LADEE Mission Seeks Answers To Moon’s Mysterious Twilight Rays

“Twilight” rays sketched and notated by Apollo 17 astronauts just before sunrise as they orbited the moon. Credit: NASA

One of the strangest things Apollo astronauts saw on the moon were pale, glowing beams of light popping up over the horizon about 10 seconds before lunar sunrise or sunset. Earth’s atmosphere and air-borne dust provide an endless variety of fascinating and colorful sunrise-sunset phenomena, but you’d never expect the same from the essentially airless moon.

Twilight or crepuscular rays on Earth are easily explained as sunlight shining through gaps in clouds across our dusty atmosphere. Credit: Bob King

Sure, the astronauts saw some things they expected  — a wedge of zodiacal light emerging from behind the moon’s edge and the sun’s outer atmosphere or corona, but something else caught their attention. The curve of the lunar horizon was shot through with rays of light resembling crepuscular rays poking through holes in Earth’s clouds. You know these; they create the familiar “sunbeam” effect.

LADEE is perched atop a converted ICBM Minotaur rocket on the launch pad today Sept. 5. Credit: NASA EDGE/Franklin Fitzgerald

Astronauts from Apollo 8, 10, 15 and 17 all witnessed these “twilight rays” and made sketches of them. The Surveyor 6 lander even took several photos of the phenomenon as early as 1967.

Tomorrow night we’ll take one big step in cracking the mystery.  At 11:27 p.m. EDT, NASA will launch the  Lunar Atmosphere and Dust Environment Explorer (LADEE, pronounced “laddie”) atop a converted intercontinental ballistic missile (ICBM) from the Wallops Flight Facility in Virginia.

The nighttime launch and flight of the Minotaur rocket over the Eastern Seaboard should be an amazing spectacle. If you live in the New York City area, you’ve got a ringside seat. Click HERE for when and where to see it in other cities.

LADEE’s path through the sky after launch takes it over the Eastern Seaboard including Manhattan and Long Island. Click to see full-sized maps for many locations. Credit: Orbital Sciences

During its 100-day mission LADEE will orbit the moon’s equator and study its exceedingly thin atmosphere, composed of argon-40, created from radioactive decay in the lunar interior, plus helium, sodium, and potassium, sputtered off the lunar surface by solar wind and micrometeoroids. It will also use a dust detector and other instruments to study dust in the moon’s vicinity believed to be the cause of the mysterious beams.

The western horizon photographed just after sunset by Surveyor 6 in November 1967 reveals the lingering glow of moon dust levitating about a yard above the surface. Credit: NASA

Scientists think they know what causes the rays. It all goes back to the moon’s long history of impact from the infancy of the solar system to this very day. Without an atmosphere to shield the surface from impacts, continual bombardment by meteorites of all sizes has churned the moon’s outer layer of crust into a jumble of shattered rocks, melted bits of rock and billions of tiny, sharp-edge dust particles that resemble finely shattered glass.

Ultraviolet light from sun made give dust on the moon an electrostatic charge causing it to levitate. This could be what the Apollo astronauts saw. Credit: NASA

Scientists hypothesize that what the astronauts saw and Surveyor photographed was electrically-charged moon dust. Ultraviolet light and X-rays from the sun on the dayside of the moon are powerful enough to knock electrons, negatively charged particles, off lunar dust particles. This leaves the particles with a net positive electric charge. Since like charges repel, the dust particles push away from one another and float in the direction of least resistance — up. The smallest particles reach the highest altitudes and then fall back to the surface, pulled down by the moon’s gravity. Others float up to replace them in a repeated cycle or rising and falling like the stream of water in a water fountain.

A balloon rubbed on your head separates positive and negative charges causing the balloon to lift up your hair.

If you’ve every tried to shake those packing peanuts loose from your hands only to see them stick on your arms, you’ve experienced first hand the power of electrostatic charging. Don’t have packing peanuts? Rub your head with a balloon. When you move the balloon away, it drags your hair with it . Rubbing transfers negatively-charged electrons to the balloon, leaving your hair with a net positive charge. Opposite charges attract.

Astronomers speculate that lunar nightside dust could float even higher than the dayside variety. Finally, there’s the interesting situation at the lunar terminator, the border between day and night.

The terminator moves to the east as the moon’s phase waxes and to the west as it wanes. It’s a constantly shifting boundary. Timothy J. Stubbs of the Laboratory for Extraterrestrial Physics at NASA’s Goddard Space Flight Center, speculates there could be “significant horizontal electric fields forming between the day and night areas, so there might be horizontal dust transport. Dust would get sucked across the terminator sideways.”

Who knew scant bits of dust could fire a burning curiosity to return to the moon?

8 Responses

  1. Andrew Kirk

    Bob, prior to our current “age” of digital photography, how did satellites and other unmanned missions make photographs and send them home?

    1. astrobob

      Great question. The early cameras carried film which was processed automatically with developer and all the rest on board the spacecraft. The developed film was then scanned by an electron beam kind of the way a TV works and then sent via radio back to Earth.

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