564 NORMAN LOCKYER LECTURE 



of an atmosphere on Mars can be proved by photographing the planet 

 in light of different colours. Photographs in the infra-red show permanent 

 markings, which are evidently surface features, whereas photographs 

 in the ultra-violet show none of these. By photographing through 

 filters which pass a narrow spectral region, it is found that the surface 

 details become more and more distinct as the wave-length of the light 

 increases. The atmosphere is extensive enough to scatter ultra-violet 

 light to such an extent that the light cannot penetrate to the surface 

 and out again. 



The images obtained with ultra-violet light are larger than those 

 obtained with infra-red light and the difference in size indicates that the 

 atmosphere extends to a height of fully fifty miles above the surface. 



The polar caps provide additional evidence of an atmosphere on Mars. 

 As the summer advances over one hemisphere the polar cap gradually 

 shrinks and disappears whilst the opposite cap, with the advance of winter, 

 forms and grows. These changes are to be explained by the melting 

 or deposition of ice, snow or hoar-frost, for the temperature is not low 

 enough for the caps to consist of solid carbon dioxide. From the rate 

 at which the caps decrease as summer advances it can be calculated that 

 they are not more than a few inches thick, so that the whole quantity of 

 water contained in them would be sufficient to make a lake of only moderate 

 size. The caps are more prominent in ultra-violet than in infra-red 

 photographs and are therefore partially atmospheric ; in winter, there is 

 a permanent cloud layer above the pole. 



Photographs by Wright, at the Lick Observatory, in light of different 

 colours have given further confirmation of an atmosphere in the occurrence 

 of clouds. The clouds are of two different types. One type of cloud is 

 most prominent in the ultra-violet photographs. Such clouds must 

 occur fairly high up in the atmosphere and must be sufficiently thin to 

 allow the infra-red light to pass through ; these clouds have a tendency 

 to begin to form at about Martian noon and to grow during the afternoon. 

 It is probable that they are produced by the condensation of water- vapour, 

 with the fall of temperature that begins at noon. The second type of 

 cloud is seen on the infra-red, but not on the ultra-violet photographs. 

 Such clouds appear yellowish to the eye. They must be at a fairly low 

 level in the atmosphere and the yellowish hue is no doubt caused by 

 atmospheric absorption. 



All attempts to detect oxygen in the atmosphere of Mars have been 

 unsuccessful. It can be concluded that the amount of oxygen is not 

 more than one-thousandth part of the amount in the Earth's atmosphere. 

 The red colour of Mars, which is unique among the heavenly bodies, 

 provides indirect evidence of oxygen, suggesting rocks that have been 

 completely oxidised. We may contrast the colour of Mars with the grey 

 or brownish rocks of the Moon, which have not been oxidised. It appears 

 probable that Mars may be a planet where the weathering of the rocks, 

 followed by their oxidation, has resulted in the almost complete depletion 

 of oxygen from the atmosphere. 



The amount of water-vapour in the atmosphere of Mars is so small 

 that it can be detected only under the most favourable conditions. At the 

 Lowell Observatory, which is at an altitude of 7,250 feet, Slipher, in 



