214 SCIENCE PROGRESS 



hydrogen molecules there is probably about 50 miles a second. 

 The earth has a lower critical velocity, namely 6'g miles a 

 second, whilst the maximum velocity of the hydrogen molecules 

 at the mean temperature of the air would be about 7*4 miles 

 per second and but little hydrogen is found free in our atmo- 

 sphere. The critical velocity at the surface of Mars is about 

 3' 1 miles per second and the temperature, as we shall see, is 

 probably not so much below that of the earth as to make 

 it likely that gaseous hydrogen is a constituent of its atmo- 

 sphere though other gases whose maximum molecular velocities 

 are less than this may well be present. 



On account of the weakness of gravity on Mars it is pro- 

 bable that though water may be scarce, yet the commoner 

 constituents of the earth's atmosphere whose molecular 

 velocities at its surface are all likely to be less than 3*1 miles 

 per second may well be common. Among these gases are 

 those which make life possible here — namely water vapour, 

 oxygen, nitrogen and carbon dioxide. We need, therefore, 

 feel no surprise when appearances on Mars indicate the 

 presence of gases which are thus shown to be theoretically 

 possible. That there are other causes besides gravitational 

 weakness operating to rob the planet of a terrestrial atmo- 

 sphere cannot be doubted : diminished pressure of sunlight 

 is perhaps the most obvious. It appears therefore that we are 

 justified in concluding that the atmosphere of Mars may be 

 like our own, though less dense and that probably it is dis- 

 appearing gradually. It will be seen later that this conclusion 

 is amply corroborated by the detailed observations of the 

 surface features and their changes. 



Our estimate of the temperature at the surface of Mars is 

 based upon the following considerations. The heating and 

 lighting power of the sun at the distance of Mars is about 

 half what it is on the earth ; but only about 40 per cent, of the 

 solar heat which the earth intercepts ever reaches the surface ; 

 the remaining 60 per cent, is thrown back into space by 

 our atmosphere. On Mars the conditions are very different. 

 Though the planet only receives 50 per cent, of the earth's 

 share, it retains a much greater proportion, for the low 

 albedo or reflecting power of Mars is an indication that 

 more than 80 per cent, of the incident light is retained and 

 hence it appears that the surface of the planet receives from 



