Apollo astronauts may be garnering another prize from their exploits of more than 3 decades ago. They left seismometers across the face of the moon to probe its interior, but no one had been able to paint a clear picture from the data the sensors collected. Now, two independent groups have reanalyzed the Apollo data using modern but very different techniques, and both teams say they have detected lunar seismologists' prime target: a core of iron that is still molten 4.5 billion years after the moon's formation.
The Apollo seismic experiment was challenging from the start. Moonquakes are sparse and feeble, the moon's impact-shattered crust garbles any seismic signals, and computers of that era couldn't handle the complete data set. Today, computers are faster, and terrestrial seismologists have developed far more powerful analytical techniques, so lunar researchers have taken another crack at the Apollo seismic data, which were recorded by the five sensors and radioed back until the mid-1970s.
Like an earthquake, a moonquake sets off ripples of motion called seismic waves that speed through surrounding rock. Both groups combed the data for signs of quakes' waves that may have reflected off the core, but each group took a very different approach. Planetary scientist Renee Weber of NASA's Marshall Space Flight Center in Huntsville, Alabama, and her colleagues analyzed four types of seismic waves—which differ in the direction of vibration—from deep quakes clustered in 38 spots. They combined seismic records from each cluster to bring out any reflected signals and filtered the combined records to remove some of the noise. Seismologist Raphaël Garcia of the University of Toulouse in France and his colleagues, on the other hand, analyzed two wave types from three moonquakes after calibrating the seismic stations.
In back-to-back talks at last month's meeting of the American Geophysical Union in San Francisco, California, the groups reported that, like Earth, the moon has a molten core. Garcia and colleagues found a liquid core with a radius of 365 kilometers. Weber and her colleagues reported a core radius of 330 kilometers, which they also report online today in Science. Given the uncertainties, the two estimates are indistinguishable. In addition, Weber found seismic reflections from a solid inner core with a radius of 240 kilometers—a feature Earth has as well—and reflections from a layer of mostly rock with a bit of magma 150 kilometers thick lying above the liquid iron outer core.
"I'm surprised they could get this much information from this data," says planetary physicist David Stevenson of the California Institute of Technology in Pasadena. If the seismic results hold up, he adds, they would be by far the strongest evidence yet for a liquid core. Then researchers could use the detailed seismic picture of the moon's interior to understand better the evolution of a planetary body assembled from the vaporous debris of a giant impact on the still-forming Earth. But they're not quite there yet. "The Apollo [seismic] data have all sorts of weirdness in them," says seismologist Jesse Lawrence of Stanford University in Palo Alto, California. "As is so often the case, more work needs to be done."