8.02.2021
Micrometeorites land on every corner of Earth. Matthew Genge is using these shards of interplanetary space to understand Earth and its place in the solar system.
A collection of micrometeorites found in Antarctica, featured in the 2020 book Atlas of Micrometeorites.
Jon Larsen & Jan Braly Kihle 2020 Atlas of Micrometeorites Vol.1.; Arthaus DGB/Kunstbokforlaget DEN GYLDNE BANAN (Norway); ISBN: 978-82-93805-00-7
Every year, roughly 10 particles of space dust land on each square meter of Earth’s surface. “That means that they are everywhere. They are on the streets. They are in your home. You may even have some cosmic dust on your clothes,” said Matthew Genge, a planetary scientist at Imperial College London who specializes in these alien dust grains, known as micrometeorites.
Round and multicolored like tiny marbles, micrometeorites are as distinctive as they are ubiquitous, yet they escaped notice until the 1870s, when the HMS Challenger expedition dredged some up from the bottom of the Pacific Ocean. (On land, the accumulation of terrestrial dust tends to overwhelm and conceal the cosmic kind.)
For a century, scientists thought that the strange spherules found on the seafloor had dripped off the molten surfaces of larger meteors as they crashed through the atmosphere. In fact, cosmic dust floats here from space rocks hundreds of millions of miles away, bearing tiny messages.
For 30 years, Genge has been deciphering those messages, one grain at a time.
He began his career just as Antarctica was identified as a bountiful new source of micrometeorites. Strong southerly winds help sweep away earthly debris, so that as much as 10% of the dust lodged in the ice comes from space. “I got to do a lot of the easy stuff,” Genge said, like figure out “what they’re made of, what they look like, what the different types are.” Since then, he and other micrometeorite specialists — a small enough community that he “knows the children of most of them” — have gleaned much more information from the dust. Recently, Genge has been interpreting messages the space dust carries, not about its origins, but about its destination: Earth at different points in the planet’s history.
The wiry, bald Brit takes Zoom calls in his London bedroom, squeezed between a bed, a wardrobe and a microscope. He brought the microscope home from the lab as lockdown was about to begin last March, along with plenty of dust. When we video chatted this winter, Genge grabbed a plastic jar from a box on the wardrobe and jiggled it in front of the camera. The jar was half-full of tawny silt — Antarctic dust, some from Earth, some extraterrestrial. As he sorts through it, Genge could conceivably come across a speck of 6626 Mattgenge, an 8-kilometer-wide asteroid near Mars named in his honor for his contributions to the study of cosmic dust.
Our conversation about his dusty discoveries has been condensed and edited for clarity.
Have you always liked meteorites? How did you get interested in geology?
I was fascinated as a kid by Arthur C. Clarke’s books of mysteries. That’s what led me to ask lots of questions. But the reason I got attracted to geology was I liked art. There were two classes I got to do a lot of drawing in: One was geology and the other was art. And as soon as I went out into the field and started drawing rocks and realized that I could use my drawings as detective stories, to work out the formation of that rock, to see events that occurred millions of years in the past, I was hooked. Then I was geology all the way. [Genge is the author of Geological Field Sketches and Illustrations: A Practical Guide, published in 2020.]
What drew you to space dust, specifically?
Astronomers have always focused on stars and galaxies. They literally are the bright sparkling bits of astronomy that everybody gets drawn to. But actually, dust is one of the most important parts of astronomy, because OK, the stars sit there producing elements that eventually make planets, but it’s the dust that delivers that stuff from stars to planets. If it wasn’t for the dust, our universe would be a pretty mundane place: flickering stars with nothing around them. The dust links stars with everything else, with all the planets, all the living things on those planets. It’s the dust that’s responsible, ultimately.
What do we know about where earthly space dust comes from?
At the beginning, in the 1990s, we had very little idea what objects in the solar system are producing all this dust. The French were very keen on the dust coming from comets; I don’t know why. We eventually worked out that the micrometeorites are largely coming from primitive asteroids. They’re similar to a type of meteorite called carbonaceous chondrites, which come from the most common type of asteroid — carbon-bearing “C-type” asteroids.
What can we learn from micrometeorites that we can’t learn from meteorites, if they both primarily come from the same source?
We can learn a lot, which has to do with the way dust is delivered to the Earth. To get a meteorite to the Earth, you have to knock it off from an asteroid, and then it floats around in space and its orbit slowly changes until eventually that orbit may cross the Earth’s. That’s quite a random process.
Whereas tiny little dust particles, when they’re blasted off the surface of an asteroid or stream off the surface, they go into space, and light from the sun affects their motion. It’s a really cool process called Poynting-Robertson light drag; I love it because it sounds so sci-fi.
This light drag basically slows down dust particles, and if you slow an object down in its orbit, it’s got to move inward, so the dust slowly spirals in toward the sun. It moves through the orbits of the planets, and it’s got a high chance of being swept up by the planets. So there’s this mechanism to deliver dust to the Earth that is a lot more reliable than the mechanism that delivers larger chunks of rock. Because of that, micrometeorites are a better sampling of what is actually out there in the solar system than meteorites; they allow you to study a lot more asteroids and comets than meteorites do.
But of course micrometeorites are tiny; each micrometeorite provides you with a tiny bit of information, whereas a meteorite will keep you busy for your entire life if you find a good one. So meteorites provide us with a lot of information about a small number of objects, and micrometeorites provide a tiny amount of information on a lot of objects. And so the two work together really well.
How does this constant influx of dust affect Earth and the other planets?
It has fallen on our planet all the way through our planet’s history. It’s fallen on Mars. It’s fallen on Venus. The origins of life may have something to do with cosmic dust, because it actually delivered most of the intact amino acids and organic molecules of the Earth during the Late Bombardment [about 4 billion years ago]. On Mars, if there is anything living in the Martian soil, it’s probably eating micrometeorites because that’s the main source of organic material to the Martian surface. You measure the amount of nickel in Martian soil and it’s several percent, and that nickel is mainly coming from micrometeorites. I like to think of them as micrometeorite munchies on the surface of Mars.
Even on Earth at the moment, micrometeorites are important in terms of the delivery of nutrients. The deepest, most remote parts of the ocean are so far removed from land that they receive very little terrestrial dust, and living organisms need a range of trace elements like iron in order to survive. And actually most of the iron delivered to the southern Atlantic and parts of the Southern Ocean are coming from micrometeorites.
You’ve said that micrometeorites are helping us figure out “what’s out there” in the solar system. Can you talk about why asteroids are so diverse? Why aren’t asteroids and planets all made of the same stuff?
If we knew the exact answer to that, I would be — well, actually, no, I probably wouldn’t be rich. I would be famous. Slightly.
So it’s a bit like baking. You get a bowl, you fill it with flour, and then you pour the sugar into the center, and then you mix it all together. And as you mix, the sugar gradually moves outward in the bowl and mixes with the flour. So over time the composition changes. Our solar system formed in a mixing bowl of chemical elements that had been building up since the Big Bang.
What we’re aiming to do when we look at meteorites and micrometeorites is to look at these different components and try and decide where they formed in the disk to reconstruct its history. How did the disk change over its 3-million-year lifetime, during which planets formed? That’s really crucial to understand, because the nature of each planet is determined by the materials that accumulate at that point in the mixing bowl to make that planet. It might be the difference between having life on the planet or not. And understanding how these protoplanetary disks work will give us the ability to predict what planets around different stars will look like and how they form.
You’ve also shown that micrometeorites can tell us about Earth, isn’t that right?
Yes, the way micrometeorites mix with the Earth’s atmosphere doesn’t just provide us with information about what’s up there, but also what’s down here. Most metal particles get all their oxygen from the Earth’s atmosphere as they’re coming through; they heat up and they react with atmospheric oxygen, so when you measure their oxygen isotopes, their oxygen just exactly matches terrestrial oxygen.
I published a paper with Andy Tompkins in 2016 in Nature on 2.7-billion-year-old micrometeorites, which we found in limestones in Australia. We recognized that all the oxygen in those spherules is coming from the Earth’s atmosphere. And so that gives you a way of measuring the Earth’s atmosphere in the past, and it’s much more direct than the ways geologists have been doing that — by looking at crystal carbonates that grew at the bottom of the ocean. There, you have a really complex process; you have to work out how much oxygen was in the water at that depth, relate that to the surface water and then the Earth’s atmosphere. It’s really difficult.
Whereas if you’re heating a piece of metal in the atmosphere over the course of 10 seconds, then you get an instantaneous absorption of oxygen, many kilometers above the ground — great way of measuring the composition of the Earth’s upper atmosphere. And so cool, as well, that you can go to rocks, collect these little bits of space dust, and they can tell you about the Earth’s atmosphere in the past. How cool is that? The great thing is, not just on Earth: If one day we find micrometeorites on Mars, we can study the history of Mars’ atmosphere.
Wow. So what did the ancient micrometeorites tell us about Earth’s ancient atmosphere?
Up until that point, people assumed there was very, very little oxygen in the Earth’s atmosphere 2.7 billion years ago. Because of those micrometeorites we found in Australia, we now know that was false; there was actually a lot of oxygen, even if it might have been tied up in carbon dioxide.
I’ve seen plots that trace the oxygen and carbon dioxide levels throughout Earth’s history and show how those levels relate to evolutionary jumps and other events.
A fun game to do is to look at several plots and notice how very different they are.
OK, so ancient micrometeorites are a way of getting some more accurate data points, so that we better understand the Earth system.
Absolutely. We’ve actually been back to Australia since. We wanted to find even older dust, so three years ago I was in the Pilbara driving around and sampling really old rocks, avoiding snakes and huge spiders. We came back with bags and bags of rocks to search for cosmic dust.
How do you go about finding micrometeorites?
One of the unfortunate things about micrometeorites is most of the fun stuff takes about five minutes. And then the rest is quite dull — thousands of hours staring down a microscope. I’m still working on a collection that I made in 2006 that took me less than five minutes to collect, in a moraine [an accumulation of rocks and debris deposited by a glacier] in Antarctica. There was a layer of dust in this moraine, and I just put it in a plastic bag and I’ve been working on it for the last — how long is it? — almost 15 years.
I guess the hard part is knowing where to scoop.
Lucky for me, I just looked where I went. I was on the Antarctic Search for Meteorites expedition and we were due to search this nunatak [a mountaintop protruding from a glacier] for meteorites. While we were there, I decided to go and look in this moraine close to the nunatak to see if I could find any micrometeorites. I just shoveled away a bit of snow and there was this lovely dusty layer sitting underneath the snow.
So I presumed there must be a lot of micrometeorites in the dust, and I was right. I collected 6 kilograms of dust, and I’m about halfway through, and I’ve got over 3,000 particles. And I probably missed quite a few, too. Materials I found in the moraine later showed that it’s been collecting dust for at least 700,000 years.
So you occasionally return to your bag of dust and sort through some more?
I’m just worried that someday somebody will accidentally throw it out.