Learning Earth’s Impact History With Lunar Craters

16:38 minutes

Our moon formed about 4.51 billion years ago and it’s been pummeled by meteorites ever since, leaving behind the lunar craters you can see on the surface today. Recently, scientists curious to know how often those impacts occurred came up with a clever way of determining the age of the craters. They discovered that many of them are relatively young—that is, the moon got hit by space rocks a lot more recently and a lot more frequently than scientists once thought. Sara Mazrouei, planetary scientist at the University of Toronto joins Ira to discuss the new research, out in the journal Science this week, and what it could tell us about Earth’s crater history.

Ever wonder what those lunar impacts might sound like as music? Check out this video by SYSTEM Sounds:

Plus, Dean Regasoutreach astronomer at the Cincinnati Observatory, tells skygazers how to see the January 2019 lunar eclipse.

Further Reading

Segment Guests

Sara Mazrouei

Sara Mazrouei is a planetary scientist at the University of Toronto in Toronto, Canada.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. 

If you’re planning to take in the super blood moon eclipse this weekend, you’ll get an opportunity to gaze at the lunar surface and its many, many craters. That totality is going to last over an hour. 

And perhaps as you watch, you’ll also wonder, as astronomers have, just how old are the craters there? For close to 4 and 1/2 billion years, the moon has been pummeled by meteorites. And scientists are curious to know how often those impacts occurred. Was the moon struck once every million years, every two million years– what? 

And how would you tell from far away here on earth? Well, some clever researchers devised a solution and discovered that many of the creators are relatively young. That is, the moon got hit a lot more recently and a lot more frequently than scientists once thought. They published their work in the journal Science. 

Here to tell us how they did it and what this could tell us about the earth’s impact history is Dr. Sara Mazrouei. She is planetary scientist at the University of Toronto. Dr. Mazrouei, welcome to Science Friday. 

DR. SARA MAZROUEI: Hi. Thanks for having me– excited to join you today. 

IRA FLATOW: We’re excited to have you. So what got you interested in studying the moon? 

DR. SARA MAZROUEI: In general, I’ve been interested in what I call airless bodies. I know it sounds funny– but asteroids, moons, things that don’t have surfaces because they can tell us a lot more about the history of the solar system. 

IRA FLATOW: Dr. Mazrouei, why are you interested in knowing the age of these moon craters? 

DR. SARA MAZROUEI: It’s been a mystery, right? We’re all wondering, how old are these craters on the moon? There’s this well-known late bombardment heavy period that started when the earth and the moon were forming. But in the past couple of billion years, we thought that the bombardment rate had been constant. So I’ve been interested to know how old the craters on the moon are because they can tell us a lot more about the earth, and our own history as well. 

IRA FLATOW: And that’s a tough thing to do if you’re not on the moon. 

DR. SARA MAZROUEI: Definitely. It would be a lot easier if we could have a bunch of astronauts collecting samples right now. 

IRA FLATOW: Would you like to be one of those? 

DR. SARA MAZROUEI: I would love to. Are you hiring? 

IRA FLATOW: [LAUGHS] Hey, you got my– I’ll write you a letter of recommendation right now if you’d like. So but you’re not there yet. So you came up with a really clever way of determining the age of these craters. Tell us about that. 

DR. SARA MAZROUEI: Yes. So actually my PhD supervisor, Dr. Rebecca Ghent at the University of Toronto, pioneered this new method to date young craters on the surface of the moon. And it’s a brilliant way. 

We’ve got the lunar reconnaissance orbiter, NASA’s mission orbiting the moon since 2008. And it’s got an instrument called diviner on it. And it measures temperatures on the surface of the moon. We use the nighttime temperature from the moon to try and determine how rocky different parts of the moon are. 

So really young craters, for example, once you’ve got a fresh impact, it excavates a lot of rocks. And these big chunks of rocks are going to sit on the surface. And in about a billion years, they’re going to get broken down into smaller pieces from future impacters. 

And this idea uses the fact that big rocks stay warm throughout the lunar night. It uses the concept of thermal inertia. It might sound fancy, but we’re all super familiar with it. When we go to the beach on a hot summer day– unlike today’s really cold weather– during the day, both the sand and the big rocks get warm. But as soon as the sun sets, the sand gets cold, whereas the big rocks stay warm throughout– they can stay warm for longer. 

It’s the same concept on the moon. The big rocks stay really warm throughout the night, whereas the sand and the stuff that has been turned down into sand are cold. So we use that to determine the ages. So as craters get older, they become less rocky. And we use how lucky they are to determine their age. 

IRA FLATOW: So if you see a crater with a lot of big rocks, you know it’s younger. 


IRA FLATOW: And what makes them? I mean, there’s no weathering, right? I mean, there’s no wind. There’s no rain on the moon to weather these rocks. Do they just crack apart? And how did they get smaller? 

DR. SARA MAZROUEI: So the moon is constantly being bombarded by stuff, things of different sizes. So future impacters of smaller sizes, they keep breaking down those rocks into smaller and smaller pieces. Also, the moon experiences big temperature changes. And that causes stress on the rocks and it helps it further break down. 

IRA FLATOW: What did you discover about the timeline for the crater impacts on the moon? From your paper, it shows that there was a little jump in action, right? 

DR. SARA MAZROUEI: Right. So our method is only able to date craters younger than a billion years on the surface of the moon. So we started collecting data for all of the big craters on the moon that are younger than a billion years. And once we started to look at their ages, we saw a jump at about 300 million years ago. We saw a jump by a factor of about two to three. 


DR. SARA MAZROUEI: Yeah, the rate has been almost tripled in the past 300 million years. 

IRA FLATOW: Any idea why there would be a jump like that? 

DR. SARA MAZROUEI: Yeah, so there is a few hypotheses for it. A big one is that there may have been a breakup in an asteroid family. So asteroids live in the asteroid belts between Mars and Jupiter. And sometimes two of these large asteroids or more collide into one another. And they get broken down into smaller pieces. And as they absorb heat from the sun and re-omit the heat back, they start to shift in their spot. They start to move around. And eventually they exit their orbit and they start moving towards the inner solar system, so towards the earth and the moon. 

You can think of it as a landslide starting at the top of the mountain. And think of the earth and the moon as a house in the valley. So we see the footprints of those broken pieces of asteroids as craters on the earth and the moon. 

IRA FLATOW: So something happened that, hundreds of millions years ago, all of a sudden you had more bombardment. 

DR. SARA MAZROUEI: Yeah, so an asteroid family broke up and they started to move towards the inner solar system. And that caused more of a bombardment. 

IRA FLATOW: You let some people use your data, I understand, to create a piece of music that represents these impacts on the moon using sound. Give us an explanation of what you did. 

DR. SARA MAZROUEI: Yeah, so System Sounds was kind enough to take our lunar data and turn it into a video and also into sound. So they’ve turned the last one billion years, the history of the impact of the moon, into sound, where every note represents the size of an impact crater. And you can hear the frequency of the bombardment. And they’ve been doing more turning into astronomy and space data into music so that they can make science more accessible, especially for those that are visually impaired. 

IRA FLATOW: Well, let’s listen to that clip now. 



IRA FLATOW: Sounds like a moon, a lunar wind chime. 

DR. SARA MAZROUEI: It sounds beautiful, except it gets a little bit scary towards the end. 

IRA FLATOW: Let’s play it again for people are just– one more time, let’s hear. This is the sound of craters on the moon. 


No, not scary, beautiful. 


IRA FLATOW: And you said this can tell us something about the earth’s crater impact history too. 

DR. SARA MAZROUEI: Right. So the moon is the closest neighbor to the earth. So both of them must have experienced the same bombardment history because anything that would hit the moon would also hit the earth, even more because Earth is larger and has a bigger gravity. 

But until now, we thought that a lot of, even the large craters on Earth– we’re talking about craters larger than 20 kilometers. We thought that they would have experienced a lot of erosion and that we wouldn’t have had a complete record on Earth. But looking back at the earth crater history, on stable terrains where we have large craters, we actually see a similar signature between the two. So looking at craters on Earth we see the same increase in the past 300 million years. So we also on Earth have been getting bombarded by more stuff in the recent years. 

IRA FLATOW: But a lot of the craters on Earth have eroded, right? Because we have all that weather going on. 

DR. SARA MAZROUEI: Exactly. So we have a lot of– we have wind. We have an atmosphere, a lot of erosion that would have affected smaller sized craters. But when we talk about 20 kilometer craters, erosion wouldn’t have played an effect on stable terrains. So the areas that we’ve looked at, we’ve obviously excluded oceans, or the Amazon basins, places where we would have experienced a lot of erosion. We’ve taken places like the Canadian Shield, where we know those rocks have been stable for millions of years. 

IRA FLATOW: We have a caller who asks, “Since the moon is tidily locked with the earth, can we learn anything about the difference between impacts on the far side and the near side?” 

DR. SARA MAZROUEI: That’s a really good question. There are some beliefs that maybe the near side and the far side, or actually the leading and trailing edges of the moon, so the heading side and the trailing side of it, would have experienced different amounts of bombardment. But looking at our data, looking at really young large craters on the moon, we don’t see a difference between the two. So the bombardment has been very similar on both sides. 

IRA FLATOW: Well, what does the other side of the moon look like? I know there’s a Chinese Rover over there now, sending back photos. Does the other side look a lot different than the side closest to us? 

DR. SARA MAZROUEI: It does look slightly different. So the lunar reconnaissance orbiter that’s been orbiting the moon has been collecting data since 2008. And we can see both sides of the moon. We can see both sides pretty well. The near side of the moon, the side that faces us, we see more of darker patches, what we call maria, which means seas in Latin. So those are darker features. But the darks– the far side of the moon doesn’t have those dark features as much, and just looks more bombarded. 

IRA FLATOW: I see you caught yourself when you were going to say dark side. Because it’s not really dark, is it? 

DR. SARA MAZROUEI: No, definitely not the dark side, definitely not, no. It’s just the far side, the other side. 

IRA FLATOW: Because it’s facing the sun. It just doesn’t face our side. 


IRA FLATOW: Can the moon shield us, shield the earth from any possible impacts from outer space? 

DR. SARA MAZROUEI: Not really. No, the moon is too small. So the moon’s cross section is less than 113th of the earth. And it’s about 60 earth radii away from us. So it would be like wanting to protect yourself with a bulletproof vest that was created for an action figure. So it would be too tiny. 

IRA FLATOW: You know, we talk a lot about possibly going back to the moon. And I know you said you would like to, but don’t send your letters now to NASA because NASA’s shut down. 

DR. SARA MAZROUEI: Ugh, I know. 

IRA FLATOW: I want to ask you a question, but first remind everybody that this is Science Friday from WNYC Studios. Yeah, NASA is shut down, right? There’s no one there to take a phone call. If we were to go to the moon, what would be the best place you would like to go to, in terms of understanding the age and being a rock hound on the moon? 

DR. SARA MAZROUEI: So on the far side of the moon, there’s the oldest terrain and there’s a big impact basin. It’s 2,000 kilometers in diameter. It’s called the South Pole Aitken basin. And because it’s so large, it’s thought that it could have brought up material from the lunar mantle. So a lot of the scientists are keen to get samples from the South Pole Aitken basin. And I would put all of my eggs in that basket if we were going to send one mission to bring samples back. 

IRA FLATOW: Are you excited about the moon eclipse, the lunar eclipse happening this weekend? 

DR. SARA MAZROUEI: Absolutely. It’s very cold here, but I’m going to bundle up and try to watch at least a few minutes of it. 

IRA FLATOW: And the reasoning– the totality lasts for like over an hour, doesn’t it? 

DR. SARA MAZROUEI: It does, yeah. 

IRA FLATOW: And that’s because why? 

DR. SARA MAZROUEI: Well, the totality lasts about an hour. But it’s also a super moon this time around. It means that the moon is at its closest orbit around the earth. So we would get to experience it for a little bit longer. But we’re also experiencing a total lunar eclipse so that the entire shadow of the earth would be covering the moon. So it would give us a lot of time to go out and check it out. 

IRA FLATOW: I guess the closer we are to the moon, the closer it is to us, the larger the shadow size. 


IRA FLATOW: It’s a pleasure talking to you. 

DR. SARA MAZROUEI: Same. Thanks for having me. It’s been great. 

IRA FLATOW: And I’m wishing you good luck on watching the eclipse. 

DR. SARA MAZROUEI: Thank you. You too. Bundle up. 

IRA FLATOW: You betcha. Dr. Sara Mazrouei is planetary scientist at the University of Toronto. And speaking of which, one last thing, speaking of the moon and this weekend’s total lunar eclipse, what’s the best way to see it? Well, of course we asked Dean Regas of the Cincinnati Observatory to give us a sneak preview. 

DEAN REGAS: The sun, moon, and earth will be lining up this weekend. We’re going to be having a total lunar eclipse. But the big question is, what day is it? A lot of media outlets are saying it’s going to be on January 21. Well, not so if you’re in the Western hemisphere. For the United States, North America, South America, it’s going to be the night of Sunday, January 20. So mark your calendars. Don’t go out on the 21st, you’ll have missed it. 

I want to share a couple tips on what to look for. I have a couple favorite things to look for and favorite times. So here’s the schedule. This is what’s going to happen. First contact, the first time you’re going to see the shadow of the earth on the moon, will be at 10:33 PM Eastern Standard time. So adjust your time accordingly. I always like this part because that’s when the eclipse starts. And you’re like, wow, it’s right on time. I love the precision of it and starting to see the shadow. 

And then the shadow will slowly sweep across the moon. And totality begins at 11:41 PM and goes till 12:43 AM. That’s when the moon is totally in the Earth’s shadow and turns all sorts of cool colors like orange, and red, and all sorts of bloody colors. And so you can check it out and see how the light changes and how the colors change from minute to minute. It’s very cool. 

And then the shadow wipes away and the moon returns back to its normal self at 150 AM on the morning of the 21st. But remember, you’ve got to start watching on the 20th. So Sunday night, get ready for this, because the next total lunar eclipse won’t happen until 2021. 

IRA FLATOW: One last thing, speaking of the moon this weekend, that eclipse that Dean Regas of Cincinnati Observatory is telling us about, it’s also a good opportunity, as we heard, as Dr. Mazrouei said, you’ve got to bundle up to go out and see it, but you can also use that opportunity to go see the other stars and the planets since the moon will be very dark. You’ll be able to see the other planets and stars coming out. So that’s really a good opportunity to do some winter watching in the cold weather. Hopefully storms coming across the country will not upset. Every time I want to go out and see an eclipse, there’s always a cloud there or something that’s getting in the way. 

Speaking of going out, just a quick note to let you know that I’m going to be going out. I’m heading off for a few weeks to Southeast Asia, going to see Angkor Wat and a lot of other sites in Cambodia, Vietnam, Thailand. And John Dankosky, who does such an able job filling in for me when I’m away, he’s going to be here filling in for me. And I’m going to be trying to send back a few dispatches from the road. So maybe catch up on what I’ve seen and share with you the stuff that’s going on. So take good care of John while I’m gone. He’s such a valuable asset to us all. I’ll see you soon.

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