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alright so as I mentioned this is the

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last of our two talks for planetary

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atmospheres I expect to see more next

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year guys seriously planetary

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atmospheres are awesome what I want to

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talk to you today about is mostly

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looking for life around other stars and

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one of the best ways that we can do that

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is by using the earth as an analog and

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so the big picture here is that we have

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this big blue ball and we know that

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there's life on it conveniently

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otherwise there would be no one here to

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listen to me talk now we can use the

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earth as a proxy by saying that life is

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here and we can see the signs for Life

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all around us and we want to do is we

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want to take those signs for life and

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extrapolate them to other places in the

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universe especially when what we're

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looking at is just the size of a pixel

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so looking for terrestrial planets

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around other stars this is about as much

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information as we're going to have any

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time in the next hundred years until we

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build some solar system size telescope

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to resolve these terrestrial planets now

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one of the biggest bio signatures in the

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terrestrial system is the oxygen that we

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are breathing now it is predominantly

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formed by biology on the present earth

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and oxygen has been suggested as a bio

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signature for basically 50 years at this

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point and attendance Upton's ups and

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downs there have been some suggestion

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that oxygen could have other abiotic

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sources but oxygen is wonderful because

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it has this big feature here at point

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seven six microns that is a pretty deep

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and we could see that so oxygen is

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visible at about greater than 1% the

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present atmospheric level so you can see

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this is 21% oxygen 1pl and as you

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decrease oxygen that feature kind of

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goes away now the nice thing is at low

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oxygen concentrations ozone which is a

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photo chemical byproduct of oxygen still

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stays visible so about point one percent

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PL o to you have a pretty

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a substantial ozone feature out in the

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infrared and so this is how we might

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detect oxygen in a planetary atmosphere

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now like I mentioned for some of the

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sources for oxygen we have transient

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sources in the present Earth's

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atmosphere including lightning so if you

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take some water and co2 nitrogen add a

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bunch of electricity you get n 0 and 0 2

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and some hydrogen or co this is

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remarkably short lived because that

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oxygen goes back to recombine with n 0

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or with h2 so it goes away fairly

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quickly another transient source is

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through co2 fatalis is for example in

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the upper atmosphere of the earth you

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can break apart co2 that loan oxygen

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goes off and finds itself a dance

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partner and makes 02 which could linger

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in the upper atmosphere more

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persistently however we have life which

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takes water in co2 and makes organic

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carbon which is buried in sediments and

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gaseous co2 which is allowed to

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accumulate in the atmosphere or through

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hydrogen loss so for example early Venus

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it probably lost most of its oceans

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through the Potala seas and subsequent

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loss of hydrogen to space leaving behind

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large amounts of oxygen which

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subsequently reacted saw planet it's

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gone today although there is a little

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photo chemical oxygen left over in the

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newsie naps here so if we're going to

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talk about oxygen as a bio signature we

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need again understand how it operates on

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the present earth and to do that we need

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to look at oxygen through time I showed

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this earlier in the intro mostly as a

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self-serving motion but if you look at

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oxygen before the gioi before the great

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oxidation event it is essentially zero

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the only sources of oxygen are going to

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be those transient or sources i

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mentioned before so co2 photolysis or

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lightning so there's very little here

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life however had different plans for the

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earth's atmosphere and so after the gioi

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oxygen is suggested to have jumped to

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between one and fifty percent of modern

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now there have been

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a number of recent suggestions that

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oxygen may have been lower so if you

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have read the 2014 klonoski at all paper

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in science they suggest that the

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proterozoic 02 so the oxygen level in

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here may have been in an order of

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magnitude or lower than an order of

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magnitude or more lower than the

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previous estimates for proterozoic

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oxygen and so what this means is that we

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could with a first generation

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terrestrial planet finder type telescope

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look for oxygen and may have detected it

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at the canonical values for proterozoic

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action or looked in the infrared for the

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ozone feature but really the earth could

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potentially be a planet without a

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significant bio signature for much of

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its history so this is bad news if we

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want to look for life elsewhere so this

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brings me to the crux of my question

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what is a false positive now as I

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mentioned before the proterozoic oxygen

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right after the first jump up was fairly

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low so it could be that in some

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situations the abiotic sources of oxygen

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actually produce more oxygen than the

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biotic sources did on the early Earth

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and so any abiotic oxygen in excess of

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and now I have to back away from this

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for a second and talk about some nuts

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and bolts just to give you guys a

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context for some of the things I will

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talk about towards the end of the talk

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when we talk about chemistry and a

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terrestrial planetary atmosphere what we

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really mean is that there's a whole slew

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of chemicals that are doing their own

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business in the atmosphere and in the

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case of co2 which I mentioned would be a

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big source of oxygen if these single

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oxygens could escape c 0 + 0 + m

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recombines those back into co2 or it

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would if that reaction wasn't spin

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forbidden and so what happens is you

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could build up CO and oh those o's could

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go off to make oxygen and we might see

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it that way however in the monitors

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atmosphere

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actually have these catalytic cycles

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that are fueled by the products of water

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vapor photolysis and so you can take CEO

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and the hydroxyl radical and through a

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bunch of other intermediate reactions

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you basically get a net result that is

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recombining CO and oh and this is very

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efficient in the prisoner its atmosphere

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that's why we get very little abiotic 02

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in the present atmosphere now this is

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particularly important if we start

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talking about other stars I know there's

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a lot of lines up here don't be scared

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the present Sun the solar spectrum is

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here in the black and you can see in

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this blow up that the the near you the

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far-uv which is these are in nanometers

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are is here and the far-uv is here the

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near-uv is here and there's actually a

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very different radiation environment for

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lower mass stars so you can see here GJ

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876 is an MSR middle M star and this is

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a dealio and gray which is another

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middle M star and those have very

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different radiation environments

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spanning this essentially a fairly

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arbitrary cutoff here so you can see

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that they have comparable amounts of

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what I'm going to call far UV and very

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different amounts of nuv up to two

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orders of magnitude less near-uv and

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this is particularly important when we

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look at the Potala sis cross-sections

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the absorption cross sections for some

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of the relevant species in the

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atmosphere so short word here we are

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getting still some fatalis asst of water

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vapor and co2 but long word than this so

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into the near-uv it's predominantly

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water vapor photolysis so that's that

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source of hydroxyl radical in an

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atmosphere now what that means is that

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i'm going to transition to a slide and

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wow sorry about that so the model in

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particular worries about the atmosphere

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as a whole and so we have to maintain

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global redox balance and I'm not going

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to get into the details because they're

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gory and kind of boring so I'm going to

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skip over them and say that if we assume

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that there are no geologic sinks for

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example on 100 million year time scales

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we

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can enforce atmospheric redox balance by

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assuming a return of reducing

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constituents to the atmosphere to

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balance the rain out of reducing and

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oxidizing species at the lower boundary

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that's a bunch of jumbled you don't have

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to worry about it if you have questions

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come see me the boundary conditions that

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are imposed are not tunable parameters

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they depend entirely on what you assume

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about the solid planet so these cases

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here where we're enforcing atmospheric

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redox balance that is in the context of

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global assumptions so I'm going to show

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you some results because that was kind

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of boring what we have here is I've

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taken the earth and I've plunked it

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around different types of stars and so

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for the Sun in the absence of life the

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oxygen mixing ratio at the surface is

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incredibly low it's our abiotic levels

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so like you saw before the gioi the

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oxygen was essentially zero for the f

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star which is slightly brighter than the

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Sun so the habitable zone moves out this

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terrestrial planet is nearly twice as

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far away from the Sun as the earth would

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be around the Sun the terrestrial planet

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on the f star is nearly twice as far

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away as it would be around the Sun but

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you can see that there's only slightly

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more oxygen in the upper atmosphere

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again that's from that Fatah lysis of

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co2 but again at the surface there's not

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so much what happens really is that

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around smaller stars where that

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radiation balance search the change is

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that we see we start to get for example

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with the K star which is slightly

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smaller than the Sun you start to see

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that the oxygen of the lower boundary

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irrespective of what you assume about

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the oxygen sinks the oxygen starts to

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build up and in the case of the M stars

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you get a range of values for oxygen

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based on your assumptions about the

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solid surface so in the worst-case

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scenario where we assume that there are

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no oxygen sinks which is likely

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unrealistic we get a few percent oxygen

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at the lower boundary which would be a

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detectable amount of oxygen in the cases

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where we assume that there is a large

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sink for oxidants

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for example the modern earth there's

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organic matter there's ferric iron

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deposition and banded iron formations so

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you could absorb a lot of that oxygen

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and actually draw it down to below that

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pollen offski at all false positive

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threshold I had suggested before and so

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to get back to this balance between the

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near you the far you the far-uv and the

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near-uv what we can say about what's

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driving the amount of oxygen is really

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this far UV to near-uv ratio and so if

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you plot these values for the stars

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based on their integrated fuv and

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near-uv fluxes you can see that the M

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stars plot way up here with the amount

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of oxygen being controlled by the amount

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of the ratio of f u v 2 nu V and the the

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g and the f star is down here and the K

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star sort of in the middle because it

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had a middling amount of oxygen but if

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you take the solar flux and you actually

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decrease the nu V so if you decrease the

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nu V we're coming over this way you can

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see that the oxygen actually builds up

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and and falls quite nicely along the

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other values for these stars and so in

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conclusion I'd like to suggest that

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there is little 02 or ozone around F and

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G type stars that there's a modest and

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potentially detectable amount of 02

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around k type stars and detectable 02

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around M stars in some cases EG if there

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are no surface sinks fro to and this is

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a lovely suite of spectra that Eddie

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schwieterman has put together from u-dub

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so not this you dub the other you dub

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and you can see that that for example

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that ozone feature in the UV is pretty

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strong for the M star and that oxygen

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feature at point seven and six microns

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is pretty strong too and so I'm going to

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leave that up I'm going to take

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questions for Sonny excellent i'm going

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to ask a naive question then so are you

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suggesting just that the takeaway is

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that if we have a detection around an

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effort a g-type star then that's

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probably fairly secure that it's biotic

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and not a biotic i would argue that if

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you do detect oxygen around an effort

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g-type star that it would likely be from

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a biological source okay of course the

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the gold standard biasing sure is oxygen

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in combination with some other reducing

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gas for example nitrous oxide there are

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very few and very small sources abiotic

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sources of n2o so those two together

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00:14:03,570 --> 00:14:02,110
would be a good bio signature any other

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questions otherwise we're right on time