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okay yesterday we had this nice panel

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discussion and this question came up

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what may I study if I want to become an

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astrobiologist so we mentioned several

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fields like biology chemistry geology

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and engineering science but one field

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was missing and this is actually my

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field so geophysics and so I think I

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have to advertise a little bit for

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geophysics in this field as a

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geophysicist you can basically come from

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two directions to astrobiology and the

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one direction is the big topic

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habitability so what does a geophysicist

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have to do with habitability for example

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if you look at plate tectonics you can

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decide or you can find out y 0 SS plate

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tectonics and why other planets do not

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have plate tectonics what about

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exoplanets what about big size planets

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about five earth masses also does five

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earth-mass planet low plate tectonics

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what about volcanism is it more likely

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on bigger planets on smaller planets and

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is there enough outgassing for example

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to maintain an atmosphere at smaller

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sized planets like Mars like or even

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smaller so this is all about the

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habitability but you and we also saw a

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nice talk by lena a few minutes ago in

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this direction but it's not all about

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habitability you also can go from

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another direction and i will talk a

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little bit about it so if you have your

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model how's yours evolves with time

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starting with the origin of the earth

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and then you you put in all the

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parameters and you start your in your

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model z us to evolve and you end up at

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present day earth and so all is nice but

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if you know do the same thing again but

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switch some parameter off and you can

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have

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this parameter you may find some

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parameter which is related to life so

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you may switch life off and you can find

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out how the planet evolves and if you

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now observe some exoplanets you can

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basically look at these differences and

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may distinguish if it's related of life

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is there life present or not and my talk

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is about this direction and basically

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it's about a continental crust

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production so so I start with a with the

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approach to form continental crust

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continents are formed in subduction

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zones point is here yep continents are

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formed in subduction zones over here

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when water is present at depths and this

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is very important because it's not very

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clear how you can get water very deep

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and we have this oceanic crust which is

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abducted beneath the continental crust

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we have sediments which are subducted

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and now I can yeah and now how do you

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get this sediment the sediments are

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basically from the continent by erosion

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and there are several effects while life

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could affect this continental erosion

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rate by weathering processes and so you

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form the sedimentary layer and if you

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know look at very shallow depths the

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first few kilometers and before the

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metamorphic reaction take place you you

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have shallow dewatering and you will

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lose a lot of your water but if you have

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sufficient amounts of clays in the

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sediments which have low permeability

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you you can suppress some of these

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shallow dewatering and this will lead to

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a more available water in greater depth

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and it will enhance your continental

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production rate and also the mental

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water regarding rate

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now as a geophysicist this in turn

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impacts the model because a mentor in

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the water in the mental reduces the

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viscosity of of the mantle convection

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and stuff like this so I sketched I I

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did not include any formulas in this

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presentation to not bore you with but i

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just want to mention is a key link again

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so was increasing by bioactivity you

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increase the continental erosion rate

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the thickness of the sedimentary layer

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also the shallow dewatering decreases

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you have more water available in greater

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depth and it's enhanced the continental

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production and mentor gassing great so I

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just skip the rest of the model how I do

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it mathematically but show the results

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and what you see here is a face plan

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which is spent by the mental water

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concentration in the continental surface

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area and each dot in this phase plane is

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representative for one state where the

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earth basically could be for example if

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you are here then your earth have a very

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wet mental and no continents at all and

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each point or in this red line shows a

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point where the continental surface area

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does not change with time anymore and

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the blue line shows each value whether a

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mental water concentration does not

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change with time so you got one point

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basically where two lines cross so you

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got a stable fixed point over there and

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now independently from where you start

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in the neighborhood you always will run

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into the stable fixed point about forty

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percent continental coverage as we are

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also observe them on earth and this self

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consistent result of the model and now

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you can have three fourths that is not a

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nice table because the transition to

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this neck but you can see we are here so

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we have 4.5 billions of years and we are

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by

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we have a biotic continental Rosen rate

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and now I can just go from from this to

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here to to go to an abiotic planet and

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you can see how the face plant shifts

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and first of all you see that the the

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stable fixed point moves up because the

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continental erosion rate decreases and

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so you can end up with more continents

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but at the same time you see that the

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second stable fixed point evolves with

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very tiny continent and a very low

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mentor water concentration and now

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suddenly depends where you start because

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you can also run into this stable fixed

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point with very low meant for water

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concentration and continents and so

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where did you end up and this is why I

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started at zero Giga years so just

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videos developed and how the phase plane

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looks like for an abiotic earth and I

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just move up and I start with 300 ppm

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enter water concentration and without

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continents and here you see all the

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continental growth rate evolves with

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time and I've I just go up and you see

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all the phase plane shifts and how the

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continental surface area of all this

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time and you see that you will end up or

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some terminate at this lower stable

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fixed point without your with a ten

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percent continent coverage and a more or

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less dry mental yeah but it's okay you

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can do the same again and suddenly you

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can for at one billions of years you can

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switch on life in the model and so you

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basically go from up to down by ya ever

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increasing the the continental Rosen

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rate and you see how the phase plane

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shift and then you you start your yeah

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you run your you continue to run your

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and you see how suddenly the whole

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system evolves into the this stable

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fixed point above there and you will

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also terminate there and you see how the

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continental surface area evolves with

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time and so this is my conclusion that

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life that there is our ways where life

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impact continental growth and we also

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find it founded transition that you may

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end up at this stable fixed point with

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low continental surface area and may

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evolve with the origin of life to thirty

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five percent and continents in turn

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helps life again because they have a

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solar energy as they helped us harvest

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solar energy because they got direct

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access to to this processor so there's a

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feedback again and we can ask ourselves

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whether our life always change the

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planet in this way that the amount of

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available energy is maximized and I want

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to end with the sexy do we have any

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no what I did take into account is that

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if you have a lot of water and you

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mentally or viscosity decreases and with

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decreasing viscosity your convection is

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faster and with faster convection also

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the subduction goes faster so and and

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this in turn also is rated that that the

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surface is that that you cannot lose so

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much water and so on and also the

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temperatures I am I included that at the

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beginning of the earth it's very hot and

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stuff it at how many points in time can

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you turn on life and still wind up with

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an appropriate amount of continental

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surface area authorial kind of good what

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is the range of times where you can turn

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life on in your models to wind up with

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the right amount of continental area ah

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you mean at one billions of years or

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right yeah how far can you lose that um

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its relative robust so I I can turn it

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off just at the beginning of the OC us

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but but you're right if I turn it too

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late on then you will evolve into the

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first ever fixed point with the lowest

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continental surface area I did not find

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out where the transition asst but there

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will be some transition over there all