Mystery of Faint Moonlight Finally Solved
Apollo astronauts left arrays of corner-cube prisms, like the one shown here, on the moon 40 years ago. Now light bouncing back from these reflectors appears to have dimmed. Scientist think dust is to blame.
Credit: UC San Diego

Astronomers have long known that light bouncing off man-made reflectors on the lunar surface is fainter than expected, and mysteriously dims even more whenever the moon is full. Now they think moon dust and solar heating may be the dirty culprit, according to a new report.

The evidence is right here on Earth, researchers said. Only a fraction of the light a team beamed at the moon from a telescope in New Mexico bounces off of old reflectors on the lunar surface and returns to the observatory.

"Near full moon, the strength of the returning light decreases by a factor of ten," said Tom Murphy, an associate professor of physics at the University of California, San Diego, and the study's lead author. "Something happens on the surface of the moon to destroy the performance of the reflectors at full moon."

Measuring the moon

Murphy leads an effort to precisely measure the distance from Earth to the moon by timing the pulses of laser light that reflect off targets left on the lunar surface 40 years ago by Apollo astronauts.

Earth's atmosphere scatters the outgoing beam, spreading it over a distance of approximately 1.24 miles (2 km) on the surface of the moon.

The scientists aim the light at polished blocks of glass called comer cube prisms, each of which is about 1 1/2 inches (3.8 cm) in diameter.

Most of the laser light misses its target, which is roughly equivalent to the size of a suitcase. Furthermore, the reflectors also diffract returning light so that it spreads over 9.3 miles (15 km) when it reaches Earth again.

So the researchers have always expected to recapture only a small portion of the reflected photons, or particles of light, that actually bounce back. On average, their instruments detect just one-tenth of the returning light, and when the moon is full, ?the results are oddly ten times worse.

Moon dust and heat

Murphy believes that the cubes are heating unevenly at full moon, and that the cause of this discrepancy is likely caused by dust.

"Dust is dark," Murphy said. "It absorbs solar light and would warm the cube prism on the front face."

Ideally, for optimum performance, the entire cube must be the same temperature.

"It doesn't take much, just a few degrees, to significantly affect performance," Murphy said.

NASA engineers went to great lengths to minimize temperature differences across the prisms, which rest in arrays tilted toward Earth. Individual prisms sit in recessed pockets so that they are shielded from direct light when the sun is low on the moon's horizon.

But, when the full face of the moon appears illuminated from Earth, the sun is directly above the arrays.

"At full moon, the sun is coming straight down the pipe into these recessed pockets," Murphy explained.

The reflective properties of the prisms, which are clear glass, derive from the shape of their polished facets. Uneven heating of the prisms, which could occur with absorption by a dust coating, would bend the shape of the light pulses they reflect, interfering with the accuracy of measurements.

Light travels faster through warmer glass, and although all paths through the cube prisms are the same length, photons that strike the edge of the reflector will stay near the surface. Meanwhile, those that strike the center will pass deeper into the cube before hitting a reflective surface.

If the surface of the prism is warmer than the deeper parts, light that strikes the edges of the prism will re-emerge sooner than light that strikes the center, distorting the shape of the reflected laser pulses.

Lunar dust dilemma

But finding the source of the problematic dust could be more difficult, Murphy said.

The moon has no atmosphere and no wind, but electrostatic forces can move dust around. A constant rain of micrometeorites might also puff dust onto the moon's surface. Larger impacts that eject material from the surface across a greater distance could also contribute to the buildup.

Murphy recently returned from a trip to Italy, where a chamber built to simulate lunar conditions may help sort through the possible explanations.

"We think we have a thermal problem at full moon, plus optical loss at all phases of the moon," Murphy said. Accumulated dust on the front surface of the reflectors could account for both observations.

If sunlight-heated dust is really to blame, the researchers should notice the effect vanish during a lunar eclipse. In other words, light should bounce back while the moon passes through Earth's shadow, then dim again as sunlight hits the arrays once more.

"Measurements during an eclipse ? there are just a few ? look fine," Murphy said. "When you remove the solar flux, the reflectors recover quickly, on a time scale of about half an hour."

The researchers' findings will be published in an upcoming issue of the journal Icarus.

Previously, the McDonald Observatory, a research unit of The University of Texas at Austin, located in the Davis Mountains of West Texas, ran similar experiments at full moon between 1973 and 1976. But, between 1979 and 1984, they had "a bite taken out of their data," during full moons, Murphy said. "Ours is deeper." This could signify that the problem may be getting worse.

So far, bad weather has prevented the project from operating during a lunar eclipse. The next opportunity for the researchers will be on the night of Dec. 21, 2010.

  • Images - Full Moon Fever
  • Top 10 Lunar Eclipse Facts
  • Images - Apollo 11 Anniversary: A Look Back in Pictures