Vapor surface density profiles. Shown in black are the steady state vapor profile when all the ice is in the gas phase (thin solid curve), and the surface density corresponding to equilibrium vapor pressure P_eq (Eq. (C.2); dashed solid curve). The snowline r_snow is located at the intersection of these two curves. The numerically-obtained profile (thick gray curve) closely follows the minimum of steady-state and equilibrium profiles. The blue curves present mathematically-convenient approximations to Σ_eq (see text), valid for r>r_snow.