Moon Has Iron Core, Lunar-Rock Study Says

Brian Handwerk
for National Geographic News
January 11, 2007
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.)

The group studied a type of lunar rock called mare basalt, which is believed to have been created deep in the moon's mantle and have retained signatures of that region. Mare basalt hails from vast, dark, flat areas of the moon's surface called mares. It is dense, dark gray, and likely formed from cooled magma.

Sinking Feeling

The moon rocks suggest that the lunar mantle is very low in elements that bond easily with iron, such as gold and platinum—like Earth's mantle, but with even lower levels of those elements.

"What happens during the formation of any terrestrial planet is that it undergoes a melting state early in its formation," Taylor said. "In that state you get the separation of metallic iron into a core."

When cores formed on Earth and other terrestrial planets these iron-loving elements were largely scavenged from the silicate mantle and transferred down into the metallic core, which would explain the relative lack of these elements in both Earth's mantle and the moon's.

"We must have had a core form [in the moon] to have [iron bonding] elements at the [low] levels we see now," Taylor said. "That's the same thing that happened on Earth, Mars, Venus, and Mercury—the terrestrial planets."

Though he doesn't discount this idea, Richard Walker, a geologist at the University of Maryland in College Park, sees a second option.

"It could be that the [amount] of these elements in the silicate portion of the impactor and the proto-Earth were quite low at the time of impact, so that when the moon formed, it simply did not contain a high abundance of the elements in question," said Walker, who was not involved in the study.

Earth's iron core can be identified through the measurements of sensitive seismographs scattered all over the planet.

During earthquakes these vibration monitors can help determine the content of the Earth's layers, based on how the movement of those layers effects waves passing through the planet.

Seismic equipment on the moon is not sufficient to recover such information, though moonquakes commonly occur.

"In the case of the moon, we've never been able to find distinct evidence for [a core]," Taylor said, "although we've always had our suspicions."

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