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[Music]

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[Applause]

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we're gonna stay pretty much on this

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same theme of looking at life within a

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serpent Knights here but from somewhat

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more of a theoretical view than what

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Alexis talked about just recently just

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in the prior talk but I'm gonna I have

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mercy on you this afternoon I only have

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one data slide in this talk so and this

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is work that I've been working on for

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several years and then have a number of

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collaborators that have worked on helped

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and me understand different aspects of

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this listed up here so we are looking at

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this retinas ation reaction I think most

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of you by now are pretty familiar with

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this idea that a mantle peridotite rocks

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containing olivine and pyroxene react

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with water at temperatures below about

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350 to undergo there's certain ization

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reaction and the reason this is of

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interest astrobiology is because one of

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the byproducts of this reaction is the

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production of hydrogen which can then go

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on and feed microbial populations in

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these surprising environments and over

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the last couple decades these several of

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these kind of environments has been the

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focus of different microbiology studies

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most of these have focused on places

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like hydrothermal vents on the ocean

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floor a lot like a place like lost city

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and alkaline springs where these fluids

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are coming up and do being discharged at

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the surface where the interaction with

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surface oxidants presents a favorable

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habitat for biology what I'm really

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going to be looking more at today is the

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subsurface part of this systems where

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you have either seawater or groundwater

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penetrating into these rocks reacting

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with the olivine and pyroxene and

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undergoing this serpent is a ssin

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reaction and what I'm looking at here is

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the capacity for the rocks to support

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life in these subsurface environments so

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below these Springs and in hydrothermal

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vents what how what can live there and

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what how much biomass it can support so

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I'm looking a couple of questions can

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autotrophic communities exist within the

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subsurface of these certain izing

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systems and if so how much subsurface

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microbial lie

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can be supported in those kinds of

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environments so I'm looking at this from

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a kind of an energy balance perspective

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where there's some in some system

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there's an energy supply and the

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organisms lived in there have some sort

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of demand of energy in order to run

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their metabolism so different

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environments can have different degrees

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of energy supply and in different

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environments organisms can have a

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different energy requirements in order

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to exist and if the energy supply far

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exists the demand then there can be

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growth of organisms but if it's not

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sufficient to support microbiology then

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life is not possible

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and then there's an intermediate level

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where organisms can exist there but

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maybe not actively grow and kind of the

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hydrothermal vents and in stand out

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plant springs environments are probably

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some up here where there's a lot of

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energy supply into the environment but

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most solar subsurface environments

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probably exist somewhere along this

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interface where there's enough energy to

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to stay there and live there and survive

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but not a lot of energy for growth so

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I'm gonna first look at the energy

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supply and these serpent sizing systems

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and ask the question how much energy is

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available for hydrogen based little

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author little otto trophy in these

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subsurface environments so we have this

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reaction where all the empiric scene is

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grazing this this serpentine mineral is

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assemblage and hydrogen and one way to

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get at how much hydrogen is potentially

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being produced in these environments is

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to look at reaction rates and here's my

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one data slide and what I've done here

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is I've compiled all the experimentally

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derived in a data for hydrogen

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production during experimentalist

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organization so experiments that started

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out with olivine or olivine pyroxene and

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measured the amount of hydrogen coming

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off this this thing so I'm plotting

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these up as a function of temperature is

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this hydrogen production rate in there

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in the units of nano moles per gram per

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day and this is something that is really

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only people have only started to

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experiment

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the last 15 or so years so there's not a

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lot of data could strain this and you'll

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notice right away that most of the data

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that is available is President have

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temperatures that are higher at about

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230 degrees another thing you'll note is

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that you know at any one particular

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temperature there's a about a four order

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of magnitude distribution or variation

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in the rates that have been measured in

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these laboratory experiments these are

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due to factors such as rock composition

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pH and we're just really beginning to

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understand what causes that kind of

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variation and understand why different

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kind of rocks and can produce different

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amounts of hydrogen in different

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situations one thing you'll notice too

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that there's a there's pretty much a

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scarcity of data at lower temperatures

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where we were looking at these these

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potentially habitable shallow subsurface

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environments and all of these data

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that's shown here with arrows on it

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those are experiments where there was

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too little hydrogen presented to measure

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on laboratory scales so pretty much at

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temperatures below about 175 degrees the

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reactant is just going to slow to be

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able to measure hydrogen production from

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certain ization on laboratory timescale

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so that's all weird giving us a clue

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that in these in these shallow low

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temperature environments a hydrogen rate

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of hydrogen delivery by this

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urbanization reactant is going to be

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pretty slow but if we go down here if we

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and if we take just brackets these

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higher temperature datas with a very

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simple preliminary and probably not very

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good model for first-order reaction

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kinetics with Arrhenius temperature

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dependencies we can just trial and some

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provisional lines here about what we

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think might be the hydrogen production

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late rate in low temperature

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environments and I should say too that

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these are all are all measured on

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laboratory conditions where you have

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powdered minerals and very favorable

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conditions for the reaction and in the

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natural system we would probably expect

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things to be going actually slower than

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but if we just take this data that we

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have and extrapolate it down temperature

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if we go down here in like the 52 30 10

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degree temperature range where we think

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things might be living in subsurface

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environments we can extrapolate to

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something like a rate of 10 to the minus

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18th or 10 to the 14th moles of hydrogen

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being produced 4 grand per day in these

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shallow subsurface environments so we

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get something like that that translates

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to something like 10 to the 6 to 10 to

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the 10th molecules per hydrogen per gram

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of rock per day and just for perspective

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on that some cells specific race for

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sulfate reduction in marine sediments

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that requires about 2,500 molecules per

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day per cell in order to maintain that

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for aerobic heterotrophs they consume

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about something like 10 to the fifth

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molecules of oxygen per day so right

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away we're getting to see that this is

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probably going to be only enough to

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support a relatively small population we

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can look at this energetically also and

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say well if we have a reaction like

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sulfate reduction that's using this

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hydrogen that produced that yields about

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10 kill a jam kelly gems kilojoules per

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mole of hydrogen this would and equate

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to an energetic power supply is

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something on the order of 10 to the 15

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to the 10 to 19 watts per gram of rock

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so that's our power supply what about

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the energy demand well this is something

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that's really not very known very well

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at all about how much energy is the

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minimum amount of organisms can survive

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on one study recently there's been

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looked at this is is this paper by a

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Doug Leier Owen yan Amin where they

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looked at looked at microbial

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populations in submarine sediments in

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the South Pacific Gyre and compared it

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with various models of the minimal

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energy supply and they came up with

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something an estimate of the basal well

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basil maintenance energy requirement to

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sell of something like 2 to the minus 10

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to the 19th watts per cell or 200 Zepto

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watts of energy so we can compare this

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with our our serpentinization supply

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of energy and it looks like there is

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enough energy being supplied to support

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less than only a few hundred cells per

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centimeter cubed and this is again this

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may be the mystic number based on these

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laboratory experiments in the actual

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hydrogen generation rate in natural

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rocks maybe somewhat lower than that but

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I'm not sure that that that kind of

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maintenance energy requirement applies

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so much these high pH serpent analyzing

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environments where the high pH may

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impose additional energetic costs for

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example I took this from some work by

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holer that's in preparations and

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preliminary calculations of what it

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costs to maintain your energy mint your

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the pH across the cellular membrane if

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you're consuming something like

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bicarbonate or sulphate with hydrogen

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and as you go up in pH the cost of

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maintaining your cellular membrane

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potential increases substantially so we

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may be looking at additional energetic

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costs just to live in those kind of

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environments more than what we see in

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the in those other estimates I showed so

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it may cost more but in any case it

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looks like very small populations can be

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maintaining the subsurface serpentine

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izing environments based on hydrogen and

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that seems to be consistent with some

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data that are beginning to come in

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looking at these environments and for

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example this is some data that came out

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last year from drill holes drilled into

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the Atlantic math seif where the slaw

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city system is hosted here so looking

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down these drill calls

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what they find is cell populations in

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those rocks on the order of ten to a

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hundred cells per centimeter cubes have

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very small populations and that may be

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all that this hydrogen producing reacts

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serpentinization reaction may be able to

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produce another environment where

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astrobiologists are interested in this

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in the solar system is Mars and because

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we see serpent Knights on Mars and we

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have a surface that really seems

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relatively inhospitable there's a lot of

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people there

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we need to go look at subsurface

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environments and serpent Knights there

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certainly and in my a good target for

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that kind of thing but these extremely

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low biomass populations may indicate

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that these impose really serious

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challenges for detection of life within

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serpent knives and rocks if you're

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trying to find a rock on Mars that only

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is supporting a couple of hundred or

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less selves per centimeter of rock it

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may be tough finding those biomarkers

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what may be more favorable as it goes

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through these to look for places like

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these alkaline springs or other areas

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where fluid flow is focusing these

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energy sources into a more confined

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environment and may be better targets

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those place may be better targets to go

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and look for for evidence of life in

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Mars and other planetary bodies beyond

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Earth anyway that's all I have today and

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happily I only have to answer any

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questions you showed a diagram of the

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rate of h2 generation versus temperature

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yes and the highest rate was around 250

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degrees and shoes however you did not I

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did not see the pH well it that that's

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one of the very pH is probably one of

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the variables that accounts for this

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this this range in hydrogen generation

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rates and actually these ones in red

300
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those are all what experiments that have

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a room temperature pH greater than 10

302
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everything else has a room temperature

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pH below s so in some cases you see

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greater faster hydrogen generation at

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higher pH some cases you don't and

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that's that's something is that is

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subjective ongoing investigations of why

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that I think is very important for h2

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generation because if

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you look at the diagram of if put upon

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the potential electric potential versus

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pH you can realize that at high

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colliding pH very high nine point five

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to fourteen and at high temperature 300

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to 350 degrees Celsius and also 10 to 25

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mega Pascal if the ferrous ion is

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transformed into ferric iron with

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emission of h2 so you you approach the

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temperature but the pressure is also

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very important

321
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yes well yeah yeah and these are all

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these experiments are almost all

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performed at about 350 to 500 bars yes

324
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the correct pressure because a 300 bars

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is 50 mega Pascal

326
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yeah I'm love I would agree I would

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agree that high pH and and these

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temperatures there there is a lot of

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thermodynamic potential to create higher

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amounts of hydrogen but these rates are

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pretty much all controlled by kinetic

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factors so you have to add that into the

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into thinking about you know that what

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what is thermodynamically favorable yes

335
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but I just wanted to show that in this

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specific domain of water which is called

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high subcritical and also where the

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silica deserves much better than at

339
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lower temperature and higher temperature

340
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the all these compounds will dissolve

341
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and from h2 and sio2 maybe if you still

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have time in five minutes I can show you

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my poster about that and I shall send

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you my articles yeah please do

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I'm going to talk about that more

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because I think we should probably move

347
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Oh quick question just a quick question

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yes I mean it's great work one thing I'd

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like to just question is when we talk

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about Mars applications from Mars you

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use in situ

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that's the argument that's what you look

353
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we're looking at the lower temperatures

354
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now maybe when you think about biomass

355
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estimations from Mars so be sure you

356
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don't think all tumbled in sich you

357
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we have gradients so if you go example

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you know think about Mars support water

359
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percolating you can have actually water

360
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mostly in contact with much water what

361
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warmer regions where you have much large

362
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production in the order of you below 100

363
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C so you can produce much more and bring

364
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it into the regions where there's more

365
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may be life so which means you've eh in

366
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you know you'd have production rates

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which are orders of magnitude larger

368
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than the lower temperature wants so I

369
00:16:03,620 --> 00:16:01,920
would be careful using the in situ low

370
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temperature as a prediction for my mass

371
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because it might be quite misleading yes

372
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and I I absolutely agree and I think a

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lot of these these systems for example

374
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the one in Oman those kinds of

375
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environments I think they're they are

376
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largely driven by higher temp long

377
00:16:21,890 --> 00:16:20,550
residence time fluid at probably higher

378
00:16:23,600 --> 00:16:21,900
temperatures that are carrying their

379
00:16:25,340 --> 00:16:23,610
their hydrogen and methane load up

380
00:16:27,050 --> 00:16:25,350
towards the surface where it begins to

381
00:16:30,500 --> 00:16:27,060
be so it's kind of a focusing mechanism

382
00:16:32,120 --> 00:16:30,510
there yes yes so that that that that of

383
00:16:36,610 --> 00:16:32,130
course changes the old picture yes yes