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[Music]

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so thanks Laura

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my name is Caitlin Loftus I'm a PhD

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student at Harvard and today I go talk

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to you about some work on trying to

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constrain the presence of surface liquid

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water via oxidized sulfur in the

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atmosphere and this has been done with

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my PhD advisor Robin Wordsworth as well

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as Caroline Morley okay that's not

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working okay so starting big picture one

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of the big sparkly questions kind of

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driving exoplanet science is a question

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of how common is life in the universe so

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very traditionally perhaps we have to

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rethink this a bit and secrets talk

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about this water paradox but we consider

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its surface liquid water pretty crucial

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for the development of life because of

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its kazmo chemical abundance and also

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because it's a really great solvent to

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get these kind of organic materials

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started so one way we can try to track

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tably try to think about how common is

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life in the universe is how common is

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surface of the water in the universe we

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want to answer this via observations

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because water distribution during

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planetary formation via models is a bit

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so we really want to constrain this

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observational e but first we kind of

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need to figure out how we can actually

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observe these EXO oceans so basically

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just from the phrase surface like a

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water if we look at these individual

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words we can put pretty basic

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constraints on where we could possibly

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find surface water first we're talking

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about a surface so we want a rocky

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planet and second we want liquid water

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so that tells us about a temperature

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regime we expect the planet to be in

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based on stellar insulation so this is

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kind of the state of the art of

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observations right now what we can tell

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about the potential for liquid water

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looking farther to the future trying to

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actually detect service like a water is

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non-trivial we're gonna be getting our

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information from various forms of

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spectroscopy right now all of the method

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methods proposed for detecting liquid

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water have to do with the radiative

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properties

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these are theoretically observable but

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are going to require very far away

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instrumentation they have some false

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positive concerns and also they're not

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going to work for every planet so if we

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want to learn about liquid water we

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really need to develop some more methods

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to constrain its presence or absence and

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today I'm going to discuss how we can

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use the sulfur cycle to propose to new

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observational methods so detecting

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liquid water via liquid water radiative

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properties makes a lot of sense now

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we're trying to throw in the sulfur

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cycle there's actually two really good

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reasons why we want to use the sulfur

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cycle to probe the presence or absence

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of oceans the first is that atmospheric

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features of the silver atmospheric

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features of the sulfur cycle are

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observable and the second which is kind

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of physically how we can tie these two

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ideas together is via aqueous chemistry

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so first I'm gonna talk about very

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briefly because I'm sure it doesn't

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surprise the majority of people in this

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room that we can observe these kind of

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we can observe the sulfur cycle and the

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atmosphere we can think about how

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sulfate haze is very clearly distinct

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from clear atmosphere so here we're

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gonna look at transmission spectra and

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increase as the blue gets darker the

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optical depth of the haze present and as

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we kind of know and love or hate we see

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this kind of distinct flattening of the

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spectra very broadly and these

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simulations were done by Carol and

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Morley additionally we can think about

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atmospheric gases and how they affect

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spectra via specific absorption lines

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so2 it's also possible to detect as we

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start adding it in high quantities or

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high trace quantities into an atmosphere

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so if we can detect it for now we need

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to couple it to the amount of water we

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have present and we can do that via

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aqueous chemistry

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so first so2 is dissolving into water in

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an amount that's proportional directly

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proportional to the partial pressure of

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so2 at the surface that will give us a

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concentration of aqueous so2

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so that's telling us in a in a unit of

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water how much so2 is present as we

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scale up that amount of water and

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maintain the same concentration that

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requires we have more and more sulfur

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present in the ocean additionally we

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have further disproportion Asian

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reactions of that aqueous so2 with water

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to get some other sulfur bearing species

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that are all in equilibrium with that

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aqueous so2 which is the only sulfur

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species that's an equilibrium with the

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atmosphere these kind of other sulfur

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bearing species that I've highlight an

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orange I'm gonna be referring to just

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kind of collectively as s4 sulfur in the

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fourth oxidation state or sulfite what's

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kind of key here is that you can see

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this H+ ion and that's gonna make this

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whole system very dependent on the pH of

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the water so if we want to see how

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sulfur is stored in the ocean over the

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atmosphere we can put some numbers

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behind these reactions and equations and

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plot out so on the on the bottom panel I

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have pH and then on the y-axis log we

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have the amount of atmospheric sulfur

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and so2 over the amount of sulfur in the

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ocean that those s4 or aqueous so2

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products and then in blue we're going to

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be varying the amount of liquid water

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present and as we can see as you as you

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add more liquid water and the spa's the

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smallest line here is just a thousandth

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of an earth some ount of water in the

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ocean and go to kind of more realistic

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neutral pH as you're driving the amount

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of sulfur that's in the atmosphere

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compared to the ocean down to very small

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quantities so down to trillions billions

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those kinds of numbers that means if we

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want to get if we want to get sulfur

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into the atmosphere in the presence of

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an ocean we have to have a lot more

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sulfur lurking in the ocean versus what

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we're actually observing in the

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atmosphere and kind of to further

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emphasize this inability to get sulfur

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into the atmosphere in the presence of

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water

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as we're kind of pumping all the sulfur

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into the ocean because of this aqueous

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chemistry these s4 or aqueous so2

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products are being destroyed in the

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water because they're thermodynamically

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unstable so that means that we're kind

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of limited to having so2 to make those

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observable sulfate aerosols and so2 gas

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all that sulfur has to be supplied by

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recent outgassing and so in ocean and

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that is fundamentally limiting the

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buildup of so2 that's needed for

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fundamental atmospheric sulfur so now

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what we want to do is kind of put

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forward this hypothesis that if we can

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observe us o2 or sulfate haze

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we don't have an ocean or a large

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significant quantity of surface of the

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water present on that planet

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of course anecdotally in the solar

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system we know this is true with Venus

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but one example does not a very

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convincing Theory make so we test this

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via quantitative model of the sulfur

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cycle so some of you who think about

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sulfur a lot might have noticed that

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I've kind of been ignoring a lot of

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sulfur species and that's because sulfur

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can occupy a ton of different redox

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States and so they kind of tend to group

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collectively what's present on a planet

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based on the red ox state right now

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we're only interested in the oxidized

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surface so we're only looking at a

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system that has sulfate aerosols so2 gas

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and leads us for aqueous products then

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how we kind of set up our model as we

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say okay in order to have an

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observational sulfur detection we have

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to have a critical amount of sulfur in

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the atmosphere for that observation we

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track that down to how much water or how

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much sulfur needs to be present both in

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the atmosphere and the assumed amount of

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water present to say this is the total

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amount of sulfur in the atmosphere ocean

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system that we need to see sulfur to

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observe that sulfur and then we need to

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say that this amount around servation

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needs to be less than our expected

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amount of sulfur which we can get very

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easily from an assumed outgassing rate

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of sulfur and an assumed decay timescale

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of sulfur it should make you kind of

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nervous and I just kind of said the word

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assumed

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that's because a lot of elements of the

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system have very very large

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uncertainties and we try to do our best

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to account for the very many ways a

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planetary system could could could could

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develop a sulfur cycle so first kind of

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a really big problem is that while it's

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very robustly demonstrated that these s4

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silver products are thermodynamically

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unstable in water we don't really have a

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good idea of either the kinetics or a

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lot of times the mechanisms for this s4

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decay so in the modern earth ocean this

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decay happens basically so fast that you

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can't measure it but as you go to more

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limited oxygen regimes were a bit less

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sure about how fast this reaction occurs

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so in order to kind of couch for that

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because that's a huge part of how we're

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doing these the calculations of what

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some silver you can observe we kind of

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put all our results in terms of this

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decay timescale and we calculate that in

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terms of our ocean parameters which

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which are pH because of how sensitive

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the system is the pH of the ocean and

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also the size of the ocean and we're

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getting that decay timescale such that

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we can have the condition here that are

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there are critical amount of silver for

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observation as people which are expected

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amount of sulfur and then other key but

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poorly constrained model parameters that

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we have inner system are set first kind

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of realistically or our best guess is

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right now from kind of Earth Venus

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analogs and then second we do runs with

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physically limited scenarios so

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basically we say we look at each

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parameter individually and set that

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parameter to the to the value that will

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make it most hard to prove our

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hypothesis so will most encourage the

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build-up of sulfur in the atmosphere

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okay so now we can start looking at the

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results so we're gonna have contour

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plots and our x axis here is pH our Y

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axis we have two options for thinking

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about the size of these oceans

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first we have just in terms of number of

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Earth oceans and this isn't log and then

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we also have a global equivalent ocean

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layer in meters just because I think

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that's kind of an easier way sometimes

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to think about the quantity of water

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involved next we kind of put this box of

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around what our kind of our ocean

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parameters of interest they kind of say

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if you have less than a thousandth of an

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earth ocean we're kind of getting it's a

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regime where we're not really willing to

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call that significant liquid water

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anymore we also set upper limit of kind

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of PHS we we think are reasonable to

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occur because at six basically just

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because of how water rock interactions

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we would expect to buffer the pH to

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higher values but of course we plot the

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rest of of the parameters you have

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interest just so if you don't trust our

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box you can see the rest next the we

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have the blooming of like what we're

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contouring in color

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and this is the log of the critical

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decay timescale of these s four products

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and I'll note this is this is a log and

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this is in in log years so at the very

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end of of this this light blue it's

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getting to ten to the ten years so we're

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getting to the timescale of the length

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of the universe and we kind of divided

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between orange and blue saying or

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interest where we kind of are saying

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okay this is kind of a reasonable decay

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time scale of these s4 products from

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what we know right now about these

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reactions hope they will be able to get

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better experimental data to constrain

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this a little better as we got to

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lighter and lighter blue on the other

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side we're saying becomes less and less

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feasible or sober to build up to an

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observable letter level and then finally

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kind of mark a - grade point at where we

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see pH equals 6 and this decay timescale

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at kind of what we pick as this white

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line here at a tenth of the year to give

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a kind of a baseline of what we could

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say I'm a maximum ocean size would be so

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first looking at observable so2 for our

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realistic model parameters

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you need less than three microns Gogol

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equivalent ocean layer in order to start

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seeing observable so2 with these

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realistic parameters for contacts we

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have about a global current layer of

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three kilometers on earth today we can

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push into our very limiting or Lemmy

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parameter so basically basically the

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system that can help this observable a

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so to build up as much as possible

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obviously the system is a bit less

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favorable but we're still very clearly

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the are regime of interest is still very

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clearly in kind of many many years of

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decay before it starts becoming feasible

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that we would get observable so2 with

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those ocean parameters next we can look

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at sulphate Hayes's so for our realistic

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model parameters we see that to get an

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observable sulfate haze layer would

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require an ocean of less than 70 microns

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global equivalent layer again this is

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really small and you can note that kind

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of at our ten to the negative three

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ocean mass threshold that whole regime

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is is still blue and finally we can look

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at our limiting parameters for sulfate

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aerosols this is a a bit less favourable

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because we're basically inconsistently

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studying every single parameter to be

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limiting when really that wouldn't

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really be physically consistent we still

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find that for our regime of interest we

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still see that as like that's

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predominantly blue and still is less

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than 5 meters both will clip on layer of

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water so to kind of wrap up with what

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I've just shown via quantitative model

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of the solar cycle either observable

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sulfate haze layers nor observable

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levels of so2 are likely compatible with

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surface significant service liquid water

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this incompatibility seems to persist

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even in the most extreme conditions to

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promote observable sulphur and therefore

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we propose the observational detection

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of sulfate haze and as such a gas as two

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new constraints on surface liquid water

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and we've submitted the paper and it's

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up on archive here you want to check it

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out and with that I can take any

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questions Thank You Caitlin let's start

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with Sara over the wall didn't realize I

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was gonna ask questions in the session

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otherwise I would have that somewhere

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else

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um this is really interesting I'm just

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wondering if you could tell us a little

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bit more about how you're actually

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deciding like when there's haze and when

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there's not haze under certain

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so basically we're making a really

377
00:16:10,819 --> 00:16:08,360
really simple model all we're saying is

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we're kind of testing via transit

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00:16:15,620 --> 00:16:13,740
spectra what's an observable optical

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00:16:17,960 --> 00:16:15,630
depth of haze when can we start to see a

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00:16:20,449 --> 00:16:17,970
significant difference in our transit

382
00:16:23,300 --> 00:16:20,459
spectra by just increasing optical depth

383
00:16:24,949 --> 00:16:23,310
and then trying to setting that as how

384
00:16:26,660 --> 00:16:24,959
much haze is present in the atmosphere

385
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to be observable and kind of working

386
00:16:34,189 --> 00:16:29,160
backwards from that so it's entirely

387
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driven by kind of a guess at kind of the

388
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amount of hazy depressing the atmosphere

389
00:16:48,100 --> 00:16:45,810
to observe this atom I was wondering if

390
00:16:50,569 --> 00:16:48,110
you'd comment slightly more on Venus the

391
00:16:52,130 --> 00:16:50,579
units of our remember numbers correctly

392
00:16:53,990 --> 00:16:52,140
it's got sort of were 20 parts per

393
00:16:56,560 --> 00:16:54,000
million water in the atmosphere I'm not

394
00:17:00,290 --> 00:16:56,570
sure if your constraint on the 70 micro

395
00:17:02,480 --> 00:17:00,300
meters of precipitable would include

396
00:17:04,280 --> 00:17:02,490
yeah would include if that includes gas

397
00:17:06,049 --> 00:17:04,290
than beams yeah so I shouldn't I should

398
00:17:07,610 --> 00:17:06,059
clarify that doesn't include any gas at

399
00:17:08,780 --> 00:17:07,620
all that's just liquid water yeah yeah

400
00:17:11,179 --> 00:17:08,790
because Venus is actually quite a bit

401
00:17:13,100 --> 00:17:11,189
above that if you do yeah yeah yeah yeah

402
00:17:14,720 --> 00:17:13,110
you actually you need to have liquid you

403
00:17:16,159 --> 00:17:14,730
need to have I should say you need to

404
00:17:17,390 --> 00:17:16,169
have water vapor present in the

405
00:17:27,919 --> 00:17:17,400
atmosphere in order to start making

406
00:17:32,380 --> 00:17:27,929
those soul phases RJ hi RJ Oxford um

407
00:17:34,820 --> 00:17:32,390
I've read a paper - that argued that

408
00:17:37,549 --> 00:17:34,830
there you might be able to like support

409
00:17:42,049 --> 00:17:37,559
a a negative geochemical negative

410
00:17:43,490 --> 00:17:42,059
feedback on early Mars with sulphate so2

411
00:17:44,690 --> 00:17:43,500
as a greenhouse gas but then there's

412
00:17:47,450 --> 00:17:44,700
other paper than argue it's infeasible

413
00:17:50,660 --> 00:17:47,460
because of the haze formation but this

414
00:17:52,850 --> 00:17:50,670
kind of just like kill that down the

415
00:17:56,300 --> 00:17:52,860
water I think it makes it really

416
00:17:58,970 --> 00:17:56,310
difficult if you have liquid water to

417
00:18:01,790 --> 00:17:58,980
build up a pre Schabel amounts of so2

418
00:18:04,460 --> 00:18:01,800
for a long period of time all of this is

419
00:18:05,510 --> 00:18:04,470
assuming iquilibrium conditions so just

420
00:18:08,050 --> 00:18:05,520
like on earth and we have like very

421
00:18:11,240 --> 00:18:08,060
large volcanic eruptions and you inject

422
00:18:12,890 --> 00:18:11,250
so2 way up into the stratosphere there's

423
00:18:15,740 --> 00:18:12,900
a period of time where you can form kind

424
00:18:17,660 --> 00:18:15,750
of optically significant cases but it

425
00:18:22,400 --> 00:18:17,670
doesn't take very long to get back into

426
00:18:29,360 --> 00:18:22,410
equi Librium I think Daniel have the

427
00:18:33,640 --> 00:18:29,370
last question very naive question but is

428
00:18:38,510 --> 00:18:33,650
there sulphur on Mars today that we see

429
00:18:41,110 --> 00:18:38,520
yes we see a lot of sulphur in the in

430
00:18:43,730 --> 00:18:41,120
the minerals on the surface at least

431
00:18:45,290 --> 00:18:43,740
lucky looking from orbiters looking at

432
00:18:46,250 --> 00:18:45,300
subject rasca P but no one's really sure

433
00:18:50,120 --> 00:18:46,260
how they got there

434
00:18:53,090 --> 00:18:50,130
so is Mars than like another data point

435
00:18:54,800 --> 00:18:53,100
besides Venus that favours this so I

436
00:18:56,630 --> 00:18:54,810
guess I guess I should clarify the

437
00:18:58,940 --> 00:18:56,640
mechanism works in the sense of if you

438
00:19:00,530 --> 00:18:58,950
observe sulfur you can say that there's

439
00:19:02,120 --> 00:19:00,540
probably not for you observed these

440
00:19:04,460 --> 00:19:02,130
atmospheric sulfur you can say there's

441
00:19:05,870 --> 00:19:04,470
probably not liquid water significant

442
00:19:07,160 --> 00:19:05,880
quantities have liquid water present it

443
00:19:09,950 --> 00:19:07,170
doesn't go the other way and say if you

444
00:19:12,290 --> 00:19:09,960
don't observe sulfur that there is water

445
00:19:16,880 --> 00:19:12,300
present liquid water present if I

446
00:19:18,890 --> 00:19:16,890
clarifies all right let's think Caitlin