On Earth, deep subsurface life extends several kilometers down and may hold a comparable biomass to everything on the surface. Bacteria have been recovered from 5 kilometers deep in continental boreholes. The upper temperature limit for known life is about 122 degrees Celsius (°C).
Could the same be true of Mars? The Martian crust warms with depth at roughly 10–20 °C per kilometer. Starting from a mean surface temperature around −60 °C, the temperature ranges between about 0 and 120 °C at depths of 5 to 10 kilometers, potentially allowing life to survive in liquid water.
In 2018, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on ESA’s Mars Express orbiter, detected a strongly reflective patch about 1.5 kilometers beneath the south polar cap. Its initial interpretation in terms of a subglacial lake of salty water was later replaced with the understanding that it is more likely a layered volcanic rock. It was also realized that perchlorate salts found by NASA’s Phoenix and Curiosity robotic missions, can pull water vapor from the air and form liquid films at night, a process Curiosity has indirectly detected. These are thin, briny, and fleeting layers, not an aquifer.
However, in 2024 the robotic NASA lander on Mars InSight, measured seismic velocities which revealed mid-crust beneath the lander, about 11–20 kilometers down, which is best explained by fractured rock saturated with liquid water. If that holds globally, the reservoir would be enough to cover Mars with a water depth of 1–2 kilometers, which would account for much of its missing ancient water in its hypothesized ancient lakes and oceans. The supporting evidence was presented in three papers here, here and here. The inference rests on one location and on model assumptions about rock porosity, so it is not conclusive without supporting data from more stations.
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