Old data can hold new information when viewed with a fresh pair of eyes. Researchers have used data originally collected during the Apollo missions to the Moon to get a picture of its interior that was previously unavailable. (Note of interest: this isn't the first time in recent memory that data originally collected during the Moon landings has appeared in modern articles.)

During the late 1960s and early 1970s, the Apollo astronauts placed four seismometers on the side of the Moon that faces the Earth (the Apollo Passive Seismic Experiment). These instruments continuously recorded the motion of the lunar surface in three orthogonal dimensions until late 1977. Given the small number of listening stations, limited geographical range spanned by the stations, and weak signals emanating from moonquakes, scientists at the time were not able to learn much about the deep interior of the Moon.

Using a modern analysis method known as array-processing, scientists were able to combine a series of small amplitude signals from the original data to identify reflections at different layers of the lunar interior. Analysis of these now-visible wave reflections revealed the Moon's inner structure.

The data shows that there's a solid inner core with a radius of 240±10 km, a fluid outer core that extended to 330±20 km, and a partially molten boundary area that spread out to 480±15 km. Further than that, and you're in the lunar mantle. One interpretation of the data is that the deepest interior regions of the Moon have similar structure to the inner regions of the Earth. Coupling the measurements and assumptions with phase diagrams of iron-sulfur alloys suggests that the lunar core is approximately six percent sulfur by weight.

That's relatively low, and the authors think that we can blame the moon's formation in a major impact event. The depletion of light elements is a "natural consequence of the lunar formation process, through high-temperature devolatilization during the Moon-forming impact: in effect, the present lunar core is likely comprised of thermally processed material from the core of the impactor."

ScienceExpress, 2010. DOI: 10.1126/science.1199375