Primitive Planetary Atmospheres 21 



There is no certain evidence that life exists on Venus, The conditions most 

 likely to prevent life are (i) the absence of water and (2) prohibitively high 

 temperatures on its surface. So far as we know the conditions on that planet 

 do not exclude its existence. 



THE ATMOSPHERE OF MARS 



The only constituent of the atmosphere of Mars that has been definitely 

 identified is carbon dioxide. From Kuiper's [23] observations on the intensities 

 of absorption of the i-6 band in the Martian and terrestrial atmospheres, 

 Grandjean & Goody [24] estimate that the CO2 content of the Martian atmos- 

 phere per cm^ is thirteen times that of the earth. The remainder of the atmos- 

 phere is probably nitrogen with some admixture of radiogenic argon. The total 

 amount of atmosphere is about 230 g/cm^ according to the measurements of 

 Dollfus [25]. 



Very small amounts of water are present on Mars. The polarization of the 

 polar caps is similar to that of ice at liquid-air temperatures [25] and Kuiper [23] 

 reports that the reflection spectrum of the caps is similar to that of frost at low 

 temperatures. Also, the morning haze is probably water. Dimham [26] estimated 

 the water as less than 5 cm atmospheres. Oxygen has not been detected. 



The maximum equatorial daytime temperature is reported to be as high as 

 30 °C but the night temperature falls to —70 °C. It is evident that there is no 

 liquid water on the planet at the present time. 



The escape of gases from Mars presents quite different problems from those 

 met in the cases of the Earth and Venus, because the mass of the planet is so 

 very much smaller. Hydrogen will escape readily from the planet if the tempera- 

 ture of the high atmosphere is only moderate. The interesting question is 

 whether oxygen will escape also. A recent study by Urey [10] in which con- 

 densation at the equatorial tropopause and diffusion in the high atmosphere are 

 considered comes to the following conclusions. If the temperature of the high 

 atmosphere, i.e. the escape layer, is at 2000 °K oxygen will escape from the 

 planet at appreciable rates and the net effect is the removal of water. (A tem- 

 perature of 2000 °K for the high atmosphere of Mars is not unreasonable in view 

 of the indications that high temperatures exist in the high terrestrial atmos- 

 phere.) Water remains near the surface but carbon dioxide is transported by 

 oxygen to the high atmosphere. Photochemical decomposition gives atomic 

 oxygen and carbon monoxide. The latter reacts with water at low levels to pro- 

 duce carbon dioxide and hydrogen. Both the oxygen and hydrogen escape. On 

 the other hand nitrogen will not be lost at catastrophic rates because it is not 

 appreciably dissociated at the escape layer and remains mostly as molecular 

 nitrogen, whereas oxygen is mostly dissociated. Thus very large amounts of 

 water may have been present in the past. If life is present on Mars, such oceans 

 must have existed in the past and only the escape of oxygen and hydrogen from 

 the planet could have removed large quantities of water from the planet. 



If large amounts of water were present on Mars in the past, a more extensive 

 cloud layer must have been present, the albedo was higher and glaciers must 

 have covered the poles and most of the surface. These glaciers must have trans- 



