Scientists believe that the moon formed shortly after Earth did, around 4.5 billion years ago. In fact, before becoming one solid mass, the debris may have settled into two moons that later crashed. The Mars-sized object that collided with our infant planet is known as Theia, and recently, data have emerged that suggest 40% of the moon is made up of material from Theia. This theory of the moon's formation has many names, including the Giant-Impact Hypothesis, the Theia Impact, the Big Splash, and the Big Whack.
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Key Facts In This Video
The Barringer Crater in Arizona is a mile (1.6 km) in diameter, and was formed 50,000 years ago. 00:38
See what it may have looked like when a Mars-sized object collided with Earth: 01:19
When it first formed, the moon was made of molten material. 02:23