Page:The Solar System - Six Lectures - Lowell.djvu/68



In the next place, its gravity could control only a much smaller velocity at its surface, thus making the critical velocity beyond which a particle would pass off into space much less. By the kinetic theory of gases, a certain number of particles will in a given time attain the critical velocity, and the more the lower the critical velocity. Thus, from the planet that hath not shall be taken away even that which it hath.

In consequence, on Mars the density of the air at the surface of the planet at the start was probably not denser than one-seventh of our own, or more rare than that at the top of our loftiest mountains, and now probably is rarer even than this, owing to the greater speed with which it has been lost.

The rate at which the different gases would be differs. The curve of probability shows that they would disappear much more rapidly than the ratio of their speeds. Water vapor would go long before atmospheric air.