Today we push our aging curiosity out into the Solar System to ask that simple question: how old is it and how do we know? What techniques do astronomers use to age various objects and regions in the Solar System?
This is part two of a series.
Meteorites – can be aged and compared to Earth ratios
Craters – how clear/worn they are determines relative age
Fraser: Astronomy Cast Episode 523: Age and Origins, Part 2, The Solar System. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute, and the director of Cosmo Quest. Hey, Pamela, how are you doing?
Pamela: I’m doing well enough. How are you doing?
Fraser: Yeah, yeah, I know. You scratched your cornea and you are so sad about your scratched cornea.
Pamela: I am. It’s just healing exceedingly slowly. And I’ve hit the point where – the problem is, I have dry eyes. And I have so much ointment in my eyes now that like the whole world is a foggy, slimy thing.
Pamela: And I’m just going to embrace the slime.
Fraser: So, I emphasized, of course, that I’m the publisher of Universe Today because I think I broke Twitter on like Wednesday or Thursday by noting that I just got accused again of plagiarizing the scripts for my videos from this website, Universe Today, written by Fraser Cain. So, that’s pretty funny that people don’t realize that I also write all the scripts for the articles and they publish on Universe Today and also on my YouTube channel. It’s all just me, but I’ve got a big piece of news that I want to share – two more pieces of news.
One, I just came back from Calgary. I got a chance to do a big presentation for the Royal Astronomical Society of Calgary, and they were wonderful hosts. It was so great to get on an airplane, fly within Canada, not have to go through a border, not spend 12 hours. It was very civilized, and I had a great time. And it was really nice to be able to connect with a Canadian audience, which I never get a chance to do. So, thank you to the Royal Astronomical Society of Calgary. I had a great time, and I can’t wait to come back.
And the second thing is, tomorrow when we are recording, Saturday, March 23, 2019, will be the 20th anniversary of Universe Today. So, I will have been doing this job for 20 years. Isn’t that crazy?
Pamela: That is amazing.
Fraser: Yeah, yeah. And it feels weird. It feels weird to be wrapping up 20 years of being a science space journalist, and I am having so much fun. I can’t wait to do another 20, 30, 40 years of this. So, stay tuned. All right.
Pamela: And if you have your robot way, you will be doing this into many more millennia.
Fraser: Yeah, a few more billion years. All right. Well, today we pushed our aging curiosity out into the solar system to ask that simple question. How old is it, and how do we know? What techniques do astronomers use to age the various objects and regions in the solar system? What techniques? How do we know how old it is?
Pamela: We calculate it.
Fraser: Where do we start?
Pamela: Basically, I –
Fraser: All right.
Pamela: – this is one of those things where – every day you and I get news stories across our desks along the lines of – this new star cluster that has been found is predicted to have formed a billion years after the universe was created. This globular cluster over here is remarkably young at only 8 billion years. And it becomes one of these, how on earth do they get at all of these different numbers?
Pamela: Because it’s not like we can go out and grab a sample of a star and do like we talked about last week and run it through a mass spec and count up the ratio of this isotope to that isotope and get at the date that the star formed. So, that’s not what we do. We’re gonna start with that.
Fraser: No, but I mean, we were gonna try to constrain ourselves to the solar system although, as you said, the solar system does include a star. So, how do you find out how old the solar system is if you can’t sample the star? If you can’t scoop up a chunk of star and ask it how old it is?
Pamela: So, here is where we have to – unlike with trees, where we assume well, when the tree formed, it had this ratio of carbon atoms. Instead, the process that we use for objects in our solar system and that we use with stars at a certain level, cosmochronography, nucleocosmochronography relies on us saying, okay, so when this particular isotope set formed; it had this ratio when it formed. And when we look at an object and we’re able to get at the amount of those atoms in the thing today, that tells us how long since the atoms formed that, that object has been around.
Now, the problem with this is, it doesn’t tell us specifically how long the thing has been around. It tells us how long the atoms have been around. And so, this is sort of a first stab at things. In our solar system, we generally assume that everything is the same age as our Sun. And so, where we start is, let’s look at the Sun using its light, spread that light out as much as possible into a high-resolution spectrum. And then, do the best we can to count atoms by looking to see how many photons they absorbed and emitted. And from that spectral signature, get at it. It’s the same thing in a much more complicated form.
Fraser: Yeah, but I guess the challenge is that if you – like here on Earth, right, if you’re looking at say a tree, you take that tree and you cut it down. You measure the amount of carbon 14 to the nitrogen that it is decaying into. And you know what it should have been when the tree started forming because it pulled that atmosphere out of the air and then started the timer.
Pamela: And to be fair, we don’t cut down trees and age them that way. When we cut them down, we just count their tree rings. It’s a whole lot simpler. But when we find a piece of wood –
Fraser: Find, yeah.
Pamela: – like a Viking ship –
Pamela: – in ice somewhere, then everything you said is true because Viking ships were made out of wood.
Fraser: Then that’s what we did, right. I can speak for all Canada and say that we don’t cut down trees everywhere, all the time.
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Fraser: But the challenge is, if I’m imagining a star being the same thing, then I’ve got some primordial atmosphere, which is like obviously, the stellar nebula, those elements are coming into the star at certain ratios. And then, those elements are decaying at a set rate over the billions of years. We can’t use carbon. We have to use something else, like uranium or whatever.
But in theory, but back to that idea, I can’t just – 1) How do I know what it should have been in the beginning? And 2) I can’t just scoop up a chunk of star, separate out the atoms, and go, count up the uranium, and thorium, or whatever it turns into, and to know what they are. So, how do I know what the initial ratios were supposed to be?
Pamela: Models, and that’s the problem with everything not on the surface of our planet when it comes to trying to figure out the age. You have to do models of, okay, you start with the Big Bang. At the end of the Big Bang, you had this amount of hydrogen, this amount of helium, these trace amounts of lithium, beryllium. And then, you have a generation of stars. After that generation of stars, what is the ratio of atoms you should have had?
Okay, so then you run forward a few more generations of stars. What are the ratios of atoms that you should have had? And it’s these models that are giving us a fair amount of error because these models get us at – what are the initial ratios of these atoms that should have existed in the stars? So, with G-type stars like our Sun, one of the things that we can do that is actually one of the best ways for age-dating a star, is we look at the thorium to neodymium ratio. And this will give us, within 9 and 14 billion years, the age of an older G-type star.
With younger G-type stars, we do have to start looking at things like lead and uranium. When we look at the ratios of these atoms, though, we have to assume that some of the thorium was already there when the star formed. Some of the neodymium was already there when the star was formed. So, we use our models to guesstimate what is the base ratio of these atoms that the star would have had when it formed, just given the universe?
And then, we take a stellar spectrum and you can get at what are the various percentages of the star’s atmosphere, which is generally unprocessed, all the nuclear processes are going on in the core for the most part. There are exceptions. Don’t “at” me. We can get at the ratios by looking at how the atmosphere of the star absorbs certain colors of light. And thorium’s one of those things that the first time I did high-resolution stellar spectroscopy, it became my enemy because it has a lot of different lines. Technetium, also my enemy. All of these higher atomic mass elements have line, after line, after line.
And it’s by very precisely measuring these lines, knowing that different isotopes will appear at slightly different places, measuring the different isotopes very, very carefully. You can do it. I measured magnesium hydride isotopic ratios. I did not enjoy it, but it can be done.
Pamela: By very, very carefully making these measurements, you can start to get at what are the percentages of these different stars in the atmosphere, what are the percentages of these different elements in the atmosphere of the star, by comparing those measured ratios to the model ratios. That difference tells you how much decay has taken place. And you want to do this with as many different elements as you can. So, we look at that thorium to rubidium ratio –
Fraser: Don’t remove that, Susie.
Pamela: – in G-type stars. We then start looking at the ratios of different kinds of uranium, specifically the 235 to 238. We then look at the ratio of uranium 238 to thorium 232. All these different ratios, they all tell us different things. Strontium, rubidium, lead 207 to uranium 235. These are all created in slightly different processes. They all have slightly different errors, but by putting all of them together, you can start to narrow in on what are the ages of things. And for our solar system, we have a sun right there.
Fraser: Right, right. But the thing is, though, is that to be off by factors of billions of years is not great. And yet, I know that our solar system is 4.54 billion years old. So, if that method of dating the sun is so inaccurate, how do we know that number to such level of accuracy?
Pamela: Well, the thing is, if each of these different things is off by a fair amount, but there is only a very narrow overlap in where all of them mesh up, we use that overlap of all of the errors to say this, this must be where it is. And with our Sun, because it’s so close, because it’s so bright, we can split its light up a bazillion times and get very, very fine-grained measurements. And it’s from those fine-grained measurements that we are able to be more precise with our own Sun than we are with anything else.
Fraser: Right. But I guess what I’m driving at is that there’s got to be some other method that astronomers have used to figure out how old things are in the solar system that’s perhaps raining from the sky, falling on our heads all the time.
Pamela: Well, we do use meteorites, yes. And this starts to get us back to exactly what we talked about last week, which is, you take the thing, you pull it apart atom by atom, and you count those atoms to figure out what are the atomic ratios. And here again, you are relying on what do you think the primordial ratios were. What are the present-day ratios in that object? Because it’s a meteor, we know that it hasn’t gone through age processes like a tree has, and this starts to get us at how old do we think the asteroids were, because it’s shards of asteroids that are raining on us. How old do we think Mars is, because it’s shards of Mars raining on us.
And one of the fascinating things about doing this is you correctly get that the Moon is a different age from Mars because it did go through kind of a resetting when the Moon and the Earth evolved out of the proto-Earth and Theia Mars-sized object that hit the proto-Earth and created a great splash.
Fraser: That’s kind of a fascinating idea that if you go back and you look at all of these objects, you look at these meteorites, and they are all – you count up the uranium atoms, and you count up the I don’t know what they became to. I’m gonna guess thorium. Who knows? Something – you count up the output, and then you can say, oh, here’s the ratio, and you know how long. And you have a pretty good idea of what the original ratio was based on, I guess, seeing enough of these, you can sort of triangulate where that initial data set should be, that you’re like oh, you look at a meteorite. You date it. Oh, 4.54 billion years old.
You find another one, oh, 4.54 billion years old. And yet, you bring a rock back from the Moon, and it only tells you, oh, 4 point – I don’t know what the number is – 3, 4.4, or –
Pamela: It’s usually three point something billion.
Fraser: So, and then it is not. And so that you know that something smashed into the Earth, turned this blob that got remixed-up, and then started from scratch again. Started the timer again, which is this just incredible idea that you can find. And so, we would think that if we could find a meteorite that was older than the solar system, for example, that would tell us that it had to have come from outside the solar system.
Fraser: It’s sort of a really amazing technique. And that it’s always the same number.
Pamela: And the amazing thing about the Moon and Mars is we’re actually seeing how old are the different rocks. So, the reason that so many of the Moon rocks are appearing to only be three billion years old is because that’s when the lava that they were made of was produced. So, we’re looking at basaltic rocks that essentially got reset yet again, even though the Moon itself is estimated to be more than four billion years old. So, when you have –
Fraser: It’s the same trick.
Pamela: – lava involved, all bets are off.
Fraser: Yeah, it’s the same trick. You can tell how old lava is here on Earth. You measure the ratio – because it’s freshly-squeezed out onto the surface of the Earth. And then, the same thing on the Moon, it’s gonna be freshly-squeezed out on the surface of the Moon. All right. So, let’s take two, and we’re sort of leading into what is sort of the next part, is actually starting to age and date specific portions of the solar system. So, how do we know how old various parts of the Moon are?
Pamela: Well, this is where, with the Moon, we’re lucky enough that between the Apollo astronauts and the Soviet landers, we have brought back a lot of space rocks. And these space rocks were gathered from a variety of different sites. And those different sites had different densities of craters on their surface. So, when we look at the Moon, you can see there are these great dark seas. There are these lighter regolith, we call that, lighter areas that just have their surface beat to expletive with all the craters that appear.
Different rocks from areas with different densities of craters we find have different ages, where areas that have the fewest craters have the youngest rocks. Areas with the greatest number of craters have the most rocks. And this is because that accumulation of craters on the surface tells us how long that surface has been exposed to the sky.
This is sort of like looking outside. And if you see different accumulations of snow in the winter, where that area of your driveway might have two feet of snow if you’re having a particularly bad weekend, but there is a rectangle next to it that only has like 10 inches. Well, that rectangle –
Fraser: There used to be a car there.
Pamela: – is probably where there was a car.
Pamela: Someone moved the car. It’s a freshly-exposed surface, and that freshly-exposed surface has had less time to accumulate snow. Well, with the Moon, it’s not accumulating snow. It’s accumulating craters. And different size craters form at slightly different rates. So, the giant craters, luckily, appear at a slower rate. And then smaller and smaller, more and more common. So, we can sort of zoom in on how old by going to smaller and smaller craters until we start to saturate.
Fraser: And actually, that process isn’t useful here on Planet Earth because of all the weathering processes that are happening, we can only count a few large craters across the surface of the Earth. The rest are weathered and gone. But on the Moon and on Mars, they are just there, and have remained there for billions of years. The processes are very slow or even nonexistent.
Pamela: And one of the other things that blights our planet, because you can look at Mars, and there is plenty of erosion on Mars. But what we have that Mars doesn’t is plate tectonics where we are resurfacing our planet by having one plate dive under another such that there are only small bits and pieces of Canada and western Australia that are truly ancient. And when we go to Mars, we do see areas that are covered in sand dunes, that are constantly eroding due to the weather. But these are smaller regions, and in general, we can use craters to get at the ages of different surfaces.
Fraser: And I would imagine that same technique could be used for some of the cryovolcanism that’s happening out there across the solar system as well.
Pamela: One of the really awesome things about cryovolcanism that I learned from the Ceres results that have been coming out from the Donn Mission is, and we all know this from looking at photos of like Hawaii, volcanoes slump over time when they’re done doing their volcano thing. So, when you have an extinct volcano, it is going to slowly slouch back into the Earth and over time, work its way back towards flat.
Well, it turns out that on Ceres, cryovolcanoes are going to do the exact same thing where we can trace the history of volcanism, cryovolcanism, on Ceres, we think. But looking at all of these different volcano-shaped mounds that have slumped a variety of different amounts, appearing to say that there has been cryovolcanism over time. And it’s traced out different patterns across the surface.
Fraser: So, what are some places in the solar system that, if you could take samples to do some kind of radioisotopic dating, would you love to get your hands on some samples so that you could then answer some questions about some interesting features in the solar system?
Pamela: Oh, man. So, for me, looking at how different objects have been processed over time, being able to go out and grab samples from asteroids at a whole variety of different distances would get two different things for us. It would tell us when they formed, which hopefully should be about the same time for everything.
But it will also tell us what was the ratio of volatiles where stuff formed. And the amount of these atoms that like to go from ice to gas instantaneously, these volatiles, their ratios are different at different distances from the sun, with fewer volatiles being present inside the things that formed near the sun, and more volatiles being present further out. I would love to be able to use volatiles to trace how things moved around after they formed.
So, this double data on where did things form, what were they formed of, will start to help us answer questions about well, when did different parts of the solar system solidify, how much did things move around, and how much have they been processed over time.
Fraser: Yeah, I think about – the correct answer to that question was everywhere, because –
Pamela: Yes, which I do think I said in a roundabout kind of unscientific kind of way.
Fraser: – pretty close, pretty close, yeah. But imagine, if you could take samples of the ice mountains on Pluto and the ice plains to see when the ammonia glaciers formed. If you could take samples from the hills on Titan, and the sand, and the seas of ammonia, if we could go back and seriously take some samples from the surface of Venus to see when some of those features formed and really understand what shut plate tectonics down on that world, it would be incredible.
And yet, each one of those, unfortunately, they are really rough to get to, and we have to send missions that will get down close and dig around in the dirt and do some of this really careful work.
Pamela: And one of the things that we’re able to do here on Earth is ice core samples where we can look at –
Fraser: Oh, yeah.
Pamela: – how old is this glacier versus that glacier by measuring the difference in atmospheric composition that gets trapped in the ice over time. Now, imagine going to Pluto and being able to do ice cores to get at the history of all these different areas.
Fraser: Or Europa.
Pamela: Europa, so many different places. Ice cores of the poles of Mars.
Pamela: There is so much out there to be learned, if only we had better low- power, low-energy requirement, completely sterilized drilling equipment.
Pamela: We have none of those things.
Fraser: No, but that would be – and unfortunately, even the plans for sending say probes to places like Europa and stuff involve some kind of nuclear reactor that radiates heat away and melts its way down through the ice, which is not a great way to take an ice sample.
Fraser: You want to have a nice clean drill that pulls up your ice samples one at a time. So, literally, this entire solar system, when you think about how busy geologists here on Earth, and hydrologists as they work with ice cores, and water, and samples and people like that, there are scientists who would love to get their hands on every nook and cranny of this entire solar system and grind it, and drill it, and age it, and date it, and figure out how old this stuff is. And to just understand the history and run it all in reverse.
Pamela: And the hope starts to be that we’ll figure out how to build a more, I don’t know, OtterBox coded cube sets for lack of a better way to put it. Cube sets are still fairly fragile, but if we can get them to the point that they can get collided into an asteroid and still come back to Earth healthy, then we can start to imagine sending out small fleets of essentially interplanetary rumbas that go up and grab samples and then fly on home without taking too much damage in the process. We’re just not to that point.
And as long as we have to rely on SUV-sized things that have more mass and thus more momentum to be transferred during a collision, yeah, you’re better off dropping something lightweight than something heavy. So, we need those rumba spacecraft now, please. Please?
Fraser: Yeah, so if anyone is concerned that there won’t be lots of things to study and discover in the future, it will never end. All right. Thanks, Pamela. Did you want to do a Part 3? Are we gonna talk about how old things are outside of the Milky Way, or we’ve got a bit about stars?
Pamela: Oh, yeah. There is so much cool stuff on how to age-date stars beyond our sun.
Fraser: Yeah, and I think about things like pulsars, white dwarfs, exoplanets, and of course, the cosmic microwave background So, who knows how long this is gonna take us? All right, thanks, Pamela.
Pamela: Thanks, Fraser.
Fraser: Before we leave, have you got some names that you want to say?
Pamela: I do. So, as always, we are here thanks to you. It is your generous contributions that allow us to pay Susie, who keeps us organized, keeps our audio edited, and keeps us in line when we wander off like lost children. So, thank you, Susie, and thank you, all of you, for your generous contributions over on Patreon. I am slowly but surely working our way through our backlog of thank yous. And today, I want to give a special thank you to Jos Cunningham, Les Howard, Dana Nori, Kajarten Svere, Helga Bjorkime, Bill Hamilton, Frank Tippin, Greg Thorwall, Richard Riviera, Thomas Sepstrup, Cory Dovall, Sylvan Westby, and Jeff Cullins. Thank you all so much. As they say on NPR, we couldn’t do this without you. So, thank you for donating well, a cup of coffee a month or more to make us happen. Thank you.
Fraser: All right, we’ll see you next week.
Pamela: Bye-bye everyone.
Pamela: This episode of Astronomy Cast is brought to you by 8th Light, Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember, that’s www.8thlight.com. Drop them a note. 8th Light, software is their craft.
Female Speaker: Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cane, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at firstname.lastname@example.org, Tweet us @AstronomyCast, Like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 19:00 UTC. Our intro music was provided by David Joseph Wesley, the outro music is by Travis Searle, and the show was edited by Susie Murph.
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Duration: 33 minutes