THE MOON: GATEWAY TO THE SOLAR SYSTEM

Teacher's Guide
by G. Jeffrey Taylor, PhD

When astronauts dug into the Moon's surface during the Apollo program, they were doing more than scooping up dry, dark dirt. They were acting as time travelers. The rocks and soil brought back by Apollo missions hold crucial clues about how Earth and the Moon formed, when and how early melting occurred, how intense the bombardment by space impacts was and how that intensity changed over time, and even about the history of the Sun.

Most of this information—key pieces of the story of planet Earth—cannot be learned by studying rocks here on Earth. That's because our planet is so geologically active that it has erased much of its own record. The clues have been destroyed over billions of years by mountain building, volcanic eruptions, weathering, and erosion. Colliding tectonic plates (the huge slabs of rock that make up Earth's outer shell and slowly shift position over time)...

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The Moon, Front and Back

The photograph at the top is a telescopic image of the side of the Moon that faces Earth, taken at Lick Observatory in California. The photograph below it was taken during the Apollo 16 mission and shows mostly the far side of the Moon—the side we never see from Earth—except for some dark patches on the left that are barely visible from our planet.

These images clearly show the Moon's two major types of terrain. The highlands are light-colored, heavily cratered regions, especially visible in the far-side photograph. The maria (dark, smooth plains) formed when lava flows filled in low-lying areas. One lingering mystery is why the far side has far fewer maria than the near side. Notice how densely packed the craters are on the far side—these enormous pits record the Moon's early, intense bombardment by space debris, a barrage that likely struck the early Earth just as hard.