The New Moon; December 2003; Scientific American Magazine; by Paul D. Spudis; 8 Page(s)

THE MOON does not yield her secrets easily. Although Earth's airless satellite was the first planetary object to be explored by spacecraft and the only body ever visited by astronauts, scientists still have many unanswered questions about its history, composition and internal structure. In recent years, researchers have called for renewed exploration of the moon; the European Space Agency and Japan are planning to send probes into lunar orbit, and NASA is considering landing an unmanned spacecraft on the moon's far side. By studying the moon, these missions may also illuminate the history of all the rocky planets in the inner solar system: Mercury, Venus, Mars and especially Earth. Because the moon's surface has remained relatively unchanged for the past three billion years, it may hold the key to understanding how the inner planets formed and evolved.

When astronomers first gazed at the moon through telescopes 400 years ago, they found that its surface consists of two principal types of terrain: bright, rugged, heavily cratered highlands and dark, more sparsely cratered lowlands. Galileo Galilei, the 17thcentury astronomer, called the lowlands maria-Latin for "seas"-because of their smooth, dark appearance. One of the biggest surprises of the space age came in 1959, when the Soviet spacecraft Luna 3 photographed the moon's far side, which had never been seen before because it is always turned away from Earth. The photographs showed that it almost completely lacks the dark maria that are so dominant on the near side. Although scientists now have some theories that could explain this dichotomy of terrain, it remains an unsolved puzzle.