ATMOSPHERES OF THE PLANETS — RUSSELL 167 



must be thoroughly frozen out of their atmospheres, leaving only 

 ammonia and methane. The ammonia, indeed, must be at the point 

 of precipitation. Dunham has obtained in this way a minimum 

 temperature for Jupiter's visible surface. The 10 meters of am- 

 monia above the surface, under the planet's surface gravity, should 

 exert a pressure of 1.5 mm (on the familiar laboratory scale). The 

 vapor tension of the solid (below the triple point) has this value 

 at —107° C. At a lower temperature the observed quantity of 

 ammonia could not exist in the atmosphere — it would partially 

 condense itself by its own weight. 



If the atmosphere consists mainly of hydrogen, this limit may 

 be lower, for the mean molecular weight is diminished, and the 

 partial pressure of the ammonia in the same proportion. With a 

 large excess of hydrogen the pressure may be reduced to one-sixth 

 of the previous value and the limiting temperature to —120° C. 



The direct radiometric observations of Jupiter indicate a temper- 

 ature of about —135° ; but this determination is complicated by large 

 and rather uncertain corrections for the absorption of infrared radia- 

 tion in the atmospheres of the earth and the planet, so that the 

 agreement is about as good as could be expected. It is, therefore, 

 very probable that the clouds which form Jupiter's surface are com- 

 posed of minute crystals of frozen ammonia. A perfectly absorbing 

 and radiating planet, at Jupiter's distance and heated exclusively by 

 the sun, would have a mean temperature of —151° C. The excess in 

 the actual temperature may be attributed partly to the fact that we 

 observe the sunlit (and warmer) side; partly to the "greenhouse" 

 effect of the atmosphere, which lets in the short-wave radiation from 

 the sun much more easily than it lets the long waves emitted from 

 the planet's surface out again ; and partly, perhaps, to some residual 

 internal heat in the planet. The existence of the latter is made 

 probable by the rapid changes in the cloud forms, which often 

 suggest the ascent of new material from below. The variety of 

 colors upon the surface, which range from clear white through pinks 

 and browns almost to black, remains unexplained. 



On Saturn, where the ammonia bands are fainter than on Jupiter 

 and the surface gravity less than half as great, the limiting tempera- 

 ture may be 10° or 15° lower. The radiometric observations indicate 

 about the same difference. 



Uranus and Neptune, being farther from the sun, should be still 

 colder. The ammonia should be frozen out of their atmospheres, 

 leaving them clear to a greater depth, which may explain the extraor- 

 dinary strength of the methane bands in their spectra. The 

 methane itself must be nearly ready to condense on Neptune, despite 

 its very low boiling point. Assuming, roughly, that Neptune has 



