Unraveling the Mysteries of Planet Formation with Harvard Geophysicist Rebecca Fischer


Rebecca Fischer, Jennifer Berglund


Jennifer Berglund 00:04

Welcome to HMSC Connects!, where we go behind the scenes of for Harvard museums to explore the connections between us, our big, beautiful world, and even what lies beyond. My name is Jennifer Berglund, part of the exhibits team here at the Harvard Museums of Science and Culture. And I'll be your host. Today, I'm speaking with Rebecca Fischer, a geophysicist and assistant professor of Earth and Planetary Sciences at Harvard, who studies the formation and deep interiors of Earth and other planets. I want to talk to her about that work and the reason why she's excited about the landing of Perseverance, the new Mars Rover, which happens on February 18. Here she is. Rebecca Fischer, thank you so much for being here. And welcome.


Rebecca Fischer 01:06

Thanks so much for having me.


Jennifer Berglund 01:15

We actually worked together on an exhibit, and it was an exhibit in the Museum of Natural History, the Harvard Museum of Natural History called Cosmic Origins, where we covered your work researching the formation of the moon. Can you tell me about that work and the theories about the moon's formation that you're modeling?


Rebecca Fischer 01:34

Sure, it's pretty well accepted in the scientific community that the Earth's moon formed as a result of a giant impact on the Earth toward the end of the Earth's formation. But the exact details of how that happened are still being worked out. And there's still a fair bit of controversy about it. So in the sort of traditional model, the Earth was about 90% of its current size, and it was struck by a Mars-sized body, and planetary scientists really like to name things, so this sort of hypothetical Mars-sized body that hit the Earth has been named Thea. And in this sort of traditional model, the moon was formed mostly from Thea, not from the Earth. And this is potentially problematic because it's been discovered, in the years after this model was proposed, that the Earth and the Moon look identical or nearly identical in terms of their isotopes for a whole bunch of different elements. They're made up of different elements like silicon, and oxygen, and magnesium, and each of those elements comes in different flavors, which we call isotopes. And they look more alike than any two bodies that we know of in the solar system, the Earth would have had a rocky surface, possibly it would have been molten at the time, which we refer to as a magma ocean if the surface of the planet is melted, and then this other body the size of Mars came in and just slammed into the Earth, and it would have been such a large and violent collision, that it would have melted a huge fraction of both the Earth and Thea and even vaporized a lot of the rock. All this vaporized rock would have been sort of caught orbiting the Earth, and then as it cooled, it would have condensed out into rock again, or magma first, and then that rocky material eventually would all sort of run into each other and coalesce to form the moon. In the traditional model where it's a Mars-sized body, you don't get that much mixing, even though it's such a big, violent collision, and it's like vaporizing the surface of the Earth, you still end up making a moon that's like 90% Thea. Basically, that there are different ideas people have to try and sort of get out of this problem, like the Earth and Thea being really similar or like a different impact that causes more mixing, and any of these could work, but the problem is that we don't know how likely they are. Like, it could work in theory, but we don't know if it actually could have happened, and so that's where my work comes in. We were specifically looking at this first possibility that the Earth and Thea just coincidentally looked really similar, and we had this sort of canonical impact with a Mars-sized Thea, and the moon is made mostly of Thea but it looks like the Earth because the Earth and Thea were so similar. So, basically what we did was we started with computer simulations of the formation of our solar system, and we looked for Earth and Thea-like bodies that formed in these simulations, and we tracked their growth, and we have a sort of geochemical model that we use to calculate their tungsten isotopes in particular. The element tungsten is one case where the Earth and the Moon are very, very similar. And so we track the growth of the Earth and Thea, and then at the end, we make a Moon out of Thea or a mixture of Thea and the Earth, and we look to see, what did the tungsten isotopes look like in the Moon versus the Earth at the end. And we do this a whole bunch of times for, like, slightly different conditions, and we look to see, like, what is the probability of making an Earth and a Moon that look identical in their tungsten isotopes. And what we found is that it's a pretty low probability. So if you're looking just at tungsten, it's like a few percent chance of a match, but if you factor in other elements too, it becomes much, much lower, way less than 1%.


Jennifer Berglund 06:17

How do you even go about doing that? What does the data look like that you input? And how do you model something from that?


Rebecca Fischer 06:25

We basically have two completely different models that we're using, one that is mostly handling the physics, and one that is mostly handling the chemistry.


Jennifer Berglund 06:36

Oh, interesting.


Rebecca Fischer 06:37

So the physics comes first. So, that's a model of the formation of the solar system. It's called an N body simulation. So, basically, we start with a few 1000 small bodies orbiting the Sun, and we just let them orbit the Sun and interact with each other through gravity, and anytime two of them collide, they, like, merge and form a larger body. And so we run these simulations, and we track them for a couple 100 million years, and it forms sort of the planets in the inner solar system.


Jennifer Berglund 07:16

So in a computer model, how long would it take to go through a few 100 million years? I know it's a super high powered computer, so you can do that in less than a few 100 million years,


Rebecca Fischer 07:27

When I ran the suite of simulations, which was, like, back in 2013, each one took a few months on a supercomputer.


Jennifer Berglund 07:39



Rebecca Fischer 07:40

I think there are new techniques now that speed it up considerably, but it's a pretty intensive calculation,


Jennifer Berglund 07:48

A few 100 million years squeezed into a few months, that's not so bad. And so this is for the physical model. And then, so what about the chemical model?


Rebecca Fischer 07:58

So once we finish running that model, we sort of look at the planets that formed and pick out the ones that look like the Earth. So then that gives us this whole, like, history of the Earth, right? It formed out of these bodies that started out in these positions in the solar system and collided at these times, and so then we have this chemical model on top of that where basically every time something runs into the Earth, there's some chemical reactions that take place between the metallic cores and the rocky mantles that changes the composition of the core and the mantle, and changes the tungsten isotopes. So we sort of step through the evolution of the Earth and calculate how its chemistry would have evolved as it was growing. I would say that it's probably more likely that either the impact happened slightly differently than we think it did, or that there was this, sort of, post-impact mixing that erased the differences. I think those are probably more likely, although we don't actually have good numbers for the probabilities of those things yet. There's a few different models for how it could have happened, and one of them the Earth is spinning really, really fast at the time of the impact, and that sort of changes what happens. There's another one where the earth and they are the same size, they're each half the size of the Earth today, and that causes a lot more mixing. There's one model where it's not just one impact, it's actually several, and then that causes more mixing.


Jennifer Berglund 09:44

What do you think happened?


Rebecca Fischer 09:46

I think one of those slightly different impacts is probably more likely, but it's sort of a problem we encounter a lot in planetary science, which is that we've shown that it's a very low probability event for the Earth and Moon to come out with the same tungsten isotopes in this canonical model, but it's not zero. It's just very low probability, and so there's always this sort of philosophical question. It's like, at what point do you say, like, that's too low of a probability. It must not have happened that way. It's hard to know because we only have the one solar system to look at, and since it only happened once, we don't know that it was the most likely outcome.


Jennifer Berglund 10:34

That's interesting you bring that up, because we've talked about this before about how the significance of studying Planet Earth in studying the rest of the planets in our solar system, right? We have the most information about Earth because we live on it, we can gather as much data as we need to without having to build a spaceship. So, probably the most significant part of your work is studying how the Earth itself formed. Can you explain what you do in terms of studying Earth's formation, and then how you use that to study other planetary bodies in our solar system?


Rebecca Fischer 11:11

Most of my work on the Earth's formation has to do with understanding how its formation influenced the chemistry of the metallic core and the rocky mantle because their initial compositions were set by chemical reactions deep inside the Earth as it was forming, and so we look at this in two different ways. We do experiments in my laboratory where we take mixtures of rock and iron metal that are sort of analogues for the mantle and core, and we subject them to really high pressures and temperatures like you find deep inside the Earth as it's forming, and we look to see what kinds of chemical reactions happen between them. And then, the other part of the research is all computer simulations, where we're sort of tracking the growth of the Earth, and using the information that we get from our experiments to calculate how the composition of the Earth's core and mantle would have changed as it grew. And then in all of this, we know something about what the Earth's mantle is made of today, so we can run our computer models for slightly different conditions of Earth's formation, and look to see which ones do the best job of reproducing and a mantle that we see today. And that lets us sort of back out some kind of information about how the Earth formed.


Jennifer Berglund 12:46

Why is it important to understand how the Earth formed?


Rebecca Fischer 12:53

Partly, it's important because it tells us about what the Earth is made of, and what's going on deep inside the Earth today. One example of this that I work on a lot in my research is trying to understand the composition of the Earth's core. So we know it's mostly made of iron with a little bit of nickel, but it also has 10% of something else in it that we don't know what it is, and we can make predictions of what that should be by this combination of experiments and computer modeling of the Earth's formation. That's important because it tells us these sort of impurities in the core are what controls the temperature in the core, and they also play a really important role in the generation of the magnetic fields in the Earth.


Jennifer Berglund 13:41

And why is it important to understand the generation of the magnetic field?


Rebecca Fischer 13:46

So there's a lot we don't know about magnetic fields, and we don't always understand why some planets have them and some don't, but they are thought to be really important to habitability because the magnetic field shields us from radiation from space that might otherwise make the surface of the Earth uninhabitable.


Jennifer Berglund 14:08

So, sort of understanding how this magnetic field works on Earth, it could potentially predict the habitability of other planets?


Rebecca Fischer 14:18

Yeah, exactly. As you said, we understand the Earth better than any other planet, and we still don't really understand what's been driving the magnetic field at different times in the past. So, you know, if we can understand that better that might help us understand what we're seeing on other planets.


Jennifer Berglund 14:45

How did you first become interested in geophysics? Was there like a particular moment that sparked your interest?


Rebecca Fischer 14:53

When I was in college, like my freshman year, I thought maybe I would go into physics, and I started taking a whole bunch of physics classes. So I was always good at physics, but I didn't really enjoy any of these classes. They were all sort of dry and theoretical, and then I took a geophysics course, and I was just sort of immediately hooked on it. It was the same kind of math and physics that I was doing in these other classes, but now applying them to understanding our planet, and all of a sudden, you know, the same kinds of concepts seemed really concrete and important. In the same quarter I was taking geophysics, I was also taking like a partial differential equations course, and in that math class, we had talked about the heat equation, which is a very, like, traditional equation to try and solve, and we were talking about it in this mostly theoretical way of like, how do you solve equations like this? And then, literally, like not more than two weeks later, in my geophysics class, we talked about the same equation, and we were applying it to understanding the amount of heat coming out of the Earth at the surface. So sort of like, people had drilled holes into the Earth's crust and measured the temperature as a function of depth, and you can understand those data with this equation, and it tells you something about how much heat is coming out of the Earth, and that's a really important problem in Earth Science because the heat flow is what drives movement inside the Earth, including the movement that causes the tectonic plates to move around on the Earth's surface. It was the application of it to understanding these sort of big picture questions in Earth science that was that really, really hooked me. Prior to that course, I had barely heard of Earth Science, like, it was never something I considered as a possible career path. You know, my experience of taking this class sort of got me really excited about it, and particularly about the interior of the Earth, and then after that, I started taking more classes, but also doing research of different types, and then that led me to sort of narrow it down to the specific type of work that I do.


Jennifer Berglund 17:26

Did you know you wanted to be a scientist when you were younger? Or is that something that kind of developed later on? Like, what did you think you would do when you were a kid?


Rebecca Fischer 17:36

I always liked science, and I was always good at it, but I was always interested in like a million different things. So I think when I was a kid, I went through dozens of different jobs I thought I wanted to have when I was an adult, and even at the point where I was applying to college, I was applying to some universities as a prospective physics major, and some as an art history major.


Jennifer Berglund 18:00

That's very interesting. Art History. What about art history interested you?


Rebecca Fischer 18:06

I think a lot of it has to do with the connection to history, and sort of how the different things that are happening in the realm of society get sort of reflected in art, but also, I think it was partly because I was sort of dabbling as an amateur artist, and so I found it really cool to learn about the artists of the past and be inspired by their work.


Jennifer Berglund 18:30

What kind of art did you do? Or do you still do?


Rebecca Fischer 18:32

I still dabble. More recently, my favorite medium is printmaking.


Jennifer Berglund 18:39

Oh cool!


Rebecca Fischer 18:40

Mostly wood cuts. Also, occasionally, painting or drawing.


Jennifer Berglund 18:45

Would you call yourself kind of a visual person? Or maybe a hands on person, visual sort of hands on person who, sort of, like ,can get a better grasp of concepts or ideas if you can picture it or handle it?


Rebecca Fischer 18:57

Yeah, definitely. Especially I think in terms of science I'm a really hands-on person. Like it's, you know, it's one thing to have someone explain it to you or even show it to you, but you don't really get it until you do it yourself.


Jennifer Berglund 19:21

On February 18 of this year, 2021, the new Mars Rover, Perseverance, will land on the surface of Mars. What about this mission is most exciting to you, and how does it relate to your research on terrestrial planets?


Rebecca Fischer 19:38

There's a lot of cool things about Perseverance, but the part I am most excited about is that it has a little drill that it is going to use to collect samples of rock from the Martian surface, and it's going to, like, seal them up and little tubes, and leave them to hopefully be picked up by a future mission, and brought back to Earth.


Jennifer Berglund 20:05

What's the timeline on something like that? And then what do you hope to learn from those samples?


Rebecca Fischer 20:11

it would be huge to have more samples of Mars. Right now, the only samples we have are meteorites. And those are awesome. But we we don't know where they came from on Mars, so we don't really have any context for them, and we also don't know how representative they are of Mars. If we have more samples, and, like, carefully selected samples, that could potentially tell us a lot more about what Mars is made of.


Jennifer Berglund 20:40

When you say representative samples, you're most excited about getting samples of the mantle. So where would you collect something like that?


Rebecca Fischer 20:48

So we're not necessarily going to find samples of the Martian mantle, but what we can find is volcanic rocks that are magmas that came from the mantle. Those kinds of rocks, their composition is not the same as the mantle composition, but we understand enough about how composition changes with melting that you can sort of calculate what the composition of the mantle would have to be for this to come out of a volcano.


Jennifer Berglund 21:20

What do you hope to learn about the formation of Mars? Or what do you think you might learn?


Rebecca Fischer 21:27

It's like the same kind of game we play with the Earth. We can model the formation of Mars, and sort of look at how its chemistry evolved, and then in the end, we're trying to see if we can match the composition of the Martian mantle. And we've been doing that based on what we know from meteorites, but we're going to know a lot more about the mantel composition if we have more samples, which will let us fine tune our models, and then that will tell us more about how Mars formed because, you know, only certain conditions of its formation are going to give us the right answer for the mantle. One of the reasons I'm particularly interested in this is that a lot of the work we do is on the Earth since that's where we have the most data, but we usually don't really have a good understanding of if we can apply the same principles to other planets. People do all the time, but we don't always know if that's really valid. And so the idea is that if we can at least do it for Earth and Mars, that at least gives us two data points instead of one, and we can see what aspects of its formation seem to be the same, and what seems to be different, and hopefully, that'll teach us more about planet formation more generally.


Jennifer Berglund 22:50

Why is it important to understand the formation of planets and the formation of solar systems?


Rebecca Fischer 22:57

I think the biggest way is that the formation of the planet is what sets the initial composition. The compositions in the different layers, like the mantle and the core, affect everything. I mean, the composition of the mantle determines what minerals it's going to be made of, which gives it different properties. It affects the geodynamics, or the movement of material deep inside the Earth, which in the mantle gives rise to plate tectonics on the surface, and in the core gives rise to the magnetic field. You know, it affects what we think the temperature is deep inside the Earth, and how the heat is flowing to help drive movement. So, if we understand formation better, it gives us more insight into all of these kinds of first order processes. One of the big applications, at least that I'm interested in, is understanding why our planet is the way it is, and specifically how it came to have the conditions necessary to support life. And there's a lot of geological processes that people think might be important, including things like the magnetic field, and plate tectonics. The idea is that if we can understand better why the Earth has these things, it might help us understand why other planets do or don't, and might let us predict whether planets should have them or not, if we can observe them directly.


Jennifer Berglund 24:35

So at its core, no pun intended, it's a question of life. It's a question of what makes a planet amenable to supporting life.


Rebecca Fischer 24:46

Yeah. You can really trace it all the way back to how the planet formed and its billions of years of evolution have all sort of created just the right conditions, and we're sort of trying to understand the most fundamental aspects of formation. Hopefully it will help us better understand how we got to where we are today.


Jennifer Berglund 25:09

In terms of your own curiosity, what do you want to know most?


Rebecca Fischer 25:14

I guess for me, the biggest part of it is just trying to understand our planet better. It just blows my mind on a regular basis how much we don't know about the Earth and how it works and what it's made of. I just want to answer some of these questions.


Jennifer Berglund 25:37

Rebecca Fischer, thank you so much for being here. This has been super fun.


Rebecca Fischer 25:41

Thanks so much for having me. It's been a blast.


Jennifer Berglund 25:46

Today's HMSC Connects! Podcast was produced by me, Jennifer Berglund, and the Harvard Museums of Science and Culture. Special thanks to Rebecca Fischer and the Department of Earth and Planetary Sciences for their wisdom and expertise. And thank you so much for listening. If you liked today's podcast, please subscribe on Apple Podcasts, Spotify, Podbean, or wherever you get your podcasts. See you in a couple of weeks.


Transcribed by https://otter.ai