Dust is underappreciated. Most people consider it a nuisance, something to be collected with a damp cloth and washed down the drain. And why not? Most household dust is made up of cast-off skin cells mixed with tiny dirt particles; nothing very inspiring. But, as it turns out, there's a reasonable chance that maybe one or two of the tiny dust particles comes from the deepest reaches of outer space.

Cosmic dust is constantly raining down on the Earth from space, and in an average year, we pick up about 40,000 tons of the stuff. That may sound impressive, but when you consider spreading that amount of dust over the entire surface of the Earth, the challenge of actually identifying one of these cosmic dust grains becomes substantial.

Can you imagine NASA scientists arriving at your home and analyzing the millions of dust specks in your house in hope of finding one of non-terrestrial origin? Talk about a needle in a haystack. But recently, scientists have been getting quite a lot better at finding these alien dust grains, and what they're discovering about them may eventually lead to clues about how life took hold on Earth in the first place.

To begin with, let's ask the obvious question: how can you tell if a grain of dust comes from space? That's actually the easy part, as cosmic dust has a very different chemical and mineral content from anything else on Earth. Cosmic dust forms far out in space, possibly from the residue of a supernova explosion, or in the upper atmosphere of a giant, cool star.

One simple cosmic dust identifier is the presence of iron and nickel in the grains. On Earth, both iron and nickel are extremely rare in surface rocks. Most of these heavy metals sank to the core of the Earth when our planet was still molten (a process called "differentiation"), and the rest of the stuff got rusted and oxidized away by the water in our atmosphere. Remember how long it took humans to discover how to smelt iron? For a long time, the only iron we had came from meteorites. It was only a few thousand years ago that we discovered how to melt down soft, reddish rocks and retrieve the pure metal.

Another smoking gun of extraterrestrial dust is the ratio of iron to nickel atoms. In our Solar System, the Sun, meteorites, and cosmic dust all contain about twenty atoms of iron for each nickel atom. On the Earth, due to differentiation, the iron atoms outnumber the nickel atoms two hundred to one.

There's other identifying chemistry as well. Some cosmic dust particles contain Helium atoms that are missing a neutron. Helium itself is extremely rare on Earth (we actually discovered it on the Sun before we found it here), but when we do find it, its nucleus contains two protons and two neutrons. However, in the solar wind, which is a stream of charged particles constantly flying off the surface of the Sun, we can detect an exotic form of Helium called Helium-3 (the 3 indicates the number of particles in the nucleus with one missing neutron). It goes without saying that any dust that contains Helium-3 atoms has spent a lot of time (as in billions of years) in space, with direct exposure to the solar wind.

But perhaps the most exciting difference between cosmic dust and your average Earth-bound detritus is the weird kind of organic chemistry we find in the stuff.

It surprises people to learn that space is full of organic molecules. Using spectroscopy, a technique that splits light into a spectrum and allows us to identify the chemical content of stuff floating around in space, we've been able to find all kinds of interesting things drifting between the stars, from water and ammonia, to alcohol and amino acids.

But there's more: on Earth, we know of only 23 amino acids. In space, as well as in meteorites and cosmic dust, we've identified over 70. Amino acids turn out to be handy things to build bigger molecules out of, but all life on Earth uses less than half of the flavors available in space. Needless to say, if a particle has one of these exotic amino acids in it, you've surely got an alien dust bunny.

So once we know how to identify cosmic dust, how do we find it? For a long time, scientists thought that we'd have to get up in space to get a good sample of cosmic dust, or at least get very high into the atmosphere. Once it hits our upper atmosphere, cosmic dust takes a very long time to settle down to the Earth's surface, so there's a good chance that any dust you find way up high might have extraterrestrial origins.

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