Page:The Solar System - Six Lectures - Lowell.djvu/67



counterparts of our own. The days are a little longer and the seasons nearly twice as long, reckoned either by Earthly or by Martian days. The orderly succession of day and night, spring, summer, autumn, and winter, are the same there as here.

Now this is no accident. It is a direct consequence of the planet's size and of its position in the solar family. That, however, the circumstances of the Earth and Mars should chance to agree so nearly in quantity as well as quality, we as yet lack the data to explain.

Size, or rather lack of it, has done something else for Mars. It has reduced the atmospheric blanket that covers the planet's body. It did this both at the start and subsequently. If the planets set out with atmospheres in proportion to their masses, a small planet having a greater surface in proportion to its mass would not have this surface so thickly covered, and its lesser gravity would further spread this out skywards.