It makes sense to start by looking for the kind of life we know about, and that means searching the solar system for locations with both liquid water and interesting carbon-based ‘organic’ chemistry.

The prior existence of life in ancient rocks on Earth, long after the cells have been destroyed, can be concluded by the presence of molecules not produced by abiotic chemistry. As well as the chemical components of cells, we can detect life by the effect it has on its environment. Life produces distinctive features, such as the high percentage of oxygen in Earth’s atmosphere, steep chemical gradients set up in lake waters or sediments, or the biasing of one isotope over another (the elements come as different isotopes, differing only in the number of neutrons in the nucleus – photosynthesis, for example, preferentially uses carbon-12 over carbon-13). In general, ‘biosignatures’ are any tell-tale molecules, structures or features that can only be produced by biological action. Proving that a distinctive aspect can only be produced by life is extremely difficult, however, and debate rages over the earliest signs of life on Earth and claimed evidence for Martian life within the meteorite ALH84001.