TEACHER'S GUIDE

GATEWAY TO THE SOLAR SYSTEM
G. Jeffrey Taylor, PhD

When astronauts dug into the Moon's surface during the Apollo program, they were doing more than digging up dry, dark sediment. They were time travelers. The rocks and sediment returned by Apollo contain vital clues to how Earth and the Moon formed, the nature and timing of early melting, the intensity of impact bombardment and its variation with time, and even the history of the Sun. Most of this information, crucial parts of the story of planet Earth, cannot be learned by studying rocks on Earth because our planet is so geologically active that it has erased much of the record. The clues have been lost in billions of years of mountain building, volcanism, weathering, and erosion. Colliding tectonic plates and falling rain have erased much of Earth history, especially the early years before four billion years ago. The Moon was geologically active in its heyday, producing a fascinating array of products, but its geologic engine was not vigorous and all records of early events were not lost. Its secrets are recorded in its craters, plains, and rocks. This guide reveals the secrets that lunar scientists have uncovered since the Apollo missions returned 382 kilograms (843 pounds) of rock and sediment from the lovely body that graces the night sky. The emphasis here is on geology. The samples returned by Apollo are the stars of the show. [See the 'Lunar Disk' activity on Pages 39-42 and the 'Apollo Landing Sites' activity on Pages 43-46.] Understanding the Moon, however, requires other geological approaches, such as geological mapping from high-quality photographs, the study of analogous features on Earth (for instance, impact craters), and experiments in laboratories.