If we are to look for life elsewhere in the universe, we need to be able to recognise life should we find it. This is not as easy as it might seem, as we only have one example to study. It has not proved possible to produce a generally accepted definition of life (though many have tried); but what we can do is describe the essential features of life as seen on Earth. This shows us that the peculiar features of water are critical to life as we know it, and that life can be found almost anywhere where there is liquid water. Thermodynamics tells us that energy flow is essential to maintain life, and in living organisms energy is delivered, carried and stored as potential energy in electrons, or temporarily as a proton gradient. The temperature range over which the life-cycle can be completed differs between eukaryotes (for whom the range is narrow) and bacteria and archaea (which have the widest range). The lower limit for life appears to be around -20 oC, and is probably defined by the vitrification of the cell interior. The upper limit is > 100 oC., but we do not know the mechanism that defines this upper limit (though it is probably related to thermal denaturation).
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