solar system. Their speed relative to spacecraft makes them a bit of a hazard. But once captured in the Earth's atmosphere, they tend to drift to the earth's surface without getting completely melted, as larger metorites are. (Micrometeorites often form a melted "rind" around pristine material.)

Tons of micrometeorites enter the earth's atmosphere every day, making them the largest source of extraterrestrial material to arrive at the earth's surface. You can actually collect micrometeorites in your own backyard , by sorting through the grit at the base of a drainspout, because they stick to magnets whereas most other earth dusts will not. Visible under a moderately powered microscope, glossy spherules with pitted surfaces are likely to be from outer space.
(Micrometeorites collected in Antarctica; picture copyright US Army cold regions engineering lab)

Systematic efforts to collect micrometeorites are made in ice pack or from ocean sediments. Glacier melt-holes will have the same sort of grit (on a much larger scale) as found under a drainspout; and in pristine environments, this will be a fairly rich source. In Antarctica, US scientists have collected them by suctioning up the grit at the bottom of a covered meltwater drinking well; whereas a French group has been carefully melting ice cores under clean room conditions.

Micrometeorites are of interest mainly because their mineralology is much more varied than that of conventional meteorites. Many contain complex organics, and even amino acids , suggestive of an origin in comets. Therefore these particles are of interest in the origin of life because they might have delivered organics to an Earth which was effectively sterilized during the late heavy bombardment. The big question is whether organics supplied by this stardust could have helped the origin of life along.

Current research focuses on identifying compounds in micrometeorites, but it is made harder by the huge chemical heterogeneity. I'm guessing that microanalytical methods, working with batches of less than 100 or so, may produce something very interesting soon.

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