How Scientists Think the Impact Occurred
Here's the scenario: A little more than 4.5 billion years ago, a young, hot Earth, constantly bombarded by thousands of asteroid-sized objects, had grown to almost its present size. Most of the iron it contained had sunk toward the center, forming a huge iron core that was much denser than the rest of the planet. Surrounding the molten core was a slowly hardening mantle of lighter rock.
Suddenly, a round, fast-moving, fully formed planet the size of Mars or perhaps larger, with its own iron core and rocky mantle, loomed from space. Traveling at a speed of 40,000 kilometers (25,000 miles) per hour, it struck the Earth a glancing blow. The object's kinetic energy (energy of motion) was instantly converted into heat that vaporized much of the object's mantle along with a good part of the Earth's. The collision produced a great, expanding cloud of fiery vapor composed of gasified rock. Thrown to a height of perhaps 22,500 kilometers (14,000 miles), much of the vapor formed a diffuse cloud that orbited the Earth. At the same time, most of the iron core of the body that struck Earth looped around the planet and struck again, this time penetrating and merging with the Earth.
Over the course of about a year, the debris in the cloud condensed into solid particles and formed a ring around the Earth. The particles slowly clumped together, forming tiny rocks, then bigger and bigger ones. For a time there were thousands of these “moonlets” orbiting Earth. But over a period of less than 100 years, the larger moonlets swept up the smaller ones, until they all merged into one large body. The ring was gone, and in its place there was the infant moon. At that point, the Earth-moon system resembled a double planet. The moon circled Earth rapidly at a small fraction of its present distance, and Earth spun rapidly, thanks to the blow it had suffered in the impact.