Steve
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Is Water Necessary for Life?

All Life as we know it requires water.  This raises a question.  What about life in forms we do not know about? The only life we know about is on or from the Earth.  People can theorise that other forms of life could exist.  Possibly liquid Methane or Ethane based life on somewhere like Titan. There are reasons to think that water is a much more suitable solventfor life than any other one, but we cannot completely rule out the possibility of other types of life based on different solvents.

However, confining the answer to life similar to what we know it, we can say that all life requires water to grow although there are many organisms that can survive drying out and revive when water is available.

Liquid Water

Further than that: we can say that all growing life (rather than just surviving in an inactive state) needs liquid water.  The other states of water, ice and water vapour, will not do.

But the last statement is not necessarily completely true.  For example, lichens can continue to metabolise down to about minus 40 degrees C (minus 40 degrees F).  The water in their cells would still be liquid.  Snow algae may be able to grow a little while the snow they are on is below freezing, although they are more active when it melts.

Lichens also seem to be able to absorb some water vapour although this is probably very limited.   On the Earth, in a moist area, this ability may be useful, but would not be so useful on Mars where there is only about 0.03 percent water vapour in the atmosphere.  Locally, the water vapour percentage may be much higher than this, but is unlikely to be high enough for any Earth organism to absorb more water than it loses.

Origin of life

Although some organisms may be able to use a limited amount of water in solid or vapour form, it is more difficult to see how life could have originated except with liquid water present.

If there is life native to Mars and still surviving, it is probably below the surface, deep enough for the water to be liquid.  But it is not inconceivable that some near surface life does exist there in limited favourable habitats like under stones, or a few millimetres below the surface where there is some protection from ultraviolet radiation, but enough light penetration for photosynthesis.  For this to happen it would probably require a thin film of liquid water.

The idea of a different form of deep-rooted plant with its photosynthetic organs seeking partially protected environments very close to the surface is not totally beyond the realm of possibility.