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so um yeah my name is eric larson i'm

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going to tell you about some

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three-dimensional modeling i've been

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doing of titan's organic haze

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and i'm going to

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give you some implications for the early

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faint young sun problem there

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this image in the background is a image

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of titan taken from cassini in true

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color and you'll notice that it's an

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orange blob

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titan's organic haze completely obscures

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the surface at visible wavelengths

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and the color of this is important

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because the fact that it's kind of an

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orangish yellow tells you that

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it's very good at a but the haze is very

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good at absorbing um

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uv and blue wavelengths

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all right so that'll be important later

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so uh let's frame our understanding

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about titan for those of you who aren't

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familiar with it with a little

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comparison for with earth

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these sizes are roughly the scale there

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might be some

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compression in this image

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but yeah titan's uh

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about 2500 kilometers

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in radius so

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a little less than half of the earth uh

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it's obviously quite a bit less massive

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so you have a very low gravity

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um with the low gravity and the thick

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atmosphere you get a very extended

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atmosphere so when i show you some plots

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where i talk about the atmosphere and

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the aerosols being hundreds of

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kilometers over the surface

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that's that's normal for titan on earth

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that's where like the you know

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international space station flies things

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like that

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um i'm not going to tell you titan's

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habitable i don't think it is but i

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think there are a lot of processes going

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on on titan um

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from which we can gain understanding

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about what the early earth might have

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been like

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uh and i'm going to go into those now

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so here's a cartoon telling you a little

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bit about the

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aerosols on titan how they're formed and

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what's going on

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we have pressure on the y-axis here and

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a

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so

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this organic haze is formed through

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photodissociation of methane and

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nitrogen

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in the upper atmosphere of titan

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this photodissociation and abundant

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photochemistry

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leads to these large organic molecules

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and at some point these organic

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molecules grow so large chemically that

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they start interacting physically so

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they start colliding with each other and

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sticking and coagulating

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at first they grow in spheres

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these spheres are charged because this

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photochemistry leads

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has an abundant

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excess of electrons

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so you get these little charged spheres

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coming down and as they start sticking

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and colliding

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they start growing

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in a fractal nature what i mean by that

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is uh

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not exactly like fern leaves but kind of

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like snowflakes they're they're

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aggregate clumps

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um these things slowly settle and

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coagulate and eventually land on the

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surface so they contribute to things

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like dunes

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titan's really cool one of the neat

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things about titan are all these active

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processes going on that really make it

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one of the most earth-like bodies in the

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solar system so not only do we have this

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this

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but it acts as seed nuclei for

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condensation of methane and ethane

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clouds

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that eventually precipitate out and make

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river valleys and seas on titan's

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the size that these aerosols

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coagulate into

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is really determined by the amount of

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charge

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that's placed on these molecules

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and

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what i'm going to tell you about is some

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modeling efforts i've done using a

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three-dimensional gcm so a global

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circulation model which calculates all

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the temperatures and winds and pressure

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profiles everywhere on the planet that's

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coupled with an aerosol microphysical

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model

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that tries to

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back out some of these numbers by

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if you guys have any questions about any

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of this just let me know feel free to

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interrupt

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so

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i'm just going to go through a few

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examples of spacecraft data that we use

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uh to

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try to constrain some of these

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parameters so the first one we'll talk

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about is the um is the phase function so

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the phase function just tells you that

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when

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a photon interacts with a particle and

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it's scattered the angle at which it's

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scattered

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can tell you something about the size of

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the particle the size of the aerosol

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so

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here's the uh the percentage of photons

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that are scattered in each direction

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here's the scattering angle so zero

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would be forward scattering that's a

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photon interacting with that aerosol

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particle and keep going forward 180

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degrees down here is just completely

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back scattered

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the solid lines in here

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are

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our

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modeling results the dashed lines are

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observations from the huygens probe that

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lined up cassini

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and

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what i've got here is i have a different

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charge ratio so this would be five

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electrons per micron radius on these

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aerosol particles

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here's seven and a half 10 15

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and you can see that some of these fit

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better than others and it's really

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somewhere between ten and seven and a

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half electrons per micron that gives us

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the best

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fit to these

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phase functions so we can use plots like

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this to try and oh it's a lot louder to

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try and um

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constrain some of these values

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and like i mentioned before these charge

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ratios really affect the size that the

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aerosols grow to and this makes a

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difference by about a factor of three or

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four in size

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um the next one we can use is we can use

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the extinction so this is how much

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or

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how much of the

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of the light is scattered or absorbed at

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every layer in the atmosphere these are

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vertical profiles at the huygens landing

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site the black dash line here is the

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huygens data this is all different

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casino cassini data sets from the

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equator

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they're at slightly different times

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but what i want to show you is that

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there's a large diversity in the upper

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atmosphere

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probably more so than the lower

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atmosphere in time anyway we can use

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these vertical profiles to help

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constrain how much aerosol we should put

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in so the more aerosols we dump in the

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more lights can be scattered and

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absorbed so we can use this to constrain

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our production rate

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and finally we can use the wavelength

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dependence of the optical depth so this

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slope here

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tells you how much of the light is

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absorbed or scattered as a function of

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wavelength and the slope of this is

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highly sensitive to the shape of that

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fractal dimension i told you about these

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aerosols these aggregate

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clumps right

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and if they're very spherical if they're

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tightly compact they're going to have a

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different slope than if they're very

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loose and stringy

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and it turns out that the slope of this

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changes as you descend farther into the

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atmosphere which indicates that the

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shape of the aerosols change

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and

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we find that a fractal dimension of two

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which is about like a snowflake

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um is what we is is the best fit for

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um high in titan's atmosphere above 80

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kilometers but as we descend toward

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towards the surface we really need to

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make the particles more spherical more

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compact

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all right so we can use these spacecraft

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observations to help constrain these

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aerosol parameters

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but why does that matter to astrobiology

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why should you care and what does that

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well not only are oh okay well not only

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can we constrain these aerosols but

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these aerosols are really important for

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all kinds of atmospheric properties

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especially temperature so these aerosols

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they absorb and scatter light

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which is going to change the temperature

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profiles in the atmosphere which is also

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going to drive the circulation

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and i'll get into that in a second

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but just to go back to this figure

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really quick now that we've we've used

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these spacecraft observation to

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constrain particles we can start

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throwing numbers into these different

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things

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so for instance these spherical

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um aerosols

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uh we call them monomers if there's

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still a sphere at the top of the

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atmosphere we get about 50 nanometers

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that they grow

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so these aerosols grow spheres until

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about 50 nanometers after that they

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start

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sticking together into these

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fractals

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we find that a production rate

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is kind of a meaningless number 10 to

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the minus 14 grams per centimeter

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squared per second

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this charge per

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on the particles of seven and a half and

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this fractal dimension of two like i

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mentioned like a snowflake

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so we can start putting numbers to these

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so we can we can understand them a

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little a little better and get more

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accurate results

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so this is important because these

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aerosols

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act as an anti-greenhouse

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on titan so you're all probably familiar

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with the greenhouse effect

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i've got a schematic of it over here the

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greenhouse effect we basically have this

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layer of atmosphere or in a greenhouse a

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layer of glass

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that lets visible light through that

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light's absorbed

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inside inside the atmosphere at the

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surface

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and then it's that energy is re-emitted

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as infrared wavelengths which is

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absorbed by your greenhouse gases or by

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the glass over a greenhouse and then

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it's re-radiated in both directions

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so you get more energy coming back and

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heats up your surface

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the anti-greenhouse from these aerosols

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is the exact opposite case

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here we have

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aerosols which absorb in the visible

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light but they're transparent

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in the infrared so this tends to cool

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the planet's surface

290
00:10:17,509 --> 00:10:14,800
and on titan it's estimated that the

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00:10:20,949 --> 00:10:17,519
aerosols cool by about nine kelvin

292
00:10:23,269 --> 00:10:20,959
well if the early earth had abundant

293
00:10:25,269 --> 00:10:23,279
organic aerosols similar to titan

294
00:10:26,710 --> 00:10:25,279
the surface of the early earth should

295
00:10:28,710 --> 00:10:26,720
have been quite a bit colder well

296
00:10:29,990 --> 00:10:28,720
there's a lot of geological evidence

297
00:10:30,790 --> 00:10:30,000
that suggests it should have been really

298
00:10:33,030 --> 00:10:30,800
hot

299
00:10:34,630 --> 00:10:33,040
so this contributes to the faint young

300
00:10:36,310 --> 00:10:34,640
sun problem

301
00:10:38,550 --> 00:10:36,320
i'm not trying to i'm not trying to make

302
00:10:40,389 --> 00:10:38,560
a claim of whether or not there was

303
00:10:42,870 --> 00:10:40,399
there were abundant organic aerosols on

304
00:10:44,710 --> 00:10:42,880
the early earth i don't know but people

305
00:10:46,710 --> 00:10:44,720
have said that and if there were this

306
00:10:47,590 --> 00:10:46,720
makes it more complicated

307
00:10:48,870 --> 00:10:47,600
and

308
00:10:51,190 --> 00:10:48,880
i'm going to give you some more things

309
00:10:52,630 --> 00:10:51,200
to think about

310
00:10:54,630 --> 00:10:52,640
so let's just take a look at the effects

311
00:10:56,150 --> 00:10:54,640
of this anti-greenhouse

312
00:10:58,069 --> 00:10:56,160
you can look at this but i can i can

313
00:11:00,389 --> 00:10:58,079
actually have modeling results that

314
00:11:01,829 --> 00:11:00,399
might show you that really it's it's

315
00:11:04,069 --> 00:11:01,839
there it works

316
00:11:06,069 --> 00:11:04,079
so here's the temperature profile

317
00:11:08,710 --> 00:11:06,079
from titan from the modeling work the

318
00:11:11,350 --> 00:11:08,720
dashed lines are observations

319
00:11:13,509 --> 00:11:11,360
black here is at 10 degrees south so

320
00:11:14,550 --> 00:11:13,519
near the equator

321
00:11:17,990 --> 00:11:14,560
red

322
00:11:22,150 --> 00:11:18,000
is at 80 degrees north near the pole

323
00:11:23,670 --> 00:11:22,160
and the solid lines are from my model

324
00:11:25,590 --> 00:11:23,680
and

325
00:11:28,069 --> 00:11:25,600
they're a bit sloppy but

326
00:11:29,670 --> 00:11:28,079
don't worry about that too much

327
00:11:32,069 --> 00:11:29,680
up at the top here i have the aerosol

328
00:11:33,430 --> 00:11:32,079
production

329
00:11:34,949 --> 00:11:33,440
this first number is the one just to

330
00:11:36,870 --> 00:11:34,959
look at this is a half and i'm going to

331
00:11:38,790 --> 00:11:36,880
go through an order of magnitude change

332
00:11:40,790 --> 00:11:38,800
in aerosol production now what you're

333
00:11:42,470 --> 00:11:40,800
going to see is that a strengthening of

334
00:11:44,150 --> 00:11:42,480
this anti-greenhouse

335
00:11:46,069 --> 00:11:44,160
so you're going to see heating in the

336
00:11:48,710 --> 00:11:46,079
upper atmosphere where we're absorbing

337
00:11:50,230 --> 00:11:48,720
that solar radiation that visible light

338
00:11:52,310 --> 00:11:50,240
and you're going to see cooling at the

339
00:11:54,470 --> 00:11:52,320
surface

340
00:11:56,949 --> 00:11:54,480
due to that anti-greenhouse effect so as

341
00:11:59,670 --> 00:11:56,959
we go through a half to one

342
00:12:03,430 --> 00:11:59,680
to three to five you can see that it's

343
00:12:04,630 --> 00:12:03,440
much warmer here especially if i go fast

344
00:12:08,550 --> 00:12:04,640
and you can see that it's quite it gets

345
00:12:11,829 --> 00:12:09,430
all right

346
00:12:14,629 --> 00:12:11,839
so the surface temperature

347
00:12:16,230 --> 00:12:14,639
is greatly affected by these um by these

348
00:12:17,990 --> 00:12:16,240
aerosols and it makes sense if you

349
00:12:19,269 --> 00:12:18,000
change the production rate the number of

350
00:12:20,550 --> 00:12:19,279
aerosols you're going to

351
00:12:22,150 --> 00:12:20,560
change the the effect of the

352
00:12:23,670 --> 00:12:22,160
anti-greenhouse but what about all those

353
00:12:24,790 --> 00:12:23,680
other

354
00:12:27,110 --> 00:12:24,800
properties that i mentioned at the

355
00:12:29,590 --> 00:12:27,120
beginning

356
00:12:30,949 --> 00:12:29,600
well we can look at the production rate

357
00:12:33,670 --> 00:12:30,959
and

358
00:12:36,150 --> 00:12:33,680
for this specific case of runs for titan

359
00:12:38,949 --> 00:12:36,160
we're getting a change in the

360
00:12:41,350 --> 00:12:38,959
in the surface temperature of about

361
00:12:44,389 --> 00:12:41,360
two and a half kelvin

362
00:12:46,790 --> 00:12:44,399
and one interesting thing is that we see

363
00:12:49,269 --> 00:12:46,800
for a low aerosol case

364
00:12:51,750 --> 00:12:49,279
a um

365
00:12:53,750 --> 00:12:51,760
a half kelvin

366
00:12:55,269 --> 00:12:53,760
equator to pole temperature gradient

367
00:12:57,190 --> 00:12:55,279
however when we get to the high aerosol

368
00:12:59,269 --> 00:12:57,200
case that equator pole temperature

369
00:13:01,590 --> 00:12:59,279
gradient completely goes away

370
00:13:03,190 --> 00:13:01,600
this is because the aerosols are

371
00:13:04,710 --> 00:13:03,200
interacting

372
00:13:06,629 --> 00:13:04,720
with the dynamics

373
00:13:08,150 --> 00:13:06,639
and the aerosols tend to accumulate at

374
00:13:09,670 --> 00:13:08,160
the poles so they're heating the pools

375
00:13:11,670 --> 00:13:09,680
which drives the circulation so they're

376
00:13:13,750 --> 00:13:11,680
all these feedback mechanisms that make

377
00:13:17,190 --> 00:13:13,760
this a much more complicated situation

378
00:13:18,550 --> 00:13:17,200
than a 1d model might suggest

379
00:13:20,230 --> 00:13:18,560
we can also look at that charge i

380
00:13:21,990 --> 00:13:20,240
mentioned at the beginning you can see

381
00:13:23,829 --> 00:13:22,000
this has one of the greatest effects

382
00:13:25,990 --> 00:13:23,839
more than three kelvin

383
00:13:28,150 --> 00:13:26,000
um surface chain or a change in uh

384
00:13:29,910 --> 00:13:28,160
surface temperature

385
00:13:31,030 --> 00:13:29,920
we can also look at things like the a

386
00:13:32,870 --> 00:13:31,040
change in the

387
00:13:35,430 --> 00:13:32,880
particle input size at the top of the

388
00:13:37,269 --> 00:13:35,440
model thank you

389
00:13:39,509 --> 00:13:37,279
so if we

390
00:13:41,269 --> 00:13:39,519
put two nanometer particles at the top

391
00:13:43,430 --> 00:13:41,279
of the model which is roughly molecular

392
00:13:45,030 --> 00:13:43,440
size molecules

393
00:13:47,030 --> 00:13:45,040
versus

394
00:13:48,150 --> 00:13:47,040
42 nanometers which are those monomers

395
00:13:51,190 --> 00:13:48,160
and the top of the model i should

396
00:13:52,550 --> 00:13:51,200
mention here is about 580 kelvin

397
00:13:54,310 --> 00:13:52,560
or we can put in something that's

398
00:13:56,150 --> 00:13:54,320
two-thirds of a micron

399
00:13:57,750 --> 00:13:56,160
so the effect of this is saying how

400
00:14:00,230 --> 00:13:57,760
large are these particles by the time

401
00:14:02,389 --> 00:14:00,240
they get to about 500 kilometers and if

402
00:14:03,269 --> 00:14:02,399
they're really large

403
00:14:04,470 --> 00:14:03,279
um

404
00:14:06,550 --> 00:14:04,480
that'll change

405
00:14:07,990 --> 00:14:06,560
you know the way the particles interact

406
00:14:09,590 --> 00:14:08,000
the amount of light that's absorbed and

407
00:14:11,670 --> 00:14:09,600
scattered at different

408
00:14:14,310 --> 00:14:11,680
um altitudes however this has very

409
00:14:16,470 --> 00:14:14,320
little effect on the uh on the overall

410
00:14:18,150 --> 00:14:16,480
surface temperature

411
00:14:20,230 --> 00:14:18,160
where we put the particles in however

412
00:14:21,829 --> 00:14:20,240
where we input that mass of aerosols so

413
00:14:24,389 --> 00:14:21,839
where the methane is destroyed and where

414
00:14:27,030 --> 00:14:24,399
it creates aerosols uh does greatly

415
00:14:29,509 --> 00:14:27,040
affect we get almost a kelvin change if

416
00:14:34,470 --> 00:14:29,519
we change the the input of the aerosol

417
00:14:37,509 --> 00:14:34,480
mass between 250 and 580 k kilometers

418
00:14:39,269 --> 00:14:37,519
all right so to sum it up

419
00:14:41,509 --> 00:14:39,279
spacecraft deals and models can

420
00:14:43,350 --> 00:14:41,519
constrain titan's aerosol microphysical

421
00:14:45,910 --> 00:14:43,360
properties however for earth these

422
00:14:47,030 --> 00:14:45,920
properties are really unconstrained

423
00:14:49,189 --> 00:14:47,040
organic aerosols create an

424
00:14:50,710 --> 00:14:49,199
anti-greenhouse effect that cools a

425
00:14:53,110 --> 00:14:50,720
planet's surface and the strength of

426
00:14:54,870 --> 00:14:53,120
this effect is affected by our choice of

427
00:14:56,710 --> 00:14:54,880
aerosol parameters

428
00:14:58,389 --> 00:14:56,720
and a sensitivity shows that these

429
00:14:59,509 --> 00:14:58,399
aerosol at the aerosol charge and

430
00:15:01,030 --> 00:14:59,519
production rate have the greatest

431
00:15:01,829 --> 00:15:01,040
surface temperature response maybe the

432
00:15:04,389 --> 00:15:01,839
most

433
00:15:05,430 --> 00:15:04,399
uh surprising one is the fact that this

434
00:15:14,230 --> 00:15:05,440
this

435
00:15:18,310 --> 00:15:14,240
account if we think about the

436
00:15:21,750 --> 00:15:19,590
and the

437
00:15:32,150 --> 00:15:21,760
organic aerosols on the early earth all

438
00:15:36,310 --> 00:15:34,470
so thanks eric maybe one quick question

439
00:15:37,910 --> 00:15:36,320
because we are slightly behind schedule

440
00:15:39,189 --> 00:15:37,920
okay uh can you guys hear me we got one

441
00:15:42,150 --> 00:15:39,199
from online

442
00:15:44,310 --> 00:15:42,160
uh this is from uh sanjoy song uh how

443
00:15:46,790 --> 00:15:44,320
does the aerosol cooling compare to

444
00:15:48,710 --> 00:15:46,800
methane greenhouse warming

445
00:15:51,030 --> 00:15:48,720
slash nitrogen pressure

446
00:15:53,509 --> 00:15:51,040
broadening on early earth

447
00:15:55,430 --> 00:15:53,519
oh on early earth well

448
00:15:57,829 --> 00:15:55,440
i'm not sure about the early earth i can

449
00:16:00,550 --> 00:15:57,839
tell you for the um

450
00:16:02,949 --> 00:16:00,560
for titan titan has about a 21 kelvin

451
00:16:04,550 --> 00:16:02,959
greenhouse and about a nine kelvin

452
00:16:08,710 --> 00:16:04,560
anti-greenhouse

453
00:16:09,749 --> 00:16:08,720
so you get a net plus 13 to what um

454
00:16:12,629 --> 00:16:09,759
to what there would be without an

455
00:16:12,639 --> 00:16:15,350
cool thank you

456
00:16:17,749 --> 00:16:16,310
okay

457
00:16:20,870 --> 00:16:17,759
sorry

458
00:16:23,030 --> 00:16:20,880
so the atmosphere on titan is really

459
00:16:24,870 --> 00:16:23,040
thick um you're modeling modeling and

460
00:16:28,470 --> 00:16:24,880
the observations i mean that's more than

461
00:16:30,150 --> 00:16:28,480
500 kilometers of depth uh i'm not sure

462
00:16:31,749 --> 00:16:30,160
what if anyone has an idea of what the

463
00:16:32,790 --> 00:16:31,759
early earth's atmosphere was as far as

464
00:16:34,629 --> 00:16:32,800
depth but

465
00:16:36,710 --> 00:16:34,639
right now i mean the carbon line's 100

466
00:16:38,310 --> 00:16:36,720
kilometers so do you think that would

467
00:16:40,629 --> 00:16:38,320
make a big difference at all in the

468
00:16:42,150 --> 00:16:40,639
early earth having a much smaller

469
00:16:44,310 --> 00:16:42,160
atmosphere so there's not as much of a

470
00:16:45,910 --> 00:16:44,320
depth of particles to pass through

471
00:16:48,550 --> 00:16:45,920
anyway

472
00:16:50,949 --> 00:16:48,560
that's a good question um

473
00:16:53,590 --> 00:16:50,959
so on one hand all your gas is

474
00:16:55,350 --> 00:16:53,600
compressed so you have this well

475
00:16:56,870 --> 00:16:55,360
titan has about 10 times the column mass

476
00:16:58,629 --> 00:16:56,880
but you would have about the you know

477
00:17:02,310 --> 00:16:58,639
you'd have about the same amount

478
00:17:04,470 --> 00:17:02,320
but what would what could make

479
00:17:06,470 --> 00:17:04,480
uh what could have an effect

480
00:17:08,150 --> 00:17:06,480
is the length of time it takes particles

481
00:17:09,270 --> 00:17:08,160
to fall

482
00:17:10,870 --> 00:17:09,280
um

483
00:17:12,230 --> 00:17:10,880
but again that's usually dependent on

484
00:17:14,309 --> 00:17:12,240
pressure as well

485
00:17:16,470 --> 00:17:14,319
so

486
00:17:17,990 --> 00:17:16,480
yeah i don't know you'd have to model it

487
00:17:21,350 --> 00:17:18,000
i don't think it would change any of the

488
00:17:22,549 --> 00:17:21,360
major conclusions about um

489
00:17:24,470 --> 00:17:22,559
yeah about

490
00:17:26,470 --> 00:17:24,480
how these different aerosol properties

491
00:17:28,069 --> 00:17:26,480
affect the anti-greenhouse effect um