But, upon the whole, as Mr.Lowell remarks, the atmosphere of Mars is remarkably free of clouds.
The reader will remember that Mars gets a little less than half as much heat from the sun as the earth gets.

This fact also has been used as an argument against the habitability of the planet.

In truth, those who think that life in the solar system is confined to the earth alone insist upon an almost exact reproduction of terrestrial conditions as a _sine qua non_ to the habitability of any other planet.

Venus, they think, is too hot, and Mars too cold, as if life were rather a happy accident than the result of the operation of general laws applicable under a wide variety of conditions.

All that we are really justified in asserting is that Venus may be too hot and Mars too cold for _us_.