Deep biospheres on earth, mars and beyond

Over the last two decades, living organisms have been found below the surface of the Earth at ever greater depths. The "deep biosphere" exploits geochemical energy sources and nutrients as well as photosynthetic organic carbon and oxidants delivered from the surface. Although the presence of liquid water on a planetary or lunar surface is traditionally regarded as the key criterion for global habitability, these discoveries suggest that planets and moons with frozen surfaces might be able to support subsurface biospheres where liquid water is maintained by geothermal heat. Here, I briefly summarize current knowledge of the distribution and diversity of Earth's deep biosphere, the possibility of a similar biosphere on Mars or in the icy moons of the outer solar system, implications for the detection of life in the universe, and forthcoming space missions that may extend our understanding.