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Thread: Why life needs a liquid (tho maybe not water) - explanation by Chris McKay

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    Why life needs a liquid (tho maybe not water) - explanation by Chris McKay

    We've had discussions here before about whether life chemistry on another world would need to be involve liquid water, and whether it would need to be based on a liquid at all.

    From the Cosmos website:

    " 'We think that life requires liquid,' says NASA Ames Research Centre planetary scientist Chris McKay. Life needs a medium that brings chemicals close enough together to interact but not so close that they canít move. Gases are too diffuse and solids too cramped... "

    Seems to me a good, simple explanation why astrobiologists like McKay are interested in looking for life on the surface of Titan, where there is rain and liquid lakes and seas, even though the liquid is a mix of methane and ethane rather than water.

    What do others think?

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    From the perspective of where we would invest time and energy looking for life elsewhere, that's perfectly understandable. But there are gas environments that could provide the functional medium McKay is alluding to, such as on (in) a gas giant planet. It may be more accurate to assert that life requires a fluid, vs. a liquid.

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    I think the logic is that chemical reactions tend to proceed much more quickly in a solvent system. Just about all laboratory and industrial organic chemistry is performed in various solvents, protic, polar, aprotic, and non-polar.

    I think that even better than a liquid would be a super-critical fluid.

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    Quote Originally Posted by swampyankee View Post
    I think the logic is that chemical reactions tend to proceed much more quickly in a solvent system. Just about all laboratory and industrial organic chemistry is performed in various solvents, protic, polar, aprotic, and non-polar.

    I think that even better than a liquid would be a super-critical fluid.
    Yes, a super-critical fluid might work as "a medium that brings chemicals close enough together to interact but not so close that they can’t move".

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    What about a magnetic environment?
    Et tu BAUT? Quantum mutatus ab illo.

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    Quote Originally Posted by Ara Pacis View Post
    What about a magnetic environment?
    Do you mean in a plasma? There's been some work done on the possibility of complex structures forming and evolving in the fourth state of matter.

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    Artificial life wouldn't necessarily need a liquid solvent.

    The probability of encountering intelligent life at a similar stage of development to ourselves is vanishingly small. Much more likely is AI or something we can't even imagine.

    But primitive life that evolves naturally, it will be easier to identify from a distance if it is similar to Earth life. It used to be said that hydrocarbons as on Titan don't fit the bill because they are not good enough as solvents. You need a polar, protic, solvent -water, or at a stretch, liquid ammonia.

    But maybe things have moved on since then.

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    Quote Originally Posted by kzb View Post
    Artificial life wouldn't necessarily need a liquid solvent.

    The probability of encountering intelligent life at a similar stage of development to ourselves is vanishingly small. Much more likely is AI or something we can't even imagine.

    But primitive life that evolves naturally, it will be easier to identify from a distance if it is similar to Earth life.
    No doubt that's right, IF other factors are equal. Other factors like how far away its habitat is (this solar system versus another), whether the liquid it depends on is abundant or scarce (I'm thinking here of the scarcity of liquid water on Mars), and whether or not there are kilometres of ice above the liquid you hope to find it in.

    It used to be said that hydrocarbons as on Titan don't fit the bill because they are not good enough as solvents. You need a polar, protic, solvent -water, or at a stretch, liquid ammonia.

    But maybe things have moved on since then.
    My impression is that there has been a gradual shift in scientific thinking, as new models have developed for what could happen in a low-temperature environment with liquid hydrocarbons, e.g. possible role of acrylonitrile (in membranes of cells or proto-cells) and polyimine formations (as versatile structures and catalysts). Acrylonitrile and polyimine are carbon-hydrogen-nitrogen compounds, not all that far removed from the CHON chemistry of life on Earth.
    Last edited by Colin Robinson; 2017-Mar-13 at 08:51 PM.

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    Regarding polarity, perhaps it's worth mentioning that water is neither the most polar nor the least polar of liquids. It is more polar than ammonia, less polar than sulfuric acid...

    Sulfuric acid may sound an unlikely liquid for living things to use, yet, right here on Earth, there are organisms (of the genus Picrophilus) which not only appreciate a good dose of sulfuric mixed in with their water, they actually need the acidity and will die without it.

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    As well as coining the word "Thalassogen", Isaac Asimov also wrote about the possibility of life chemistries involving a liquid other than water, including non-polar liquids such as methane.

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    Quote Originally Posted by Colin Robinson View Post
    As well as coining the word "Thalassogen", Isaac Asimov also wrote about the possibility of life chemistries involving a liquid other than water, including non-polar liquids such as methane.
    That's an interesting article. No doubt I read it as a kid but it hadn't stuck in my mind like the Thalassogens.

    It's something that has to be born in mind I think. All effort at finding exolife is directed at liquid water. The possibility of surface liquid water on a planet defines its habitability in modern thinking.

    But here we have a list of different temperature regimes where life not-as-we-know-it might exist.

    QUOTE:

    <list of life chemistries, spanning the temperature range from near red heat down to near absolute zero:

    1. fluorosilicone in fluorosilicone
    2. fluorocarbon in sulfur
    3.*nucleic acid/protein (O) in water
    4. nucleic acid/protein (N) in ammonia
    5. lipid in methane
    6. lipid in hydrogen

    Of this half dozen, the third only is life-as-we-know-it. Lest you miss it, I've marked it with an asterisk. >

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    Quote Originally Posted by kzb View Post
    That's an interesting article. No doubt I read it as a kid but it hadn't stuck in my mind like the Thalassogens.

    It's something that has to be born in mind I think. All effort at finding exolife is directed at liquid water. The possibility of surface liquid water on a planet defines its habitability in modern thinking.

    But here we have a list of different temperature regimes where life not-as-we-know-it might exist.

    QUOTE:

    <list of life chemistries, spanning the temperature range from near red heat down to near absolute zero:

    1. fluorosilicone in fluorosilicone
    2. fluorocarbon in sulfur
    3.*nucleic acid/protein (O) in water
    4. nucleic acid/protein (N) in ammonia
    5. lipid in methane
    6. lipid in hydrogen

    Of this half dozen, the third only is life-as-we-know-it. Lest you miss it, I've marked it with an asterisk. >
    I suspect that there are some more interesting, exotic combinations out there: super-critical carbon dioxide, hydrogen sulfide, etc

    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



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    I think the reason you need the fluid is for small vortices--and they can get small indeed:
    http://www.spacedaily.com/reports/Re...fluid_999.html

    Tornadoes often have sub-vortices--I've seen an image or two where they form a double helix.

    Here is one in beer:
    https://www.youtube.com/watch?v=Gj1TIhRfeNQ
    https://www.youtube.com/watch?v=azxdArBl4O0

    My own idea is that DNA is an after image of early vorticity near smokers--that provide an updraft--like how an old ABC coke plant ner me spawned a vortex near the ground--what with the steam plume far aloft.

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    Quote Originally Posted by Colin Robinson View Post

    Seems to me a good, simple explanation why astrobiologists like McKay are interested in looking for life on the surface of Titan, where there is rain and liquid lakes and seas, even though the liquid is a mix of methane and ethane rather than water.

    What do others think?
    In relation to Titan specifically, and any similar energy challenged environment, it seems in addition to the balance between solid and gas in the form of liquid, one also needs an underlying balance in energy level. Water appears to represent that balance.
    "It's the rabbit hole that matters, not the blackhole, the wormhole or any other hole," the wolf said.
    "But, what's in the hole, Mr. Wolf?", came the question after a long pause.
    "Infinite approximations deciding everything exactly with just enough uncertainty." The wolf howled with some difficulty.
    "What about the rabbit?", was the next question.
    "Oh, he's long gone!", the wolf declared instantly.

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    Quote Originally Posted by Canis Lupus View Post
    In relation to Titan specifically, and any similar energy challenged environment, it seems in addition to the balance between solid and gas in the form of liquid, one also needs an underlying balance in energy level. Water appears to represent that balance.
    In what sense is Titan "energy challenged"? I know it is further from the Sun than Earth or Mars, but its methane-rich atmosphere is very good at absorbing the solar energy it gets, producing chemical energy which living organisms can use.

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    Quote Originally Posted by Colin Robinson View Post
    In what sense is Titan "energy challenged"? I know it is further from the Sun than Earth or Mars, but its methane-rich atmosphere is very good at absorbing the solar energy it gets, producing chemical energy which living organisms can use.
    In the sense that the elements and compounds present in their respective forms indicate low energy. H20 is frozen hard as any rock on Earth, for example.

    Titan's surface temperature is about 94 K (−179.2 įC). At this temperature, water ice has an extremely low vapor pressure, so the little water vapor present appears limited to the stratosphere.[51] Titan receives about 1% as much sunlight as Earth

    https://en.wikipedia.org/wiki/Titan_(moon)
    ^ Summed up "low energy".

    Although in the context of the search for life, I prefer to think of it as an unbalanced candidate worthy of dismissal.
    Last edited by Canis Lupus; 2017-Mar-25 at 11:17 PM.
    "It's the rabbit hole that matters, not the blackhole, the wormhole or any other hole," the wolf said.
    "But, what's in the hole, Mr. Wolf?", came the question after a long pause.
    "Infinite approximations deciding everything exactly with just enough uncertainty." The wolf howled with some difficulty.
    "What about the rabbit?", was the next question.
    "Oh, he's long gone!", the wolf declared instantly.

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    Quote Originally Posted by Canis Lupus View Post
    In the sense that the elements and compounds present in their respective forms indicate low energy. H20 is frozen hard as any rock on Earth, for example.



    ^ Summed up "low energy".

    Although in the context of the search for life, I prefer to think of it as an unbalanced candidate worthy of dismissal.
    I think you are confusing available energy with temperature. Two quite different things. Of course the atmosphere of Titan near the surface has a lower temperature than (for instance) the atmosphere of Mars near the surface. But the atmosphere of Mars is very close to chemical equilibrium, whereas the atmosphere of Titan is not. Which means chemical energy is readily available on Titan for living things to use...

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    Quote Originally Posted by Colin Robinson View Post
    I think you are confusing available energy with temperature. Two quite different things. Of course the atmosphere of Titan near the surface has a lower temperature than (for instance) the atmosphere of Mars near the surface. But the atmosphere of Mars is very close to chemical equilibrium, whereas the atmosphere of Titan is not. Which means chemical energy is readily available on Titan for living things to use...
    We are using different models rather than disagreeing on anything.
    "It's the rabbit hole that matters, not the blackhole, the wormhole or any other hole," the wolf said.
    "But, what's in the hole, Mr. Wolf?", came the question after a long pause.
    "Infinite approximations deciding everything exactly with just enough uncertainty." The wolf howled with some difficulty.
    "What about the rabbit?", was the next question.
    "Oh, he's long gone!", the wolf declared instantly.

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    Quote Originally Posted by Canis Lupus View Post
    We are using different models rather than disagreeing on anything.
    But don't simply dismiss Titan because it's "too cold". It's too cold on the surface for liquid water, that is certainly true, and it's also true that the only life known (on Earth) is based around liquid water as solvent.

    Life on Titan would be based on liquid hydrocarbon as solvent. It would be chemotrophic, using the energy from breaking down organic molecules produced by sunlight in the atmosphere.

    The energy budget for life on Titan would be way below what is available on Earth of course.

    It's a long shot, I certainly wouldn't bet money on it, but it could happen.

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