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[Music]

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Thank You Alec for the lovely

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introduction talk what I'd like to talk

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to you about today is some of the work

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we've been doing funded by a NASA

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emerging world proposal to look at the

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atmospheric evolution of Venus

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especially in its early period when we

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expect that it may have lost most of its

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ocean and to give you guys some broad

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context obviously here's the earth and

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here's Venus to scale and like Alex said

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we often think of Venus as Earth's twin

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if earth had a hotter twin they have

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very similar reservoirs for example of

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volatile carbon most of it on Venus is

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stored in the atmosphere whereas on

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earth it's stored in like things like

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carbonates in the near surface and a

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little bit in the atmosphere similar

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amounts of nitrogen we think and it has

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a similar mass and radius and one of the

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things that sort of drives us to ask

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questions about Venus is as a

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comparative planetology perspective how

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do we go from an object that's beautiful

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and blue and has liquid oceans to

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something that's a hellish landscape

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that you can not let on and then it

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rains acid this transition is really

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poorly understood and it's often seen in

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the perspective of comparative

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planetology when we look at something

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like the habitable zone and this is a

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figure of my own design we can talk

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about many variations later on but

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essentially what you can see here is we

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have this solar system for some sense of

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scale and this is the x-axis tells you

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about the amount of starlight on plow

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planet relative to the sunlight on the

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earths of Earth here is that one Venus

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receives about twice as much sunlight as

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the earth does and Mars receives about

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30 48 percent and a number of Kepler

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objects and one radial velocity

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measurement are on this for to give you

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some sense of the

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diversity of exoplanets we expect and

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have found in around other stars and you

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can see that that the the way we've

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defined the conservative habitable zone

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and the optimistic habitable zone are

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these yellow and red lines which are

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directly related to what we think is the

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underlying process that has driven

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Venus's evolution here in the yellow

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line is a moist or runaway greenhouse

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where the server temperatures heat up to

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the point where you evaporate some or

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all of your surface ocean it gets broken

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apart and you lose it and this recent

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Venus limit is a consequence of the most

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recent evidence we have for Venus's

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resurfacing which will come up again in

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the talk but this was a it could be you

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know about half a billion or a billion

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years ago and this is often based on

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some older estimates of when that

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resurfacing happens so I'm going to use

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a billion years just as a conservative

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estimate for that mile milestone and so

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the the thing I want to point out is

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that the we know that the earth is

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habitable conveniently otherwise there

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would be no one here to listen to my

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talk but somewhere between here and here

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there's either one point or several

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points or a number of underlying

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processes that drives you from beautiful

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and blue to bad things have happened and

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so one of the things that we asked about

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Venus is how did all the water get out

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of the atmosphere like Alec mentioned it

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has a very high D H ratio which suggests

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that it lost a lot of water

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it currently has incredibly small

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amounts of trace water in its atmosphere

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but not anything on its service and the

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runaway greenhouse atmosphere takes you

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from something that may be temperate

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either co2 or n2 dominated and as that

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water evaporates the atmosphere becomes

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water dominated if you evaporate one

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earth ocean you get something like 300

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bars of water vapor in your atmosphere

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it goes it's an incredibly disastrous

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effect in that sense and what you can do

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once you put the water into the

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atmosphere is you destroy it by

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photolysis either in the UV where you

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break it in the hydroxyl and atomic

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engine or if you're looking at the

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extreme ultraviolet instead of just

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breaking apart the water you can

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actually shatter it into its component

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pieces so you might be left with atomic

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oxygen and a couple of atomic hydrogen's

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if you're pummeling it with an e UV flux

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much larger than you see in the modern

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and so just to come back to that extreme

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ultraviolet the at smaller extreme

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ultraviolet fluxes for example less than

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10 times the modern solar EUV at the

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earth we find that atomic hydrogen is a

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minor component in the literature and

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that the majority of the hydrogen is

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bound up as h2 or h2o now conversely for

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higher UV fluxes you would expect that

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most of the atmosphere is actually going

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to be dominated by atomic hydrogen again

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because you're sort of pummeling this

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water vapor into its constituent pieces

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and that brings us to sort of three

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points we can think about in Venus's

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timeline where we might expect that the

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intersection between the runaway

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greenhouse process which is a function

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of the total flux that the planet is

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receiving versus the escape processes

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which are often driven by this EUV

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heating can intersect with one another

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where there's some overlap and so here

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on this lovely diagram i've made for you

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this is for example the formation of the

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solar system about four-and-a-half

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billion years ago this is today and this

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is a Venusian timeline we can think

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about the amount of EUV the extreme

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ultraviolet that Venus is receiving as a

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function of time and we can think about

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the amount of sunlight that Venus is

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receiving with time like I mentioned

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earlier Venus is receiving about twice

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the flux in both UV and integrated flux

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as compared to the earth these are

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relative to earth values and one of the

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suggestions is that Venus lost its water

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very early in its history it accreted

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went into a runaway greenhouse as part

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of that accretion process and never left

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and so that hamana at all result from

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2013 would have driven the escape of

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Venus's water in the first hundred or so

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million year

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of its evolution where the installation

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was actually relatively lower it's about

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135 percent which is still interior to

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the runaway greenhouse limit from the

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classical habitable zone but the e UV

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flux is anywhere between 10 to 20 or

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more times larger than the modern a UV

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flux and so you're talking about again

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this this regime where you're sort of

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pummeling the atmosphere which may be

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predominantly water vapor shattering

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those water vapor molecules so you can

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drive that escape and as you move

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forward in time you have to go from this

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EUV flux that's greater than 20 times

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modern to something like 2 times what

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the earth receives and this insulation

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actually increases in the opposite

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direction so the e UV is tailing off but

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the insulation is increasing and so the

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installation drives the runaway

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greenhouse process but the e UV drives

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the escape and to other points we can

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think about for example a billion years

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into Venus's history the e UV has

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started to tail off the installation is

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increased by about 20 percent this is

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really that sort of knee I talked about

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when you get below about 10 times modern

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solar e UV where the atmosphere goes

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from being potentially atomic hydrogen

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dominated to something like molecular

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hydrogen dominated if you are

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dissociating water vapor in the

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atmosphere and that atomic hydrogen can

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recombine

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and as I mentioned before the last

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resurfacing was about a billion years

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ago or about three-and-a-half billion

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years in the Venus's evolution that's

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the last possible time that we may have

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had water it might likely did not but if

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Venus was resurfaced we would have lost

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any evidence that it had liquid water

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before that time and as you can see the

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UV flux is approximately modern and the

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insulation is about 175 percent of the

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Earth's insulation which is still well

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inside the runaway greenhouse limit now

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the thing that the atmosphere does is it

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absorbs and repurposes solar UV air is

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actually the most I'm sorry the most

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absorbed wavelengths in the air are the

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e UV wavelengths which

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run from basically you know ten or so

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nanometers up to about a hundred and

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twenty-one nanometers which is that

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lyman-alpha line a little bit beyond

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that 125 nanometers and across that

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whole region most of the atmospheric

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constituents that we're worried about

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for example co2 and n2 which are the

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predominant components of Venus's

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current atmosphere absorb very strongly

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in that region and as you can see the

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fluxes vary across a couple orders of

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magnitude from a very young Sun to

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something more like the modern Sun now

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the e UV that's driving this process is

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going to essentially move the Venusian

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atmosphere from genes escape mode where

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individual molecules are reaching escape

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velocity and just ballistically exiting

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the atmosphere to a regime where there

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is flow of the atmosphere the atmosphere

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is moving as a fluid and that's why we

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call it hydrodynamic escape to a point

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where it's no holds barred everyone out

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the door escape you're just scooping

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large portions of your atmosphere off

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thermally and driving it away from the

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surface of your planet this is this is

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much more cataclysmic than you might

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expect for this regime this is like you

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are disintegrating your planet bad news

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so we I'm going to really focus on this

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middle one here where you sort of have

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this nice calm regime of things just

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flowing away in a mild setting so like I

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mentioned before there's there's three

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regimes of thermal escape the jeans

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escape limit where you have high-energy

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molecules on the tale of the Boltzmann

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distribution reaching escape velocity

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and exiting the atmosphere there's the

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energy limit where you assume that all

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the energy all a UV that's poured into

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the atmosphere is used to drive escape

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there's the diffusion limit where you

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have some escaping constituent that must

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move through a heavy background gas and

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that limits the amount of material you

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can supply to the upper atmosphere that

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can escape and then lastly you have

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something called the radiation

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recombination limit which I will not

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touch on but that's basically you're

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limited by essentially the pummeling

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that you're giving to the atmosphere and

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it's photo ionization

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balanced by radiative recombination and

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most of the rest of talk is going to

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look at these two limits which are at an

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interplay in these cases and as a brief

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note one Earth Ocean lost in 100 million

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years is a flux of about two times ten

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to the thirteen molecules per centimeter

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squared per second so I'm going to throw

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up some numbers and some graphs and

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that's really sort of the benchmark here

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for the evolution of the atmosphere the

277
00:11:40,480 --> 00:11:39,020
diffusion limit is a strong function of

278
00:11:42,670 --> 00:11:40,490
composition so if you have heavy

279
00:11:44,620 --> 00:11:42,680
constituents as your heavy background

280
00:11:46,510 --> 00:11:44,630
gas and you have a light escaping gas

281
00:11:49,090 --> 00:11:46,520
your flux is much higher than if you had

282
00:11:51,910 --> 00:11:49,100
for example a light background gas and a

283
00:11:55,510 --> 00:11:51,920
lighter escaping constituent I will say

284
00:11:57,790 --> 00:11:55,520
that this point here is sort of a bad

285
00:12:00,070 --> 00:11:57,800
thing to do on a plot like this this

286
00:12:01,750 --> 00:12:00,080
there is no diffusion limit when it's H

287
00:12:04,060 --> 00:12:01,760
and h2 dominated it's you are just

288
00:12:09,190 --> 00:12:04,070
limited by photons at that point there's

289
00:12:11,950 --> 00:12:09,200
no diffusion the research the literature

290
00:12:15,550 --> 00:12:11,960
on this type of project often gets up to

291
00:12:17,500 --> 00:12:15,560
about ten times modern solar in UV for

292
00:12:19,030 --> 00:12:17,510
earth cases because we're worried about

293
00:12:20,800 --> 00:12:19,040
the evolution of the early Earth and a

294
00:12:23,440 --> 00:12:20,810
lot of these circumstances and you can

295
00:12:26,500 --> 00:12:23,450
see that by and large these cases are

296
00:12:29,110 --> 00:12:26,510
all limited by diffusion except when you

297
00:12:30,610 --> 00:12:29,120
roll over here and that's really when

298
00:12:33,190 --> 00:12:30,620
you're reaching that energy limit you

299
00:12:34,900 --> 00:12:33,200
have used as much energy as possible to

300
00:12:37,290 --> 00:12:34,910
drive escape but you cannot get to the

301
00:12:39,850 --> 00:12:37,300
point where you are limited by diffusion

302
00:12:46,750 --> 00:12:39,860
so this is an energetic limit and this

303
00:12:49,330 --> 00:12:46,760
is just a physical chemical limit so the

304
00:12:51,280 --> 00:12:49,340
model is solving these coupled equations

305
00:12:53,500 --> 00:12:51,290
for several components in the atmosphere

306
00:12:55,000 --> 00:12:53,510
at this point there's no chemistry so

307
00:12:56,650 --> 00:12:55,010
the two components are basically what I

308
00:12:58,990 --> 00:12:56,660
tell them to be either atomic hydrogen

309
00:13:02,170 --> 00:12:59,000
molecular hydrogen and then whatever

310
00:13:05,350 --> 00:13:02,180
heavy background gas but the mass

311
00:13:07,240 --> 00:13:05,360
continuity momentum continuity and the

312
00:13:09,340 --> 00:13:07,250
energy equations couple these two

313
00:13:13,690 --> 00:13:09,350
components so that the exchange energy

314
00:13:17,440 --> 00:13:13,700
and velocity over time and so if we use

315
00:13:19,600 --> 00:13:17,450
this model to explore the parameter

316
00:13:21,190 --> 00:13:19,610
space for the earth which is always a

317
00:13:23,440 --> 00:13:21,200
good place to start since there's a lot

318
00:13:26,320 --> 00:13:23,450
of literature on it we see that

319
00:13:28,990 --> 00:13:26,330
the model reproduces fairly well to

320
00:13:31,740 --> 00:13:29,000
within a factor of two or three results

321
00:13:33,820 --> 00:13:31,750
from the literature for the earth and

322
00:13:35,950 --> 00:13:33,830
then we get to the point where we can

323
00:13:37,660 --> 00:13:35,960
start to ask well what about Venus early

324
00:13:39,790 --> 00:13:37,670
in its history here we're talking about

325
00:13:41,470 --> 00:13:39,800
fluxes that are on the order of ten

326
00:13:43,990 --> 00:13:41,480
times modern because remember this is

327
00:13:46,030 --> 00:13:44,000
Earth so if you park a planet twice's a

328
00:13:48,550 --> 00:13:46,040
little bit closer you get twice the flux

329
00:13:50,590 --> 00:13:48,560
which is what the case for Venus if you

330
00:13:53,320 --> 00:13:50,600
do that what you see is that the

331
00:13:55,720 --> 00:13:53,330
Venusian atmosphere early in its history

332
00:13:56,830 --> 00:13:55,730
remember we're at 20 times solar UV so

333
00:13:58,900 --> 00:13:56,840
it's going to be atomic hydrogen

334
00:14:01,150 --> 00:13:58,910
dominated and if you assume the

335
00:14:02,890 --> 00:14:01,160
background atmosphere is a runaway

336
00:14:05,260 --> 00:14:02,900
greenhouse case where it's water vapor

337
00:14:06,880 --> 00:14:05,270
and co2 dominated you find that it's

338
00:14:09,490 --> 00:14:06,890
essentially diffusion limited across a

339
00:14:12,040 --> 00:14:09,500
broad range of potential hydrogen

340
00:14:13,330 --> 00:14:12,050
concentrations which covers a lot of the

341
00:14:15,220 --> 00:14:13,340
parameter space you can start in a

342
00:14:17,560 --> 00:14:15,230
runaway greenhouse way up here where

343
00:14:19,420 --> 00:14:17,570
your hydrogen's being borne by water

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00:14:20,860 --> 00:14:19,430
vapor and then as you lose that

345
00:14:23,200 --> 00:14:20,870
atmosphere you still follow that

346
00:14:25,720 --> 00:14:23,210
diffusion limit down across this whole

347
00:14:27,730 --> 00:14:25,730
parameter space and this is for Venus

348
00:14:30,130 --> 00:14:27,740
and its early history if for example we

349
00:14:33,190 --> 00:14:30,140
worry about when Venus transition across

350
00:14:34,690 --> 00:14:33,200
that the recombination limit where

351
00:14:36,910 --> 00:14:34,700
you're actually looking at the escape of

352
00:14:39,760 --> 00:14:36,920
molecular hydrogen from a water vapor

353
00:14:41,530 --> 00:14:39,770
co2 atmosphere these limits decrease and

354
00:14:44,530 --> 00:14:41,540
you actually come off the diffusion

355
00:14:46,600 --> 00:14:44,540
limit much sooner so if you drop below a

356
00:14:48,640 --> 00:14:46,610
few tenths of a percent water vapor in

357
00:14:50,320 --> 00:14:48,650
the atmosphere for example it becomes

358
00:14:54,040 --> 00:14:50,330
much harder to get the rest of it out in

359
00:14:56,320 --> 00:14:54,050
this hydrodynamic flow regime and then

360
00:14:57,640 --> 00:14:56,330
lastly if you look at it about a billion

361
00:14:59,350 --> 00:14:57,650
years ago you get the same sort of

362
00:15:00,910 --> 00:14:59,360
behavior where it just touches the

363
00:15:02,440 --> 00:15:00,920
diffusion limit at one point but its

364
00:15:04,150 --> 00:15:02,450
energy limited through much of the

365
00:15:06,340 --> 00:15:04,160
higher hydrogen concentrations and then

366
00:15:11,650 --> 00:15:06,350
it's well below the diffusion limit for

367
00:15:13,000 --> 00:15:11,660
the rest of that and so in conclusion we

368
00:15:14,440 --> 00:15:13,010
think that the radiation environment for

369
00:15:16,450 --> 00:15:14,450
early Venus would have favored rapid

370
00:15:18,130 --> 00:15:16,460
escape of the photons as products and

371
00:15:19,180 --> 00:15:18,140
the parameter space where Venus is

372
00:15:20,680 --> 00:15:19,190
atmosphere would have experienced

373
00:15:22,960 --> 00:15:20,690
diffusion limited escape is diminished

374
00:15:24,940 --> 00:15:22,970
with time composition composition

375
00:15:26,800 --> 00:15:24,950
matters chemistry matters and that's why

376
00:15:37,040 --> 00:15:26,810
that's a second part of this proposal

377
00:15:43,130 --> 00:15:39,180
just a to point out these are all

378
00:15:47,880 --> 00:15:43,140
pictures from as of AB grad Con 2012

379
00:15:55,380 --> 00:15:47,890
2013 2014 2015

380
00:16:00,360 --> 00:15:55,390
is it apps icon folder 20 no I wasn't at

381
00:16:07,110 --> 00:16:00,370
2016 is it apps icon I've been doing

382
00:16:09,360 --> 00:16:07,120
this too long yeah so the blow-off point

383
00:16:11,940 --> 00:16:09,370
yeah what would that look like on a time

384
00:16:14,970 --> 00:16:11,950
scale like how fast does that occur once

385
00:16:17,790 --> 00:16:14,980
it reaches that point it depends on your

386
00:16:19,350 --> 00:16:17,800
your model parameters it tends to you

387
00:16:21,269 --> 00:16:19,360
often see that it's sort of episodic

388
00:16:23,699 --> 00:16:21,279
where you'll heat up the atmosphere and

389
00:16:25,920 --> 00:16:23,709
it'll go through sort of a phase where

390
00:16:27,870 --> 00:16:25,930
it's pushing off atmosphere and then

391
00:16:29,880 --> 00:16:27,880
it'll cool off because of that adiabatic

392
00:16:32,639 --> 00:16:29,890
expansion and collapse a little bit and

393
00:16:34,740 --> 00:16:32,649
then it'll just pulse but tip if you're

394
00:16:36,660 --> 00:16:34,750
driving that if you're putting enough

395
00:16:38,130 --> 00:16:36,670
energy into it to drive that sort of

396
00:16:49,280 --> 00:16:38,140
blow off regime it tends to be fairly

397
00:16:56,340 --> 00:16:54,179
3:40 okay there we go um first off so

398
00:16:57,929 --> 00:16:56,350
all those pictures of our Predacon I

399
00:16:59,759 --> 00:16:57,939
think I may have taken most of those you

400
00:17:01,499 --> 00:16:59,769
took all those and I did not credit you

401
00:17:02,730 --> 00:17:01,509
and I apologize oh no it's just like

402
00:17:07,409 --> 00:17:02,740
wait these look familiar

403
00:17:09,990 --> 00:17:07,419
um secondly um so at one point in my

404
00:17:13,049 --> 00:17:10,000
earlier academic career I spent a bit of

405
00:17:15,299 --> 00:17:13,059
time focusing on potential habitability

406
00:17:17,220 --> 00:17:15,309
of Venus in the upper atmosphere it's

407
00:17:19,529 --> 00:17:17,230
kind of earth like a being oh hey

408
00:17:20,819 --> 00:17:19,539
well if Venus had a lot of water maybe

409
00:17:22,769 --> 00:17:20,829
it had an ocean in there for maybe like

410
00:17:24,689 --> 00:17:22,779
got establish which then migrated to the

411
00:17:27,329 --> 00:17:24,699
upper atmosphere as the greenhouse

412
00:17:28,680 --> 00:17:27,339
effect kicked in from the sounds of it

413
00:17:31,830 --> 00:17:28,690
I'm guessing that's probably less likely

414
00:17:35,580 --> 00:17:31,840
given how early it sounds like things

415
00:17:37,649 --> 00:17:35,590
kind of went downhill yeah so I I

416
00:17:39,779 --> 00:17:37,659
personally like the mono at all result

417
00:17:41,549 --> 00:17:39,789
because it it's really clever and how it

418
00:17:44,970 --> 00:17:41,559
interprets the the evolution of the

419
00:17:46,830 --> 00:17:44,980
solar system and it's from their efforts

420
00:17:48,510 --> 00:17:46,840
it has looked like there's a very sharp

421
00:17:51,029 --> 00:17:48,520
divide between their type 1 and type 2

422
00:17:52,350 --> 00:17:51,039
planets so I think that that Venus and

423
00:17:54,480 --> 00:17:52,360
anything that's close to Venus is going

424
00:17:56,250 --> 00:17:54,490
to go through essentially it's going to

425
00:17:57,600 --> 00:17:56,260
form be in this stuck in this runaway

426
00:18:01,860 --> 00:17:57,610
greenhouse and it's going to lose all of

427
00:18:03,690 --> 00:18:01,870
its water very quickly so they said any

428
00:18:06,149 --> 00:18:03,700
nice talk maybe this is something I

429
00:18:08,039 --> 00:18:06,159
didn't understand but what's like the

430
00:18:09,870 --> 00:18:08,049
limit of how much water you could have

431
00:18:11,880 --> 00:18:09,880
start with such that you would have lost

432
00:18:13,830 --> 00:18:11,890
all of it are you saying that it

433
00:18:16,680 --> 00:18:13,840
happened so fast that you're not really

434
00:18:20,970 --> 00:18:16,690
limited by your initial amount of water

435
00:18:23,399 --> 00:18:20,980
so it really so there's a couple things

436
00:18:26,330 --> 00:18:23,409
in there right so when we talk about the

437
00:18:28,320 --> 00:18:26,340
DTH ratio for example that gives us a

438
00:18:30,210 --> 00:18:28,330
conservative estimate on the amount of

439
00:18:32,039 --> 00:18:30,220
water that Venus could have had but then

440
00:18:33,419 --> 00:18:32,049
if escape rates were higher you could

441
00:18:35,399 --> 00:18:33,429
have just dragged off the Tyrian as well

442
00:18:39,090 --> 00:18:35,409
which would have basically erased your

443
00:18:42,750 --> 00:18:39,100
HD to H ratio the amount of water you

444
00:18:46,680 --> 00:18:42,760
could lose over Venus's lifetime is

445
00:18:49,230 --> 00:18:46,690
essentially if you look here the if

446
00:18:51,690 --> 00:18:49,240
you're if you start with like 10 earth

447
00:18:53,610 --> 00:18:51,700
oceans and you're in this very early

448
00:18:55,169 --> 00:18:53,620
regime where you're close to the

449
00:18:57,539 --> 00:18:55,179
diffusion limit for that whole parameter

450
00:19:00,360 --> 00:18:57,549
space then you can lose 10 earth oceans

451
00:19:02,159 --> 00:19:00,370
in a billion years okay so it's

452
00:19:08,220 --> 00:19:02,169
prodigious the amount of

453
00:19:09,869 --> 00:19:08,230
you could lose Thanks hello yep

454
00:19:11,070 --> 00:19:09,879
thank you for that talk I was wondering

455
00:19:13,859 --> 00:19:11,080
if you might be able to share your

456
00:19:15,960 --> 00:19:13,869
opinion regarding what you may think

457
00:19:18,539 --> 00:19:15,970
about tidally locked planet surround and

458
00:19:19,950 --> 00:19:18,549
inkay dwarf stars man that's a whole

459
00:19:20,970 --> 00:19:19,960
nother session that's not just a

460
00:19:24,210 --> 00:19:20,980
question

461
00:19:26,669 --> 00:19:24,220
so like the lugar and Barnes result for

462
00:19:28,139 --> 00:19:26,679
basically dehydrating em star planets

463
00:19:30,419 --> 00:19:28,149
because of their pre main sequence

464
00:19:32,369 --> 00:19:30,429
lifetime and then tidal locking you can

465
00:19:34,739 --> 00:19:32,379
go into these moist greenhouse States

466
00:19:36,299 --> 00:19:34,749
even when you're in the hab the

467
00:19:39,479 --> 00:19:36,309
conventional a bubble zone because of

468
00:19:41,070 --> 00:19:39,489
the the dayside cloud cover it's I think

469
00:19:42,659 --> 00:19:41,080
M star planets are really hard and I

470
00:19:44,220 --> 00:19:42,669
think they're really cool and we should

471
00:19:45,359 --> 00:19:44,230
look at them definitely but I don't know

472
00:19:46,499 --> 00:19:45,369
that we should go into it thinking that

473
00:19:50,159 --> 00:19:46,509
any of them are going to be even

474
00:19:52,160 --> 00:19:50,169
potentially habitable so yeah with that