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.
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 Mercurythe 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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