Scientific studies indicate the solar system formed about 4.6 billion years ago and that about 4.5 billion years ago an object about the size of Mars hit the Earth, blasting away material that eventually cooled into what is now our moon. (Read more in Science News: "The Solar sytem's big bang")
Rocks brought back from the moon contain zircon crystals. Zircons crystallize only after 80 to 85 percent of a volume of molten rock has solidified. By understanding how uranium within the zirconium breaks down into lead, scientists believe they know when the crystals formed with an error margin of less than 4 million years. The oldest zircons from the moon are about 10 million years older than the oldest yet discovered on Earth. The ages of lunar zircons identified in other studies hint that small amounts of the moon’s crust remained molten for another 200 million to 400 million years.
"Alexander Nemchin and his colleagues have used a uranium-lead dating technique to scrutinize a 0.5-millimeter-wide zircon embedded in a moon rock collected by Apollo 17 astronauts in December 1972. The crystal — which, although small, is rather large as zircons go — is about 4.417 billion years old, the researchers report online January 25 in Nature Geoscience."
Science News: "Oldest zircon fine-tunes history of moon's formation"