There now remains Mars. This planet approaches the Moon inasmuch as it reflects only 15.4 per cent. of the sunlight arriving to the orb. Everything points to the conclusion that the atmosphere of Mars is very thin. Lowell estimates, on somewhat meagre grounds, however, that on each square metre of the planet rests only 22 per cent. of the mass of air supported by each square metre of the Earth’s surface.
It would naturally be very interesting to ascertain the amount of sunlight our Earth throws back into space. This we cannot measure, as we cannot place our instruments outside of the Earth’s cloud-mists nor can we read them there. Not less than 52 per cent. of the Earth is covered with clouds, whose whiteness (Latin: _Albedo_) is 65. Thus the clouds alone return 0.52 × 0.65 = 0.338 parts of the sunlight. Of this portion a fraction amounting to about 4 per cent. is extinguished in the air above. The remainder is 0.325. Atmosphere and suspended dust reduce the sunlight over the cloud-free part, _i. e._ 48 per cent. of the Earth, by 60 per cent, half of which returns to space, while the other half reaches the ground in the form of light from the sky, and of this fraction again about 4 per cent. is reflected into space; these two items added give 0.15. Finally, the 40 per cent. sunlight directly received by the Earth’s surface is reflected to the extent of 6 per cent. by the oceans and by the generally moist ground; deserts and bare rocks reflect about twice as much, but their total area is comparatively small; of this 6 per cent. reflected light, 70 per cent. reaches outside space; thus we obtain 0.48 × 0.40 × 0.06 × 0.70 = 0.008. In all, therefore, the amount of reflected sunlight is 0.338 + 0.15 + 0.008 = 49.6 per cent. If the air were free from clouds, the reflexion-number or Albedo would be 33 per cent., or considerably higher than that of Mars. When now half or a little more (52%) of the Earth’s surface is overcast with clouds and this portion therefore has the whiteness of Venus, the figure 49.6 (Russell calculates the figure 45) for the entire Earth naturally falls closer--almost 3.6 times--to 59, the figure of Venus, than to 15.4, the figure for Mars. We may also compare the value 33 per cent., which applies to the cloud-free portion of the Earth, with the value 15.4 per cent. for Mars, which is almost without clouds, and with the value 7.3 per cent. for the Moon, which has neither clouds nor dust, because it lacks an atmosphere. We can then conclude that the atmosphere of our Earth holds almost three times as much dust suspended over each square metre as does Mars, and this in spite of the smaller gravitational force on Mars, which is about 37.5 per cent. of that on Earth. Taking proper account of the low temperature on Mars we may easily compute, by means of a formula given by Stokes, that a particle of dust should sink 2.3 times slower on Mars than on Earth. When, nevertheless and in spite of frequent but thin mists, so few particles of dust float in the atmosphere of Mars, the conception inevitably comes to our mind that the air on that planet must be extremely rarefied so that the wind-puffs have little power to raise the dust from the ground. Lowell estimated the barometric pressure at the surface of Mars to be about 64 mm. (2.52 inches), and Proctor gives about twice this figure. There appears to be ground for considering already the former value too high; both are very uncertain. If we accept that of Lowell, we find that each square metre of the surface of Mars supports a column of air, whose mass is only about one-fifth of the mass resting on each square metre of ocean surface on Earth.