Life requires the right ingredients to get started, but it also needs the right conditions. Searching for life elsewhere in the Universe can help us learn about whether life is common or rare, as well as understand more about how life on our own planet began.

In our search for life beyond Earth, we may begin close to home, by considering the prospects elsewhere in our Solar System. Our dry, airless Moon is not suitable for life, but there are other places where the conditions may be (or once may have been) right. Detailed observations of the planet Mars from orbiting spacecraft and from robots on its surface (in particular the Spirit, Opportunity, and Curiosity rovers) have provided evidence that liquid water flowed on the Martian landscape earlier in its history. Could life have arisen on this warmer, wetter Mars? So far, there is no definitive evidence, but investigating this question is a major focus for NASA and international space agencies.

Elsewhere in our Solar System, Jupiter’s moon Europa, as well as Saturn’s moon Enceladus, are thought to have a liquid ocean beneath a thick surface layer of ice. And Saturn’s largest moon Titan is rich in organic compounds, including methane — although if liquid water exists on that moon, it must be deep beneath its frozen surface. These moons are all in the distant Solar System, where the Sun provides little heat, but other processes such as tidal heating and volcanism may provide the energy to sustain life there.