What conditions allow significant bodies of liquid water to persist deep beneath the Martian south pole today?

Answer

Geothermal heat and the pressure exerted by overlying ice and rock

The water found kilometers below the surface remains liquid because the immense pressure from the overlying layers of ice and rock, combined with geothermal heat, prevents it from freezing solid.

What conditions allow significant bodies of liquid water to persist deep beneath the Martian south pole today?

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