Cosmic dust bunnies
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.
Scientists started flying unmanned aircraft and balloons up to about forty miles altitude and exposing sticky collection plates to sample the dust up there, and they've had pretty good luck finding cosmic dust. But even at that height, there's plenty of contamination from Earth dust, so the grains have to be picked through one-by-one to find the cosmic dust grains.
An interesting side discovery of this search has been the detection of living microbes at these super-high altitudes. To give you an idea, the International Space Station orbits the Earth at an altitude of approximately one hundred miles. These microbes are living at the very edge of our atmosphere, almost half the way to the space station!
For the moment, no one really knows how the microbes survive up there, or even how they got there in the first place. Some people have suggested that aircraft toilets might have been the source of these microbes (think about that the next time you go to the bathroom at 30,000 feet), while some scientists have even gone so far as to theorize that these microbes don't come from Earth. To be sure, most biologists think the presence of alien microbes in our upper atmosphere is extremely unlikely, and indeed, some of these high-flying microbes have already been identified as terrestrial bacteria and funguses. But it does warrant further study, and plans are underway to catch some of these high-altitude micro-bugs.
Another good source of cosmic dust can be found in exactly the opposite direction: the bottom of the sea. This may seem hard to believe at first, but the deep sea bed is a very pristine place. Hundreds of miles out at sea, there is very little wind-borne dust, and the sediment on the sea floor accumulates at the rate of only one meter per million years. Scoop up a few feet of deep sea sediment, and you've got a record of what's fallen into the sea and drifted to the bottom over a huge amount of time. And since cosmic dust contains a higher abundance of iron than normal, dragging a magnet over a dried sample of the sediment can often separate and identify the precious cosmic grains.
Another great place for cosmic dust-hunting is Antarctica and the ice sheets of Greenland. Especially near the South Pole, thousands of miles from exposed, dust-producing soil, you've got to wonder where any little bit of grit you find came from. There, the strategy is to melt a good volume of ice and see what settles to the bottom.
Some scientists even got the clever idea to check into what was at the bottom of their well at the South Pole research station. Miles away from any un-frozen water, the "well" worked by melting ice to produce drinking water for the South Pole residents. Over a few decades, a huge amount of ice has been melted, and has a reasonable amount of grit has built up at the bottom. Sure enough, grain-for-grain, there's a large abundance of cosmic dust at the bottom of that well.
All this begs the question: why go to so much trouble to find space grit? Why should we care that whoever does the universe's dusting has been falling down on the job?
For one thing, cosmic dust is a fairly unaltered sample of what our solar system formed out of about five billion years ago. Cosmic dust may even be leftover bits of the dust cloud that formed the Earth itself, and as such, may give us clues about how the whole process worked.
But even more intriguing is the organic chemistry going on in the dust. Is it possible that complex organic molecules like amino acids arrived here from space? Perhaps it's not a coincidence that we seem to be built of molecules that are quite common in cosmic dust. Does cosmic dust seed the building blocks of life on new-born planets? Are we, in fact, what you get when you let cosmic dust sit around for a few billion years?
At least it's something to think about, and there's a lot more we don't understand yet. So, as you go about your day, don't forget that we live in a gentle rain of dust from outer space, falling on your head, on your cat, in your corn flakes. And maybe, from now on, look at your dirty dust mop with a little more respect.
Dr. Michelle Thaller is an astronomer at the California Institute of Technology. A massive-star specialist by trade, she currently dedicates most of her time to education and public outreach for the Space Infrared Telescope Facility.