A discussion of life elsewhere therefore naturally begins with a review of some key aspects of the biosphere here on Earth. On Earth’s surface, there is something like a thousand trillion kilograms of carbon locked up in the living things that we see easily with our naked eye—plants, animals, and fungi. Most of this “biomass” is in trees.
But in the past couple of decades, we’ve also learned that there seems to be a similar biomass of microscopic organisms living in the oceans, and another comparable biomass—this learned from deep-Earth drilling projects—of microscopic organisms living underground, down to depths of at least several kilometers. It appears that at least a small fraction of this subsurface biosphere is independent of surface conditions—that is, there are microorganisms living underground today that would likely continue to thrive even if the Sun were to go out, and photosynthesis shut down, tomorrow.
This is not true for a great deal of subsurface life, much of which directly or indirectly depends on the energy harvested from sunlight at Earth’s surface—e.g., because it depends on the organic molecules produced by photosynthesis, or depends on the oxidized molecules resulting from the oxygen liberated by photosynthesis. But it appears that some microorganisms—such as those that make their living by combining hydrogen (produced from subsurface water weathering rocks) with carbon dioxide dissolved—might really represent ecosystems that are independent of the surface.
As long as liquid water would persist in Earth’s interior—and this will be the case as long as there is enough internal geothermal heating to sustain some layer in Earth’s rocks where liquid water exists—it seems likely that there will be a subsurface biosphere.