Space Science in the Twenty-First Century

Imperatives for the Decades 1995 to 2015

Planetary and Lunar Exploration

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Introduction

The solar system consists of nine known planets orbiting the Sun, along with a larger number of moons, asteroids, planetary rings, and comets. Among the mysteries that have occupied human thought throughout history are the mechanisms by which the solar system came into existence, the laws and physical processes that shape the evolution and behavior Of planets, and the relationship of the solar system to the wider cosmos. These questions continue to stimulate modern planetary science.

In addition to illuminating some of these long-standing scientific questions, planetary studies have additional significance from a human perspective. Planets are likely to be the only bodies in the universe capable of supporting advanced life. Among its other objectives, planetary science seeks to understand the formation of life-supporting planets and the conditions under which life arises and develops. The answers to these questions promise to help shape human perceptions about our origins and about our place in the universe.

The Sun and planets occupy a flattered, disk-shaped region of space extending some 40 times farther from the Sun than does the orbit of Earth. All of the planets go around the Sun in nearly circular orbits, in the same direction, and nearly in the same plane. The planets near the Sun—known collectively as the terrestrial planets—are comparable in size to Earth; like Earth, the terrestrial planets are composed largely of rock and metal. Most have atmospheres; some have (or once had) hydrospheres. The planets in the outer solar system—known as the jovian planets—are much larger than the terrestrial planets and composed largely of gas and ice. Many of their satellites resemble terrestrial planets, but with a much larger endowment of low-temperature and icy material.

According to our present understanding, the Sun and planets formed together, some 4.5 billion years ago, from a flattened nebula of interstellar gas and dust. The overall pattern of the solar system, including the arrangement of planetary orbits and the variation of planetary mass and composition with distance from the Sun, is thought to have arisen as a natural consequence of the structure and evolution of the protoplanetary disk. The phenomena that led to the formation of this planetary system are thought to be representative of the processes prevailing in star formation. This suggests that many stars may have similar planetary systems associated with them.

The planets have evolved considerably since their formation. Large sources of internal heat—trapped gravitational energy and energy liberated by radioactive decay—have driven a continuing evolution in most of the. planetary bodies. Tidal energy is important for some satellites. As a consequence, many of the planets have thoroughly differentiated, their heavy constituents having sunk to their centers. To this day, the continuing escape of heat from planetary interiors drives convection, which is responsible for the observed dynamical behaviors of planetary surfaces and interiors. On Earth, volcanoes, earthquakes, sea floor spreading, and continental drift are contemporary consequences of Earth's thermally agitated interior.

Through a combination of gas trapped at the time of planet formation and volatiles released during subsequent planetary evolution, most of the planets have developed atmospheres. On Earth, the water-rich atmosphere has played a major role in the formation of terrestrial life—still the only known instance of this remarkable phenomenon—which has existed for more than 3 billion years. During that time it has developed into an extremely complicated system. Many of the characteristics of Earth's surface and atmosphere result from the effects of biological activity. Conversely, Earth's atmosphere and hydrosphere remain dominating factors in the existence and continuing success of life on Earth.

The extreme evolutionary processes through which the major planets have passed have resulted in the obliteration of most of the primordial characteristics of planetary bodies and of their constituent material. Thus, from the planets themselves we can glean few clues about the detailed processes of their origin. What information the planets do communicate about the solar system's origin is contained in their orbits, their masses, their bulk compositions, their rings, and their satellite systems.

However, the solar system contains any small objects that have not suffered such severe processing since their formation. Comets are believed to be relatively well-preserved accumulations of matter left from the time of the solar system's birth. Comets are rich in volatile matter, suggesting that they formed in the relatively cold outer reaches of the early solar system. Asteroids seem to have formed closer to the Sun than did the comets—in the general vicinity of their present location, mostly between Mars and Jupiter. Because most asteroids are small, they have not undergone continuing evolution in the manger of planets, although they do show evidence of early evolutionary processing. Thus asteroids are thought to preserve evidence of processes, and perhaps sources of heat, that are no longer active in the solar system.

The solar system is the largest region of space available to humans for in situ measurement and experimentation. Much of our understanding of more remote astrophysical systems is based on the knowledge gained from investigation of solar system objects, which has revealed the actual behavior of the physical laws in complex, large-scale systems.