THE PLANETS MARS AND VENUS 361 



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 to the depth of a very few feet, but not entirely 

 through, except, perhaps, very late in the season, and 

 then only right at the equator where the crust is thinnest. 

 So cold must be the planet that the sun would be power- 

 less to cause evaporation were it not for the accidental 

 circumstance mentioned a moment ago that, on account 

 of the diminished surface gravity and lower atmospheric 

 pressure, the boiling point of water there is reduced from 

 our 212 F. to about 100, and the thawing point corre- 

 spondingly. 



The process of evaporation takes place from the sur- 

 face of this glacial crust, or rather from the surface of 

 the thaw-sheet submerging that crust, during the day- 

 light hours of Mars' protracted northern spring of 199 

 days and his scarcely shorter summer of 183 days. The 

 nights are relatively cold, and during them much snow 

 falls upon the thaw-softened surface, thus giving birth 

 in the course of time to a vast superficial litter of disin- 

 tegrated ice extending far into the high latitudes. 



When this condition reaches a maximum, this super- 

 ficial sheet of thaw-water is of itself sufficiently deep (or 

 perhaps is sufficiently reinforced by the subjacent waters 

 breaking through the softened crust at or near the equa- 

 tor) to reflect in its movements the tidal effect of the sun. 

 Waves of wide scope are formed, which progress with a 

 high degree of regularity over the smooth, solid ice shelf, 

 sweeping the glacial detritus before them until, having 

 reached the limit of their range, they deposit their car- 

 goes in long, geometrical tidal lines, which then form em- 

 bryonic ridges, or dams, for future waves and new car- 

 goes to respect and add to ; until in the end the accumula- 

 tions grow to a width capable of arresting our attention 

 even over this vast abyss of space. Veritable congealed 

 tidal waves are they, following, by the very law of their 

 formation, the great circles of the planet, and supported 



