150 WATER VAPOUR ON MARS. 



The air pressure on Mars being considerably less than on the 

 highest mountains of this Earth, the average temperature should 

 theoretically, from this cause alone, be below zero, as occurs on 

 the summits of the Andes or the Himalayas ; and the planet 

 should be a globe of ice. And this, without taking into account 

 other reasons, such as the greater distance from the Sun. But, 

 as a matter of fact, what do we find ? In the words of Marsden 

 Manson* 



" The observed phenomena are correctly interpreted by saying that the 

 Martian chmate is on an average milder than that of the Earth." 



And to substantiate this statement I need quote but one 

 instance, the behaviour of snow on Mars. We find that the snows 

 are limited in extent, var}/ with the seasons, and in general are less 

 extended than on the Earth. In Mars, the snow in winter extends 

 from the Poles over an area some 40° to 50° in diameter : in 

 summer, the snow-belt shrinks until it girdles only some 4° to 5°, 

 occasionally disappearing entirely at the South Pole ; whereas 

 our snows in winter extend over a region some 90° in diameter, 

 while in summer the}^ still embrace an area as large as obtains 

 in the Martian winter. 



To what can this be due ? No doubt, the smaller extent, or 

 the absence of the seas, and the purity of the skies in Mars are 

 factors that go towards the making of this genial climate ; but 

 by far the most potent agent, to my mind, is the water vapour 

 whose presence in large quantities has been demonstrated. Let 

 us take these in turn. 



One of the factors which counts for a good deal is the pecu- 

 liarity that there are not on Mars as on the Earth, extended masses 

 of water — oceans — constantly absorbing a large proportion of 

 the solar radiation without themselves being strongly heated, 

 and so heating the circumambient atmosphere ; and that prac- 

 tically all the heating power of the sun's rays is employed in 

 warming a continental surface, for the greater part unprotected 

 by vegetation — a surface to all intents an unmitigated Sahara. 



Again thg great purity of the Martian sky, nearly always un- 

 obscured by cloud, allows the solar radiation to reach the soil 

 without loss, thus compensating in a measure for the greater 

 distance of the source of heat. This fact becomes more significant 

 when we bear in mind that it is estimated the clouds on our 

 Earth intercept, on an average, 50% of this radiation. 



Another factor is the longer duration of the Martian summer, 

 aliowing a larger accumulation of heat in the season, being 381 

 da^s in Mars against 186 on Earth. The inclination of the axes 

 ot Earth and Mars to the Ecliptic being approximately the same, 

 the obliquity of the sun's rays for different latitudes at the different 

 seasons is practically the same for both planets and can be 

 eliminated as far as our present enquiry is concerned. 



But by far the greatest and most pregnant factor in producing 

 a temperate climate on Mars is without doubt the amount of 

 aqueous vapour in the Martian atmosphere. 



* Astronomical Society of the Pacific, 1895, VII. 



