Culture
Building Earth-like Planets: from gas and dust to ocean worlds.
In this episode, we explore the fascinating process of building Earth-like planets from dust and gas, focusing on the essential role of water in creating habitable ocean worlds. Join us as we delve in...
Building Earth-like Planets: from gas and dust to ocean worlds.
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Interactive Transcript
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So first, a very heartfelt thank you to Mr. and Mrs.
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LaBonna Frostowski for this honor.
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I'm really pleased to be here to give the first of these lectures.
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And also a tremendous thanks to Oxford as a whole for the warm welcome I've had this week.
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And particularly to Tamsen-Mayder for leading the charge to get me here as the astrophelow
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for this year at Oxford.
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I'm really very honored and it's been such a pleasure.
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And I've learned so much since I arrived and certainly my knowledge of noble gases has
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gone up by a thousand fold, which is a great thing actually very very useful.
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So thank you all very much indeed.
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So this is what I'll talk about today indeed.
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Building Earth-like planets from dust and gas to Earth-like ocean worlds.
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And the photograph I'm showing here is really in honor of Mr. LaBonna Frostowski.
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This is the Angara River in Russia.
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And the reason I'm showing it is because it's a beautiful image of water and life and rocks.
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And that's really what I'm going to try to talk about.
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So I'm always telling my postdocs and I've also generously told your postdocs here this week
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that they should be trying to answer the largest question that they can.
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And of course as scientists we can only meaningful and remake incremental steps.
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But we need to be aiming towards some large question.
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And so the large question that I like to say that I'm working on is are we alone?
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That's a very fundamental question of humankind.
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And so to begin to make progress toward that as a planetary scientist
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I ask myself what are the things that we know with a certainty are required by every kind of life we've discovered so far?
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And one of them is water.
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And so I'm going to take a line from NASA's line of book of quotations and say we're going to follow the water today.
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And what I'm hoping to convince you of by the end of this talk is that it is possible indeed likely
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for any rocky planet to accrete with enough water that it should have oceans almost immediately
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and become habitable in very short order without any stochastic by chance addition of water later on.
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And so what I'm going to try to do is track the water through the accretion of planets from the smallest particles
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up to planetary size objects talking about what I consider to be the critical bits of chemistry and physics along the way
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that allow them to retain water such that they could have oceans almost immediately.
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That's the goal.
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So the first question might be where are we in time?
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I always like to think about things in terms of time.
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So here's a cartoon of our solar system's development from on the left hand side 4.568 billion years ago.
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And I hope that that's a number that for many of you looms large in your mind.
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And if not then for the rest of you I hope that you'll remember it forever after this lecture.
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I used to tell my undergraduates at MIT that if they could not recite to me 4.568 they failed.
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And so that there will be no exam I promise.
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But that is the age of the very first solids that condensed out of the dust and gas cloud that became our planets.
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And so that's our beginning point.
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And sometimes people say that's the age of the earth but of course that's absolutely not true.
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And I hope you'll understand the ways that that's not true if you don't grasp that already.
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That's really just the first little pebble size object that condensed in our solar system.
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And so you can see very very shortly after that something that I'm terming here the moon forming impact.
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So this is the last giant accretionary impact that built our earth.
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And we think also through off enough debris to create the moon.
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And certainly there were many impacts to the earth after that but none of that magnitude.
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So that's kind of where our earth begins in a sense with the moon forming impact.
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Very shortly afterwards the first evidence of water oceans on the earth very very early.
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And then there's a sort of a late heavy bombardment that you see in cartoon.
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And then there's the gradual development of tiny life.
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And then finally the rise in multi-cellular life and the rise in oxygen.
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And then extinctions that of animals large enough that we can easily recognize this in the fossil record.
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And so at the other point I want to make with this slide before going on to zero in on the bit of time we're going to talk about today.
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It's that as we know and of course at Oxford and indeed and other universities there's an unbelievable heritage of the work that's been done on ages in that last bit during those extinctions when we have lots of fossils.
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And we've really been able to finally tune our understanding of the geochronology in the last say 500 million years.
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We understand a lot about the rate at which things happen and exactly when they happened.
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And then there's sort of what several of my friends and I call the boring billions in the middle where really we actually don't have as many landmarks as we might.
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And we don't know as much about rates and processes and events.
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And the other thing that I hope to impress upon you if you didn't already think this way is that just as we have great fidelity in our timeline very recently we actually have tremendous fidelity in our timeline way back where we're going to be talking about right now.
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And that's I think quite a remarkable achievement for science.
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So I'm going to show you next slide that just goes from 4.568 just to the moon forming impact and a little bit beyond through to the ocean.
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And now it's now it's vertical. I hope you'll forgive me for this.
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This is the very bottom of the geological timeline from Martin Van Crane a dog.
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And you can see on the right at the bottom 4.568 4.568 billion years the first solids in the protoplanetary disk.
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And then right after that 4.404 earth's oldest crustal material.
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And these are minerals called zircons that have weathered out of sedimentary rocks and have been found in western Australia.
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And they date to as oldest 4.404. So first let's look at 4.568.
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This is a calcium aluminum inclusion writ large. It's actually about a half of a millimeter across.
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And it is an example of the earliest solids to form in our protoplanetary disk out of dust and gas.
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And they can be dated quite accurately.
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And they come primarily the oldest ones come from a meteorite called Ayende that fell in Mexico in the 1960s.
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And happens to hold some of the very oldest of these calcium aluminum inclusions.
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They're virtually tiny pebbles. Some of them are centimetric in size.
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And they have the oldest ages of anything that we found.
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And so we have them because fragments of this unprocessed material falls to earth as meteorites.
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And it's really just the record of these earliest times.
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And so this is obviously not in real color. I wish they really were colored like this.
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But they're colored according to the composition. It's magnesium and titanium and calcium that have been colored in this slide.
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And the things that magnesium, titanium and calcium have in common is that they condense at the highest temperatures.
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So as the gas and dust is cooling, these are the first things to condense out. And they give us that earliest age.
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Now here are the zircons that give the next age 4.404 billion years ago.
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And they are from the Jack Hills in Australia. And you can see the scale bar. They are 100 microns.
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So that's a tenth of a millimeter scale bar. And I think this is, I'm not sure is this cathode luminescence or this is some sort of back scattered electron image.
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But what it's showing you is composition in color. And so you can see how the compositions of these zircons have changed over time as they formed.
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And then you can see the many, many holes where they were individual ages were obtained. And some of these ages are as old as 4.4.
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And so for those of you who have not actually experienced the wonder that is a zircon, it's zirconium and silicon and oxygen.
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And they like very much to crystallize with uranium in their matrix. And uranium decays to lead and gives us a radiometric way to measure their age.
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And the reason they're so good at this is because they will accept the uranium into their crystal. But they will reject any lead in their community.
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And so all the lead that ends up in them is from the decay of their uranium. So it's a very high fidelity clock. And they're also very beautiful.
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And so I will tell you that the earrings I'm wearing are natural zircon earrings from India. And I hope they're not very radiogenic.
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But they're quite a wonderful mineral. And so these have given us the oldest ages of an earth created rock that we have. And there is none older for many reasons the crust in between has been destroyed.
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And so that gives us this span of time. 164 million years to go from pebbles to an earth that has oceans.
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Because some of these zircons record the presence of liquid water on the surface of the earth as they were forming in their oxygen isotopic composition.
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And so that is a very short amount of time. And that's the amount of time that we're going to traverse in the next 35 minutes or so.
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All right. So here's a cartoon of the disc of dust and gas that I mentioned in my title from which are condensing these little fragments.
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And out of which are then growing pebbles and boulders and then what we call planetesimals. These miniature planets and tens to hundreds, maybe a thousand kilometers and radius growing larger into planetary embryos and finally all the way into planets.
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And that's the series we're going to traverse. But what is the material that we're starting with? I'd like to talk about this because it points out two things.
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One is one of the fundamental differences between physics and earth science and that is the use of the word metal, which is used entirely different in the two fields, differently in the two fields.
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All right. So this is a table from Anna Lauder's paper in 2003 in which she is looking at the composition of the sun.
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The composition of the sun is, I think, a probably very accurate proxy for the composition of the bulk material that made all of our solar system.
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And so it's very interesting to see what the sun is made of. And I did not know this until I came back onto the faculty at MIT and started working with astrophysicists that the sun and other stars are measured in terms of three components, X, Y and Z, which are hydrogen, helium, and everything else.
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And so this is how much of everything else that there is. And they're all called metals. If it's not hydrogen and helium, it's a metal, which is different from our use in earth science.
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But what is the point of this? Why am I showing this? Because we only have between 1 and 2% of everything else to make the planets out of.
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And we do have plenty of hydrogen, but not as much helium on our earth. But the vast majority of what the earth is made of and what you and I are made of and what is required to create light is in that Z. It's in that other category.
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So when we try to trace what happens to water, which is only the tiniest part of the Z, which is already the tiniest part of the whole, it becomes a really different exercise, difficult exercise, because you're just tracing the trace of the trace through these processes.
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Because most everything is hydrogen and helium. And the rest of the Z is almost everything else. So it's just a trace of a trace that allows a planet to be habitable.
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And so the point of this is that it only takes a trace of a trace to make oceans of water. And that's really the message in the end.
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So one pointed out to me once that if you look over the year of publication from 1984 to 2003, the amount of Z in our Sun is dropping precipitously.
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But of course that's a measurement error, not measurement error, but a changing in the accuracy in the techniques of measurement, not in an actual change.
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So what happens to this material? So on the left in this cartoon that I've drawn, we have the little pebbles and bits of dust and the things that are creating in the disc.
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And what is happening is they form into these bodies in the arc in the middle called planetesimals.
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And you can think of a planetesimal as an asteroid. And I think that some asteroids probably really were planetesimals. And many of them are just fragments of planetesimals.
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Vesta, the asteroid Vesta is a beautiful example of a planetesimal. So these are bodies tens to hundreds of kilometers in radius that are then going to gravitationally interact and build up into planets.
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And I want to make an important distinction here, which for some of you is second nature and you understand exactly what I'm drawing.
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And for others of you is maybe a new idea. And that is the topic of differentiation.
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Now it's a critical, critical step in planets. And that's when you take the metal part of what's building up and the silicate part of what's building up.
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And you allow the metal to sink into the middle and make a metallic core surrounded by silicate mantle, which is the structure of our earth and the structure of mercury and Venus and the moon and Mars.
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But not the structure of the original, caundritic, undifferentiated primitive, those are all adjectives that apply to the same class of materials, undifferentiated primitive material in which the metal and silicate are mixed on an intimate scale.
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So you take a lot of that intimately mixed metal and silicate and you put them together.
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And the first thing that happens on the left of this arc is they begin to heat up. And that's primarily because of the decay of a short-lived radioisotope of aluminum that really is only around in the first one and a half million years of our 164 million year window of opportunity.
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But it causes these planetesonals to be hot. And if they heat up sufficiently, the metal fraction leaks into the metal and forms the core.
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And so we think that there were planetesonals like on the left that are heated in some onion skin way, but not actually differentiated into a core and a mantle.
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And then others, you can see with the red hot metallic core, have differentiated. Some have retained a caundritic primitive crust and others have melted all the way through.
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Now, why is this important? Because this is the first important step where we need to trace what might happen to the water.
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So you have some amount of water in this dust that's creating into planetesonals. But if the planetesonals are heating up to 900 or 1,200 degrees Celsius or in fact much hotter if they're allowed to because of this aluminum 26, how can they possibly retain water?
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And so that's the first question that we have that I'll show you the beginnings of an answer in a moment.
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What happens to these planetesonals is many of them are swept up and become part of planets and some of them are broken apart and later delivered to us as meteorites.
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And so that's the far side. That's the library of material we have from which we can imagine to make planets.
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And so here is a graph of data on this library of meteorites that have fallen to the earth that are really just a selection of some subset of the material that was making planets early in the solar system.
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So here's a graph that shows you water in weight percent from 0 to 18 weight percent on the vertical axis and carbon from 0 to 4.5 on the horizontal axis.
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So here are two critical things water and carbon. And these are bulk compositions of these meteorite samples of many different kinds, both the primitive undifferentiated and the differentiated processed types of meteorites are shown here.
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And of course there's some you see that go up to almost 18 or almost 20 weight percent of water. And let me tell you that as many orders of magnitude more than you need to make a planet with oceans.
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So that's more water. In fact, Kevin's only always says the problem is getting rid of water, not adding water. And I certainly have come around to that opinion as well.
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So looking back here, the very wettest ones are the ones on the left that are these onion skin undifferentiated ones.
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And the ones that have differentiated are drier, but they're not perfectly dry. And so how would a body like a planet test will hold on to water so that it would own some water as it went to a creed into planets.
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It would still bring water to those planets. So I've been doing quite a bit of work on this in the last few years. And this is a paper that is being led by Roger Foo, who's, he's just a third year graduate student at MIT.
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He's mostly working with Ben Weiss and we did this project on the side. And he's someone to watch for. He is very, very bright guy. He's mostly doing paleo magnetism. He's a person who just recently measured HED meteorites from Vesta and showed that Vesta had a core dynamo in a magnetic field.
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So he's doing some quite interesting work. And it is copious spare time as a PhD candidate. He did this project with me on the side. And our question was, you start on the left here with a large early accruing planetesimal.
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And with progressive radiogenic heating from the left to the right, the first thing that's going to happen is any isses or water bearing minerals are going to be heated to the point that they break down and they give off their water as a free fluid.
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And this free fluid is going to start to percolate upward through buoyant flow in the planetesimal. And Ed Young at UCLA has done a lot of work on this. And he said that this percolation in bodies less than about 40 kilometers in radius is not very efficient.
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And actually that will get recirculated and the mineral grains will rehydrate and so that body will hold onto its water. But in bodies larger than 60 kilometers, the fluid loss is relatively rapid.
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But then these fluids may freeze in the extremely cold boundary layer against space near the surface. So you would end up with a body that was dry on the inside and wet on the outside.
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And so then we've asked the further question, if the silicate part starts to melt, what's going to happen with that melt? Is the melt going to erupt onto the surface? Is it going to remove all the remaining volatile to space? Or can in fact this planet has to retain both a wet crust and it silicates in the interior?
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And so what Roger and I have done is we have calculated the density of the melts that would be expected from many different classes of meteorites. And we found that some of those melts are buoyant and they should erupt on the surface, allow volatiles to go free into space and dry their parent body.
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But others are actually denser than their coexisting solid and they would be expected to stay inside and you would keep a planet decimal with a bit of a wet crust.
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And so these are the kind of processes we're trying to follow through a planetesimal evolution. I actually think this is a really interesting field and I have a lot of ideas for more work on planetesimals.
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There's so critical to understanding what kind of planet you can build. You have to make a planetesimal first. And in fact at my department on Monday and Tuesday we're having a big workshop on planetesimals. We have about 65 people coming from Europe and Asia and Australia and all over the US to come talk about this problem.
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So the question is how do you form a core on a little body like this? How does melting proceed? What kind of volcanism could you have and can you keep hold of the water? And the reason it's so interesting is because first of all there's very low gravity and the melting process is entirely unlike anything that happens on earth.
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So for those of you here who like to study earth melting processes I'll just talk about this for a moment. I think it's really interesting.
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So on the earth melting happens in several ways. One is by having pressure released from it. If you have packages of the mantle that are rising up toward the surface, the pressure on it goes down and down and it may then be able to melt through the release of pressure.
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And another way that melting happens on earth is by the addition of water in a subduction zone. You add water to the hot mantle and it melts because the water lowers its melting temperature.
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Neither of those things happen on planetesimals. What happens on planetesimals is they are heated through their internal heat, through radiogenic decay.
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And so the heat is hottest in the middle of the body and the least on the outside. The melting starts at the bottom of the mantle instead of at the top of the mantle.
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And because heating begins progressively from the temperature of the disc up to the melting temperature of the silicates, they are able to pass through the different thermal regimes on their way, lose their water, for example, dewater all their hydros minerals and become a dry melting source before they melt as silicates.
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So for those of you who like to study melting I think that's quite an interesting regime and not one we're used to.
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But the point of this slide is just that we think that there are different ways that even fully differentiated planetesimals could hold onto some water but it would be heterogeneous in that body.
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And so here's the same part of the library of meteorite samples to see if we might be right or wrong. Here's water on the vertical axis as before but only to 1 weight percent, not to 18 weight percent.
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And on the horizontal axis carbon only up to 0.5 weight percent. So we're looking at the extreme corner of the previous slide that I showed you right down near the origin.
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And the points that I've highlighted in orange are samples from differentiated planetesimals, ones that have been through this core forming process, ones that have been through some degree of melting and might be expected to be fully dry but in fact they're not fully dry.
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There's one out there up there at about 5 or 6,000 parts per million of water that is 0.5 or 0.6 weight percent. But even the ones down near the origin are in the vicinity of 0.1 weight percent of water which is 1,000 parts per million in its bulk composition.
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So 1,000 parts per million that doesn't sound like very much but I'm sure many of you realize that if you were to take all the water on the surface of the earth and mix it back into our silicate mantle.
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It's only between 200 and 250 parts per million because that's the equivalent of four oceans right there on a sample that's fairly dry.
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So that's a part of my reasoning for saying that planetesimals retain water as they're going into the planet building process. And so we have this picture like this.
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So we have planetesimals and they some are differentiated, some are not differentiated. They are colliding together and forming embryos which are these larger bodies and you can see I'm showing on the embryo that some of these big impacts actually melt a portion of the embryo.
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And now we have a magma ocean on the surface of the embryo. This is a miniature planet that's say 1,000 or 2,000 kilometers in radius.
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Some people say Mars is an embryo that was stranded without anyone to continue colliding into it and making it into a proper size planet. So it's sort of an embryo it's a little embarrassing for Mars I think.
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And then these embryos collide together and make planets. And so here's a summary of where we are so far.
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So on the left we have the calcium aluminum inclusion. Oh sorry, on the very far less we have gas. This is just the gas disc around our growing young star.
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And then there's the calcium aluminum inclusion at 4.568 billion years ago. And on the right you see the planetesimals and the embryos growing up into planets in the middle you see an asterisk which is the universal scientific symbol for then something magical happens.
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Because I don't know if any of you are working on this. I didn't happen to talk to anyone who was but there are a few people trying to bridge that gap.
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And here's the problem. If you have got dust and tiny, tiny pebbles and they have a little bit of a differential velocity but they're colliding together in that spinning disc.
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They can stick partly through electrostatic forces. I mean it's easy to say it's very much like the dust bunny under your bed at home. These things kind of clump together naturally.
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And when you get up to things that are hundreds of kilometers or tens of kilometers in radius gravity is now important and they can actually collide in a creek.
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But for those middle-sized bodies, electrostatic forces are not strong enough and neither is gravity. And so there's really sort of an open question. How do you get from the dust to the planetesimals?
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So we know for a fact that it happens because here we are. And so there are a lot of ideas having to do with for example, kelvin Helmholtz instabilities and turbulent compaction. But that's a really wonderful unsolved problem.
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So now to skip on to the accretion part. Here's a beautiful figure from Raymond in 2006. And it's illustrative of many models that have been made of the accretion part of planet building, the gravitationally driven accretion.
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So let me explain what we're looking at and why. What they've done is they've made a dynamical code that imagines something like a thousand and 54 planetesimals going around the sun. And the code tracks every single one.
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And when they have a certain proximity and relative velocity, they're soon to have accreted into a planet. But sometimes they're not quite that close. And so they might gravitationally bend each other's orbits and cause planetesimals to mix in or out or cause planetesimals.
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Orbits to become less round that is more eccentric. And so the question is the question we're trying to solve at this point is how much mixing is there radially when you're making a planet? Traditionally it was thought that the earth accreted from material at the radius of the earth. And Venus accreted from material at the radius of Venus.
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And now we know neither of those things is true. Not only do planets accreted from material at a wide range of radii, but the planets themselves probably migrate since they were made. And so it becomes more complicated. Why is this important before we walk through this? Because ostensibly, and there seems to be good evidence for this, the farther away you get from the sun, the wetter your little planetesimals are, the more ice.
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And so if you want mercury to start with some water, it would be good to have some mixing inward while it's accreting. And so the question is how much mixing is there? So there are six time slices here. And we'll start at the top left at zero million years. And the vertical axis is eccentricity that is how far out of round is the orbit. And the horizontal axis is semi major axis. And so you're going out to just, I guess, five AU. And they do have a Jupiter put in out there past the edge of the planet.
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So if you're looking at the surface of the screen, so to speak, because those gravitational perturbations are very important. And then the color scale is water. How much water? So you can see in the first slide you start with dry things near the sun and wetter things and then wetter things going further out. And you can see to the right at 0.1 million years. There's been a lot of gravitational interaction and a lot of the orbits have become quite eccentric. And some mixing radially is occurring. And as you step through, you see more and more radial mixing of material as orbits are perturbed by neighboring energy. And so as you see, I'm going to have a little bit of a sense of how the
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planet has moles of material from far out in the disk is thrown inward and
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material from inward is thrown outward.
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Until finally in the last disk you see that this particular run has made four
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planets, each one of which is wet. There's no planet that's all red.
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Even the innermost planet has had material mixed into it.
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And so I think that you can just never claim to make a perfectly dry mercury by
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only accruing material from next to mercury.
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There's so much efficient radial mixing.
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And I could have showed you a paper from 2013 or 2005 and from
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a number of different research groups they seem pretty in agreement about this.
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So if we think that we can retain some water and even relatively dry planet
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decimals and then even mixing of radial mixing of planet decimals adds water
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to planets. So then we're I think upping the chances that these planets end up
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with water. And so what happens? Now here's where we get to the stuff that I love
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the best. What happens when you have an embryo, a big planetesimal,
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coming in and smacking into another embryo to make a proper size planet.
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And so this is a cartoon of this process made by Casey Liss at the University of
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Maryland. And so here's this innocent hapless embryo going around a star.
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Unbeknownst to it being pursued mercilessly by this other embryo.
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Of course this is just an artist's image of what happens, but I like it because
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it's visceral. And so what happens is the energy of accretion is often sufficient
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to melt the entire target body to some depth. And you end up with a magma ocean,
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which is one of my very favorite things to study. And also I think I think really
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very poetic, isn't it? To think of a planet during its growth, having so much energy
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input that it should become just a ball of liquid magma in space. And so there are
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many dynamical simulations. I could have showed you computer models of this instead
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of this cartoon that with careful energy balance do indicate that for example in the
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moon forming impact for the earth that that impact would have been sufficiently energetic
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to melt the earth to between 2,000 kilometers depth or all the way down to the core.
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And so then the earth would have started again as a magma ocean. And so starting, I don't
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know 10 or 15 years ago or something, I started to ask myself if I want to make progress
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in understanding how terrestrial planets form. I need some kind of good starting point
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for a forward model. Everything I've showed you up to this point, you can model bits of
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it and we begin to get a hold of it. But it's hard, it's very stochastic or it's sort
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of unknown conditions. We don't really know how things solidify on planetesimals. But
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this is a clean starting condition. We know a lot about how silicate melt solidifies.
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Maybe not enough, we don't know enough, but we know a lot. And so it makes a nice starting
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position to make forward models and predictions of the planets that might result. But I would
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say that the idea of a magma ocean is quite old. Lidenits who we like to think of as
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one of the fractious co-founders of calculus started his career as a mining engineer in
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the Hartz Mountain Silver Mines in Germany. And his job was to keep the mines clear of
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water so that the mining could go on. But being an inquiring mind, he noticed that the
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water he was pumping out was quite hot and that the farther down he went into the mine, the
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hotter it got. And so he made a simple first-order inference, which was the earth is a sphere.
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It's cold on the outside. It's hotter in the middle. Therefore, through time it's been
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cooling through its surface boundary condition. Therefore, if you go back in time, it was hotter
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yet. And he posited that the earth had actually condensed from some kind of a vapor or a plasma
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and then gradually cooled so that its interior was hotter than its outside. And in fact, it
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was his work and this wonderful book, Protogaya, which you can still buy and read if you've
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not, that was later read by Fourier, which inspired him to produce the heat transfer equations.
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And so I'm very fond of saying that earth science is at the very basis of mathematics. It's
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not usually a popular thing to say, but I say it anyway. So there we are. So even before
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we really knew what an atom was, we had the idea of a magma ocean from a different path,
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I suppose. So are you asking yourself, hopefully I've taken you to this point and you're convinced
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that planetesamoles might have some water, embryos might have some water, we have some chance
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of bringing water to the planet. But when we look at the energy that's involved in a giant
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impact and the fact that the entire planet is melted, how can you possibly retain volatiles
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through that process? And there are, of course, people who still say that you cannot, but
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I think that the evidence is that you can. And I'll give you some actual observational evidence
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before I argue with you on a theoretical basis. So here's some fabulous data from the messenger
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of mission to Mercury. Now Mercury, before we get into the specifics of this graph, Mercury
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is a planet, as I'm sure that you know, that has a very large metallic core in comparison
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to its thin, silicate mantle compared to the other planets. And the going model for that
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is that Mercury had a very large, creationary impact late in its formation. Actually, it wasn't
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a creationary, I don't know what the right word is, it was destructive. And it blew off a
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lot of mercury silicate mantle that was then lost into the sun, leaving a very thin mantle
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behind. So if that's so, then Mercury is surely the poster child for the giant impact.
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But even if it's not so, Mercury itself was also built by giant impacts because that's
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inevitably how planets are built. And there's the one real problem I have with that particular
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hypothesis for Mercury is that you have to remove the silicate mantle from Mercury's orbit
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immediately. Otherwise Mercury will sweep off its silicate mantle again as it continues
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around. So that I think is an unsolved problem, which perhaps you could go solve would be
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good. So this is a compilation of a lot of different data. And so why are we looking
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at potassium on the vertical axis and thorium on the horizontal axis? Potassium and thorium
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are two things that the Messantra mission could measure with its instruments. And their
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importance is that potassium is volatile. So it's going to act like water. It would like
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to go into a gas phase sooner than its neighbors. And thorium is not volatile. And so it's going
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to act more like something that wants to stay on the planet. So the very low slope line
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with the little blue triangles, that's all lunar material. Lunar material is notoriously
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dry. It was thought for a long time that the moon was absolutely perfectly dry on an
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almost part per billion basis. And you can see it in this slope of this line that for a
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given thorium, not volatile content, there's relatively little potassium or volatile. So
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it's low volatile content. And then on the much steeper line, you'll see a tremendous
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amount of data from Mars in black. Down at the very bottom if you can see it, there are
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in the earth line on the same line. They have about the same amount of volatile elements
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for non-volatile elements. And the surprise of Messinger, a messenger data relies on that
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little red circle. Mercury is about as volatile rich as Mars in the earth, which is kind
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of amazing. So mercury held onto its volatiles despite its most violent birth and its position
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very close to the Sun. And so that's point number one. There's observational data from
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Messinger that planets that have been through giant impacts hold onto their volatiles.
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Now in 2008, was it Alberto Saul at Brown University and Eric Hauri in my department at
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Carnegie and some other people started measuring materials from the moon and looking for water.
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And I will tell you, being friends with these people and in the community, people thought
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it was kind of a silly thing to do. But then they found water and then they became very
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famous. And so if they were not already, it was a huge fine. They measured little volcanic
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glass beads, which I've also measured during my PhD. I remeasured a whole bunch of them
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from Apollo 15. But I never thought to measure them for water. Isn't that a pity? Now
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I look back on it. And they found water in these little volcanic glass beads. So it was
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melting inside of the moon and fire-fountaining eruptions that threw magma onto the surface
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which froze in tiny beads and it had water in it. And they've since gone back and measured
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additional volcanic glass beads. Now why would the moon be dry? Everyone thought it was perfectly
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dry. There's metal in the samples that are brought back by Apollo, so there can have been
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high oxygen activity would have oxidized all the metal. And the moon is thought to be the
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detritus through into the largest giant impacts. And so it should have been dried, right? Turns
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out it wasn't dried. Isn't that amazing? So here's data from their latest paper, 2011.
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And these are three graphs with fluorine sulfur and chlorine, which are also volatile elements.
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And on the horizontal axis water. And I don't know if you can read it, but it goes from 0,500 to
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1,500 parts per million of water. And in the big blue blob is mid ocean ridge basalt from the earth.
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And so what they're showing here is that the orange dots of the lunar glasses plot more or less
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in the mid ocean basalt field. So they're about as wet as mid ocean ridge basalt's from the earth.
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Now I hope that there are not very many people who think that the whole moon is that wet. It's
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thermodynamically, it does not make sense. You can't make some of the rocks that you see.
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It's that wet all the way through, but there's at least a part of the inside of the moon that's
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that wet. All right, so here's my little summary slide of wetness on the moon, which is a variation
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of one that I made for physics today a couple of years ago. So on the vertical axis is water or
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hydroxyl, which is OH in parts per million by mass, parts per million. And it goes from point
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001 parts per million up to 100,000 parts per million. Now in the left hand gray box where it says
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volcanic glass is an appetite mineral grains, these are measurements of volcanic products from the
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up to about 7,000 parts per million. And then in the next little box, it are from the lunar surface,
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surface grains that as measured from orbital spacecraft and shadowed crater soils from orbital
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spacecraft. And I especially wanted to show this here because this when this data was released
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a few years ago, even my graduate students were confused by the headlines that said the moon has
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water. And I don't know if any of you were confused or taken aback by those headlines, the moon has
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water. But my graduate students actually in lab group, they said, well, do they mean like there
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were rivers like on Mars in the past? No, no, no. And so for comparison next to that over on the
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right is Sahara Desert sand. And so the surface of the moon is approximately as dry as the Sahara
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desert. It's pretty dry. But it's just not zero, not zero. So that can't. So when people say damp
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and wet in geology, it means all kinds of different things, doesn't it? Yeah. And so here at the
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very bottom, a little picture of the moon down at .001. And that was everyone's favorite bulk moon
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water content after the Apollo. I shouldn't say everyone. It was a favorite bulk moon content
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after the Apollo era. And now, how many people think it's more like 100 parts per million in the
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bulk moon? 100 parts per million is the equivalent of a half of an ocean on the earth. So damp, I don't
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know what adjective would be best. And then on the right, you see the bulk moon possible range. We
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don't know how much water is dissolved in the rocks in the mantle. It might be another ocean. It
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may be five more oceans. Some people think ten more oceans, but I don't know. I think that's a
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little extreme, but there are arguments in many different directions. But the point of this is that
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even in the moon, which is supposed to be the driest body round, is damp. And so then the question
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becomes, now we've built up our earth. And I hope that I've convinced you that it could have retained
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some water along the way. So did the earliest times of the earth do it look like life magazine said in
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1952, did it deserve the name Hadeen at that time? I love this because it's got the giant moon
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looming right over the horizon because the moon was formed on the a couple of earth radii away
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from the earth. And so it was huge and raised giant tides on the earth and vice versa back then. And
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or so. And if you have not read it'll oak, halvinos, short story and cosmic comics about when the
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moon was close to the earth, you should read it right away. It's quite lovely. He talks about how
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in the old days, when the moon was close to the earth, they would go out in boats on the ocean and
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they'd set up ladders and they'd climb up and they'd jump and then they would reach the moon and
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they would mine it for moon milk. And it's just gorgeous, gorgeous story, gorgeous. So the
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question is, or did it look like this? Did it in fact, did it look like the New York Times said
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that it did in 2008 and this was based on the Zircon evidence that the oxygen isotopes in these
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Zircons show evidence for liquid water in the very earliest time of the earth. And I think that this
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is the answer. I think that very, very rapidly the earth looked like this and it was not violently hot
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and rapidly convecting. So I just have I think three more slides to take you through the last bit of
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this talk because I don't like to go over and trying to be careful. So I spend a lot of my time
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making models of magma oceans and this is a wedge cross section of a hypothetical planet with a
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core and a magma ocean. And so I set up both the chemical and the physical equations to simulate
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solidification of the magma ocean. So cooling is happening at the surface boundary layer,
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cool drips are going downward, they're depositing solids at the bottom. And I,
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a priori, I decide the sequence of minerals that are going to form. So the bottom is going to be
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perhaps post-probskyd if it's a deep magma ocean or provskide in Magnesia, Wustite, for those of
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you who like this glossary of mineral names. And there's some experimental data on how trace
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elements or bits of water like hydroxyl or carbon might partition into those solidifying minerals
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and out of the liquid. And so I calculate the equilibrium composition, both composition of the
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minerals and I partition into them these trace elements. And then as solidification continues,
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things like water that really don't like to go into these minerals, little bits of them go
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into the minerals, but more of them stay behind in the evolving liquids. And so as solidification
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progresses, the evolving liquids become richer and richer in incompatible elements that don't
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like to be in these minerals, including water. And so water content in the evolving liquids go
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up and up. And then we allow the water and the carbon dioxide or whatever the phases we're looking
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at to digass into an atmosphere. And then we track energy transfer out of the atmosphere. And
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that way we get both a composition and a time scale for solidification of the planet. And so
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here's the sequence of events in the simplest case. The simplest case sequence is solidification
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happens from the bottom upward, but slower and slower as solidification continues because
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there's a thicker and thicker atmosphere which slows down heat transfer to space. So in the
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beginning heat transfer is very fast, solidification is very fast, but then as gases join the atmosphere
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it slows down. Now here's a really key step that I haven't mentioned before. The solidification of
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a magma body like this produces in its later steps heavier minerals than it does in the beginning.
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And partly that's because all these minerals have got many of them have A site that will take
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either magnesium or iron. And they all prefer magnesium. And so they build up a magnesium rich
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layers and iron is enriched in the meltheads that's excluded from the fractionates. But then eventually
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the minerals are forced to take in more and more iron. So the last ones are quite iron rich and
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therefore denser than the first ones. They're also a little wetter because they partitioned in a
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little more water. And maybe there's also more titanium or chromium or other dense minerals. And so
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so so now what we've done is we've solidified the magma ocean and we've made a pile of a of a
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solid mantle which is unstable the top of its dense and it's undergoing an undergrow solid state
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overturn and the dense stuff is going to sink to the bottom and the bottom stuff is going to rise
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to the top and then the planet will cool down to what I hope will be that ocean on the in the
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New York Times. And so this is what the models look like. So I apologize for the size of the font.
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It didn't occur to me to the last minute I had to make it bigger. On the vertical axis is the
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log of atmospheric water partial pressure. So it's just how much steam is in the atmosphere,
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what's the pressure of it. And so if you look at zero on that vertical axis to the right is a
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little red E which is where earth plots right now. If all of its atmosphere was water we're pretending
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it's all water. And on the horizontal axis is planetary surface temperature. So if you start over
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at the right at time zero in red or blue two different models you can see that the planetary
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surface temperature drops very fast. Those curves straight to the left as temperature drops.
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But then the atmospheric pressure goes up and the rate of cooling goes down and it ends up
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completely solid at T1 in between 10,000 and a million years. So depending on how much water
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there is to degas it happens quite fast maybe even just tens of thousands of years to solidify
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the entire mantle of the earth. But now you recall it's unstable and the dense stuff at the top
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is going to sink down and the hot stuff at the bottom is going to rise up and it's going to melt
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a little bit again and you end up up there at T2 on the right at a hot temperature next to two
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period overturn. And then in these models the surface just cools through its radiate of atmosphere
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straight to the left. I'm not assuming any atmospheric loss in this. They're very simple models.
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But in my simple models very simple models and in other people's much more sophisticated models
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that do the chemistry and the radiate of a convective atmospheric transfer. I've worked with
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a group at Jet Propulsion Laboratory and with Erykayav and Lamar in Austria. We all seem to agree
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that we get into the liquid water field order one two three million years. Incredibly fast.
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Incredibly fast. And so another thing that I like to think about is this cools in all of my
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models. The pressures end up to be right through the critical point. And so you would end up with
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this point where you have super critical fluid over the whole surface of the earth as it's
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collapsing down into an ocean which I think is a really interesting thing to think about. And so
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here's that same figure again water from zero to 0.7 weight percent in this case and from zero
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to 0.4 carbon. And the same library of meteorites with both the differentiated and undifferentiated
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ones. And so the thing that's going to happen to that atmosphere as it passes through the critical
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point and collapses is it's going to make a water ocean. And so it turns out that if you start with
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0.1 weight percent of water, a thousand parts per million, that's the equivalent of making a 5 kilometer
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deep water ocean over the entire surface of the earth. But if you start with this little is 100
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parts per million, you still get hundreds of meters of deep ocean. And so what we think happened
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is that the earth probably went through several giant accretionary impacts to reach its current size
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with magma oceans of varying depths and water contents of varying amounts that would within
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two or three million years cool all the way down, collapse into a water ocean over the whole
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surface of the planet. And then along would come the next accretionary impact. So not only do I
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think that there were oceans on earth very early, but I think there might have been several oceans
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on earth very early. All right so very briefly a little advertisement for Simone Marci and Bill
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Bocchi and I are working on models for that tail of accretion. So how much crust can survive
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through these different ages and how much of the earth's surface is covered by giant impacts.
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I think it's extremely interesting work that will bear on what happens to the early atmosphere.
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And this is really just an advertisement friend who's coming to the AGU meeting in December
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and would like to talk more about what might happen with a series of impacts. And so then here's
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my final slide where I just want to sum everything up. And on the vertical axis, on the horizontal
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axis, there's no vertical axis really. On the horizontal axis is billions of years before the present.
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And I'm sure you can now recite with me the age of that red line 4.568 billion years ago. That's
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our CAIs, our calcium aluminum occlusions. At the very top in purple, those are some ages that we
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know. These are actually radiogenic dated ages for core formation in planetesimals for the formation
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of some of these differentiated meteorites. Planetesimals form very very early. And then models
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show that they could still have molten interiors after some tens of millions of years if they have
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a cold crust against space. So the next thing here in yellow are lunar crustal ages. The moon, as I'm
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sure that you know, when you look up at the moon, you see craters impact basins that are filled with
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basaltic rock. So that's the most basalt you can see with the human eye in the solar system.
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And the white material around it is a flotation crust. This is Pledge of Clay's Feldsbar that
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floated to the top of the lunar magma ocean and made what were called rock burgs. I can't
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remember it was by Smith or by Wood in 1970 when they posited the magma ocean on the moon immediately
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upon return of these white rocks by Apollo. So these are the ages that those flotation crust
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rocks on the moon. They've spanned a very long time 200 million years. In green are geochemically
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determined ages of differentiation of the earth's mantle. That is that solidification process
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of the mantle that created different silicate domains that happened very early. Then on the right
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is the earth's surface. These zircons that date to as far back as 4.4. The ones that have
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unequivocal evidence for water is date to about 4.3. So what that gives us at the bottom is this
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purple line. So this is the time of earth accretion from pebble size CAIs through the asterisk
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miracle up through planetesimals and embryos to make the earth through multiple giant impacts
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that I think created multiple oceans and cooling events. And then I think the latest possible
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moon forming impact might have been about 4.41 to give the earth sufficient time to cool down for
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a final ocean to make these zircons that are preserved today. And so the reason I like this graph
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and I've had a couple of versions of it in a couple of papers in just the last couple of years is
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I love thinking about the evolution of the moon and the evolution of the earth on the same scale
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because what this means is that you could have been standing in a New York Times version of
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a watery landscape at say 4.36 if you could breathe the air and not you know all the rest of it that
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would be unpleasant but you could be standing with your feet in something that looks like an ocean
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and looking up at the moon and knowing that inside the moon was still an annulus of magma ocean
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that was yet unsolidified. And the reason that the moon would still be a little molten inside
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is because it has that flotation crust on the outside and the conduction of heat through that
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flotation crust is the rate limiting step for heat loss and it hasn't managed to cool all the way
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and the earth has because it didn't have a flotation crust and in fact we are completely ready
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for habitability to begin by about 4.3 billion years ago and so this is what I mean when I say
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the fidelity of the timeline in the very first part of the solar system is getting almost as
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exciting as the fidelity of the timeline that we have near the present. So thank you very much.