Love numbers -- measures of how much a planet's surface and interior move in response to the gravitational pull of nearby bodies -- may indicate that the Moon has something like a molten slush surrounding its core, say researchers at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

This false-color photograph is a composite of 15 images of the Moon taken through three color filters by Galileo during the spacecraft's passage through the Earth-Moon system on December 8, 1992. The false-color processing used to create this lunar image is helpful for interpreting the surface soil composition. Areas appearing red generally correspond to the lunar highlands, while blue to orange shades indicate the ancient volcanic lava flow of a mare, or lunar sea. Bluer mare areas contain more titanium than do the orange regions. Mare Tranquillitatis, seen as a deep blue patch on the right, is richer in titanium than Mare Serenitatis, a slightly smaller circular area immediately adjacent to the upper left of Mare Tranquillitatis. Blue and orange areas covering much of the left side of the Moon in this view represent many separate lava flows in Oceanus Procellarum. The small purple areas found near the center are pyroclastic deposits formed by explosive volcanic eruptions. The fresh crater Tycho, with a diameter of 85 kilometers (53 miles), is prominent at the bottom of the photograph, where part of the Moon's disk is missing. Credit: NASA

The Moon's surface, pulled by the Sun and Earth, may bulge out and dip in as much as 10 centimeters (almost four inches) over 27 days. Love numbers show how elastic the Moon is, giving clues to the material under the surface. The newly calculated Love numbers support the idea, first suggested by Apollo program scientists, that a partially melted zone lies above the core.

"Finding out what's inside the Moon isn't simple," said Dr. James Williams, a research scientist at JPL. His team will present results at the annual Lunar and Planetary Science conference, to be held next month in League City, Texas. "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 measured Love number tells how its gravity field changes due to the tidal pull of the Sun and Earth. The Moon's Love number is .0266. Earth's is .3, showing that our planet's bigger, rocky surface may move as much as a half a meter (about 20 inches) in a day in response to the pull of the Moon and Sun. The Moon's Love number is tiny compared to Earth's, and it takes huge planetary bodies to stretch and squeeze the rocky Moon. Venus' surface, with a Love number of .3, may move as much as .4 meter (about 1 foot) from the pull of the Sun.

NASA's Apollo missions noted moonquake waves lost energy if they went deeper than 1,000 kilometers (620 miles) or over halfway into the center of the Moon. This could indicate that the Moon's depths are at least partially melted, Williams said. After the Apollo measurements of moonquakes ended in 1977, two decades passed without new measurements of the deep lunar interior.

Researchers calculated the Love numbers from data gathered by the Lunar Laser Ranging Experiment, using retroreflectors left on the Moon's surface 30 years ago by U.S. and Russian missions.

A laser pulse is fired from Earth to the Moon, bounced by a reflector and returned back to Earth. The round-trip travel time gives the distance between the two bodies with accuracy better than 2 centimeters (.8 inches). Unlike the other scientific experiments left on the Moon, the reflectors require no power and are still functioning perfectly after 30 years.

Scientists who analyze the data from the Lunar Laser Ranging Experiment have measured, among other things, that the Moon is moving away from Earth and that the shape of Earth is changing. They have also used the experiment to test the validity of several predictions of Einstein's Theory of Relativity.

Love numbers are named after Augustus E.H. Love, an Oxford mathematician who worked on mathematical theories of elasticity and waves in the late 1800s and early 1900s.

JPL, a division of the California Institute of Technology, manages numerous space missions for NASA's Office of Space Science, Washington, D.C.