Press Release

Moon’s Heart Melted, Say Lunar Love Numbers

By SpaceRef Editor
February 14, 2002
Filed under ,

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.

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

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

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.

SpaceRef staff editor.