 Who here can get into rocks? Well, they're here for the right time. Join me in welcoming graduate student in Earth and Space Sciences in University of Washington, Michael Kepin. Thank you astronomers and astronomy enthusiasts for having a humble geologist out. I will try not to make a fool of myself. For here it goes on. What if I were to tell you to forget everything that Josh told you in the last talk and probably all of your common sense about this field of astrobiology. And if I were to tell you that we actually do have evidence of alien life. And it is on the surface of our Earth and it's unlike anything that we see living on our planet today. And what's more, the reason you have not heard about it is that it's sitting buried in the north woods of Russia. I'm being dead serious and I'm not talking about this. I feel I need to defend that we are actually doing science here. And so I am not talking about that but I am dead serious, I swear. What I'm talking about is this rock right here and many others like it. What you are looking at is a two billion year old stromatolite, in other words a microbial mat made of photosynthetic microbes that grow on top of each other. It's not the prettiest specimen here but this is basically a columnar stromatolite that's growing upwards this way. Here's a hand sample of a smaller example of the same thing. You see these layers here. I've outlined them for you. These are photosynthetic microbes growing on top of each other and a trapping mud that is dissolved in shallow water in between their cellular matrix. And this type of life, like Josh was saying in the talk before this, was prevalent on Earth for much of Earth's history. In fact almost all of it, about 90% of it. Here's another example of the same thing. This is a 1.3 billion year old stromatolite. You find these things everywhere. If you look if you're at rocks that are more than a billion years old. They have much nicer layers. And then what you're seeing is that they're thicker on the top. This is how we know it's not just some random set of entry structure. In fact the fact that these are thicker on the top is diagnostic because if this was just sediment, say filling into a little divot, it would be thicker on the sides, shallower on the top. But this sort of anti-gravity effect tells you that these are microbes that have a competitive advantage when they grow on the top of the mat because they need sunlight to do photosynthesis. This is just to say that when we look back at Earth's history, the vast majority of it was another planet that contains life unlike we see it today. And just to put this sort of into a more quantitative context, if we're thinking about looking at planets out orbiting other stars, we're going to look at them somewhere in their period of their history. We don't exactly know where. You saw on Josh's postcard of the planets. It could be plus or minus a few billion years. If we look at Earth's history four and a half billion years ago to today, of course we know that humans are newcomers. We've been around and bought for less than two million years. Animals in general are also newcomers. We only had animals on Earth, so just about 600 million years. But we think that microbes have been around for nearly all of Earth's history, all the way back to basically the oldest rocks we can find. And this brings another question which I'm trying to pick up where the last talk left off on, which is why was Earth a microbial planet for the vast majority of its history? And this has to do, again, with oxygen. I think that you and I breathe every living animal on the surface of Earth uses its metabolism. We think that there is less oxygen in the Earth's atmosphere, early in its history, even in its middle history, and only recently has there been a lot, perhaps that's what limits animal life. Because what we do as animals is we use oxygen to respire carbohydrates. For instance, you eat a Crunchwrap Supreme, it's loaded with carbohydrates amongst many other chemicals. You react with oxygen that you breathe as you slurp through your straw and drink your Baja Blast. And you get energy, believe it or not, out of that reaction. You also produce some byproducts and it can be potentially explosive. But in all seriousness, without the oxygen, you're not eating Crunchwrap Supreme or any food, seeds, plants, whatever. And so what we can do is look at these inflection points and think of oxygen and say, did one of those enable animals to persist? So the first of these, we call the Great Oxidation Event. Yeah, it's extraordinarily creative. And before this point in our history, we think there's essentially zero oxygen in the atmosphere. Just trace, trace amounts. And we see, accordingly, evidence of microbial life. This here is another stromatolite. This one's three and a half billion years old, found in Western Australia, the oldest stromatolite on the earth. Although that is a point of contention. And I'll leave it at that. Now, after this point, perhaps we would get more oxygen. Maybe we'd start to see more complex life. But in fact, it's more microbes all the way through. Up until through this whole middle history, you can find these stromatolites. This is, again, the one that's 1.3 billion years old. And it's not until about 600 million years ago that we have an event that we sometimes call the Cambrian Explosion or, in broad terms, the rapid diversification of animal life where it goes from basically no animals to abundant and diverse animals. You get things like trilobites. From this point forward, all of the motile, multicellular, intelligent, and even today, blitheringly unintelligent life is only possible because of oxygen. So it is both to thank and it gives and takes. This gives us a picture where perhaps there is some threshold, some level of oxygen that you must exceed in order to make animal life possible on a planetary surface. And it stands to reason maybe we crossed that threshold around 500 or 600 million years ago. This jives very well with our reading of the fossil record where the oldest animals show up right around this time, but we have microbes going back nearly as old as the Earth itself. This begs sort of two questions. The first is, given this disparity that the Earth only looked like it does today for a very small sliver of Earth's history, would we recognize the Earth as inhabited if we looked, say, 2 billion years ago as a, for instance, a remote river, say, picking the Earth just over 2 billion years ago? Could we say, A, that it wasn't inhabited? And, B, in taking it a step further, could we say anything about whether it had complex life or not? We know, in the case of the Earth, even if it didn't have, say, animal life at this point, we know it was a planet that came to support animal life. Might we miss identifying such a planet if we were to find it when we were doing our remote observations? So that's a question I want to motivate to talk with. And I'm going to approach it in the following way. So the goal here is to take what we had as a picture of this broad-stroke picture of atmospheric oxygen levels and try to get into the nitty-gritty, get a very precise record of oxygen and then see how that corresponds to the timing of these evolutionary events. And we have a tool for doing this. We call them redox-sensitive elements. And if your winders are going on right now, there's one slide to explain what that means, that you can refill on a beer if you care not to listen to it. But I use clip art from the figure, so it can't be that bad. So, what is a redox-sensitive element? It is, as the name suggests, an element that's only mobilized during weathering in the presence of oxygen. So basically, they hang out in the crust. Without oxygen, they won't be solubilized. What are some examples of them? All of the stuff in the middle of the periodic table that you don't probably think about, except for back in high school chemistry, molybdenum, selenium, rhenium. You probably thought about rhenium at some point, so that's good. Now, the basic premise here is that these elements, again, they are only mobile in the presence of oxygen. And if we look at, say, marine sediments, the mud at the bottom of the ocean, but clutch the stuff that falls out of the ocean, we should see more of these redox-sensitive elements in those sediments at times, and there's a lot of oxygen in the atmosphere. If there's a little oxygen, there shouldn't be much. Again, if you're more for pictograms, here they are, here are the elements in the crust. If you get weathering without oxygen, they stay clear. If you add oxygen to the mix, it reacts with them, they become soluble, and they can get enriched in marine sediments. So, for those who did not pay attention, actually, I could have just done it in one sentence. The high levels of redox-sensitive elements in a marine sediment means high oxygen levels in the atmosphere at that time. All right. So that's our basic premise. We're going to use that thinking, and if only we could find marine sediments that go from the modern era all the way back through its old history, then we could just measure those elements and we should get a really nice picture of oxygen levels. The problem with that is that old rocks, among them, old marine sedimentary rocks, are exceedingly rare. How rare are any of these? Here is a rough approximation of what the Earth looks like, of a recall, you know, not larger than Africa. And Mr. Besslin, I could find a good foot part there. If we wanted to look at rocks that are from the first half of Earth's history, so more than two and a half billion years old, we can find them in these places. That's not too bad. There's a fair number of places. For whatever reason, they happen to be really remote areas, but that's good news to a geologist. We like to just camp out in the middle of nowhere. That's just fine. But if you just take it a step further and you go to the first one third roughly of Earth's history, rocks older than three billion years old, it's a much more dire situation and it gets worse and worse the further you go back. So we have these precious few archives of the rocks that we are desperately sampling to put together this history of atmospheric oxygen. And so what do we do? What does it involve to do this work as a geologist? Sometimes you get to go out to the side of a mountain and you just hack off a piece of the old rock. Here's some black shales. This is formerly an organic rich marine sediment. This one's 260 million years old. It's in Wyoming. Now, this is all well and good, but these rocks aren't exposed like this. These rocks get weathered over time to get covered by the soil and the trees and just all the stuff that the geologist doesn't want. So this, for instance, was only exposed because they built a highway and they blew dynamite into the side of the mountain. So that was great. A lot of times that's not the case. And so what we're doing is we drill into the continental interior to recover these archives of ancient marine sedimentary rocks. So what we're doing here is a drilling project to recover 2.6 billion-year-old marine sediments. This is University of Washington Professor Roger Buick hands up because something apparently went right finally in the drilling process. So that's actually a huge castle of the whole enterprise getting the samples, but then what happens? So we get them and we get these little chunks of rock. We cut them with a rock saw. Then you put your fancy lab stuff on. You dissolve it in really harsh acids. You purify the element of interest. You take that solution. You put it in a fancy machine, a mass spectrometer. It's tuned to give you a very precise measurement of a single element in many cases. And then you get your data and you sit back and you have a beer and you think about it. And that's what we're doing tonight. We're going to show you some data and we'll talk about it. And then this could go on indefinitely if you sign up for something called a PhD. But let's talk about some data. So I'm going to show you three examples of elements that people have looked at to try to piece together this history of oxygen, three different studies. The first one is molybdenum. I don't know why they started with that. I don't know if they did. This is from a paper about a decade ago now, published in Nature, and it was the first to really blaze this trail of taking this approach. They gathered hundreds, if not I think a few thousand samples spanning Earth's history looking at the concentration of molybdenum. And what they found was pretty neat. These are their data. And you'll notice one thing that really stands out is this huge increase in the enrichment of molybdenum around, say, five or six hundred million years ago. That was when we, like I was saying I think that this rise to near-modern oxygen levels happened. Maybe you can pick up on a little increase here between two and a half and two billion years ago. And in fact, this data is part of the framework of data that we need to put in this picture together. That there is little oxygen, there is a stepwise increase, and then you reach the modern levels about six hundred million years ago. We're calling this one's a log scale, this one's not. So this looks huge here, but you may want to look at it. So that's all well and good. And then it stands to reason we should be able to look at other elements and see something similar. Or if it's different, it's because maybe they're slightly different. The next one people looked at, or at least in this exhaustive fashion, is uranium. And they put together even larger data. It's several thousand measurements. And here's the data that they generated, papered five years ago. Now, they see this large increase around six hundred million years ago. Instead of this stepwise increase here, they actually see this between two and a half to two million years ago. And they sort of postulated then that maybe we're in fact not looking at a stepwise increase. Maybe there is transient period of higher oxygen and then it got lower and then it came up again. Anyways it was an interesting idea. It would be nice to see that support in other systems. And that's what we tried to do. We picked a third element, selenium. It was the one that the dart did when they threw it at the board. And we said let's get several hundred samples together. We actually did it all once and compiled a lot of other data that had been published. And what we saw was this trend, which is another case of this peak here between two and a half to two million years ago. And then again the large increase around five to six hundred million years ago. So this all supports the idea maybe that there is this slightly higher levels and then it got a little lower. But there's more. We followed up on this and just recently we analyzed some examples actually from that same area in Russia that I showed you in the picture at the beginning and we see this. In fact we get to modern or even exceeding modern levels of selenium and ratio. And this is actually also seen in other elements. Now this is really something interesting. This may be telling us that we are approaching or at basically modern levels of atmosphere adoption at this time. What this would tell us then is that this curve is a little over simplified and in fact the history of oxygen enters atmosphere looks something like this. And maybe you don't really care if it's this one or the other one. Why is that an important finding? Recall back to this early slide I showed you the history of biological evolution here. We used to have this neat story where it was that there was low oxygen and then it got high enough across the special and then there were animals. But now it appears that there is this time where there may have been high oxygen levels even approaching modern. Now we can't really say precisely how close. Maybe not even with an ordered magnitude. But it stands to reason it's approaching modern levels and we don't have any fossil evidence or chemical evidence of animals being around. And this may have persisted for geological timescale such that to a remote observer there's some chance that you could catch us in that speak. So what does this mean then if we're going to use this as a framework to understand exoplanets? Recall like Josh mentioned the James Webb space telescope is going to launch in a few years. We've heard that before but I think it's going to finally launch in a few years. And it's going to capture some photons that come from distant stars pass through the atmosphere of some exoplanets beam that data back to Earth and it may potentially give us something that qualitatively at least tells us something like what this diagram is showing. If you draw your attention to the bottom spectrum this is looking at a stellar absorption that has these bytes out of it. These are certain molecules in the atmosphere that are absorbing this outgoing radiation. And in this case it allows us to detect perhaps oxygen. And as Josh was mentioning if you can carefully scrutinize other gases that are present in this atmosphere and even ascribe biological origin to the oxygen that would be really cool. We could maybe start to get to the point where we might feel good saying there's life or potentially life in other planets. Now this would require a lot more follow up work but let's even say we get to the point where we are certain that it's biological oxygen. There's a caveat here that even very high oxygen levels like those of the modern Earth even if they are confidently interpreted as biologically produced we can't take that as evidence that the biosphere that may be persisting on a planet has complex life as we would define it because we now know that there is likely this period in Earth's history where oxygen is fairly high but we don't think that there are any animals. Now that's just one vignette and I'm trying to now take a step back to the bigger bigger picture and say that that's one thing that studying the earlier can tell us about the search for life on exoplanets. Generally speaking I think this sort of way of viewing Earth's long history is instructive if not essential for understanding what we will be seeing in the atmospheres of these distant planets because while we do only have a single example Earth as the only inhabited planet we know in the universe in fact if you look at the four and a half linear history of Earth it effectively gives us these alternative Earths if you will different faces of the inhabited planet, the different points of its evolution and by studying these and knowing what they look like we will have a much richer catalog of what the habitable planets can look like in terms of their atmospheric composition which will really come in handy when we try to interpret what we're seeing. And so I hope that both this talk and the one before it indicates that not only we need astronomers in this coming era of exoplanet science and we want to say whether any of these planets are in habit but I think it will also require collaboration with field geologists, geochemists, geochronologists so on and so forth to make this happen. And just very last thing I want to close with maybe an inspiring note which is something that I heard an astronomer from Harvard say a few weeks ago which is that only one generation will ever get to make the discovery that there's life beyond our planet. We have the telescopes going up. We have this understanding of Earth's long history coming together. We have the tools at our disposal to make this a reality in the coming decades and so let's be that generation. Thank you. Please come close if you have a question. All right, so the question is that these events, these increases in oxygenary history, were they perhaps caused by some extra-terrestrial impact or something like that. The prevailing theory is that it was in fact a Earth-based cause because the amount of material flux basically to get that change to happen requires anything perhaps hundreds of millions of years of continued oxygen production and burial of reduced matter and so if you had to conceive of a single extra-terrestrial that could shift the whole system in that way but if you were to aggregate things like that at a time it could be some some contribution but you can see that's much more smaller than Earth-based causes. So the question was I focused a lot on animal life, but what about plant life? Is it perhaps even possible that plants producing oxygen preceded the animals that came from the Ural Islands? Something that that effect was very, very likely to be true in two ways. One is not necessarily to do with plants but with the first oxygen-like photosynthesizers which are cyanobacteria those are essentially required to get oxygen to accumulate in the atmosphere and something about their evolution either directly or evolution in the long-distance results in the around certain natural first rise of oxygen. The second one, what I'm showing you is a team of modern upstream worlds that actually I didn't go into here but the very very way to say it is that there are two pulses to that. The first one did not quite get to very modern bodies and we could not reach nearly exactly the amount of oxygen that we have today until plants were widespread because they are able to produce much more oxygen and very much more carbon. And that occurred probably four years ago in the post-nation of the development of the Ural Islands. That's why land plants took off trees Yeah, so why are the old rocks so scarce and hard to find? The reason the main reason they are so scarce has to be because dynamic profitability to recycle itself. So the first thing anything that's possible in the deep ocean oceanic crust is going to be subducted. And the subductions are on the off the coast of the Tascadia margin of Washington to do. There's oceanic crust going into the earth, it's gone. So none of what we're looking at was on the deep deep ocean. Some of it's on the continental crust that's underwater. So all of the stuff that stands on the continental crust that then is coming to the region. And when things do arrive it's typically when they are bought by the fire-raising slaves. Not really but in the curves, in virgins and plates of great relief. There's a lot of potential in that stuff that wants to get brought back down into the marine spaces. And so if this marine truffling recycling erosion is removed it's been erased most of the rock wreck that's in the western parts. Essentially just locked the things in the sea that could be on different continents. So how do we locate where those places are? Well, this has been that sort of map up to the aggregated work of generations of field geologists going out into the field. We map what is there to get a feel for what types of rocks there are. We don't know any chance that relatively you can figure out the succession of what falls below or beneath the rocks. But we don't know the absolute age that's going to take the same amount of time. You can get a sense of geotechnology you can measure some isotopic system at the pace of time and you can figure out the absolute age of the rocks. So that map is the only constant that creates an angle. So questions about why we exceed the spikes in 11 or this union. If they are produced by the same means it's still a nucleus of that process that's in parallel with all of these. The earth has the same amount of time to begin with by releasing these spikes at certain times. That just has to be the ability on the surface. And so what it's telling us is that at that point it's not going to bolt first. All of a sudden it has more of a lift going on. It's that more of it is being transported to a new set of elements. Whereas before that it's standing up to that crust. So the question where does the movement of all these elements before it's mobilized is present in different post-minerals in the crust. So most of it will be broken into some of the instances where present sulfide minerals cross a pirate in the most common but you get sense of the future of the movement of civilization. That's how it gets sensitive to oxen. Thank you.