When the solar system was young, its inner regions were crowded with debris -- big chunks of rock and metal and ice that were left over from the birth of the planets. Some of these chunks were as big as cities or states. And like icebergs floating across the North Atlantic, they were dangerous. They frequently rammed into the newborn planets and moons.
There's no hint of this bombardment on Earth. Wind, rain, and the motions of Earth's crust have wiped out all traces. But there's plenty of evidence on the Moon. In fact, it's as plain as the nose on its face.
All of the dark features that make up the "man in the Moon" are plains of volcanic rock. They formed around four billion years ago, when some of the giant pieces of debris hit the Moon. They formed wide basins, which filled with molten rock from below the surface.
Scientists confirmed this scenario by studying the rocks brought back to Earth by Apollo astronauts. Rocks from the dark plains are volcanic, and they're billions of years old. They helped scientists piece together the violent history of the early solar system.
And you can see the plains for yourself tonight. The Moon, which is just past full, rises in mid evening. Regulus, the brightest star of Leo, the lion, is just below it, and the planet Saturn, a little farther below; more about Saturn tomorrow. As you look at the dark lunar features, keep in mind that they're the scars of an ancient bombardment.
Script by Damond Benningfield, Copyright 2007
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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