A Glancing Blow: How the Earth Got Its Moon

9:28 minutes

Crater Daedalus on the moon. Credit: NASA
Crater Daedalus on the moon. Credit: NASA

Looking up into the night sky, one can determine exactly where and when the moon will appear—the moon and the Earth are locked in a predictable pattern. Scientists still debate how this lock-step pattern evolved.

One longstanding hypothesis, called the Giant Impact Theory, says that the moon formed in a collision between the Earth and another planetary body. The theory explains certain aspects of the system, but it doesn’t explain why the moon orbits the Earth at an angle. Planetary scientist Sarah Stewart and her colleagues wanted to address this detail. Reporting in the journal Nature, the team says that at the time of impact, the Earth may have been tilted—sending the moon off into its current orbit.

Segment Guests

Sarah Stewart Johnson

Sarah Stewart Johnson is an associate professor at Georgetown University, a visiting scientist at NASA’s Goddard Space Flight Center, and the author of The Sirens of Mars: Searching for Life on Another World.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. On Monday, the moon is having, as they say in Hollywood, its close up. So close that the moon will be 14% brighter than usual. And not only will the moon appear bigger and brighter, the steadfast satellite will be the closest it’s ever been to Earth in nearly 80 years.

It’s widely believed that the moon was created when a Mars-sized body smashed into the earth billions of years ago and ripped away enough material to form it. But something about how the earth and the moon are locked in their orbital dance suggest that there’s something missing from this theory. And now a team of researchers thinks it has an answer published now in the journal Nature. Sarah Stewart is one of the researchers who’s also a professor of planetary sciences at the UC Davis. Welcome to Science Friday, Dr. Stewart.


IRA FLATOW: The giant impact theory, the idea that the moon came from something colliding with the earth– it’s been around for a while. What doesn’t that explain?

SARAH STEWART: The giant impact theory tries to explain the large size of our moon compared to the earth and the chemical similarity between the earth and the moon. And the key feature of the original theory was that it also set the current length of day on the earth. But as scientists looked into the theory, the details of the chemistry of the moon and the orbit of the moon didn’t match the original model of a Mars-sized object hitting the earth.

IRA FLATOW: So instead, how has your idea updated what we should be thinking?

SARAH STEWART: So we turned the giant impact theory on its head a little bit and changed all of the details about the impact. Instead of having a Mars-sized body, we ask for the impact to deliver a lot of energy, enough energy to mostly vaporize the earth. In addition, we want the impact to give the earth more spin so that it would spin about twice as quickly just after the moon formed compared to the original theory. And then the new kicker in this paper is that we start with the earth tilted way over so that its tilt from the orbit of all the planets is somewhere between 60 and 80 degrees instead of the 23 degrees that we see today.

IRA FLATOW: So the impact smacked the earth and put it back up to where it is today from basically being sideways to a little more horizontal?

SARAH STEWART: We’re proposing that the impact put the earth sideways and the paper explains how to get it back to where it is today.

IRA FLATOW: How do you model this to come out the way it is in your paper?

SARAH STEWART: So Matija Cuk, the lead author, wrote a computer code to model the details of the orbit of the moon with time. And when you start with the earth tilted over and the moon forming in the earth’s equator, normal tides like what we have today move the moon further and further away from the earth. And at about 20 times earth radius from the earth, the moon starts to feel the sun’s gravity pull more than the earth’s gravity pull. And in that transition, there’s a resonance between the earth, moon, and the sun that tilts the earth back up but leaves the moon’s orbit tilted more than 30 degrees from the ecliptic plane.

But that’s not where we see the moon today.


SARAH STEWART: So the moon today is tilted only five degrees from the plane. But in the standard model, the moon should have no tilt compared to the orbit of all the planets. And the tilt of five degrees was a big puzzle in the original theory. So in our new model after the first big transition where the moon is left at a 30-degree tilt or more, there’s a second transition at about 30 earth radii, about half the distance that the moon is from the earth today. And in that transition, the orientation of the moon changes and the tilt of the orbit is lower to be consistent with where we are today.

IRA FLATOW: How much of a change in the cataclysmic first impact do you have to make for this to happen?

SARAH STEWART: We basically changed everything. The only thing that’s the same are the words “giant impact.” And we’ve made it 10 times more energetic, twice as much angular momentum, which gives us the length of day, and we’ve tilted the planet way over compared to the original model.

IRA FLATOW: So do you have to look for new evidence to back up the theory, or are you saying the evidence we have is more in line with what we think now?

SARAH STEWART: So we would say that this model is the first model that explains these key features about our system. And there are other competing models out there but none of the pieces all hang together to be consistent with all of the data that we have. That’s not a proof that our model is correct. It means that we think we’re going in the right direction and we are looking for tests in the model. And one of the tests is to look at the thermal history of the earth and the moon because these events that changed the whole orientation of the system deposit heat in the earth and the moon. And that’s something we can look for in the rock record.

IRA FLATOW: When the earth and the moon were really, really close after the collision, how much heat does that leave over in the earth? Just the tidal forces would also heat the earth up, would they not?

SARAH STEWART: Yes. Earth’s early history was very interesting. It wasn’t hospitable at all. About 10% of the rock of the earth was actually rock vapor and it extended all the way out to where we think the moon formed. So the earth itself was several times bigger. And if you could see it just after the impact, it would be this huge ball of rock gas glowing like lava glows. And the moon would be forming from this vapor. And if it’s tilted over, the seasons would be completely different and that early tidal evolution would have kept the earth hot, and it would have taken longer to cool.

IRA FLATOW: Hot enough to boil the oceans?

SARAH STEWART: There were no oceans. There were no oceans, so the rock and the water would all be gas mixed into a big, thick atmosphere around the earth.

IRA FLATOW: Wow. Now, I understand that after you figure out all the math for all of this stuff that you have, you also have a lab where you can physically test out these ideas by shooting particles at each other, dust particles?

SARAH STEWART: Yes. I have what’s called a gas-gun lab. And if you imagine a couple of cannons, we launch small bullets at planetary materials and study what happens when they are subjected to the high pressures and temperatures during planet formation. And in our lab, we can reach impact velocities of about eight kilometers per second. And that’s how fast the space shuttle had to go to get into orbit.

IRA FLATOW: Wow. So you can reproduce these impacts.

SARAH STEWART: We can now in laboratory facilities, in my lab and others, reach all of the pressure and temperature conditions during planet formation.

IRA FLATOW: All right. So let me ask you then about– because I am very fascinated. We’re going to have this big moon coming up. What’s your favorite part of that story, the big moon story?

SARAH STEWART: Well, I like to run and I love seeing the moon up in the sky. And it is so much bigger when it’s at perigee. You mentioned that it was 14% bigger in diameter. That’s 30% bigger in area. So if you’ve been glancing up at the moon when you’ve been out and about, it really is bigger than it normally is. And it makes a beautiful view at sunset with this coming up super moon on Monday.

IRA FLATOW: How come we only have one moon?

SARAH STEWART: Ahh. Remember when I said the moon was unusually large compared to other moons around other planets. Our moon tends to be a bit of a bully. There could have been other moons early in Earth’s history, but the dynamical interactions between our moon and the earth would have led to their demise.

IRA FLATOW: Let me end our conversation by getting back to the big moon coming up on Tuesday. What’s the best way to see it?

SARAH STEWART: So at sunset in most places you’ll be able to see that it’s really big. But it is at its fullest just before dawn on the west coast. And so you look early morning or just before you go to bed Sunday night.

IRA FLATOW: Is it going to be a full moon?

SARAH STEWART: The super moon is the coincidence of a full moon and the moon’s closest approach in its orbit. Its orbit is not a perfect circle but a little bit of an ellipse. And so when the full moon is aligned with its closest approach, it can appear 30% larger than at other points in its orbit

IRA FLATOW: Cannot wait. Thank you for–

SARAH STEWART: It’ll be gorgeous.

IRA FLATOW: Yeah. That’s great. I always love talking about the moon. And thank you for your research and good luck.


IRA FLATOW: Sarah Stewart, professor of planetary sciences at the University of California at Davis.

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