THE ATMOSPHERES OF THE PLANETS 565 



1908, by comparing the spectra of Mars and the Moon when at the same 

 altitude under conditions of exceptional atmospheric dryness in the 

 winter, found that the water-vapour absorptions were sHghtly stronger 

 in the spectrum of Mars than in that of the Moon. 



Carbon dioxide has not been detected in the Martian atmosphere, 

 which is not surprising since carbon dioxide must be present in large 

 quantity before the absorptions in the region of the infra-red available 

 for investigation can be detected. There is some evidence of the existence 

 of vegetation on Mars. Seasonal changes in form and coloration of the 

 dark areas, light green changing to a darker green, and then to yellow and 

 brown, seem to be reasonably well established. The interpretation of 

 these changes as due to the seasonal growth of vegetation is plausible. 

 The presence of some carbon dioxide in the atmosphere may therefore 

 be inferred. Mars appears to be a world in the state that the Earth will 

 ultimately reach when the oxygen in the atmosphere will have been almost 

 entirely exhausted by the progressive weathering and oxidation of the 

 rocks. 



The major planets, Jupiter, Saturn, Uranus and Neptune, may be con- 

 sidered together. They are large massive planets, of low mean density, 

 whose visible disks are considerably oblate. Their masses range from 

 317 times the mass of the Earth, in the case of Jupiter, to 15 times the 

 mass of the Earth, in the case of Uranus. The mean densities of Jupiter, 

 Uranus and Neptune are not greatly different from that of the Sun, 

 which is 1-4 times the density of water; Saturn has the lowest mean 

 density of any of the planets, only seven-tenths that of water. The 

 velocities of escape from all the major planets are so high, from 21 km./sec. 

 to 60 km./sec, that extensive atmospheres are to be expected containing 

 an abundance of the light constituents, hydrogen and helium, which have 

 been lost from the atmospheres of the medium-sized planets. 



The telescopic appearance of Jupiter and Saturn confirms the existence 

 of dense atmospheres. Markings in the form of belts parallel to the 

 equator may be seen ; these are of complex structure and their details 

 are continually changing. Photographs in the infra-red show many 

 differences from those in the ultra-violet, due to the greater penetration 

 of the long-wave radiations into the atmosphere, but again the recorded 

 features are continually changing, so that the infra-red light does not 

 penetrate to the surface. Uranus and Neptune are too distant for detailed 

 study of their surfaces, though faint belts parallel to the equator may be 

 seen on Uranus. 



Some theoretical results of interest have been obtained from the 

 oblateness of these planets and the changes in the orbits of their satellites 

 produced by the equatorial bulges of the parent planet. From investi- 

 gations of this nature, Jeffreys concluded that these planets consist of a 

 core of rock, generally similar to the inner planets in its constitution and 

 of about the same mean density, surrounded by ice-coatings of great 

 depth, above which are very extensive atmospheres. If these conclusions 

 are accepted, some inferences may be derived about the thickness of the 

 ice-coating and the depth of the atmosphere. 



According to the calculations by Wildt, the rocky core of Jupiter has 

 a radius of about 22,000 miles, so that it occupies only one-eighth of the 



