Dose rate in water per primary flux function for protons (a) and ⁴He (b) on Mars under different surface pressures. Solid lines are the total dose rate while dashed lines represent only the upward secondary contribution. Black solid lines mark the deep space dose per flux while black dashed lines represent half of the deep space case. The gray shaded area marks range (140–190 MeV/nuc) of the atmospheric cutoff energy under different surface pressures.