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  • Over the years, centuries, and millennia, we have been watching the moon and wondering:

  • Where did it come from? What is it made of? And what events created the distinctive pattern

  • of light and dark on its surface?

  • To find out, we have sent satellites out to crash onto its surface, astronauts to comb

  • its craters and hillsides and collect rocks, and high-tech spacecraft to map its nooks

  • and crannies.

  • A half-century of study has brought us closer to the answers. Many scientists now believe

  • that the moon was born in a monumental collision between Earth and a Mars-sized body early

  • in the history of the solar system, some 4.5 billion years ago.

  • From the remains of the impact, a giant ball of magma coalesced in Earth orbit.

  • Gravity sculpted this hot mass into a sphere. In time, its surface cooled, forming a hard

  • crust with magma just underneath.

  • Around 4.3 billion years ago, a giant impact battered the moon's south pole, sending debris

  • as far as the opposite side of the moon.

  • The impact formed the Aitken basin. At roughly 2,500 kilometers in diameter and 13 kilometers

  • deep, it is one of the largest known impact craters in the Solar System.

  • Its formation marked the beginning of a period of large-scale changes to the moon's surface.

  • Over several hundred million years, the lunar terrain was rocked by a succession of heavy

  • impacts. Some formed large basins that would eventually fill in to become the dark colored

  • patches of the moon known as maria.

  • These impacts punched enormous holes in the relatively thin lunar crust. Because the moon

  • had not yet fully cooled on the inside, lava began to seep out through cracks opened up

  • by the impacts.

  • Lava spread throughout the craters, gradually filling them in and cooling. Because of the

  • high iron content of this lava, the mare regions reflect less light and therefore appear darker

  • than the surrounding highlands.

  • Around one billion years ago, volcanic activity ended on the near side of the moon as the

  • last of the large impacts made their mark on the surface. The impacts did not cease,

  • although they were much smaller than the ones that formed the largest basins.

  • Some of the largest and best-known impacts from this period formed the Tycho, Copernicus,

  • and Aristarchus craters. They feature distinctive "rays" that stretch out from the crater sites,

  • formed by material blasted out at the moment of impact.

  • Finally, after billions of years of relative quiet, we arrive at the moon we see today.

  • Though its surface continues to be affected by impacts, the bombardment has slowed dramatically.

  • The features we now see on the Moon's surface are a permanent record of its early history.

  • Within them, too, we are finding clues to the evolution of Earth itself.

Over the years, centuries, and millennia, we have been watching the moon and wondering:

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