Page 1 of 3 123 LastLast
Results 1 to 30 of 71

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

  1. #1
    Join Date
    Aug 2010
    Posts
    2,078

    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 cant 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?

  2. #2
    Join Date
    Jan 2017
    Location
    Kennewick, Washington
    Posts
    52
    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.

    Sent from my SM-G920R4 using Tapatalk

  3. #3
    Join Date
    May 2007
    Location
    Earth
    Posts
    9,580
    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.

    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.



  4. #4
    Join Date
    Aug 2010
    Posts
    2,078
    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".

  5. #5
    Join Date
    Aug 2005
    Location
    location
    Posts
    12,323
    What about a magnetic environment?
    Et tu BAUT? Quantum mutatus ab illo.

  6. #6
    Join Date
    Aug 2010
    Posts
    2,078
    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.

  7. #7
    Join Date
    Apr 2005
    Posts
    2,164
    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.

  8. #8
    Join Date
    Aug 2010
    Posts
    2,078
    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.

  9. #9
    Join Date
    Aug 2010
    Posts
    2,078
    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.

  10. #10
    Join Date
    Apr 2005
    Posts
    2,164

  11. #11
    Join Date
    Aug 2010
    Posts
    2,078
    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.

  12. #12
    Join Date
    Apr 2005
    Posts
    2,164
    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. >

  13. #13
    Join Date
    May 2007
    Location
    Earth
    Posts
    9,580
    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.



  14. #14
    Join Date
    Feb 2005
    Posts
    10,711
    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.

  15. #15
    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.

  16. #16
    Join Date
    Aug 2010
    Posts
    2,078
    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.

  17. #17
    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 C). 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.

  18. #18
    Join Date
    Aug 2010
    Posts
    2,078
    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...

  19. #19
    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.

  20. #20
    Join Date
    Apr 2005
    Posts
    2,164
    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.

  21. #21
    Join Date
    Aug 2010
    Posts
    2,078
    Quote Originally Posted by kzb View Post
    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.
    Is it such a long shot?

    I agree that energy available for life on Titan is less than on Earth, which is why people like Chris McKay think in terms of microbes.

    On the other hand, if we compare Titan to Mars (instead of Earth), and consider three factors of chemical energy (basic thermodynamic requirement), organic molecules (the stuff life is made of), and liquid solvents... Compared to Mars, Titan has more of each of these.

    (Available chemical energy on Titan is due to the combination of organics and hydrogen gas, not organics alone.)
    Last edited by Colin Robinson; 2017-Mar-29 at 02:35 AM.

  22. #22
    Join Date
    Dec 2007
    Posts
    1,642
    It's, perhaps, worth mentioning that microbes have been found living miles beneath the surface of the oceanic crust. Their metabolism, and energy consumption, is extremely low compared to closeky related surface dwelling organisms. Life on Titan will almost certainly be very simple - probably not much more than the methane drinking equivilent of protocells or rna strands - but I don't think the low energy budget is a showstopper, based on what we know so far....

  23. #23
    Join Date
    Apr 2005
    Posts
    2,164
    I thought it was a long shot, because the only known example of life uses water as its solvent. There is no proof that life in other media is possible. Proposals for detecting exolife are based around planets where liquid water can exist on the surface.

    I've no objections to life on Titan on thermodynamic grounds, and yes I think the possibility is worth considering.

  24. #24
    Join Date
    Aug 2010
    Posts
    2,078
    Returning to the quote in the opening post of this thread, if liquids (including liquid methane and ethane) are "a medium that brings chemicals close enough together to interact but not so close that they can’t move"...

    Perhaps it's worth thinking about what else this implies... What sort of carbon chemistry could we expect to find on Titan if it turns out not to have life?

    Compare Titan to the carbonaceous asteroids, which are rightly considered scientifically very interesting objects because of the range of organic compounds they contain.

    Titan, which has a range of organic compounds, plus hydrogen gas, plus surface liquid, is going to be chemically even more interesting than the carbonaceous asteroids.

  25. #25
    Join Date
    Dec 2011
    Posts
    2,935
    Titan's atmosphere is all but bereft of oxygen, or oxygen containing compounds (like water). Virtually every organic molecule of known biomolecular interest contains some oxygen - and that includes every amino acid, every protein and enzyme, every sugar, every fatty acid and DNA. If there is no credible way to incorporate oxygen into the chemical chain in the atmosphere, (and there isn't), then Titan's organics are destined to remain as virtually sterile hydrocarbons and nitriles and from that, you can't make life's chemistry work.

    McKay is fantasizing ... (yet again!)

  26. #26
    Join Date
    Apr 2005
    Posts
    2,164
    Quote Originally Posted by Selfsim View Post
    Titan's atmosphere is all but bereft of oxygen, or oxygen containing compounds (like water). Virtually every organic molecule of known biomolecular interest contains some oxygen - and that includes every amino acid, every protein and enzyme, every sugar, every fatty acid and DNA. If there is no credible way to incorporate oxygen into the chemical chain in the atmosphere, (and there isn't), then Titan's organics are destined to remain as virtually sterile hydrocarbons and nitriles and from that, you can't make life's chemistry work.

    McKay is fantasizing ... (yet again!)
    We're talking about life that is very different at a fundamental level. If you look at the Asimov article linked to upthread, there is the idea of lipids in methane as a basis for life.

    OK lipids contain a little oxygen, but there is carbon dioxide and carbon monoxide reported in Titan's atmosphere.

  27. #27
    Join Date
    Aug 2010
    Posts
    2,078
    Quote Originally Posted by Selfsim View Post
    Titan's atmosphere is all but bereft of oxygen, or oxygen containing compounds (like water). Virtually every organic molecule of known biomolecular interest contains some oxygen - and that includes every amino acid, every protein and enzyme, every sugar, every fatty acid and DNA. If there is no credible way to incorporate oxygen into the chemical chain in the atmosphere, (and there isn't), then Titan's organics are destined to remain as virtually sterile hydrocarbons and nitriles and from that, you can't make life's chemistry work.
    Selfsim, your comment is very similar to something said by Wayne Zimmerman, Jonathan Lunine, and Ralph Lorenz in their 2004 paper "A Surface Science Paradigm for a Post-Huygens Titan Mission". You have read their paper, I suppose?

    May I remind you that they go on to mention that nitriles and tholins are chemically related to amino acids, and can in fact be converted into amino acids by liquid water? And that, although water on Titan's surface is not normally liquid, it may be converted to the liquid state by cryo-volcanism or impacts from meteorites?

    I would also point out that one of the authors of that paper Jonathan Lunine later reached the conclusion that nitriles themselves (even without addition of liquid water) are NOT necessarily "sterile" in Titan conditions.

    In fact, Lunine was part of the team which found (in a study completed in 2015) that one compound in the nitrile category, namely acrylonitrile, can function in Titan surface conditions in a similar way to lipids in Earth conditions, forming membranes like those of cells or protocells...

    McKay is fantasizing ... (yet again!)
    And Jonathan Lunine is fantasizing also, I suppose?

  28. #28
    Join Date
    Dec 2011
    Posts
    2,935
    Actually, I was reading from the Lorenz/Mitton book: "Lifting Titan's Veil: Exploring the Giant Moon of Saturn" and the text also appears as unreferenced dialogue in it.

    Moving on from the trivial references issue however, from the Lunine paper you linked to:

    Quote Originally Posted by Lunine, Lorenz
    Were there no way to incorporate oxygen into thechemical chain in the atmosphere then the organics thatmade in Titan's atmosphere would be sterile nitriles andhydrocarbons. On the other hand, experiments haveshown that tholins and nitriles are readily hydrolyzed by liquid water into amino acids; e.g., Titan tholin yields about 1% amino acids by mass on hydrolysis [4]. Hence, if the accumulating organics on Titan's surface areexposed to liquid water, an entirely new step in chemical synthesis is introduced. It is of keen astro-biologicalinterest to find locations on the surface that bear evidence of past episodes of liquid water.
    Note my emboldenments and underlines .. hydrolysis is required to produce amino acids and the point still stands .. ie: where is the measured evidence of the necessary atmospheric liquid water producing processes on Titan? Because without it, all there is, is untested (outside of lab conditions) hypotheses and untested models, and no evidence of the necessary oxygen needed to speak of bio-molecules (and hence life).

    Unless I am mistaken, the figure of an almost trivial amount of 8 ppb atmospheric water vapour at 400km altitude, measured in 1997, remains as the key pertinent figure. (Perhaps Cassini updated that figure(?)).

    If McKay expects scientific thinkers to buy into his speculations, he needs to describe how his 'hypothetical Titan methanogens' can exhibit 'life' functions when its key molecules exhibit an apparent absence of the necessary oxygen. His quatum leap from 'liquid water' to 'liquid methane/ethane' as a solvent, glosses over the incorporation of oxygen issue, by simply ignoring it.

  29. #29
    Join Date
    Aug 2010
    Posts
    2,078
    Quote Originally Posted by Selfsim View Post
    Actually, I was reading from the Lorenz/Mitton book: "Lifting Titan's Veil: Exploring the Giant Moon of Saturn" and the text also appears as unreferenced dialogue in it.

    Moving on from the trivial references issue however, from the Lunine paper you linked to:



    Note my emboldenments and underlines .. hydrolysis is required to produce amino acids and the point still stands .. ie: where is the measured evidence of the necessary atmospheric liquid water producing processes on Titan? Because without it, all there is, is untested (outside of lab conditions) hypotheses and untested models, and no evidence of the necessary oxygen needed to speak of bio-molecules (and hence life).

    Unless I am mistaken, the figure of an almost trivial amount of 8 ppb atmospheric water vapour at 400km altitude, measured in 1997, remains as the key pertinent figure. (Perhaps Cassini updated that figure(?)).
    It is well established that Titan has only a trace amount of atmospheric water vapour. Water at the surface is a rock-like solid, but could be melted by cryo-volcanism or meteor impact.

    If McKay expects scientific thinkers to buy into his speculations, he needs to describe how his 'hypothetical Titan methanogens' can exhibit 'life' functions when its key molecules exhibit an apparent absence of the necessary oxygen. His quatum leap from 'liquid water' to 'liquid methane/ethane' as a solvent, glosses over the incorporation of oxygen issue, by simply ignoring it.
    Selfsim, "simply ignoring" is exactly how you seem to be dealing with the most important point in my last message, about the work completed in 2015 regarding acryonitrile a molecule which contains carbon, hydrogen and nitrogen but not oxygen.. In a medium of liquid methane/ethane, acrylonitrile can form membranes comparable to those formed in liquid water by lipids. One of the authors of the 2015 study is Jonathan Lunine the same guy who wrote in 2004 about how important oxygen is.

    Lunine also co-wrote a paper published in 2016 about the behaviour of another CHN compound, polyimine, in Titan surface conditions. In which they made the following statement about whether life molecules need to contain oxygen...

    Quote Originally Posted by Rahm, Lunine, Usher and Shalloway
    Regardless of the specific chemistry involved, life
    requires polymorphic molecules that combine flexibility with the
    ability to form the organized metastable structures needed for
    function, adaptation, and evolution. This, almost certainly, requires
    extended molecules capable of intermolecular and intramolecular
    hydrogen bonding, but such bonds need not involve oxygen; nitrogen
    is a potential surrogate.

  30. #30
    Join Date
    Dec 2011
    Posts
    2,935
    Quote Originally Posted by Colin Robinson View Post
    It is well established that Titan has only a trace amount of atmospheric water vapour. Water at the surface is a rock-like solid, but could be melted by cryo-volcanism or meteor impact.
    .. and then rapidly refrozen. The surface temperatures are just too low for too long on Titan to integrate oxygen into long chain molecules, and then become capable of performing life functions. There is no evidence that Titan's organics come anywhere remotely 'close' to supporting life's definitions.

    Quote Originally Posted by Colin Robinson
    Selfsim, "simply ignoring" is exactly how you seem to be dealing with the most important point in my last message, about the work completed in 2015 regarding acryonitrile — a molecule which contains carbon, hydrogen and nitrogen but not oxygen.. In a medium of liquid methane/ethane, acrylonitrile can form membranes comparable to those formed in liquid water by lipids. One of the authors of the 2015 study is Jonathan Lunine — the same guy who wrote in 2004 about how important oxygen is.
    Important? I don't think so. That's just a hypothetical model .. nothing of real consequence until its put to the test on Titan ..

    You asked:

    Quote Originally Posted by Colin Robinson
    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?

    I personally think he's off with the fairies in la-la land ...
    And I'm not sure why we should be too concerned with looking for explanations, (simple or otherwise), for why Chris McKay is interested in looking for life on the surface of Titan.

    Quote Originally Posted by Rahm, Lunine, Usher and Shalloway
    Quote Originally Posted by Colin Robinson
    Lunine also co-wrote a paper published in 2016 about the behaviour of another CHN compound, polyimine, in Titan surface conditions. In which they made the following statement about whether life molecules need to contain oxygen...
    Regardless of the specific chemistry involved, life requires polymorphic molecules that combine flexibility with the ability to form the organized metastable structures needed for function, adaptation, and evolution. This, almost certainly, requires extended molecules capable of intermolecular and intramolecular hydrogen bonding, but such bonds need not involve oxygen; nitrogen is a potential surrogate.
    How can we ignore 'the specific chemistry involved', when the specific chemistry involved produces the functions we call 'life' (including 'function, adaptation and evolution')?
    Let's see them produce life's functions, (including adaptation and evolution), by substituting oxygen for nitrogen!
    So easy to say .. and so difficult to produce!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •