READING THE MOON'S
One of the biggest lunar questions has been where did such a body come from?
Some scientists dub the current theory for the
Moon's origin "The Big Whack." It emerged in 1984 after more than 10 years' study of rock and soil samples taken from the six Apollo landings and it still offers the most complete account of the birth of the Moon. According to this theory, the proto-Earth formed alone. By the time Earth had reached nearly its present size, it had melted throughout and separated into a core and mantle.
At this point, roughly 4.5 billion years ago, Earth was struck a glancing blow by another nearly formed planet about the size of Mars. This impact sped up Earth's rotation, probably tipped its axis, and heated its surface intensely. The force of the collision vaporized the impactor and sent a great spray ofsuperhot vapor into orbit around Earth. This vapor quickly cooled into a ring of debris and in only a few tens of thousands of years, the fragments coalesced into the Moon.
But the newborn Moon looked very different from the one you might gaze at tonight. It loomed much larger in the sky because it orbited less than half as far away. A thin, brittle crust covered a global ocean of molten rock. Bright glowing wounds opened with every meteorite impact—and such impacts were constant at first, as the new Moon swept up the remaining swirling debris.