THE PLANET MARS 213 



ochreous areas ; they are the " canals " of which so much has 

 been heard. 



In seeking an explanation of the general appearance of the 

 planet we may recall first that it is amply proved by spectro- 

 scopic study that the known chemical elements are the common 

 property of the visible universe. With a few exceptions, 

 perhaps, all the elements that exist in the stars are to be 

 found on the earth and vice versa. We therefore need have 

 no hesitation in drawing on terrestrial experience when in- 

 vestigating Mars, which differs from the earth mainly in being 

 more distant from the sun and of smaller mass. These two 

 differences alone suffice to explain most of the contrasts that 

 are evident on comparing the surface of Mars with that of 

 our earth. 



Again the kinetic theory of gases provides a criterion by 

 which we may judge of the probability of the presence of an 

 atmosphere and its possible nature. According to this theory, 

 the molecules of all gases, at any given temperature, move with 

 velocities characteristic of each gas ; though the molecules of a 

 given gas move with varying velocities, both the mean and the 

 maximum speeds are functions of its molecular weight. 



The power of a celestial body to retain a gaseous atmosphere 

 about itself depends at any given temperature upon the force of 

 gravity at its surface, the which force is a function of its size 

 and mass. This gravitational force is capable of controlling and 

 retaining particles or molecules which move with a speed less 

 than that which would be attained by a particle falling from 

 infinity to the surface of the planet under consideration ; this 

 velocity, for the sake of brevity, is called the critical velocity for 

 the planet because particles moving faster than this, in the right 

 direction, must inevitably fly off the planet and escape into space. 



If it can be shown that the molecules of a given gas at the 

 surface of a planet would move with a maximum velocity higher 

 than the critical for a given planet, the conclusion is inevitable 

 that the planet cannot have permanently an atmosphere com- 

 posed of this gas. 



An example or two will make the operation of this law clearer. 

 It has been found that. the sun possesses an atmosphere largely 

 composed of hydrogen and this is in harmony with the fact that 

 the critical velocity at the sun's surface is something over three 

 hundred miles a second, whereas the maximum velocity of 



