104 FROM NEBULA TO NEBULA 



may be frozen over, in which state they can as easily 

 carry markings as the most rigid land, and more reg- 

 ular ones besides. 



It would be an error to jump to the conclusion that 

 Mars' oceans must be frozen solid, for decidedly they 

 are not. For one thing, the planet could not rotate 

 unless the water were fluid, as I have previously shown ; 

 and, for another, the gravistatic heat generated by the 

 planet is sufficient to avert total congelation. 



That Mars' gravistatic heat is by no means a neg- 

 ligible quantity may be proved by a simple calculation. 

 Mars' superficial gravity being .38, one \vould have to 

 descend toward its center about 150 feet for each degree 

 of increase in temperature (this distance increasing 

 with the diminishing gravity), which would give a maxi- 

 mum theoretical temperature at the center of about 

 40,000 degrees, or an average of 20,000 for the whole 

 mass. This is a much lower temperature than our earth 

 theoretically generates, and seems insufficient until we 

 call to mind that Mars' rarer atmosphere and lesser 

 gravity both ope rote to greatly lower the freezing point 

 of the water, and that ice is a bad conductor of heat. 



The excessive external cold must evince itself some- 

 how, and this it does by freezing a thick crust over 

 a major portion of the ocean surface. Neverthe- 

 less even such a surface may experience seasonal 

 chanc/es, and this is the real key to the whole mystery. 

 It remains for us now only to reason by natiiral steps 

 just what would take place under such conditions. 



Going toward our own north pole we come eventually 

 to a parallel of latitude of perpetual frost. In the case 

 of Mars, this critical parallel is nearer the equator than 

 it is with us. During the warm season the ice-crust 

 thaws partly through, but not entirely so except very 



