Page:The Solar System - Six Lectures - Lowell.djvu/65

 III

MARS presents us one phase of planetary development; Mars another, quite different. The two represent stages in world-life as distinct as those of gray hair and brown in human life.

Whatever the absolute ages of the several planets, their relative ages, as measured intrinsically, decrease pretty steadily with their distance from the Sun. Mercury is old; Mars, middle aged; Jupiter young.

World-life has its earmarks of time as human life has, and betrays them quite as patently.

Lack of atmosphere, colorlessness, changeless attitude toward the Sun, are the signs of old age in a planet. Mercury shows all these tokens of senility. Mars presents a very different picture.

Color is a telltale trait; for it is a sign that surface development still goes on. Lack of atmosphere alone prevents vegetation, and this, coupled with unalterableness of face presented to the Sun, weathers the surface to a neutral gray. Such a