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[Music]

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[Applause]

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firstly just wanna thank dr. de mer and

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the rest of the conveners of the session

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for inviting me to give this talk which

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is going to be as Bruce mentioned on the

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fates of nucleobases in warm little

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ponds delivered by meteorites and

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interplanetary dust particles so the so

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called exogenous delivery hypothesis can

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I use to change this thing okay okay so

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let's motivate this why pons why we want

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to deliver to pons why not deliver to

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the desert why not deliver to the ocean

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well there's really two main reasons the

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first being that these are actual

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locations where you might be able to do

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concentrated prebiotic chemistry the

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chemists in the room could probably

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attest that you can't really do

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prebiotic chemistry with parts per

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trillion level concentrations you need

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something more substantial and these are

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some locations where you could possibly

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get them just due to the lower volume

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within ponds and the second and most

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important is that these are locations

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that have wet and dry cycles which is

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key for polymerizing RNA and dr. Paul

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Brocker told us this morning about the

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condensation reaction and how in order

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to make photo diaster bonds you need to

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remove water so unless you have these

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alternating wet and dry environments

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then it's hard for you to make those

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bonds okay so the driving question of my

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research for a long time has been you

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know how did nucleobases arise in ponds

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on the prebiotic earth why why are we

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interested this question well in the RNA

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world hypothesis we're really trying to

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get to a self-replicating RNA molecule

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so maybe one of the most obvious

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questions you could ask is well then

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where did the building blocks of RNA

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come from nuclear bases are these

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characteristic molecules in in the

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building blocks and nucleotides and

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there they are attached to the ribose

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phosphate backbone I skipped over some

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details there it is not trivial to get

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to a nucleotide in a pond from just a

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nucleobase ribose and phosphorous source

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but I want to point your attention to

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some recent experiments that many of you

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probably know about which may have had

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success in this this is namma Dell these

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are both PNAS articles where they have

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reacted nucleobases ribose and

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phosphoric acid in micro droplets in

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order to produce nucleosides and then

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there has been a wealth of experiments

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in the past which have have had some

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success in phosphorylating those

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nucleosides and i point you to a review

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by gullet al 2014 for those results so

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back to the main question how did

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nuclear bases arise in warm little ponds

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what are the sources you can really

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break these down into two types of

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sources you have your delivery from

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space so your exogenously livery

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hypothesis and then you can also have

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the production on the planet itself for

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exhaustion is deliver your two options

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are meteorites and interplanetary dust

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particles meteorites have been analyzed

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to contain nucleobases and IDPs are

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thought to also perhaps contain the same

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molecules and then for endogenous

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production you have photo chemistry in

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the atmosphere UV photons can be

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absorbed by methane and nitrogen

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molecules to break them apart and those

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radicals that form from that react form

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hydrogen cyanide which once it dissolves

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into rainwater and falls into ponds

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could possibly through aqueous chemistry

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produce nucleobases lightning chemistry

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can do the exact same thing the only

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difference is you're changing the energy

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source which is creating those radicals

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which react to form hydrogen cyanide so

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this is what I'm working on right now

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this is a work in progress we've built a

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chemical Network a hydrogen cyanide

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chemical Network from the ground up

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using quantum computational chemistry

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methods and we found many reactions

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which have not been which have not been

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seen in literature literature before so

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I'm hoping to show you some results from

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that sometime in the very near future

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but for today let's focus just on the

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meteorites and interplanetary dust

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particles so why are these sources why

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think about nuclear bases being

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delivered by these sources well as I

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mentioned before many works have

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analyzed them for nuclear bases and we

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know they can reach the surface intact

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the most obvious reason is because in

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two people literally go and search for

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them and pick them up so we know that

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they landed there intact but other

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hypothetical models have found that if

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you're a meteoroid entering the

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atmosphere and you're about 80 40 to 80

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meters in diameter you're kind of the

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key size for you to break up into

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fragments and actually reach terminal

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velocity and kind of fall down and ponds

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just just intact a major study benchmark

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study by Carl Sagan and and Christopher

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Kaiba found that IDPs are actually a

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major source of organics so it's

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important to explore these as possible

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sources as well so what do we do well we

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built in numerical sources and sinks

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model of a warm little pond in order to

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ask the question what concentrations of

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nucleobases can you actually reach in

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ponds you know is it comparable to what

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chemistry experiments can actually do so

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here we have a schematic on the right

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and on the left if you prefer looking at

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things in tabular form I'll just walk

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you through these source in the sinks

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for pond water we we have precipitation

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and the sinks or evaporation and seepage

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four nucleobases we're comparing

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meteorites and IDPs as sources and for

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sinks we have hydrolysis seepage and foa

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sociation we get the rates for these

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sources and sinks mostly from

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experiments however in the case of

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precipitation we use the historical

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precipitation record and for the mass

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delivery by IDPs and meteorites we use

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the lunar crater in record so let's go

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through the results on the left here we

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have the pond water level and what we

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notice is there's actually a lot

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degeneracies in our model for instance

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if we model as as an analogue for the

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early Earth Columbia today at 65 degrees

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it actually overlaps with Thailand at 20

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degrees so instead of trying to cover

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all different temperatures for a

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hypothetical early Earth and

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precipitation amounts we just modeled

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three different environments which have

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many redundance which may have many

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degeneracies we call them a wet and

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intermediate and a dry environment and

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notice in the intermediate dry

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environments you actually get natural

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annual wet and dry cycles so that pops

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out just do the data we didn't impose

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that that's just due to the changing

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precipitation

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on the earth everywhere on the earth

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today on the right you can see the

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adenine concentration from just IDPs

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within one little ponds and in red you

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can see you can follow there's three

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features here and on in blue you have

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the the wet/dry cycle so you can see

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what's happening in the pond at the same

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time what happens is as your pond dries

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up you get to your maximum concentration

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of adenine and then the pond dries UV

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foud sociation is turned on and it

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blasts away your adenine to an amount

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where your incoming adenine rate matches

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your photo destruction rate then your

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pond fills up again you dilute and then

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this pattern continues on an annual

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basis and it never changes it never gets

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any more concentrated now notice what it

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says on the y-axis there that says

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adenine mass fraction ppq that's not a

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typo that's parts per quadrillion which

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is completely negligible so what this

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says is that you're never gonna get a

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concentration of nucleobases and warm

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little ponds from IDP sources they

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simply rain down on to the surface too

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slowly and the destructive forces then

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the pond are too efficient at removing

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them but what about meteorites here we

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compare meteorites to IDPs for the pond

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concentration IDP curves are on the

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lower heart lower half and the meteorite

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curves are on the upper half and notice

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the stark difference in your maximum

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adenine concentration between these two

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sources so what's happening here well

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meteorites deliver a large sum of

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nucleobases on the on the order of

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milligrams in one single event those

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nuclear bases outflow from the pores of

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the meteorite and get up to parts per

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million level concentrations and then

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begin to dilute from there and parts per

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million is still quite low but it's on

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the lower end of what you can actually

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do perhaps a chemistry experiment with

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but there's still some issues notice the

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red curve there sharply declines at some

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point that's when photodissociation

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turns on and it's completely destructive

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so you need a solution any surface model

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on the early Earth needs a solution for

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the photo destruction problem we like to

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think of sediments as being a possible

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solution sediment is everywhere in the

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wind

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gonna fall into ponds and it's gonna

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create a layer at the base which could

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possibly shields nucleobases from faux

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destruction it only takes about a

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millimeter to completely attenuate UV

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lights the second most destructive sink

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in the pond is seepage through pores in

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the base of the pond and that's what

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you're seeing in that blue curve there

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so we like to think of a solution for

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this as encapsulation something that dr.

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Christine Keating was talking about this

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more at this morning's plenary so if you

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if you take fatty acids which also

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happened to be in meteorites in fact

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fatty acids are the most abundant

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organic and meteorites you put those

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into a pond and all the all the fatty

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acids that are 8 carbons in length or

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longer are going to create spiracle

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compartments called vesicles and the

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average diameter of vesicles is larger

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than your average pore diameter so this

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might be a potential solution for

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avoiding seeping through holes in the

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base to your pond ok the last thing I'm

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going to talk about is actually how

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often did meteorites land in ponds on

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the early earth just the carbon-rich

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ones we ran a probability analysis which

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is based on a geophysical model of the

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rising continental crust you can see in

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the orange line there from 4.5 to 3.7

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billion years ago which is the interval

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in which we know life emerged and we

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used three different mass delivery

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models which are all based on the lunar

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crater in record now if you're at dr.

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Nicole's elders talk yesterday you know

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that there's now evidence that there's

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mass delivered to the moon before 3.9

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billion years ago so we can go ahead and

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ignore this black curve right there

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which is the single Cataclysm model and

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just focus on the to sustain declining

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bombardment models the red and the blue

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with the truth probably being somewhere

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between those two models we ran the

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probability based on the geometric

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probability of landing a meteorite into

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a pond and what do we find 15 to 4,000

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carbon-rich meteorites landed into warm

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little ponds on the early earth from 4.5

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to 3.7 billion years ago so this is good

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we know that this did happen and the

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truth is probably somewhere in between

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probably in the thousands regime all

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right that's all I should talk about

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I'll just summarize my results quickly

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we find meteorites or a plausible and

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probable source four parts per million

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level concentrations of new

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Leigha bases and ponds IDPs however the

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the nucleobases delivered by these

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00:11:24,359 --> 00:11:22,179
produce negligible concentrations within

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ponds so we can forget about IDPs as

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being a source of nucleobases for for

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pond environments we get natural

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seasonal wet and dry cycles and ponds

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just from seasonal precipitation changes

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the two main problems with this surface

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model are UV foe dissociation and

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seepage we think possible solutions for

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those are our sediment and encapsulation

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and finally we find thousands of

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terminal-velocity carbon-rich meteorites

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landed in ponds on the early earth

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giving life thousands of opportunities

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to fail before perhaps succeeding in one

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of these ponds okay thank you we have

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time for one quick question well dr.

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Deemer comes up so I was just wondering

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you're showing wet/dry cycles on the

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order of years and what we've been

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talking about most most people have

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shown wet/dry cycle on the order of days

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and so if you did have wet/dry cycles

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daily how long would it take for

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photodissociation to occur so every time

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you dry up your pond in our model we

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turn on photo station and it only takes

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a matter of hours for it to destroy

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pretty much everything in your pond so

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in the case if you had a daily wet/dry

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cycle and you landed a meteorite in that

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pond the day before it's gone by the

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next day yeah all right we have to close

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that up word for time dr. Deemer if you

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00:12:56,560 --> 00:12:55,480
want to come up Thank You Ben

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[Applause]