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MacArthur Fellow And Planetary Scientist Sarah Stewart Discusses How The Moon Was Formed


Sarah Stewart likes to think about what happens when planets collide. She uses two actual cannons to simulate those massive impacts. Here's one firing in her lab at UC Davis.

UNIDENTIFIED PERSON: Firing in three, two, one.


SHAPIRO: Her work earned her a spot in this year's class of MacArthur Fellows. Many of us call it the genius grant. For years, experts have thought that Earth's moon formed after a large collision knocked off a bunch of rock. Stewart told me her research suggests a different story.

SARAH STEWART: During planet formation, when two bodies collide, there's so much energy released that most of these bodies are vaporized. That means that a rocky planet like Earth is mostly rock vapor.

SHAPIRO: What is rock vapor, and what does that have to do with our moon?

STEWART: Rock vapor is taking the rocks that we stand on and heating it up to the point where it becomes a gas. And when that occurs, the Earth becomes much larger because vapor is much less dense. And it extends out into this enormous object hundreds of times larger than the Earth today. And we proposed that our moon grows within the rock vapor of the Earth after a giant impact.

SHAPIRO: So the moon actually came from the Earth.

STEWART: The moon grows within the rock vapor of the Earth. And that gives the moon the same chemistry as the Earth.

SHAPIRO: How much of a revelation was that? I mean, how different was that from what scientists used to think about where the moon came from?

STEWART: It was hard to imagine that things could be so different. And it was a mental roadblock to realize that giant impacts transformed a planet into an entirely new type of object and that could explain one of the biggest mysteries in the solar system.

SHAPIRO: Did you have to overcome some mental roadblocks of your own given what you had been taught coming up through academia?

STEWART: Absolutely. And this is when we had a bona fide eureka moment. My graduate student and I were sitting in front of a computer, looking at the calculations that we had done and realizing that all of the assumptions were actually violated by what we were looking at.

SHAPIRO: I think in any field there's a degree to which people have to be taught to trust themselves. And maybe women even more than men are taught not to trust themselves. Was it difficult for you to have confidence that, yeah, what I've demonstrated here actually is real?

STEWART: Scientists are terrified of being wrong.


STEWART: So of course I didn't want to put something out there that was really a crazy idea. And so we did have to convince ourselves first.

SHAPIRO: How long did that take you, to convince yourselves?

STEWART: Well, (laughter) from the eureka moment to the published paper was four years. So...


STEWART: ...(Laughter) I think we had a lot of internal debates amongst the co-authors and a lot of debates at conferences on the origin of the moon.

SHAPIRO: So do you expect if 10 years from now I pick up a high school textbook, it's going to have the results of your research there in the chapter on how Earth's moon was formed?

STEWART: I certainly hope so. I am very excited about this idea. And I hope that it leads to tests where we can prove whether or not the moon actually formed in the Earth's vapor.

SHAPIRO: Why are these kinds of questions relevant to ordinary people who are not scientists? Why does it matter where the moon came from, for example?

STEWART: The creation of the moon was the last major event in the creation of the Earth. And it was such a violent act. It left the Earth in a state very different from today. Understanding how Earth became the habitable planet that it is is directly linked to this final event in its creation.

SHAPIRO: So you're saying it's part of our story.

STEWART: Absolutely.

SHAPIRO: How did you get interested in planetary collisions?

STEWART: I wanted to study planet formation. And in the process, I learned that collisions were the major physical process to grow planets. And I had the opportunity to study them in the lab. And I've been doing it ever since.

SHAPIRO: Were you the kind of kid who was always laying in the backyard, staring at the stars?

STEWART: I had my nose in books imagining space travel. I very much enjoyed science fiction.

SHAPIRO: So for you, is studying planets a fallback to actually traveling through space to planets?

STEWART: I might be too terrified to actually go up in space. But...


STEWART: ...Exploration in our imagination and through the laws of physics has been exciting enough for me.

SHAPIRO: Sarah Stewart is a planetary scientist at the University of California, Davis. And today she was named to the new class of MacArthur Fellows. Congratulations, and thanks for talking with us.

STEWART: Thank you. Transcript provided by NPR, Copyright NPR.

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