These micrometeorites were recovered from the ocean floor. They average about a millimeter across. Credit: Donald Brownlee/University of Washington
As I look down into a Petri dish of sand through a binocular microscope, the larger grains become obelisks, with smaller ones forming avenues between them. It’s a microscopic echo of the Chicago skyline that I can see through the window when I look up. I’m at the Field Museum of Natural History, home to the Robert A. Pritzker Center for Meteoritics and Polar Studies. The museum has many large items: Dinosaurs, elephants, and some very substantial meteorites. But today, I’m thinking small. The meteorites I’m looking for are hidden in the sand beneath my microscope. Maybe.
The sand is from the Sør Rondane Mountains in Antarctica, brought here by Maria Valdes, a research assistant at the Museum, who journeyed there with an expedition in search of meteorites. They found several, including an extra-large space rock weighing 16.7 pounds (7.6 kilograms). To find a meteorite that big is a rarity; but for this particular project, she needs something small: micrometeorites.
Cosmic origins
About 40,000 metric tons of extraterrestrial material rain down on planet Earth every year. We can often see bits of it streaking through the night sky as meteors. If any part of a meteor survives its fiery descent through the atmosphere to reach Earth’s surface, we call it a meteorite. Sometimes those meteorites are relatively large: boulder-sized chunks of iron or small, blackened rocks. But most of the material arrives here in the form of micrometeorites, usually less than 0.04 inch (1 millimeter) in size.
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To meteoriticists, the scientists who study meteorites, these are cosmic messengers, carrying information about their origins in the solar system. Most meteorites originate in the main belt, a region of rocky bodies orbiting the sun between Mars and Jupiter. Over the course of time, some of these collide and break up, flinging pieces of debris toward the inner solar system, where their paths can intersect with Earth’s. Pieces from a single collision may land on our planet in widely separated places and at times millions — or even hundreds of millions — of years apart. But they still have one thing in common: They retain the characteristics of the asteroid they came from, called the parent body.
Chromite is a mineral often found in meteorites. A durable combination of chromium and oxygen, it is able to withstand the heat of entering the atmosphere and long periods of weathering on Earth’s surface. This makes it a good memory bank for the properties of its parent body. Though all samples of chromite are chemically the same, the nuclear properties of the oxygen they contain can differ. All oxygen atoms have eight protons, but they can have eight, nine, or 10 neutrons, resulting in the isotopes we call oxygen-16, oxygen-17, and oxygen 18, respectively. Oxygen found on Earth consists of these isotopes in the ratio 99.76: 0.04: 0.20. In asteroids and meteorites, however, this ratio can differ. What’s more, it can differ between one parent body and another, allowing meteoriticists to associate a meteorite with its parent body by measuring the ratio of the oxygen isotopes using a mass spectrometer.
Over time, meteoriticists have assembled a record of isotopic ratios for different parent bodies, so that a newly found meteorite can be checked against it and its origin can often be identified. But Valdes sees a possible problem with the technique. What if a meteorite’s oxygen content has been altered by its descent through our oxygen-rich atmosphere? Small meteorites, which might be mostly or even entirely melted on the way down, would be especially susceptible to this. Her project — the one that has me peering into a Petri dish and poking sand grains with tweezers — is trying out a different way of identifying where meteoritic material originated.
Besides chromite, meteorites commonly contain minerals with calcium. Calcium also has several naturally occurring isotopes, so measuring its isotopic ratios offers an alternative technique for studying meteorites. “The method for isolating and measuring calcium isotopes is well established for meteorites,” says Valdes, but doing these measurements on micrometeorites is new.
Although the mass spectrometry for measuring oxygen isotopes largely spares the micrometeorite from damage, the calcium testing destroys it altogether. That means establishing the calcium ratios requires looking at a lot of micrometeorites. And poking through a lot of sand.
Grains of sand
So why go all the way to Antarctica to get sand? There’s plenty of it right outside, by Lake Michigan. And it too contains micrometeorites. Unfortunately, it also contains the soot and slag of several centuries of human habitation. These particles can confuse the picture. In my hunt for micrometeorites, I’m looking for objects the size of a grain of sand, but that don’t look like sand. With the microscopic debris of civilization is mixed in, it’s even more like looking for a needle in a haystack.
The Sør Rondane mountain range of Antarctica, however, is still a pristine location essentially uncontaminated by human activity. In its sand, micrometeorites are more likely to stand out. “It’s still like looking for a needle,” says Valdes, “but at least there’s no haystack.”
The shapes I’m looking for are spheres, not needles. When stony debris shoots through the atmosphere, the frictional heat melts its surface, creating a blackened crust about a millimeter thick. If the object itself is only a few millimeters across, then it might melt entirely through. When this happens, the molten material assumes the most aerodynamic shape that it can: a sphere. And that helps it stand out from the blocklike sand grains I’m looking through.
In fact, I see one that looks kind of round. Push it with the tweezers. Nope, not a sphere. Keep looking.
After several days in the lab, I’d plucked 15 roundish objects from the Antarctic sand and placed them carefully on epoxy pads in plastic sample cases. I doubt that all of them are really micrometeorites. Honestly, I can’t swear that any of them are. Valdes tells me that they will have to be examined with a scanning electron microscope to determine what they are.
I hope I found at least a few micrometeorites, but even if I didn’t, there’s still plenty of sand left.
