Average temperature of the subsurface over one comet rotation. The solid blue curve corresponds to a constant heat conductivityof κ = κ₀ = 0.002 W m⁻¹K⁻¹. The solid red curve corresponds to purely radiative conductivity, κ = κ_TT³. The dotted red curve indicates the average of T⁴ in a quartic root. In the case of radiative conductivity, the average temperature increases with depth, while the average of T⁴ is nearly constant. The average temperature in the subsurface with constant conductivity is nearly constant.