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Apollo’s Rocks

By Andy Boyles, Highlights for Children Science Editor

Ever since humans first looked up at the night sky, the Moon has been tempting us to reach for it. On July 20, 1969, humans did reach it. The National Aeronautics and Space Administration (NASA) landed on the Moon. Between 1969 and 1972, a total of twelve astronauts left their footprints in the lunar dust. Their adventures have been told many times. But we don't often hear the story of the Moon. The Apollo astronauts brought that drama back with them. Before astronauts reached the Moon, scientists had many unanswered questions. How old was the Moon? Where had it come from? What created the bright and dark areas that make the ? The Apollo astronauts brought back 840 pounds of Moon rocks, along with photographs and other information. Out of that evidence came the story of the Moon. Rocks from the highlands (the bright areas) ended once and for all the idea that the Moon’s craters had been made by volcanoes. Instead, the highlands are hills of rocks and dust that were tossed up as asteroids and blasted craters in the Moon’s crust. There was a time when meteorites battered these hills. Newer craters overlapped and even erased older ones. Rocks were broken apart by one , then jumbled up and fused together when others struck. Scientists have named this period the Heavy Bombardment, and they say it ended about four billion years ago. They take that date from the ages of the rocks from the highlands. The Dark Plains The plains are the dark areas that form the eyes and mouth of the man in the moon. They began to form near the end of the Heavy Bombardment. At that time, huge impacts shattered the Moon’s crust, creating giant craters and letting lava seep out of the as liquid rock. Many lava flows filled those giant craters, and when the lava cooled and hardened, it became the plains. The plains were never pounded as heavily as the highlands were, so the Heavy Bombardment was over before the plains were fully formed. Since the Apollo missions, scientists have found signs of the Heavy Bombardment throughout the solar system. Some Moon rocks were puzzling. The highlands contained a lot of a mineral called . Scientists wondered what could have happened while the Moon was forming that could have concentrated this light mineral so neatly in the outer layer of rock. Their best idea is that—long before the Heavy Bombardment began—the Moon was covered by an ocean of molten rock, which scientists have called an ocean of magma. In that free-flowing soup, light minerals like feldspar would have risen to the surface. As the Ocean of Magma cooled, feldspar would have naturally become part of the Moon’s crust. Birth of the Moon These two ideas—the Heavy Bombardment and the Ocean of Magma—helped scientists accept a surprising idea about how the Moon was born. Before the Apollo missions, scientists had three different ideas about the birth of the Moon. But each of these ideas left questions unanswered. Some scientists thought the Moon had been a wandering planet that was captured by the Earth. But how could a small planet like Earth capture such a big moon? Other scientists thought the Earth and Moon had formed from the same rotating cloud of rocks and dust. But if they did, why didn't the Moon have a big, heavy core like Earth’s? A third idea was that a chunk of the Earth had split off and formed the Moon. But if the Earth were once spinning fast enough to lose a piece of itself, then it should still be spinning much faster than it is today. The Giant Impact A fourth idea, called the Giant Impact Hypothesis, had been proposed and then ignored for many years. According to this idea, an object about the size of Mars struck the young Earth. The impact destroyed the Mars-like object, blasting a huge amount of material into space. Some of it remained in orbit around Earth and clumped together, forming the Moon. After the Apollo missions, this idea began to make sense. The Heavy Bombardment had shown that big impacts were common in the past. A giant impact might have released enough heat to create the Moon’s Ocean of Magma, and it could have given the Earth its tilt and spin. Also, the Mars-like object’s heavy core could have become part of Earth’s. That would explain why the Moon’s core seems small and light compared to Earth's. The helped change our ideas about the Moon. Before Apollo, the Moon was a mysterious rock. Now it’s a reminder that the Earth and Moon are still reeling from the collision that made them. These samples revealed the Moon’s violent past. This lunar sample, 15415 (from the mission), is a rare piece of the Moon’s original crust. Scientists think it formed as the Ocean of Magma cooled, and was broken out of the crust later by a meteorite impact.

This moon rock, brought back by the astronauts, was formed as meteorite impacts fused together different types of rocks.