Ep. 363: Where Did Earth's Water Come From?

Where on Earth did our water come from. Well, obviously not from Earth, of course, but from space. But did it come from comets, or did the water form naturally right here in the Solar System, and the Earth just scooped it up?

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This episode is sponsored by: Swinburne Astronomy Online, 8th Light

Show Notes

  • SpaceX Hard Landing on Drone Ship
  • Did Comets Bring Water to Earth? — EarthSky
  • Rosetta’s Instrument Directs Scientists to Look Elsewhere for Earth’s Water — Universe Today
  • Earth’s Water Probably Didn’t Come From Comets — JPL
  • Is that an Asteroid or a Comet? It’s Getting Harder to Tell — Wired
  • Jonathan McDowell
  • Minor Planet Center
  • Dawn Mission
  • OSIRIS-REx mission
  • Evidence of a Late Heavy Bombardment in Another Solar System — Universe Today
  • 51 Pegasi — BBC
  • More Water on Europa Than On Earth — io9
  • Transcript

    Transcription services provided by: GMR Transcription

    Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the worlds longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
    Fraser Cain: Astronomy Cast. Episode 363. Where did the Earth’s water come from? Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos to help you understand not only what we know, but how we know. My name is Fraser Cain. I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University, Edwardsville, and the director of CosmosQuest. Hi, Pamela. How ya doing?
    Dr. Pamela Gay: I’m doing well. How are you doing Fraser?
    Fraser Cain: Good. And we are, at this point, at the time we are recording, we do not know what happened with SpaceX launch, and if its going to land on a floating platform in the ocean, but by the time you listen to it you will know if it happened. So, or too bad.
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    Fraser Cain: So, where on Earth did our water come from? Well, obviously, not from Earth, of course, but from space. But, did it come from comets or did it form naturally right here in the solar system and the Earth just scooped it up? And this is – just to give a little preamble to the show which is we often are asked to update people on some science, and we’re surprised how most of the changes are fairly, I guess, incremental in the science. It’s not like something that’s really super different now, but thanks to the Rosetta mission, and some of the analysis of Comet 67P we may have a little bit of an update to where Earth’s water came from. So, let’s kind of go back and set the stage here.
    Where did the Earth’s water come from and why is this even a question that I think we want to ask, right?
    Dr. Pamela Gay: Well, let’s start with why, why, why are we asking.
    Fraser Cain: Yeah, why are we asking this question? Isn’t it obvious? The sky.
    Dr. Pamela Gay: Yeah, no, not so much. The reason we have to ask this is in the early days of our solar system there was this waterline, and the Asteroid Vesta is on the dry side of the waterline. The Asteroid Ceres is on the wet side of the waterline. And the line, if you haven’t guessed, passes right through at the distance of the asteroid belt.
    Fraser Cain: I’ve gotta – I have a great analogy for the waterline which is – I don’t know if you guys ever get this, but we get frost here in Canada, and the sun will shine, and you get shadows of rooftops and stuff, and you will get frost, and then the shadow of the sun, as it moves, there’s no frost. And, it is the frost line in the solar system, and if you’ve ever seen that, that’s what’s going on. The sun is at literally one point, it is making the water go away, and at the other part the water is able to stay frozen.
    Dr. Pamela Gay: And the issue here is if you were inside the waterline the sun was blasting you just a little to hot to hold onto your volatiles, so anything that likes to become gaseous and highly energetic, when heated, went away. And, so the early Earth, well inside that waterline was a molten, hot, nasty, awful, but volatile free place to be, or at least largely volatile free. And, so any water that formed with the planet Earth that was on the surface, it went away during the early solar system. The sun just baked us. So, that raises the question, we are now a water covered world, and that water had to come from somewhere. And the story we’ve been using for a long time is, it came by comet. Comets bombarded the planet.
    They made the oceans, they melted and made water, and that was a happy story. It was easy. It was simplistic. We make comets in our lab classes. Nicole makes prettier ones that I do. And, they melt into water and carbon dioxide and this is stuff the Earth has.
    Fraser Cain: Right. And, I guess history – and time has been around for a long time. You’ve got four point five billion years of history of the Earth although comets are fairly rare we, you know, if you add them all up over billions of years, you would get a significant amount of water [inaudible].
    Dr. Pamela Gay: And, so we like to have data to back up our theories, because theories are, they are just sort of fairy tales that may or may not be true, and we’re not sure. And, unfortunately the data here is being confusing. So.
    Fraser Cain: So, sorry, just one thing. So, the one theory is the comets. What are alternative theories for where that water could have come from?
    Dr. Pamela Gay: I – well, it had to originally fall out of the sky, but not out of our sky like outer space, but on the planet Earth somewhere. So, someplace not Earth.
    Fraser Cain: Right.
    Dr. Pamela Gay: Water originated that someplace came to Earth and crashed onto Earth. So, that kind of means asteroids are really the only other option.
    Fraser Cain: But, isn’t there another theory just that the water formed in situ. That is was essentially that water molecules floating around in space and the Earth just kind of crashed into them?
    Dr. Pamela Gay: Not so much. You can’t really explain all the water that is on Earth one molecule at a time.
    Fraser Cain: Okay.
    Dr. Pamela Gay: So, the other theory that’s out there that doesn’t seem to quite ring true is that there could have been reserves of water deep inside the Earth that didn’t get baked out by the sun and have since migrated toward the surface. Again, can’t seem to come up with it in large enough amounts to account for our atmosphere, our oceans, and everything else. There is reserves of water deep inside the Earth, but, yeah, that theory doesn’t quite, as we’ve written it, seem to match reality right now.
    Fraser Cain: So, the Earth could have protected the water from the blasting – water could have formed with the Earth, but then the Earth could have protected the water from the blasting radiation from the sun, and then it might have somehow percolated up to the surface and –
    Dr. Pamela Gay: Yeah, and so a better way to think of it is that when the Earth formed it was this motley mix of different compositions and materials and there was water scattered throughout all depths of the early planet Earth, but the stuff on the surface got baked out, and there were reserves deep inside that the sunlight wasn’t able to bake out. So, if you think about baking things in a kiln, if you don’t bake them long enough you end up with incomplete ceramic. This is why it’s easier to have hollow objects that solid objects. The solid objects you always end up with pockets inside that leads to broken ceramics eventually.
    Fraser Cain: Right. Okay. And so– I guess where – so at this point everybody was pretty certain it was comets, but then – what was the thing that Rosetta discovered at 67P?
    Dr. Pamela Gay: Well, and it wasn’t just Rosetta. Rosetta’s just the most recent issue. So, with Rosetta there is a spectrometer of board that is capable of analyzing the composition of stuff, so it can go through, it can scoop up. The instrument is called ROSINA, which is the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis. And, this particular instrument is able to capture ions, capture atoms, molecules floating around near the comet that came from the comet and analyze there composition. And what it did that was it analyze the composition to see what the ratio in the water of normal H2O to heavy water which is deuterium. It has an extra neutron.
    What the ratio of deuterium to compare that ratio to the ratio here on Earth. So, here on Earth its about one in every 10,000 water molecules in your average sea water is heavy water. And, unfortunately, or fortunately depending if you like mysteries or solutions, the amount of heavy water found in the sample from CP – 67CP, or CG rather, it had way, way too much heavy water to match the Earth’s atmosphere. But, this was a single measurement.
    Fraser Cain: On a single comet.
    Dr. Pamela Gay: It’s a single measurement on a single comet and this is the third comet we’ve made this is the sort of sample for, so we’re now looking at the situation where Comet 103P/Hartley 2 has a deuterium to hydrogen ratio that perfectly matches the planet Earth. So there we have one comet from the Oort cloud that matches. We now have 67P/CG which totally doesn’t match, and we’ve also sampled a comet that came from the Oort cloud, and it totally didn’t match.
    So, having now looked at three comets with not that much data on the three comets, we’re sort of left scratching our head, but the thing is heavy water and regular water are physically very different, and it’s possible that this is simply a sampling problem. That with Comet Hartley 2 with it’s sample it was much more active than CP66 – 67. I’m going to totally going –
    Fraser Cain: 67P.
    Dr. Pamela Gay: Yeah.
    Fraser Cain: 67P.
    Dr. Pamela Gay: 67P/CG. It was much more active when that sample was taken than 67P/CG currently is. So, there’s a chance that if you have a fully engaged, fully active, you are getting a representative sample comet detection that you’ll get that it matches ratio. Whereas, this – the comet is just waking up, we’re taking the first sample off the surface. There’s a chance that we’re dealing with different shaded material where the heavy water is what melted first. And we don’t know. And this is where you have to start looking at the differences between heavy water and regular water.
    Fraser Cain: And, so do we – I mean, are we fairly certain that 67P – this is its first trip into the inner solar system?
    Dr. Pamela Gay: We’re pretty sure, but we cannot be completely sure.
    Fraser Cain: Right. Right. And, so – I mean you can get these long period comets and they may take say a million years to make their orbit around –
    Dr. Pamela Gay: But, that would be an Oort cloud object. This is a Kuiper belt object, that’s not to say that we didn’t just miss the sucker on the last pass.
    Fraser Cain: Right. So I guess you could – and then you can say – and so – if the sun has somehow been acting on the surface of the comet, what impact has that had? Has that been somehow been breaking down the ratio between water and heavy water. So, these are all a million questions, but you can probably imagine, again the way our stories worked on the universe today over the last couple of years. Comet Hartley confirms Earth’s water came from comets, right? Comet Rosetta –
    Dr. Pamela Gay: Right.
    Fraser Cain: Rosetta’s comet throws question into where Earth’s water came from. It’s – this is how science works.
    Dr. Pamela Gay: And, it’s definitely a confusing tale to look at, and it definitely starts to get at the frustration of not fully understanding how these objects formed in the past, so we do have these large blocky objects, and the thing about looking at the 67P/CG is this is some sort of a modified shape. It is either contact binary. It is two objects that are loosely held together. It is – something happened that made it the shape of a rubber duck. Weirdly rotating rubber ducks probably don’t form naturally in the early solar system. And so, when we look at it we have to wonder how much heating went with that. And the thing about heavy water is that it thaws at a warmer temperature than – it doesn’t thaw.
    It rather freezes at a warmer temperature than regular water does. Regular water you have to get all the down to 0 Celsius before you start getting ice cubes. Heavy water with the deuterium you just have to get down to 3.82 degrees Celsius which is about 39 degrees Fahrenheit. And, at that warmer temperature it will begin to freeze. One of the interesting facets about water is it forms a crystalline structure as it freezes. And, so ice cubes rise to the surface, so you have the heavy water has a freezing point that will because it to freeze at a warmer temperature, ice rises to the surface than light water. And, we know nothing about the processes which would have formed the comet.
    Nothing. So, it’s possible that based on melting and freezing histories that you could come up with different satiation between where the heavy water is, where the light water is, you have organics forming on the surface. All of this creates a complex picture where it is utterly reasonable to think, “Well, that last set of material we got came from a different satiation section of the comet.”
    Fraser Cain: Now, this isn’t the first icy object. Comets are the first icy objects that we’ve been able to take a look at. We’ve got Cassini evaluating the Saturnian Systems where all of those moons are icy. And then, of course, there’s the Dawn mission which is going to be – which is approaching –
    Dr. Pamela Gay: Ceres.
    Fraser Cain: I – which is approaching Ceres right now, so what’s going to happen there?
    Dr. Pamela Gay: Well, when we get to Ceres there is this question of does Ceres have water geysers? There have been some observations made at far too great a distance, otherwise known as “from Earth,” that hint if you over process them enough that there could be, could be maybe, maybe, over process the image enough water geysers on Ceres. And, if that’s the case then we have the opportunity to start getting a sense of how much water maybe inherent in these asteroids that are at a greater distance. And the amount of water that asteroids have today is likely to be a lot less than they had in the past.
    So, if we start to find asteroids that have a fair amount of water today, and we know that like the Earth the asteroids experienced a warmer past. Well, it could be that in the past during a very heavy bombardment where we were getting hit with asteroids as well as comets, it could be that some of those asteroids, that maybe they are responsible for bringing water. We just don’t know.
    Fraser Cain: We’re really starting to blur that line between what is an asteroid and what is a comet. That there asteroids with very comet like attributes, and there are comets with very asteroid like attributes. We’ve had asteroids sporting tails, and even when you look at 67P, it looks like an asteroid, doesn’t it?
    Dr. Pamela Gay: Well, it’s very frozen at the moment. It’s very frozen.
    Fraser Cain: It’s very frozen in that it’s all covered in dust.
    Dr. Pamela Gay: Yeah.
    Fraser Cain: And, it just looks very rocky.
    Dr. Pamela Gay: And, this was one of those things that I actually got into a conversation with Jonathan McDowell and he and I went down the Google rabbit hole trying to figure out what the International Astronomical Union uses to define minor planet, asteroid, comet, minor bodies, small body. Well, there’s the Center for Minor Planets, and that appears to take into account comets and asteroids, and it appears that with the nomenclature both types of objects do have their own definitions, but it is definitely becoming systematically more difficult to differentiate between the two types of objects.
    Fraser Cain: Right. And so, I guess best case scenario, back to the Dawn mission, let’s say that – I mean, because you know it’s going to be doing a lot of this kind of analysis of the water. It’s really looking for water at Ceres. What do you think would be the best case scenario of what we would learn with this mission?
    Dr. Pamela Gay: Best case scenario is its spectrometers look at the water that is nicely frozen and easy to observe. Places in shadowed craters on the surface and goes, “Huh. That is exactly what we have here on Earth,” but yeah. It’s trying to get those sorts of detailed observations. It’s a small spacecraft. It’s not necessarily going to have all the instrumentation we need to completely, definitely say, “Yes.” Ideally what you want to do is scoop up a handful of dust and ice and measure it in a lab, but Ceres isn’t going to get landed on this time.
    Fraser Cain: Right. But amazingly there is star dust, there has been analysis and return of samples from a comet, so this isn’t entirely impossible.
    Dr. Pamela Gay: Well. And what we’re actually looking forward to is OSIRIS-REx which is actually going to do a sample return mission of the Asteroid Bennu. So, while the Dawn mission is in – definitely making massive strides in terms of imaging and regional spectroscopy, it’s not going to land and grab a handful of surface, but the OSIRIS-REx mission is going to do exactly that. So it’s a slow and gradual process. When NASA and ESA explore our solar systems they do it very incrementally. We started out with Mars’ exploration with Viking’s which kind of landed and looked around where they landed. And then we had the Pathfinder which was a little tiny rover dude.
    Then we went to Curiosity – then we went to Opportunity and Spirit which were much more freewheeling explorers, but they didn’t have all the instruments one might want. Now we have Curiosity. We’re following a similar incremental exploration plan of the asteroids. Dawn is just one of many stops that we’re going to be taking and OSIRIS-REx is really the next big step that we’re going to be taking.
    Fraser Cain: The other thing that I’m not sure if you’ve prepared for this, so feel free to Google if you need to, is that astronomers have done a lot of analysis of other solar systems. One of the kind of amazing things is that extra-solar planets researchers have also detected Oort clouds, vast clouds of water and even ice around other solar systems which is kind of mind-bending if you thing of this as even possible. So, in addition to looking just in the solar system astronomers are also looking out into other solar systems and they are able to see them at different phases of evolution. They can see brand new solar systems that have just formed and more ancient one, and – What does that tell us?
    Dr. Pamela Gay: Well, when we look at other solar systems it starts to give us snapshots and understand how other solar systems form. This has, in some cases, confirmed our understanding of early planets sweep out these bands in the dusty disc of the early solar nebula. In other cases it has left us scratching our heads how the super Jupiter’s migrate up to right next to their suns, like the hot ones, like the 51 Pegasus. So we’re in this weird situation of where we’re confirming parts of our understand of how planets form. We’re confirming things like how asteroid belts are normal. We’re starting to find rocky worlds. We’re starting to understand planets exist in places we never imagined.
    Really hot stars it turns out has them. Really tiny stars it turns out has them. The only stars that we can’t find them at are those that don’t have a lot of metals, and that makes sense because if you don’t have metals you have nothing to form planets out of. But, the reforming part of the solar system where it goes from that solar nebula to migrating their planets all over kingdom come we’re still very confused about how that happens and a lot of work has been done here on Earth. We’re able to look around and say, “Hey, Jupiter and Saturn were in resonance at some point in the past that lead to a great rearranging of our solar system.”
    And, unfortunately when it comes to the basic understand of how does the ion ratio in different places, how does the isotopic ratio in different places vary? Solar systems are faint. In order to differentiate between deuterium and regular water, H2O versus D2O, you need to have giant spectrographs on giant telescopes. And you need to have systems brighter than we’ve seen so far if you want to get images that show what the ratio is snuggled up to the star versus farther out. We don’t have the technology yet.
    Fraser Cain: But, I think – I know the solar system rearranging fascinates and haunts your dreams, and –
    Dr. Pamela Gay: Other things haunt my dreams, but it does fascinate me.
    Fraser Cain: Yeah, but this is – just this idea, right, that how could you possibly get an object as large as a super Jupiter that close to a star where an orbit is –
    Dr. Pamela Gay: And not in the star.
    Fraser Cain: Yeah, but –
    Dr. Pamela Gay: That’s the problem –
    Fraser Cain: – that fraction of –
    Dr. Pamela Gay: – why does it –
    Fraser Cain: – the fraction of –
    Dr. Pamela Gay: Yeah.
    Fraser Cain: – from it’s [inaudible] than even Mercury. And so you can – once you’ve got these gigantic vast solar system rearrangements then that’s gotta say that all bets are off. That everything’s on the table again. Look at Europa. Europa’s got more water than Earth does. You just – as a Jupiter moves toward the sun or as is – these planets interact with each other and they kick out a world you could imagine one of these collide with Earth in the ancient history and providing all the water in one go. So, all bets are off.
    Dr. Pamela Gay: Well, we – all bets are off currently, but there is the possibility of saying that when we have a more realistic sample from 67P/CG, oh, hey, its like Hartley 2 does, actually match the planet Earth. We need that second, third, fourth, fifth measurement as the comet gets more active. Right now we know that even just one of these high deuterium ratio objects hitting the Earth would have thrown off our ratios. So, because it’s so easy to pollute the amount of water we have with just one comet – yeah, all bets are off the table, but at the same time we can also say some theories are off the table.
    Fraser Cain: Yeah. So if you had to make a guess right now based on sort of what you’ve synthesized from your reading –
    Dr. Pamela Gay: Yes.
    Fraser Cain: Where do you – what do you feel is the most likely theory of where the Earth’s water came from?
    Dr. Pamela Gay: My gut is telling me that when we get more data from 67P/CG we’re likely to see a different ratio of deuterium and hydrogen, and we’ll find that a combination of Kuiper belt and asteroids can account for the water, but that’s my gut. My gut is not data. My gut occasionally believes in fairy tales.
    Fraser Cain: Yeah.
    Dr. Pamela Gay: So, I – my brain is saying, “More data, more data, more data, more data.”
    Fraser Cain: Yes.
    Dr. Pamela Gay: And, until we have more data opinions are nothing more than, well, fairy tales.
    Fraser Cain: Absolutely. All right. Well, thank you very much, Pamela.
    Dr. Pamela Gay: Thank you.
    Male Speaker: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at info at astronomycast.com. Tweet us at astronomycast. Like us on Facebook, or circle us on Google Plus. We record our show live on Google Plus every Monday at 12:00 p.m. Pacific, 3:00 p.m. Eastern, or 2000 Greenwich Mean Time. If you miss the live event you can always catch up over at CosmoQuest.org. If you enjoy Astronomy Cast, why don’t you give us a donation. It helps us pay for bandwidth, transcripts, and show notes.
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