The Solar System

Abstract

The layout and visual appearance of the planets in the solar system make it obvious that they fall into two groups—the inner, terrestrial (Earth-like) planets and the outer, Jovian (Jupiterlike) planets. The two groups not only have different properties; they are likely to have rather different futures. From an Earthbased perspective, the inner planets—Mercury, Venus, Earth and Mars—are more immediately interesting. We can lump in with them the Earth’s Moon and the two small satellites that circle Mars. As it happens, the Earth and the Moon between them nicely bracket the properties of the terrestrial planets. On the one hand, the Earth is active, with things happening in its interior, on its surface, and in its atmosphere all the time. On the other hand, the Moon is dead, with almost no atmosphere and with few changes occurring in its interior or on its surface. We can characterize the other terrestrial planets in terms of where they lie on this scale of activity: closer to the Earth, or closer to the Moon?