The conductive heat flow through ice is given by
$$F = \frac{\lambda \Delta T}{\Delta z}$$
where $\lambda$ (= 2 W/m/K) is the thermal conductivity of ice, $\Delta T$ is the temperature difference between the top and bottom of the ice layer, and $\Delta z$ is the thickness of the layer. We know that the current geothermal heat flux, $F$, is about 0.09 W/m². Assume that $F$ had this same value at 0.6 Ga, but was twice as high at 2.4 Ga. Assume also that the top of the ice is at temperature $T_i$ and that the water below the ice has a temperature of -2°C (the freezing point of water). What is the thickness of the ice at 0.6 and 2.4 Ga?