The sun is at an altitude of 30 degrees to the horizontal. What is the corresponding air mass? What is the physical significance of air mass? Mars has a very thin atmosphere containing primarily CO_2 and dust particles. If you were to calculate the corresponding "air mass" for Mars, what difference would you find? If the density of Mars' atmospheric gas is one thousand times less than that of the earth, calculate the irradiance at AM1.5. What is the spectral distribution of solar irradiance on the surface of the moon?