March 28, 2008

Meteoric Dust

Stupid Question ™
March 14, 2002
By John Ruch
© 2002

Q: I read that a trillion meteorite particles hit Earth daily. How much does this stuff weigh? Where does it all go? Does it affect the planet’s rotation?

A: Earth indeed constantly attracts meteoric material, mostly debris from comets and asteroids. While the big ones get all the attention, the vast majority are in the form of microscopic dust. Even the average “shooting star” is only about as big as a sand grain.

The only real answer to “how much” is “a whole lot.” But scientists like to quantify, and they’ve made a series of educated, but nonetheless wild, guesses.

The first measurements were made in the 1950s, using magnets to draw iron-containing material out of dust. (Many meteorites contain iron.) They came up with huge figures like 14 million tons a year. But they also had a huge margin of error due to the amount of iron-containing debris we and the planet both create.

The current best guess, published in 1995, is about 44,000 tons a year, or 120 tons per day, mostly in the form of particles around 0.2 millimeters in diameter. This figure was drawn from a broad range of sources: particles from more-pure ocean and polar-ice sediments; dust collected by planes in the upper atmosphere; analysis of moon dust; and especially the debris that bombarded NASA’s Long Duration Exposure Facility satellite (in orbit from 1984 to 1990).

And even this figure was reported with an error margin of 22,000 tons.

This figure is a broad average that describes the normal influx of medium to small meteorites. It doesn’t count the rare gigantic impacts.

Also, the figure specifically describes how much material enters the atmosphere, not just what hits the ground. The smallest dust slows down so rapidly in the upper atmosphere that it actually floats there, probably for years at a time, before getting washed down by rain. An estimated 1 to 10 percent of all dust in the upper atmosphere is meteoric.

Meteoric debris eventually wears away like any other material. It tends to wear faster than terrestrial rocks because of its purer elements. A lot of the dust is eaten up in the air by sulfuric acid droplets.

Atmospherically, meteoric dust probably serves as nuclei for raindrops, and is suspected of causing noctilucent clouds (high clouds that glow after dark) and “sprites” (lightning that shoots out of the tops of clouds).

Some meteoric dust is very carbon-rich, and there are still-controversial claims that it was the basis of the rich soil called loess, of petroleum, and possibly of the organic material that was the basis for life itself.

While that would be a big deal, the sheer weight of the debris isn’t. Earth is huge—about 611,000,000,000,000,000,000 tons. The weight of all the water on the planet is less than 0.02 percent of that. And the weight of all the meteoric debris (figuring 44,000 tons a year for 4.5 billion years) would be about 0.02 percent of the water’s weight. Completely negligible.

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