Earth-Moon Combination Is Likely Very Rare, Study Confirms

Some 4.53 billion years ago --- roughly 300 million years into the history of our early solar system --- a Mars-sized planetary impactor slammed our young planet virtually head on. The collision cast dust and debris and a sizeable portion of our mantle and crust into Earth orbit and eventually coalesced into our Moon. But what are the odds that such an event would compositionally resemble our present-day Earth-Moon system?