The Age of the Earth - or, How to Build a Planet

Jesse Reimink: Welcome to planet geo the podcast where we talk about our amazing planet, how it works and why it matters to you.

All right, Chris fer BHU

Chris Bolhuis: that's that? Yeah. Yeah. That when somebody says that on the phone, that's when I hang out.

Jesse Reimink: that's my Siri. I think I've said this before, maybe, but when I

Chris Bolhuis: Is that

Jesse Reimink: when I tell Siri to call you, I say, call Chris Bauch and then she know.

Chris Bolhuis: Hmm. yeah, mine says the big idiot. Yep.

Jesse Reimink: the big doctor idiot.

Chris Bolhuis: Yeah, professor idiot. I get lots of you

Jesse Reimink: lots of good names. What's going on, Chris?

Chris Bolhuis: not a whole lot. It's good. It's a good, it's a good day. It's a good

Jesse Reimink: It's a good day. You

Chris Bolhuis: good to be alive.

Jesse Reimink: uh, I mean, when is this coming out? You probably are outta school and having a good summer That's exciting.

Chris Bolhuis: it's a great it's. Yeah, it is. I'm gonna have a great summer. Can't wait, let's go.

Jesse Reimink: So

Chris Bolhuis: Hey, so Jesse, what are we talking about today?

Jesse Reimink: we, well, this is gonna be a great exercise in Chris, keeping Jesse out of the weeds. I

Chris Bolhuis: I'll do my best. I apologize right now. I'm gonna, I'm gonna try

Jesse Reimink: we're talking about the age of the earth and this is near and dear to my heart. there. So I don't even know where to begin Chris. Like I, I don't know. Where are we beginning here? You could maybe start by keeping me outta the weeds.

Chris Bolhuis: we're just gonna go ahead and jump right into the episode is how old is the earth? And we're gonna get into how do we know this? And I mean, this is, this is what you do. And you've done this for a long time now. so you are definitely the expert in the room. I'm really looking forward to this episode. I mean, cuz this is, it's an important topic

Jesse Reimink: Yeah, so I think a lot of people know that the earth is four and a half billion years old roundabout. Right. And. people kind of know that number, roughly some people might say four, some people may say 5 billion something. Right. Would you agree with that for the

Chris Bolhuis: but they don't know how we came upon that number. Like how was that done? Right. And that's what today's all about.

Jesse Reimink: and then there's also a couple really interesting. questions in that because people assume, oh, that's the age of the earth. And then, oh, that must be the age of the solar system probably too. And those are two very different things and we know them to very different levels.

Chris Bolhuis: however, what you just said, first of all, I don't agree. Most people assume that the solar system is the same age as the earth. I like, I don't, I don't think a lot of people have that connection. And also you always are quick to point out that they're two really different things. However I wanna say, and this is really a big part of our episode today is how actually things in the solar system and how the solar system formed plays into how we got the number. Of how old the earth is.

Jesse Reimink: That is exactly right. \ So, Chris, the question we have to start out with is how old is the earth and what, so what is that? When did we figure this out? You know, give us a little intro to the history of.

Chris Bolhuis: So it all happened around or not really around 1955. Up until that point, we really didn't know. I mean, some people thought it was a billion years old. Some people thought it was 6 billion years old, but then we used techniques and methods that were developed during the Manhattan project. And I think most people are familiar with the Manhattan project, the development of nuclear energy, right. New the development of nuclear bombs. And. We really honed in on uranium to lead the decay of uranium, 2 38 to lead 2 0 6 and uranium 2 35 to lead 2 0 7. We really, really honed in on those rates of decay. And we also used the same elements or isotopes, I should say, in meteorites during that same time.

Jesse Reimink: Yeah. And this was, the real revolution in nuclear weapons and using uranium for nuclear weapons and nuclear energy is that we really got to understand how those two isotopes uranium, 2 38 in uranium, 2 35, the two flavors of uranium, how those decay and break down and end up in lead. And then this is. A massively useful tool for my life today in geochronology. I mean, that is what we measure in the lab, uranium and lead. And those two things are clocks and we can use those two clocks together, leverage them against each other and measure ages. And so what people

Chris Bolhuis: But those two clocks, I'm sorry. I gotta interject those two clocks run at different rates and they run independently of each other, which is hugely important. So we can like cross check. Using those two

Jesse Reimink: Yeah. And if you wanna have a, a more detailed rundown, I don't think we're gonna get too much more into detail about uranium led today, Chris. But if you wanna have more detailed rundown, we've covered this before in the ancient NUS episode of, I don't know, probably a year and a half ago, maybe something like that. So go back to that. We cover how nuclear reactors work and, some really cool stories about that and get into the details of uranium and how it breaks down to lead. But. In 1955 after round about a decade of work, a research group led by Claire Patterson, determined the age of the earth. I'm gonna throw some numbers out here, Chris, you said before, we didn't know at that point in time. Whether the earth was a billion years old or 6 billion somewhere in there, that's a pretty big range. They determined the age, huge huge range. They determined the age of the earth to be 4.5, 5 billion years old. So that's 4,550 million years plus or minus 0.07 billion years. So 4.55 plus or minus 0.07. So that 0.07 billion years. That's a 70 million year uncertainty, which. Is a lot,

Chris Bolhuis: Okay. I gotta put this in perspective. 70 million years is a ton of time. 65 million years ago, our planet got hit with a game changing meteor that was 65 million years ago. That's the time that the dinosaurs met their extinction. And so we're talking about a plus or minus here in the age of the. That exceeds that difference. That's a lot of time now, Jesse, you and I like we're used to dealing with numbers that are in billions and millions, and I think we get desensitized,

Jesse Reimink: Well, first let's consider that before this number, we're gonna kind of rip on this number a little bit, but before this number. We had a range between 1 billion and 6 billion, which is a huge range. Basically we had no idea. Now it's 4.55 plus or minus 0.07. So a lot better, but let's put this into some real life context. The average American lifespan is round about 79 years. So if we shrink the age of the earth down to 79 years, that uncertainty 70 million year uncertainty at four and a half billion years ago is basically like knowing. When you'll die down to the closest one year, two months and 12 days. So that's like saying I'm gonna die 79 years from the day I was born, but I might die one year, two months and 12 days before my 79th birthday, or I might die one year, two months and 12 days after my 79th birthday. So you kind of have an age range of two and a half years. So you might die on your 79th birthday. Somewhere within two and a half years on the either side of

Chris Bolhuis: Yeah. I'm it does. It makes great sense, actually. And I'm a little conflicted by that. Like

Jesse Reimink: would you

Chris Bolhuis: I, I am, because a part of me is like, wow, that's pretty good. But then the other part of me is like, well, that sucks. don't, I don't know where to go on. like,

Jesse Reimink: And actually, I, I think it's better if I just, for me personally, it's better if I just don't know when I'm gonna die. I think for me, for my mental sanity, it's better,

Chris Bolhuis: I was just gonna say that the interesting thing is, is that we have not improved. On the age of the earth. Since that time, 1955, we've really like what the hell jesse, what's going

Jesse Reimink: So our methods, just to kind of give you a sense of how much our methods have improved, that number 4.55 plus or minus 0.07 billion years, that took years to develop the lab techniques in order to make those measurements, they were reproduced in multiple labs. So it's a solid number. Our lab at Penn state, you know, we don't do the exact same stuff, but we can make an age measurement with the uranium led system in 45 seconds. So we can do hundreds of analyses a day. So our lab techniques have improved massively since 1955, as you would expect. Right. But you're exactly right. Our understanding of the age of the earth has not really improved since then. I could talk about this in what we know, what we do, know what we have improved upon in the Earth's oldest rocks. Some of which I studied during my PhD are 4.0 billion year old rocks, and we know the age of those rocks. To plus or minus 800,000 years. So that's 4.030 billion years, plus or minus 0.0 0, 0 8. much, much better. Very accurate, very precise. That's a solid age. There are some rocks in Northern Canada. People have suggested are about 4.2 8 billion years old. Now there's a bit of debate about that. I, myself I sort of generally agree with the idea that those are old rocks, but there is some debate about whether these rocks are actually 4.28. So the oldest, well accepted aged rocks are 4.02, which is 500 million years younger than the earth. And that's one ninth of earth history. And that's almost as much as we've had multicellular life on earth. So this is a huge age range. Like this is a massive amount of time so we do have very old pieces of earth though, that are not rocks. We have actually what are called detrital mineral grains. So these are zircon. It's a certain mineral that is eroded and deposited in beach sands we have individual zircon grains that are 4.3, 8 billion years old. Now those Zon crystallized in a magma 4.3, 8 billion years ago, that rock. Then got eroded and deposited in older sediments. So we don't have the rock itself, but we have little tiny fragments of the rock. And those are the oldest pieces of earth, which are still 200 million years younger than the earth. And so this kind of begs the question, how do we know the age of the earth? And so I need to go back to that 1950.

Chris Bolhuis: Well, okay, Jesse, I gotta interject a second then, because I think we need, I need you to do this cuz you're the expert in the room. And I know the answer to the question, but I'm gonna ask it, what starts. The clock or what can reset the clock, cuz you're talking about, ages that are, are in the oldest possible grains, those de arterial grains of 0.4 0.35 billion years that still doesn't get us to 4.5, 5 billion years. So what resets the clock? We need to talk about that.

Jesse Reimink: Yeah, absolutely. Great, great question. What resets the clock depends on what type of material you are looking at for individual grains? It's either heating 'em up to high tempera. Or melting them, re crystallizing the grains with a rock, basically mostly, any kind of really high temperature, metamorphism or melting. So if you melt a rock, it resets all the clocks in there and you basically reset the chronometers re you know, flip the sand, dial over and start it again. So this

Chris Bolhuis: So can you, can you explain that a second though, to Joyce, my mom, why does melting a grain reset the clock? R you know, you got 30 seconds go. Why does that

Jesse Reimink: Yeah, these particular grains are very useful because they have a lot of uranium, no lead. And so any uranium that starts there decays away to lead. Most of the lead that we see in the grain is by radioactive decay, which means it's a very accurate, precise clock. As soon as you melt that grain, all those uranium and lead atoms from that grain mixed back in with the magma get mixed up again. And the new grain grows. Grows with a lot of uranium and no lead again. So it kind of overprints this clock it's just like the sand dial. The sand in the top is uranium. The sand, as it trickles down is turning into lead at the bottom of the sand dial. When you melt it, you'd flip that thing over. you just have uranium in the top and the sand is down in the bot. It starts to fall back down into the bottom. So you basically reset this entire clock by removing the product or the daughter isotope the lead in this case.

Chris Bolhuis: Hey well done. I liked that. You've never busted that one out before the little, I don't think you've ever done the little sand dial thing and Joyce, I think can understand Joyce gets it. Yeah.

Jesse Reimink: good on you, Joyce. I'm glad. so all, all this conversation kind of begs the question. How do we know the age of the earth? Like where did this 4.5, five number come from? So I'm gonna spend again, 30 seconds on that really quick, Chris, just to, to get

Chris Bolhuis: But you've take, you've gone over your allotment. So hurry up. Let's go.

Jesse Reimink: I'm over time. Um, so this 1955 age was basically doing uranium led geo chronometers, except comparing the earth, the crust of the earth, the mantle of the earth to the core of the earth, except it wasn't the core of the earth. Cause we can't sample that. It was using meteorites to sort of. Predict what the core of the earth would look like. And so if you use uranium led chronometers on the cross, the mantle and the core, you get an age, basically a planetary what's called a planetary isochron, but you get an age for the entire earth for this. And that gives us that 4.55 plus or minus 0.07 billion year number, which we have not improved upon since 1955. but Chris I've described how we made the age of the earth measurement. I've. What we know about the oldest rock and minerals. Why do we not know more about the age of the earth since 1955?

Chris Bolhuis: well, the earth doesn't have any original rocks. It doesn't have any rocks that exist at the surface that are gonna be 4.5 5 billion years old. And that has to do with, well, Perhaps the most like famous event that happened is the forming of the moon. Okay. Our moon. And this happened when a planet that's the size of Mars. So roughly half the size of earth collided with earth between 4.5 and 4.3, 5 billion years ago. So early on in the formation of planet earth, there was a, just this devastating collision that happened. So what happens when something like that occurs? This event is so big that the entire surface of the planet is resurfaced. In other words, it melt. and then begins to cool. And like you said, and we already hit him on this. This is why I asked the question earlier, is that when you melt rocks, it resets the clock. So if that event happened 4.3, 5 billion years ago, no rocks will show an age older than that.

Jesse Reimink: That's right. Just a planetary resetting event, right there. Just completely. Overprinted the entire, almost the entire planet.

Chris Bolhuis: Which is super interesting because, we know that this happened as many as eight times actually, where earth was just blasted during a part of our formation, which is called the heavy bombardment era. Where we got blasted with such big impactors that it resurfaced our entire planet. when an object, the size of Mars slams into us, that's a massive ordeal. and so that's why we're never gonna be able to like, improve upon the age of the earth with earth based rocks. S, you know, so we have to look then to other things. And so Jesse, let's talk about meteor rights then. we have like, you know, big meteorites. We have little tiny meteorites all the time that are adding mass to our planet. What do they say in terms of age? when a meteorite forms, nothing really happens to it after it forms, there is no resetting. So the age we get is the age that, that, thing formed.

Jesse Reimink: Yeah. there's no plate tectonics on most meteorites. this over printing that happens on earth all the time. This resetting of clocks. Of rock clocks all the time. So there are tiny parts of meteorites, the earliest bits, we can date many different stages of meteorite formation from melting. The first melting from to core formation, to the first solids that formed in the solar system and the most primitive ones. The first solids that really condensed in the solar system, given age of 4.5, six, eight, 2 billion years, plus or minus point. 0 0 0 1 7 billion years old. So let me say those numbers a little bit differently. This

Chris Bolhuis: That's crazy. I want everybody to listen up because

Jesse Reimink: Okay.

Chris Bolhuis: This is

Jesse Reimink: I'm gonna say these differently. This is 4568.2 million years plus or minus 0.17 million years. So that's an uncertainty going back 4.5, six, 8 billion years ago, an uncertainty of 170,000 years. So that's plus or minus 170,000 years. So let me put that into our age of a human being context. Chris here is that we were talking about a lifespan of 79 years. You would know it to. Plus or minus one year, two months and 12 days, this. If you have this precision in your life, when you're going to die, you're gonna know that you're gonna die on your 79th birthday. You're gonna actually know the day you die two within one day in one and a half hours. So that's, you might die on the day before your birthday or the day after your 79th birthday, but you're gonna die one of those three days, right? That's spectacularly precise. That's

Chris Bolhuis: yeah. Yeah. It is incredible. And again, it's important to highlight two things with this, that's how precise our methods are. I mean, this is what you do. it's exceedingly precise. Okay. The other thing is why are we able to do this with meteorites? And I know you said it, I just want, I wanna come back to it , because such an important point. Meteorites, nothing happens to them. They form and, and there's, they don't get remelted ever again, but on earth, Jesse, you as a guy that dates, rocks dates, old shit. You have to have a love, hate relationship with plate tectonics.

Jesse Reimink: Yeah,

Chris Bolhuis: And your rocks because, plate tectonics is what makes this an amazing planet. It's what makes our planet so beautiful, but it makes it so difficult for you to do your job because it constantly recycles rocks. and so we got melting. We have metamorphism which can reset clocks depending on what method you're using. Um, It

it adds a,

Jesse Reimink: it makes it very frustrating, Chris. Very frustrating. Sometimes. Absolutely.

Chris Bolhuis: And you don't get that with meteors, which is, or meteorites, which is why we're able to use those and get a better approximation the age of the earth.

Jesse Reimink: And that really brings up the main question I think here, Chris, which if you're sitting there listening to this, alarm bells should be kind of going off in your head, right. You should be thinking, okay. We can know the age of meteorites, really, really precisely, but we haven't improved on the age of the earth since 1955. You should be going like, well, how does a planet actually form, right? I mean, I think that's like the sort of natural follow on to this is like, what the heck does the age planet actually mean? If the earth, the proto earth got hit by this Mar size impact or in totally reset the planet, like what is the actual age of the earth? So, Chris let's, let's wrap up this episode on. Theme. we gotta start a little bit earlier than a planet though. First to kind of get to this, right.

Chris Bolhuis: So this is really difficult Jesse, to do, to talk about, you know, the formation of the solar system in a couple minutes is a tough task, but look, all comes down to what's called the nebular theory, the solar Nebula theory. And the idea is that. Our solar system formed from a Nebula, this cloud of gas and dust that was contracting due to its own gravity. And know, you get the formation of the sun and then everything else in our solar system, the planets, the Kiper belt, the asteroid belt, And the idea is that it started to contract, as this Nebula began to contract, it began to spin and to rotate. And you know, you and I were talking about this earlier and it's, it's kind of like a, somebody taking a, ball of dough. like pizza dough, you know, they roll it into this ball. And what happens when you take that ball of, dough and whip it into the air and, and rotate it? What happens to it?

Jesse Reimink: Yeah, you get it flattened out. Right? You can picture the person in the pizza parlor, you know, throwing the dough, tossing the dough up and spinning it out into a pizza shape. Right. It's kind of what's going on here in this collapsing cloud of gas and dust. And then the next thing happens, which is the star ignites. Which starts to produce a lot of heat. And that basically moves all of the stuff that has a low condensation temperature, meaning it likes to be in the gas phase, think CO2 or water. in hydrogen that all gets kind of pushed out away from the star, kind of gets blown out until it gets cool enough for that stuff to condense. That line defines what's called what's called the frost line or the ice line in some textbooks.

Chris Bolhuis: line is what It's called.

Jesse Reimink: It's basically the

freezing point.

Chris Bolhuis: sequence.

Yeah.

Jesse Reimink: It's basically the freezing point in the solar system. And the inner part of the solar system will be what we call the Rocky planets. The ones made of mostly rock. The outer ones will be the gas giants. The ones made mostly of gas like Jupiter and Saturn in our solar system. And so, you know, as this dust. Cloud now this is mostly rock dust. Now those little dust particles will electrostatically cling together and form larger dust particles. And those will get a little bit larger. Then those will get a bit, a little bit larger. At some point they'll reach a stage, something larger than the pebble size, where they start to have their own gravity. Then they'll start to attract each other and then you can get one big one that has more gravity attracts, more little ones, which makes it bigger, which means it has more gravity, which attracts more little ones and it grows exponentially. And this is this runaway growth phase, which basically starts to build a planet.

Chris Bolhuis: eventually it becomes a planetesimal. It gets to this certain size where it begins to grow due to its own gravity. It begins to draw things in from outside. If it becomes a planetesimal, then astronomers say, well, then it's destined to become a planet. and this is what really never happened to Pluto. For instance, a lot of people get emotional about Pluto getting demoted to a planetesimal instead of, or, or to dwarf planet, I should say, instead of a planet. Well, one of the criteria that we have sat for something to be classified as a planet is it has to clear its own orbit. You know, it has to clear out the vast majority of the debris in its orbital plane, Pluto. Hasn't done that. There's a ton of stuff in its orbital plane. And so I just wanna say that, like, if this is how it happened, you take this, ball of, pizza dough and spin in the air. How does that tie into our solar system? Well, There's a bunch of stuff. all of the planets are revolving in the same direction and they're all revolving in the same plane too. I mean, it's a very, really small variance in terms of the plane at which they orbit relative to the sun, all but Venus and Uranus rotate in the same. Everything rotates counterclockwise, except Venus and Uranus, all the moons revolve and rotate counterclockwise. these are also predictions that are based on this solar Nebula theory. And sometimes in science, Jesse, like, I think you'd agree with this. Sometimes the predictions. become more important than the theory itself.

Jesse Reimink: yeah, exactly. Those are the things that become testable in the future. And those are the things that can go forward and test. So you nailed it perfectly there, Chris, and this leads us really perfectly to what does the age of planet mean? Cuz this process we're describing is a potentially a long process. This could take a while. And so there's a couple other key parts here and I'm gonna put some numbers on some things. So as this planetesimal gets big in the early solar system, there's lots of radio activity around. There's a lot of stuff. Think of like a nuclear meltdown, Chernobyl the reason why nobody can really live by that thing is cuz there's so much radioactivity around there. That's the same thing that was going on in the early part of our solar system, loads of radioactive elements, all decaying away, producing heat. When you get this planetesimal size, you have a lot of heat inside that can't escape quickly. So the planetesimal melts that melting event means that you can start to form a core in the core formation is what some people consider to be really like a planetary. Age event that is this melting that can reset the clocks and can give you this 4.55 plus or minus 0.07 billion year old age for the age of the earth. Now. That process can take a while the core formation process on planetary bodies, the size of earth in our solar system kind of is around 30 to 80 million years after the start of the solar system. So this whole process we've been describing from Nebula theory forward takes about 30 to 80 million years. There's some disagreement about that, but it's kind of in that range for the most part. And the earth is thought to be about 50% of its current mass by. That age range, 10 to 50 or 60 million years after the solar system forms. So that's about 4.5 billion years old.

Chris Bolhuis: And that's why then meteorites date. That same timeframe, older than earth because earth took time to cool. Whereas meteorites being much smaller, they cooled much faster. Right. And so, their age is gonna be really indicative of the age of the solar system. Whereas earth is gonna be a little bit younger than that. It's kinda like this. If you take a, a coffee cup and fill it with hot water and let it sit on the counter and you heat it up to like 104 degrees, the temperature of a hot tub, but you compare that to filling a whole hot tub up with a hundred, four degree water. And now just let 'em both cool. Turn off the electricity. Let 'em cool. The cup of coffee's gonna cool much faster. in terms of like age of rocks that cooling faster, turning into rock sooner is means it's gonna show up as being older, even though they like formed at the same time, the clock is gonna be older than the clock of earth. Does that make sense?

Jesse Reimink: absolutely. And earth is still warm. It's still melting. There's still a lot of heat inside of earth that is melting. And so it still hasn't cooled down yet four and a half billion years

Chris Bolhuis: That's right. Which, which is why earth still has plate tectonics. Mars is, you know, volcanically inactive, it's cooled off. It's half the size of, earth. And so there's very little activity going on there. Mercury, same thing. Now look, I'll eat my desk in front of. If Venus does not have vulcanism on it right now. Right now I'll I'll, I'll eat it. I'll eat the whole thing. Okay. That's how confident I am say, because Venus is about the same size as earth. And so it hasn't cooled off yet either. It's still an active planet and it's gonna have vulcanism then, because of that

Jesse Reimink: Oh, I can't wait to, see what these missions to Venus end up, uh, returning, cuz it's like the most interesting data point in our solar system is how does Venus compare to earth? So Chris, I, this is sort of a wrap on the information, but I just wanna kind of summarize here, like there's, to me, there's , a few key take home points. First of all, the age of the earth is about 4.55 plus or minus 0.07. We've said that number a whole bunch of times. And we haven't improved on that since 1955. And we kind of discussed the reasons why we know the age of the solar system, much, much more precisely than we know the age of the earth and that's 0.1 and two. And that brings us to point number three, which is. Building a planet is a long, complicated, and often violent process. And that's really why we don't have a great idea of the age of the earth specifically, but we know the age of the solar system. Very precisely. That I think is a wrap. Chris, this was really fun. I, I, she said, oh, it's near and dear to my heart. I hope I didn't blather on too much and spend too much time down in the weeds about it. But it's really interesting. And I think it drives home a couple really important points about, you know, how planet forms put that 4.5, five number into some context for people, you know, at your, at the next dinner party, which everybody loves talking about the age of the earth at dinner parties. Right? You can pull out this little.

Chris Bolhuis: equipped right now to go that's right.

Jesse Reimink: gonna be rocking it at the next dinner party. Oh man. All right, dude. Well,

Chris Bolhuis: be a true test to see if she listens to our

Jesse Reimink: that's right. That's right. You're gonna get like a, a phone call and a Facebook comment from your mom. That'll be great.

Chris Bolhuis: that's right. That's right.

Jesse Reimink: all, man. Well, you can follow us on all the social medias at planet geo cast, send us an email. We love hearing listener questions and we love, uh, interacting in that way. Our website, planet geo cast.com. Check it out. There's some really funny and excellent pictures of Chris bull heist in there. And, give us a rating review. That stuff really helps the algorithm. We love that.

Chris Bolhuis: But the most important thing is to share our podcast with somebody that somebody else that loves our planet and, uh, wants to know more about it.

Jesse Reimink: and should that's okay. You can harass me, feel free to harass people and say, Hey, listen to this.

Chris Bolhuis: Yeah. Yeah,

Jesse Reimink: Try this. try the easy to get signed for size.

Chris Bolhuis: That's right. That's right. All right. Cheers.