LIFE IN SPACE AND TIME I3 



Nicholson at Mt. Wilson. The heat which reaches us from 

 a planet is, however, of two kinds. One is carried by the 

 solar rays which have been reflected from its surface or 

 atmosphere, along with the hght by which we see it. The 

 other comes from the solar energy which has been absorbed 

 by the planet's surface, has heated it up, and is then 

 re-emitted in virtue of the temperature of the surface. To 

 distinguish between the two is practicable, because the 

 reflected radiation (speaking broadly) is carried by short 

 waves, and the ''proper" radiation of the planet by waves 

 ten or twenty times as long. A suitable screen, such as a 

 plate of glass, or a cell containing water, is transparent to 

 waves of the first sort, and stops those of the second, so that 

 the efl"ects of the two can be separated. From the observed 

 percentage of transmission it is possible to calculate the 

 temperature of the planet's surface (with some reservations). 



When this process is apphed to Jupiter or Saturn, it 

 is found that practically all the heat which we receive 

 from them is carried by the reflected sunhght. Unless the 

 surface temperature is more than ioo° below zero, a readily 

 measurable amount of planetary radiation would be super- 

 posed upon this; where we find hardly any; and the detailed 

 computations lead to a temperature of about — i30°c. for 

 Jupiter, and —150° for Saturn. These are but little above 

 the temperatures at which the radiation of the Sun, enfeebled 

 by distance, would suffice to keep the surface. It is therefore 

 clear that, whether or not the interior of the planet is hot, 

 very little heat must leak out to the surface. Uranus and 

 Neptune, being farther from the Sun, are doubtless still 

 colder, and direct observations of the former indicate that 

 this is so. 



Higher temperatures may prevail in the sunless interior, 

 but the low density shows that this interior is either so hot 



Fig. I . Mars. Actual photographs of planet Mars taken through large telescope 

 showing successive stages of development during Martian summer. Dates 

 given with photographs are seasonal dates on Mars which correspond to our 

 calendar dates. Note gradual decrease of snow at pole and darkening of 

 planet's tropics with advance of Martian summer. This gradual darkening 

 of certain regions of planet in his summer season and their subsequent fading 

 in winter are best explained by assuming that darkening is due to growth of 

 vegetation. (By E. C. Slipher, Lowell Observatory.) 



