LIFE IN OTHER WORLDS — JONES 155 



Another proof of an atmosphere on Mars is the appearance of 

 clouds. These are of two kinds. Some are shown clearly in the 

 ultraviolet photographs and not in the infrared ; such clouds must be 

 high up in the atmosphere and sufficiently thin to allow the infrared 

 light to pass through. Other clouds are seen in the infrared and not 

 in the ultraviolet photographs; they must be low-lying clouds of 

 water vapor, seen through a yellowish atmosphere. 



Endeavors to detect oxygen and carbon dioxide in the atmosphere of 

 Mars have so far been unsuccessful, and it can be concluded that the 

 amounts of these gases in the Martian atmosphere are less than one- 

 tenth of 1 percent of the amounts in our atmosphere. Water vapor 

 must undoubtedly be present for, although there are no open seas on 

 Mars, the polar caps must be caps of ice or snow. Water vapor has 

 been detected, in fact, at the Lowell Observatory, by comparing the 

 spectra of Mars and the Moon at the same altitude, under conditions 

 of exceptional atmospheric dryness. The reddish color of much of 

 the surface of Mars is probably due to the oxidation of iron-bearing 

 ores by atmospheric oxygen and is in marked contrast to the gray 

 unoxidized rocks on the Moon. It is probable that there is still a 

 little, though not much, oxygen in the atmosphere. 



The extreme tenuity of the Martian atmosphere is responsible for 

 great diurnal variations of temperature; a scanty atmosphere, with 

 very little moisture in it, does not have much blanketing effect. Near 

 noon, in the equatorial regions, the temperature rises to about 50° F. ; 

 but in the afternoon, as the Sun gets lower, the temperature falls 

 rapidly. After sunset the cold becomes intense and the minimum 

 temperature at night is about 130° below zero. With such an enor- 

 mous daily range of temperature, the conditions for any form of life 

 must be very trying. Whether animal life can exist seems doubtful, 

 though it is impossible to assert that life may not have evolved to 

 suit those conditions. In Mars we see a world where conditions 

 resemble those that will probably prevail on our Earth many millions 

 of years hence, when most of our present atmosphere will have been 

 lost. Mars appears to be a planet of spent or nearly spent life. 



This brief survey of the planets in the solar system has not given 

 any clear indication of life elsewhere than on the Earth, with the 

 probable exception of some vegetation on Mars. It seems that Mars 

 is a planet where life may be on the verge of becoming extinct and 

 that Venus is a planet on which life may be on the verge of coming 

 into existence. Elsewhere there is no life of any sort. 



PLANETS IN OTHER SYSTEMS 



But what is the likelihood that other stars have planets associated 

 with them and that life may exist on some of these? To assess the 

 probability that other stars have systems of planets we must be able 



