Signals from seismic sensors left on the lunar surface by Apollo astronauts in 1971 have revealed that the Moon has a liquid core similar to Earth's.
Scientists at Nasa applied contemporary seismological techniques to the data being emitted from sensors placed by their colleagues during the U.S. space program's heyday.
The new research suggests the Moon possesses a solid, iron-rich inner core with a radius of nearly 150 miles and a fluid, primarily liquid-iron outer core with a radius of roughly 205 miles.
Where it differs from Earth is a partially molten boundary layer around the core estimated to have a radius of nearly 300 miles.
The data sheds light on the evolution of a lunar dynamo - a natural process by which our Moon may have generated and maintained its own strong magnetic field.
Core knowledge: Nasa applied contemporary seismological techniques to data being emitted from sensors left on the Moon in 1971. Scientists now think the Moon has a solid, iron-rich inner core and a fluid, primarily liquid-iron outer core
Uncovering details about the lunar core is critical for developing accurate models of the Moon's formation.
The core contains a small percentage of light elements such as sulphur, echoing new seismology research on Earth that suggests the presence of light elements - such as sulphur and oxygen - in a layer around our own core.
The research, published in the online edition of journal Science, used extensive data gathered during the Apollo-era Moon missions.
The Apollo Passive Seismic Experiment consisted of four seismometers deployed between 1969 and 1972, which recorded continuous lunar seismic activity until late 1977.
Dr Renee Weber, lead researcher a Nasa's Marshall Space Flight Center in Huntsville, Alabama, said: 'We applied tried and true methodologies from terrestrial seismology to this legacy data set to present the first-ever direct detection of the Moon's core.'
The team also analysed Apollo lunar seismograms using array processing, techniques that identify and distinguish signal sources of moonquakes and other seismic activity.
The researchers identified how and where seismic waves passed through or were reflected by elements of the Moon's interior, signifying the composition and state of layer interfaces at varying depths.
Although sophisticated satellite imaging missions to the Moon made significant contributions to the study of its history and topography, the deep interior of Earth's sole natural satellite remained a subject of speculation and conjecture since the Apollo era.
Scientists had previously inferred the existence of a core, based on indirect estimates of the Moon's interior properties, but many disagreed about its radius, state and composition.
Old technology: The Passive Seismic Experiment, a component of the Apollo Lunar Surface Experiments Package deployed on the Moon by the Apollo 14 astronauts during their first extra-vehicular activity
A primary limitation to past lunar seismic studies was the wash of 'noise' caused by overlapping signals bouncing repeatedly off structures in the Moon's fractionated crust.
To mitigate this challenge, Dr Weber and her team employed an approach called seismogram stacking, or the digital partitioning of signals.
Stacking improved the signal-to-noise ratio and enabled the researchers to more clearly track the path and behaviour of each unique signal as it passed through the lunar interior.
Dr Weber said: 'We hope to continue working with the Apollo seismic data to further refine our estimates of core properties and characterise lunar signals as clearly as possible to aid in the interpretation of data returned from future missions.'
Future Nasa missions will help gather more detailed data. The Gravity Recovery and Interior Laboratory - or GRAIL - is a Nasa Discovery-class mission set to launch this year.
The mission consists of twin spacecraft that will enter tandem orbits around the Moon for several months to measure the gravity field in unprecedented detail.
It will also answer long-standing questions about Earth's moon and provide scientists a better understanding of the satellite from crust to core, revealing subsurface structures and, indirectly, its thermal history.
