The moon may not only be a chip off the home planet. It may also have formed an Earth-like core, new research suggests, a finding that has implications for understanding the moon's history and evolution.
Using seismic analysis tools developed for probing Earth, scientists took another look at data collected by four sensors positioned on the lunar surface by NASA astronauts during the 1969-1972 Apollo moon program.
The Apollo Passive Seismic Experiment recorded motions of the ground from moonquakes and other activities generating sound waves until late 1977. The network was too limited to directly monitor waves bouncing off or scattered by the moon's core, leaving scientists dependent on more indirect techniques, such as measuring minute gravitational changes, to craft a picture of the moon's interior.
Those models turned out to be pretty accurate, says lead scientist Renee Weber, with NASA's Marshall Space Flight Center in Huntsville, Ala.
The new research confirms the existence of a solid inner core and liquid outer layer, similar to Earth's. Unlike Earth, the moon also has a partly melted, mushy layer over that.
The study supports the commonly held theory that the moon formed after a large object smashed into Earth about four billion years ago, creating a cloud of debris that gradually coalesced into our satellite companion.
Knowing details of the moon's layering gives scientists sharper insight into what those layers may be made of, which in turn answers questions of how they got there.
"The possibilities hardwire you to a handful of possibilities about the origin of the moon," Arizona State University geophysicist Edward Garnero told Discovery News.
Weber's team determined that the moon has a solid, mostly iron inner core that extends about 240 kilometers (149 miles) from its center. Surrounding the inner core is a 90-kilometer-wide (56-mile-wide) layer of fluids, and then a 150-kilometer-wide (93-mile-wide) molten layer.
Weber hopes to ferret out more details of the layering with additional analysis of the Apollo seismic data. A pair of lunar gravity probes, due to be launched this year, also could provide independent verification of Weber's results.
"It's important to keep working with the Apollo data to get as much out of it as we can. It's the only data set we have on a planetary body besides Earth," Weber told Discovery News.
"The finding was very interesting," added Massachusetts Institute of Technology geophysicist Maria Zuber, the lead scientist for NASA's upcoming lunar gravity mission known as Gravity Recovery and Interior Laboratory, or GRAIL.
"It will allow us to go in and test it, to see if we get a solution that is consistent with a core size that they see," Zuber told Discovery News.
The study also shows how seismic data can be used to collect information about other planetary bodies, such as Mars, added Garnero.
The research appears in this week's issue of Science.
