Date: July 28, 2010
Title: The Origins of the Moon
Podcaster: The NASA Lunar Science Institute NLSI
Description: How did the moon form and what can it tell us about the history of our solar system? Lunar scientist Bill Bottke from the Southwest Research Institute, who has been studying the formation history of planetary bodies, discusses the different theories there have been over the years for the moon’s formation, and how our current studies are revealing some secrets about the Moon – kind of like snooping through your Grandmother’s attic.
Bio: NLSI brings together leading lunar scientists from around the world to further NASA lunar science and exploration.
William F. (Bill) Bottke is a planetary scientist specializing in asteroids. He works at the Southwest Research Institute in Boulder, Colorado. Bottke received his undergraduate degrees, in physics and astrophysics, at the University of Minnesota in 1988. In 1995, he received his PhD in planetary science from the University of Arizona for research on asteroid dynamics.
Bottke’s research focuses on modeling various properties in asteroid dynamics. He has published extensively on the importance of tidal disruption and the Yarkovsky and YORP effects on the physical structure and orbits of asteroids, and the early solar system, particularly the Late Heavy Bombardment.
Nancy Atkinson is a science journalist and is the Senior Editor for Universe Today
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Voice: You are listening to the NASA Lunar Science Institute podcast which highlights the latest news information of the Moon, on the Moon and from the Moon. It is produced from the NASA Lunar Science Institute at the Ames Research Center in Moffett Field, California.
Nancy Atkinson: Where did the Moon come from? Hi this is Nancy Atkinson for the NASA Lunar Science Institute. Although its appearance changes from night to night, the Moon has been a constant companion to us and our planet. But how did the Moon form and what can it tell us about the history of our solar system? At this year’s annual Lunar Forum, hosted by NLSI at NASA’s Ames Research Center, I had the chance to talk with scientist Dr. Bill Bottke from the Southwest Research Institute, who has been studying the formation history of planetary bodies. We talked about the different theories there have been over the years for the moon’s formation, and how our current studies are revealing some secrets about the Moon – kind of like snooping through your Grandmother’s attic.
Bottke: There used to be several models of how the Moon formed. These were very popular back in the Apollo days. If you go back to the old encyclopedias about how the Moon formed one idea, was called rotational fission. What that really means is a complicated way of saying that they thought the Earth was spinning so fast in the past that it eventually lost a big glob from around the equator and that went on the make the Moon. And there are some advantages to that idea. The problem is modeling this over time suggests there’s just not any way to get enough stuff off the Earth to make the Moon in that fashion. So ultimately that idea has fallen by the wayside.
The second idea is that the moon was captured. What would happen there is that very early on in the solar system’s history, the same time you were making the Earth, there were lots of things flying around and maybe you could capture something into that would ultimately end up in orbit around the Earth. But we found that would be very hard to do because you would have to take a big body like the Moon and somehow you would have to dump enough energy to get it into a stable orbit around the Earth and ultimately evolve into the orbit that we see today. It turns out that is pretty tough to do.
So the third idea is that maybe the Moon just formed in place. So, essentially, just the same way the Earth formed, the Moon formed very close to it and just formed from a disk surrounding the Earth. And this isn’t a crazy idea because we think the satellites around Jupiter and Saturn, many of them probably formed in this same way. But the disadvantage to this is that making an object in that fashion doesn’t really fit some of the parameters that we know about the Earth. Essentially, we think that the Earth and the Moon formed together the Moon was very close to the Earth and the Earth was spinning very quickly. And that communicates to something we call angular momentum. And the angular momentum of the Earth-Moon system is constant, and so if they both formed in place we have a difficult time making the angular momentum budget we have.
So all those models have certain advantages and disadvantages but none of them really work. So back in the 80’s at a conference, a large number of people thinking about this for awhile ultimately decided on a hybrid of a number of these things and they suggested that maybe what happened is that a giant impact took place on the Earth. So this would be an object about half the size of Earth, or so, slamming into the Earth and creating a big disk of debris and that debris would ultimately end up re-forming into the Moon that we have today.
This has a lot of advantages. First of all it can explain the angular momentum budget of the Earth, because you’re hitting the Earth and causing it to spin rapidly, which is what we think takes place. Most of the projectile that hit the Earth – if it had any things like iron and otherthings would go into the core of the Earth, and so the Moon that forms would actually be metal free for the most part, and have a very small core and that’s consistant with what we see. The Moon would melt – we think there’s evidence that the Moon has undergone what we call a magma oceans and it has completely melted to some degree. And there are other parameters that would fit this very well, as people have examined this model more closely. And in some ways it takes on the best attributes of all many of the models I described before. So this has become the dominant model, and it has become such a dominant model that no one really questions it any more.
Nancy: So why is it important that we know about the moon’s origins?
Bottke: Well, there are several different reasons why it is important. First of all, the Earth is our home and somehow it’s evolved in a way that has allowed life to exist on it. But the very, very early history of the Earth is almost lost to us because of all the things that have happened. We have erosion, tectonics, all these things that are very good at erasing history. It’s sort of like wondering about your ancestors and you’ve lost all the record from Ellis Island and you’d like to know where they come from but you don’t have any evidence. And to some degree that’s what’s happened on Earth. But fortunately, the Moon acts like our attic, sort of like finding your Grandma has notes where she came from on Ellis Island up in her attic somewhere. So, the Moon is our attic. It tells you about primordial Earth and it tells you all the things that have happened in the solar system, essentially, to the Earth, because the Moon is our neighbors. So if something hits the Earth, if there’s a population so things that hit the Earth, the Moon gets some of that as well. So the Moon tells us about the primordial days of the Earth and how the Earth evolved; what it initially was like. If we understand the Moon we understand ourselves.
The second thing is that our Earth and Moon weren’t just isolated bodies, they are part of a whole solar system wide process that ultimately caused our solar system to come into being. So understanding the Earth-Moon system you can learn a lot about how planets formed. That’s important if we want to understand ourselves and the nature of the solar system but also what is going on in other solar systems, as well. If you want to understand life in other solar systems first you have to start here, and you can learn a lot of understanding the Earth and Moon.
Nancy: What are some of the missions either current or upcoming that will help us to determine definitively – and do you think we’ll ever definitively determine how the Moon formed.
Bottke: Well, there are number of interesting missions to the Moon. One is actually in place right now. It’s called the Lunar Reconnaissance Orbiter. It is taking fantastic images of the Moon, its probing the nature of the Moon in different ways. It was originally planned to be a mission to help tell us where astronauts should go on the Moon, and it still may be used for that, but now its being passed over to the science side, so now we’re going to get a chance to do several years of science with this mission. There’s another mission upcoming which is called GRAIL, and what GRAIL is going to do is very precisely probe the gravity environment on the Moon. That sounds boring, what is so important about gravity? Well, if you can get that information precisely enough it actually allows you to probe the interior of the Moon which is something we need to understand. There’s a lot we can learn by understanding the interior of the Moon and how it formed, and ultimately again, this relates back to Earth.
Some other missions that are important for the Moon, which you might not think so at first thought, we have missions going to asteroids. Asteroids are the essentially the first cousins of the Moon. Asteroids are the left over building blocks of what formed the planets. So if you want to see the baby pictures of for the Earth and Moon, you go to the asteroid belt. The Dawn mission is going to be reaching a very interesting asteroid next year called Vesta. Vesta is an asteroid which we think is completely melted, so in many ways it is like our Moon. But it has important differences. So we can learn things by going to this asteroid and comparing and contrasting it to the Moon, and ultimately learning how that relates to the Earth. So we can learn from issues in the asteroid belt also because the asteroid belt is a big source of impactors that end up hitting the Earth and Moon. And also there are things that have happened to very early on in solar system history in terms of the building blocks of planet formation ultimately bombarding the Earth and bombarding the Moon. A lot of that we can learn by going to the asteroid belt and seeing what happened out there. It’s one big complex family that makes up the inner solar system and all the bodies are related. It’s like going to a big family like the Brady Bunch, and you talk to Cindy. Cindy can tell you a lot about what her Mom and Dad are like. So by going to the asteroid belt we are learning a lot about Mom and Dad.
Nancy: Ok, great! Thanks a lot Bill!
Voice: To find out more about this topic, visit our website at www.lunarscience.nasa.gov. Any opinions expressed are the individuals and do not necessarily reflect the opinion of NASA or the NASA Lunar Science Institute. This podcast is produced for educational purposes only. On behalf of the NASA Lunar Science Institute, thanks for listening.
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365 Days of Astronomy
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