Ice-cap minimal thickness (in m) needed to reach melting temperature at the bottom of the ice sheet as a function of the temperature at its surface and internal heat flux. The white area represents the region where conditions are too cold for liquid water to exist, even at high pressure. This internal flux could be caused by release of primordial heat content, radioactive decay, and tidal heating. The top scale shows the eccentricity needed to provide the given flux for Gl 581 c assuming Earth tidal response (see text).