GARY HINCKS / SCIENCE PHOTO LIBRARY GARY HINCKS / SCIENCE PHOTO LIBRARY
Moon's interior and composition, artwork. At left, a segment of the Moon is shown from crust (top) to the glowing core. At right, part of the Moon highlighted in a white box on the left diagram is shown enlarged. On the surface, there is a transition from mountainous highlands (right) to a flat sea (mare). The seas were formed when lava flowed into huge impact craters early in the Moon's history. The curve of the crater is seen below the surface, and it is filled with layers of solidified lava. The Moon's crust is covered by a rocky powder called the regolith, formed by countless meteorite impacts over the 4.5 billion years of its existance.
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