The journey from space to Earth is not an easy one for most meteorites. But post-arrival plans are more comfortable for the lucky ones collected by Cari Corrigan, planetary geologist at the Smithsonian National Museum of Natural History.
In this month’s “Meet a SI-entist,” Corrigan discusses her work gathering meteorites in Antarctica, those specimens’ scientific value and what happens after they reach the museum’s National Meteorite Collection.
You’re a research geologist who studies meteorites from Antarctica. What led you down this path?
As an undergraduate student, I took an astronomy course that led me to take a geology class. My professor in that class told me about this field called planetary geology. So, I declared geology as a major and it turns out my advisor was the only person in this university who dabbled in planetary geology. He helped me do independent study, which led me to an internship working at NASA’s Lyndon B. Johnson Space Center on meteorite research. The scientist I worked with there had been to Antarctica. It was the first time I had heard of anyone going to Antarctica to collect meteorites.
During that summer, I also met Tim McCoy, the NMNH’s current curator-in-charge of meteorites, who was a post-doctoral researcher back then. All the people that I met that summer ended up being the people I’ve worked with ever since. It was a crazy, life-changing experience and one of those “right place at the right time” kind of things. Never did I think to myself when I was first getting my degree that I would get a job studying meteorites and get to go to Antarctica.
Why do you go to Antarctica for meteorites? Don’t they fall elsewhere as well?
Meteorites fall everywhere on Earth. Of course, 70% of them fall in the water somewhere, because 70% of our planet is ocean. We lose a heartbreaking number of specimens that way. But meteorites are easier to find in Antarctica, because of the environmental conditions.
Sometimes, you’re on the ice where there are not any other rocks around besides meteorites. That’s because Antarctica’s structure is like a big dome with the South Pole roughly in the middle. Gravity makes ice flow out to the edges of the continent and the Trans-Antarctic Mountain Range runs across the middle of the continent. In some places, you’re above those mountains and the ice is so thick that any rocks you see have to have come from above. There are no terrestrial rocks to be found.
Meteorites have been falling and being buried by snow and ice for thousands of years. The ice flows down towards the coasts and gets stuck against the Trans-Antarctic mountains. The dry winds and sublimation remove the ice, leaving meteorites stranded on the surface. We call these areas stranding surfaces and we don’t totally understand why the meteorites are concentrated there. It isn’t like one meteorite came in and broke up. It’s all different kinds of meteorites.
This second type of meteorite collection location — these stranding surfaces — can also have terrestrial rocks. How do you spot the difference between those and meteorites?
The stranding surfaces are found on the glaciers. That glaciers’ movement has scraped the rocks off the sides of these mountains, so there are certainly places where you have terrestrial rocks. But the difference can be obvious.
There's something called a fusion crust that forms on meteorites as they pass through Earth’s atmosphere. They’re going so fast that friction melts the outside of the rock, which ends up with a layer of glassy crust. It’s pretty easy to spot that on meteorites. Also, your eye gets used to looking for the differences. Spend a day looking in a giant field of rocks on ice and you’d also be able to spot the meteorites really quickly.
What types of meteorites are there lying around?
Most of them are ordinary chondrites. The reason they’re called chondrites is because they have small objects in them called chondrules. Each chondrule was a molten droplet out in the solar system over 4.5 billion years ago and those came together to form asteroids. Roughly 98% of all meteorites are chondrites of some sort. There are also some that have a little more carbon in them, which are called carbonaceous chondrites.
There are also a few from the Moon and Mars. We know the lunar meteorites are from the Moon because we can compare them to the rocks recovered during NASA’s Apollo missions. We know the Martian ones are from Mars because of NASA’s Viking Landers that went to Mars in the 1970s. The Viking Landers measured the composition of the atmosphere of Mars which can also be found as gases trapped in the melted glass of these meteorites.
Most of the meteorites are golf ball or fist sized. A lot of the time they have broken up on their journey through the atmosphere. Sometimes we find pieces on the ice next to each other that can be put back together like pieces of a puzzle.
That’s a physical puzzle, but what scientific puzzles can meteorite research in the national collection help us solve?
Each meteorite collected by the U.S. Antarctic Meteorite Program comes to the museum and our job is to figure out what kind of meteorite it is. All of them can provide a piece of the greater puzzle to help us understand how the solar system formed. They can tell us how asteroids and planets came together. In the meteorites, there are often melted minerals that can help us learn more about impact processes. There are also iron meteorites, which come from the cores of asteroids that met a grizzly end and were blasted apart. Examining those is one of the ways we’re able to understand the Earth’s iron core.
Scientists have also found pre-solar grains, or particles older than our solar system, in meteorites. These are grains that had to have formed under extreme conditions that may have come from a nearby star going supernova. The carbonaceous chondrites are some of the meteorites that those grains have been found in.
Every spring and fall, we put out a newsletter which contains all the new meteorites we’ve obtained and classified. In 2019, we classified over 400 meteorites. In a pandemic year, it might only be 50 new meteorites.
Anybody in the world can request these specimens for research. Twice a year, a panel evaluates proposals that people have submitted to study meteorites. For example, if someone requests 10 meteorites and we approve their request, then our job is to help them get what they need from the specimens. The point of classifying the meteorites is so that they can be available to everyone for research.
This interview has been edited for length and clarity.
Meet a SI-entist: The Smithsonian is so much more than its world-renowned exhibits and artifacts. It is a hub of scientific exploration for hundreds of researchers from around the world. Once a month, we’ll introduce you to a Smithsonian Institution scientist (or SI-entist) and the fascinating work they do behind the scenes at the National Museum of Natural History.
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