DISCOVERY 



43 



artificially drawn from the atmospheric circulation of 

 the planet, or, alternately, that fogs might be arti- 

 ficially localised by electrical means — a theory which 

 would obv'iate the necessity of believing in a world-wide 

 system of irrigation. In 1918 he put forward another 

 hypothesis, in which he seeks to explain the canals 

 without having recourse to the theory of intelligence. 

 Accordingto this "aerial deposition theory," "the water 

 from the melting polar cap is deposited on the three 

 main depressions on its border, and is then evaporated 

 and carried by the aerial circulation of the planet along 

 the curved lines which in accordance with Ferrel's 

 theory of the winds it must necessarily follow. The 

 storm clouds, which in that rare atmosphere should 

 condense at night, would in this manner carry the water 

 and deposit it in the elongated marshes which we see 

 and call canals." This hypothesis, however, can 

 hardly be called wholly satisfactory, and it must be 

 admitted that Lowell's theory — startling though it is 

 in its implications — would seem to be more capable 

 of explaining the observed facts than the alternative 

 hypotheses. 



Recent astronomical research has emphasised the 

 difference rather than the analogies between Mars and 

 the Earth. Thus we know that the surface of Mars is 

 comparatively flat — that there are no mountains worthy 

 of the name on our neighbouring world ; nor are there 

 any large bodies of water on the Martian surface ; if ever 

 there were permanent oceans they have long since dis- 

 appeared. The planet's atmosphere, too, is very rare — 

 " thinner at least by half," according to Lowell, " than 

 the air upon the summit of the Himalayas " ; and its 

 surface-pressure, Pickering believes, is likely to lie 

 between one-half and one-tenth of that at the Earth's 

 surface. The Martian atmosphere is much clearer 

 than ours. It is not, however, absolutely cloudless. 

 " Clouds," Pickering wrote in 1914, " are nearly 

 always visible on the disc, but they are not usually 

 reported because they are difficult to observe. ... In 

 the polar regions the clouds are sometimes so white as 

 to be with difficulty distinguished from the snow. 

 After they have disappeared at the poles, the ground 

 is often seen to be white with freshly-fallen snow. 

 Sometimes cloud-masses partially conceal a whole 

 hemisphere, and cloudy nights appear to be frequent." 

 During the opposition of 1920, the observers at the 

 Lowell Observatory were impressed by the imusual 

 haziness and cloudiness of the Martian atmosphere. 

 But we may say that Mars is usually a world of blue 

 skies and bright sunshine. 



Despite the greater distance of Mars from the Sun, 

 the difference in temperature does not appear to be so 

 great as might be expected. It has been calculated 

 that the theoretical mean temperature is nearly 0° F. — 

 considerably below the freezing-point of water. But 



this estimate is totalh^ at variance with the observed 

 facts, and the presence of vegetation, and the obvious 

 existence of water in a liquid state, indicate a much 

 higher temperature. For of the existence of water 

 there can be no doubt, the lines of water vapour having 

 been identified in the Martian spectrum by Huggins and 

 Vogel, and conclusively by V. M. Slipher at the Lowell 

 Observatory in 1908. In his last book, Lowell, from a 

 theoretical conclusion based not only on distance 

 but on the transparency of the Martian atmosphere, 

 found the mean temperature of the planet to be about 

 48° F., which agrees with observation. 



Mars differs from the Earth in the absence of any 

 satellite of appreciable size. Until about half a century 

 ago, indeed, it was believed that Mars possessed no 

 moons, and the discovery by Asaph Hall at Washington 

 during the memorable 1877 opposition, of two tiny 

 satellites, was quite unexpected. These two moons 

 are very small, appearing as stars of about the tenth and 

 twelfth magnitude. Phobos, the larger and nearer of 

 the two, would appear to have a diameter of about 

 thirty-six miles, and Deimos, the more distant, of about 

 ten miles. Obviously, these little satellites belong to an 

 order of bodies altogether inferior to our Moon, and it is 

 not impossible that they may be asteroids which have 

 come in the course of ages within the sphere of influence 

 of Mars and thus been captured. 



Vex us 



Venus is our nearest planetary neighbour, and has 

 been defined as the Earth's sister world. Nevertheless, 

 our actual knowledge of our nearest neighbour is very 

 scanty. When nearest to us, Venus is directly between 

 the Earth and the Sun and is invisible ; and in addition 

 we never see the planet as a fully illuminated disc. 

 The thick atmospheric envelope, too, with which 

 Venus is surrounded renders the discover^' of surface 

 detail a matter of great difficulty. For over three 

 centuries astronomers have scrutinised its surface, 

 yet to-day even the length of its rotation period is a 

 matter of uncertainty. Cassini, in 1666, concluded 

 that his observations pointed to a rotation period of 

 about 2;^ hours ; and this estimate was confirmed bv the 

 German astronomer Schroter in 1789 and again in 

 1811. and in 1844 by Di Vico, of Rome, who fixed the 

 length of the planet's day as 23 hours 21 minutes 

 22 seconds. These results, however, did not inspire 

 much confidence. For Herschel, by far the greatest 

 planetary observer of his day, was quite unable to see 

 the spots which Schroter had used to determine the 

 rotation period. The diurnal motion, he said, "on 

 account of the density of the atmosphere of this 

 planet, has still eluded my constant attention, so far 

 as concerns its period and direction." 



In 1877 Schiaparelli turned his attention to the 



