Deep down, the moon may be more like Earth than scientists ever thought.
A new moon-rock study suggests the satellite has an iron core. The findings add weight to the theory that the moon formed from debris thrown off when a Mars-size object collided with a young Earth (related: "Moon Derives From Earth, Space Object, Study Says" [August 8, 2003]).
"This is the most positive evidence so far that the moon contains a core," said Larry Taylor, director of the Planetary Geosciences Institute at the University of Tennessee in Knoxville.
"It's looking more like a planet every day."
The moon's core could be a clue to its ancient origins, which have long puzzled astronomers.
"Our moon is too big to be a moon," Taylor said. "It's huge compared to the moons we see around other planets, so it has always been suspected that there was something strange in its origin."
The Big Whack
The leading moon-creation theory among astronomers is known as the "giant impact" or "big whack" theory.
An object about the size of Mars—half the size of Earth—slammed into our planet very early in its formation, the theory says.
"This impactor hit, and everything was thrown every which way," Taylor said. "Material was shattered, melted, vaporized, and thrown out into orbit. Some of that material condensed and aggregated into the moon."
It's believed that some of the impactor's remains became part of the moon, as did large parts of early Earth's mantle (the layer between core and crust), which were hurled spaceward.
Rock samples from NASA's Apollo 15 and Apollo 17 moon missions of the early 1970s have now shed more light on the moon's origins, according to Taylor and colleagues' study, to be published in the tomorrow's issue of the journal Science. (Related photo: boulders on the moon.)
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