8.2 Life, what is it?
How can life be defined and detected? What conditions does life require? Which moons are most likely to contain habitable environments?
To find out if something is alive, we usually look for a few key signs. Does it breathe? Does it move? Does it eat? Does it excrete? What is its temperature compared to that of the surrounding environment? But these criteria do not apply to all living things. In plants, with an entirely different metabolism from ours, we look for the colour green as an indication of life. What would you say are the criteria for determining whether or not something is living? Share your ideas.
In searching for life on moons we would not expect evolved life forms, but much simpler, microbial life – perhaps organisms analogous to bacteria and archaea – and would be surprised to find anything more complex than fungi and lichen. These were among the earliest the forms of life on Earth, and they continue to inhabit the most extreme environments on Earth.
For biologists the two criteria for life are being able to self-replicate and to undergo Darwinian evolution.
Beyond Earth we cannot yet study objects in sufficient detail to form judgements on these two criteria. And so space missions have to look for other signs of life: for the waste products of metabolism, for changes in the environment and, if we’re lucky, for observations of shapes and movements. To date, no mission has found any sign or trace of life. Missions have, however, found habitable environments, in other words environments that could harbour life.
The focus is on microbes, because they are known to live in widely diverse environments – from very salty lakes to dilute groundwater, from the cold of the Antarctic to the heat of hot springs, in rocks and in other biomass. Whatever the environment on moons, these simple forms of life stand the best chance of being capable of living there.