WATER VAPOUR ON MARS. I49 



has been deduced what is known as the critical velocity at the 

 surface of any planet.* 



By " critical velocity " is meant a certain definite speed in any 

 body projected from the surface of a planet, which that planet 

 shall be able to control : if the body start with a faster speed 

 it will permanently escape from the planet's control. With re- 

 ference to the planet's action we might not inaptly use the 

 expression of Professor Lane Poor " speed of control," while 

 " speed of escape " would denote the same phenomenon with 

 reference to the moving body. 



Now, for a given temperature and pressure, the molecules of 

 •each gas travel with a certain definite average velocity, each ac- 

 cording to its kind ; this velocity is slower for heavier gases than 

 for the lighter ones. The average velocity of a gas is increased 

 by rise of temperature ; and the countless collisions of the mole- 

 cules impinging on one another also tend to increase their speed 

 until a maximum velocity is attained. 



Thus, while the average velocity of the hydrogen molecule 

 is I •14 miles per second, its probable maximum speed is 7-4 miles. 

 The probable speed of control on the Earth being only 6-9 miles 

 per second explains the fact known to every student of chemistry 

 that, for all practical purposes, hydrogen does not exist on the 

 Earth in an uncombined state. 



If therefore it can be proved that the " speed of control " on 

 Mars is less than the " speed of escape " of the vapour molecule, 

 then there can be no water vapour present in the Martian at- 

 mosphere. But the " speed of control " on Mars is 3-13 miles 

 per second, and the average velocity of the water vapour mole- 

 cule equals "38 miles per second, while the probable maximum 

 speed is only 2 '5 miles per second, leaving a safety margin of 1.6 

 miles per second at the ^lartian surface. It is true that this is 

 a safe margin on the average : nevertheless, in the outer fringes 

 of ihe Martian atmosphere, the " speed of control " will neces- 

 sarily be smaller, and the outermost layers of molecules must 

 frequently escape control and be lost for ever to the planet's 

 atmosphere. 



This continually takes place, but in what proportion we are 

 unable to determine : and small though it may be, its effect must 

 be appreciable in the course of ages. This constant leakage 

 is doubtless one of the causes which have apparently converted 

 the greater part of the Martian surface into a desert. 



The fruitful consequences of the presence of this large amount 

 of water vapour in the [Martian atmosphere are extremely im- 

 portant for our knowledge of the planet ; and I propose to add 

 a word on the more important of them. 



* Johnstone Stoney, " Atmospheres upon Planets and Satellites," see 

 supra : also, Astrophysical Jouynal, Jan., 1898, and May, 1900. — S. R. Cook, 

 " On the escape of gases from planetary atmospheres according to the kinetic 

 theory," Astrophysical Journal, Jan., 1900. — Bryan, " Proceedings of the 

 Royal Society," 1900. — C. Lane Poor. " The Solar System," see supra. — 

 P. Lowell, " Mars," London, 1900, in appendix, p. 213. — Du Ligonnes, 

 " Les atmospheres des planetes," Bulletin de la Societe Astronomique de 

 France, June, 1903. 



