Laser test suggests molten moon interior

The Galileo probe used three color filters to shoot this composite image. Reds correspond with highlands, blues and oranges with ancient lava flows.

(CNN) -- Using lasers to measure the gravitational tug from celestial neighbors on the moon, NASA scientists have determined that the lunar interior contains something akin to molten slurry.

The surface of the moon expands and contracts by as much as 4 inches (10 cm) every 27 days, during which it experiences both high and low "gravity tides" from the combined influences of the Earth and sun.

James Williams, a researcher at NASA's Jet Propulsion Laboratory in Pasadena, California, said this week that reckoning what was inside the rocky heart of the moon was no easy task.

"Since we can't go inside the moon we have to use indirect methods to learn its hidden secrets. In this case, we were able to use the tidal distortion of the moon."

The periodic moon bulging was measured by firing ground-based lasers at reflectors deposited on the moon by U.S. and Russian manned and unmanned missions three decades ago.

By timing the return trip of the laser beams, the lunar ranging experiment calculated the distance between the Earth and moon to within 0.8 inches (2 cm), NASA said.

The bulges and dips in the lunar surface helped scientists come up with so-called Love numbers, used to estimate the viscosity of the interior of celestial bodies.

The new data bolsters the idea that a partially melted zone surrounds the lunar core. Apollo researchers advanced the theory after studying moonquakes with seismometers left on the moon by Apollo astronauts.

The NASA moonwalkers also left numerous retroflectors on the surface, as did several unmanned Russian missions. Those devices were used in the recent laser ranging experiment.

Love numbers are named for Augustus E.H. Love, a British mathematician who worked on elasticity and wave theories in the late 19th and early 20th centuries.