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Substance of Life

  1. Aug 9, 2007 #1
    Why is there such a focus on water (H2O) being the substance needed to sustain life. The only example of life we have is our own, but water is the most abundant resource on our planet. Why don't they look to methane or any other substance to sustain life. Wouldn't it be expected for a life form to adapt to the substances most abundant in its biosphere?
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  3. Aug 9, 2007 #2
    well, earth has plenty of methane, why don't we see methane-based life here? in any case, water isn't all that important, but a significantly polar solvent probably is for the reactions of life to take place, and water is pretty much the simplest substance you can make that fits the bill (methane is non-polar)
  4. Aug 10, 2007 #3


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    Well, to me that makes water important, but....yeah.

    Another possibly important factor is that water gets less dense when it freezes. This prevents lakes, rivers, and oceans in temperate zones from freezing solid and possibly killing any budding life.
  5. Aug 14, 2007 #4
    Because water is the only chemical that we know is necessary to sustain all known forms of life. You can't use the presence of other chemicals to suggest the presence of life because there are known examples of living organisms that do not require it. Methane is an example. So is oxygen.

    To see what I mean, say you found oxygen and water in the atmosphere on a distant planet. You could then surmise that aerobic organisms similar to our own might be living there in some way shape or form. If you found just water on a planet then you can rule out most organisms we know, BUT it still could be possible that anaerobic bacteria (such as the kind that exist near deep sea hydrothermal vents) like organisms can exist on such a planet. However, and here is the key difference, if you found a planet that *only* had oxygen on it and no water, you could not find an example of any living organism on our planet that could survive on it. It doesn't mean that there is no life on that planet, but what it means is that the life could not be related to the life on our own (and therefore you would have a hard time justifying your assertion that life was there as opposed to on some other planet).
  6. Aug 14, 2007 #5


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    There are two classes of chemical to look for in searching for extra-terrestrial life.
    Water is not an indication of life but since water is needed for most of the life on our planet looking for water is a good start in looking for planets that might have life, although the presence of water doesn't mean there is life.

    Oxygen and a few other chemicals are direct evidence for life, these cannot remain for long in a typical atmosphere and so must be continually replenished by living systems. Of course other life could use other chemicals so not finding these does not mean no life.
  7. Aug 14, 2007 #6
    "Oxygen cannot remain for long in a typical atmosphere"
    why not?
  8. Aug 14, 2007 #7


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    Oxygen is fairly reactive. The primordial earth contained lots of oxygen but most of it is bonded to iron or silicon in rocks or carbon in the atmosphere. Since free oxygen will rapidly combine with these other chemicals the presence of oxygen in the atmosphere is evidence of a chemical dis-equlibrium which usually means life.
    Last edited: Aug 14, 2007
  9. Aug 16, 2007 #8
    Seems like this topic is Biology.... :wink:

    It could be that methane as well is fairly imprtant (at least in the formation of life) as all life forms we know of are based on carbon. I've heard that primitive Earth was fairly methane rich. Although I am sure there are plenty of other sources of carbon as well.
  10. Aug 16, 2007 #9


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    There certainly is an astrobiology area of study, but I agree. The questions here have more to do with straight-forward biology. So I'm moving it to the Bio forum.

  11. Aug 16, 2007 #10


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    It's more than this ... provincialism. There are some excellent objective reasons why life is very likely to require water - and that means anywhere in the universe, not just here.
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