What defines the primary heating mechanism for the liquid water reservoirs on Mars, contrasting with the icy moons?

Geological processes involving high pressure and the presence of salts lowering the freezing point.

The stability of liquid water beneath the Martian surface is not maintained by solar warmth, unlike Earth's surface oceans. Instead, the liquid state is supported by internal geological factors. Specifically, the intense pressure exerted by the overlying layers, coupled with the presence of dissolved salts (brine) which act to depress the freezing point, keeps the water liquid. This contrasts sharply with the oceans on moons like Europa, where the primary energy input comes from the external forces of tidal flexing generated by their massive host planets.