The journey from space to Earth is not easy for most meteorites. But plans after arrival are more comfortable for the lucky ones picked up by Curry Corrigan, planetary geologist at Smithsonian National Museum of Natural History.
In this month’s “Meet a SI-entist,” Corrigan talks about his work collecting meteorites in Antarctica, the scientific value of these specimens, and what happens after they arrive at the museum. National collection of meteorites.
You are a research geologist studying Antarctic meteorites. What led you down this path?
As an undergrad, I took a course in astronomy which led me to take a course in geology. My teacher in this class told me about this field called planetary geology. So, I declared geology as a major and it turns out that my advisor was the only person in this university who was interested in planetary geology. He helped me do some independent study, which led to an internship 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 someone going to Antarctica to collect meteorites.
During this summer, I also met Tim McCoy, the current curator in charge of meteorites at the NMNH, who was then a post-doctoral researcher. Everyone 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” things. When I graduated, I never thought I was going to get a job studying meteorites and going to Antarctica.
Why are you going to Antarctica for the meteorites? Do they not also fall elsewhere?
Meteorites are falling all over Earth. Of course, 70% of them fall somewhere in the water, because 70% of our planet is an ocean. We thus lose a heartbreaking number of specimens. But meteorites are easier to find in Antarctica, due to the environmental conditions.
Sometimes you are on ice where there are no other rocks except meteorites. This is because the structure of Antarctica resembles a large dome with the South Pole roughly in the middle. Gravity causes the ice to flow towards the edges of the continent, and the Transantarctic Mountain Range runs through the middle of the continent. In some places you are above these mountains and the ice is so thick that any rocks you see must be 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 descends towards the coasts and wedges against the Transantarctic Mountains. Dry winds and sublimation remove the ice, leaving meteorites stranded on the surface. We call these areas stranding surfaces and we don’t fully understand why meteorites concentrate there. It’s not like a meteorite came in and shattered. These are all kinds of meteorites.
This second type of meteor collection place — these grounding surfaces — can also have terrestrial rocks. How do you tell the difference between these and meteorites?
Grounding surfaces are found on glaciers. The movement of the glaciers has scraped the rocks on the sides of these mountains, so there are definitely places where you have land rocks. But the difference can be stark.
There is something called a fusion crust that forms on meteorites as they pass through the Earth’s atmosphere. They go so fast that the friction melts the outside of the rock, which ends up with a layer of glassy crust. It’s pretty easy to spot on meteorites. Also, your eye gets used to looking for differences. Spend a day staring into a giant field of rocks on the ice and you can spot meteorites very quickly too.
What types of meteorites are there?
Most of them are ordinary chondrites. The reason they are called chondrites is that they contain small objects called chondria. Each chondrule was a molten droplet in the solar system over 4.5 billion years ago and these came together to form asteroids. About 98% of all meteorites are chondrites of one kind or another. There are also some that contain a little more carbon, called carbonaceous chondrites.
There are also a few from the Moon and Mars. We know that lunar meteorites come from the Moon because we can compare them to rocks recovered during NASA’s Apollo missions. We know that Martians come from Mars because of NASA’s Viking Landers that went to Mars in the 1970s. The Viking Landers measured the composition of Mars’ atmosphere which also ends up in the form of gases trapped in the molten glass from these meteorites.
Most meteorites are the size of a golf ball or a fist. Most of the time they separated during their journey through the atmosphere. Sometimes we find pieces on the ice next to each other that can be put together like the pieces of a jigsaw puzzle.
It’s a physical puzzle, but what scientific puzzles can meteorite research in the national collection help us solve?
Every meteorite collected by the US Antarctic Meteorite Program comes to the museum and our job is to determine what type of meteorite it is. All of them can provide a piece of the larger puzzle to help us understand how the solar system formed. They can tell us how asteroids and planets came together. In meteorites there are often molten 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 destroyed. Examining them is one of the ways we can 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 must have formed under extreme conditions that may come from a nearby supernova star. Carbonaceous chondrites are among the meteorites in which these grains have been found.
Every spring and fall we publish a newsletter that contains all the new meteorites that we have obtained and classified. In 2019, we classified over 400 meteorites. In a pandemic year, there might only be 50 new meteorites.
Anyone in the world can request these specimens for research. Twice a year, a panel evaluates the proposals people have submitted to study meteorites. For example, if someone requests 10 meteorites and we approve their request, our job is to help them get what they need from the specimens. The point of classifying meteorites is to make them available to everyone for research.
This interview has been edited for length and clarity.
Meet a SI-entist: The Smithsonian is 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 he’s doing behind the scenes at the National Museum of Natural History.
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