LADEE and Lunar Twilight Rays

Back in the 60s and 70s, Apollo astronauts circling the Moon saw something that still puzzles researchers today. About 10 seconds before lunar sunrise or lunar sunset, pale luminous streamers would pop up over the gray horizon. These “twilight rays” were witnessed by crewmembers of Apollo 8, 10, 15 and 17.

Back on Earth, we see twilight rays all the time as shafts of sunlight penetrate evening clouds and haze. The “airless Moon” shouldn’t have such rays, yet the men of Apollo clearly saw them.

Later this week a NASA spacecraft is going back to the Moon to investigate. Slated for launch on September 6, 2013, the Lunar Atmosphere and Dust Environment Explorer (“LADEE” for short) will seek out twilight rays and other mysteries of the lunar atmosphere.

“Yes, the Moon does have an atmosphere,” says Richard Elphic, the project scientist for LADEE at NASA Ames. “It’s just much more tenuous than ours.”

The Moon’s atmosphere is so flimsy — about ten thousand billion times less dense than Earth’s — that a good sneeze would rip through it like a hurricane. “Lunar air” is a gossamer mix of argon-40, which seeps out of the ground due to radioactive decay in the lunar interior, plus elements such as helium, sodium, and potassium, sputtered off the lunar surface by solar wind and micrometeoroids.

None of these gases appear in sufficient quantities, however, to explain the twilight rays.

“We’re missing something,” says Elphic.

The missing piece might be dust. When sunlight falls on the Moon, solar UV radiation electrifies the unprotected topsoil, possibly causing lightweight grains of moondust to rise off the ground, joining the gases already there.

“This electrically charged dust may be what the astronauts saw,” says Elphic. LADEE’s Lunar Dust Experiment will collect and analyze dust in the Moon’s atmosphere to test this hypothesis.

Researchers have a special name for atmospheres as fantastically thin as the Moon’s: an exosphere. On Earth, molecules in the thick air are constantly bumping into each other, spreading pressure and heat in all directions. In an exosphere, however, molecules are so far apart they rarely collide.

“Instead of bumping into each other,” says Elphic, “they bump into the lunar surface.”

Lunar twilight rays sketched by Apollo 17 astronauts.
Air molecules coming into contact with moondust are expected to stick, briefly, before moving on again. Hop and stick, hop and stick. At any given moment millions of molecules could be hopping like bunnies across every square inch of lunar terrain. Ultraviolet, visible light, and mass spectrometers on board LADEE will inventory the molecules present and determine how they behave.

“The dusty, flimsy mix of atoms and molecules in the lunar atmosphere is sure to have alien properties that our experience on Earth has not prepared us to anticipate,” says Elphic.

To find out, LADEE will be working on a deadline. On April 15th of next year, the sunset-colored shadow of Earth will envelop the Moon for a lunar eclipse. It will be a grand sight from Earth, but bad news for LADEE. The spacecraft is solar powered and requires sunlight to charge its batteries. An eclipse could end the mission.

"The current plan," says Elphic, "is, before the eclipse, to guide the spacecraft into the surface of the moon for a final impact that we can study. We’ll be taking data until the very end."

Video credit: NASA

The Moon by LRO

Video credit: NASA/GSFC/Arizona State University

Note: For more information, see A Unique View of the Moon.

Bright Explosion on the Moon

For the past 8 years, NASA astronomers have been monitoring the Moon for signs of explosions caused by meteoroids hitting the lunar surface. "Lunar meteor showers" have turned out to be more common than anyone expected, with hundreds of detectable impacts occurring every year.

They've just seen the biggest explosion in the history of the program.

"On March 17, 2013, an object about the size of a small boulder hit the lunar surface in Mare Imbrium," says Bill Cooke of NASA's Meteoroid Environment Office. "It exploded in a flash nearly 10 times as bright as anything we've ever seen before."

Anyone looking at the Moon at the moment of impact could have seen the explosion--no telescope required. For about one second, the impact site was glowing like a 4th magnitude star.

Ron Suggs, an analyst at the Marshall Space Flight Center, was the first to notice the impact in a digital video recorded by one of the monitoring program's 14-inch telescopes. "It jumped right out at me, it was so bright," he recalls.

The 40 kg meteoroid measuring 0.3 to 0.4 meters wide hit the Moon traveling 56,000 mph. The resulting explosion1 packed as much punch as 5 tons of TNT.

Cooke believes the lunar impact might have been part of a much larger event.

"On the night of March 17, NASA and University of Western Ontario all-sky cameras picked up an unusual number of deep-penetrating meteors right here on Earth," he says. "These fireballs were traveling along nearly identical orbits between Earth and the asteroid belt."

This means Earth and the Moon were pelted by meteoroids at about the same time.

“My working hypothesis is that the two events are related, and that this constitutes a short duration cluster of material encountered by the Earth-Moon system," says Cooke.

One of the goals of the lunar monitoring program is to identify new streams of space debris that pose a potential threat to the Earth-Moon system. The March 17th event seems to be a good candidate.

Controllers of NASA's Lunar Reconnaissance Orbiter have been notified of the strike. The crater could be as wide as 20 meters, which would make it an easy target for LRO the next time the spacecraft passes over the impact site. Comparing the size of the crater to the brightness of the flash would give researchers a valuable "ground truth" measurement to validate lunar impact models.

Unlike Earth, which has an atmosphere to protect it, the Moon is airless and exposed. "Lunar meteors" crash into the ground with fair frequency. Since the monitoring program began in 2005, NASA’s lunar impact team has detected more than 300 strikes, most orders of magnitude fainter than the March 17th event. Statistically speaking, more than half of all lunar meteors come from known meteoroid streams such as the Perseids and Leonids. The rest are sporadic meteors--random bits of comet and asteroid debris of unknown parentage.

U.S. Space Exploration Policy eventually calls for extended astronaut stays on the lunar surface. Identifying the sources of lunar meteors and measuring their impact rates gives future lunar explorers an idea of what to expect. Is it safe to go on a moonwalk, or not? The middle of March might be a good time to stay inside.

"We'll be keeping an eye out for signs of a repeat performance next year when the Earth-Moon system passes through the same region of space," says Cooke. “Meanwhile, our analysis of the March 17th event continues.”

Footnote: (1) The Moon has no oxygen atmosphere, so how can something explode? Lunar meteors don't require oxygen or combustion to make themselves visible. They hit the ground with so much kinetic energy that even a pebble can make a crater several feet wide. The flash of light comes not from combustion but rather from the thermal glow of molten rock and hot vapors at the impact site.

Video credit: NASA

Apollo 16 Lunar Rover Grand Prix

Apollo 16 astronauts John Young and Charlie Duke take the lunar rover for a spin on the surface of the moon in this film footage of what became known as the "lunar rover Grand Prix". This footage was shot on 16mm film and is silent.

Video credit: NASA

Tour of the Moon

"Tour of the Moon" takes viewers to several interesting locations on the moon. Tour stops included in this breathtaking journey across the moon's surface are: Orientale Basin, Shackleton crater, South Pole-Aitken Basin, Tycho crater, Aristarchus Plateau, Mare Serenitatis, Compton-Belkovich volcano, Jackson crater and Tsiolkovsky crater.

Video credit: NASA; text credit: NASA.

Evolution of the Moon

"Evolution of the Moon" explains why the moon did not always look like it does now. The moon likely started as a giant ball of magma formed from the remains of a collision by a Mars-sized object with the Earth about four and a half billion years ago. After the magma cooled, the moon's crust formed. Then between 4.5 and 4.3 billion years ago, a giant object hit near the moon's South Pole, forming the South Pole-Aitken Basin, one of the two largest proven impact basins in the solar system. This marked the beginning of collisions that would cause large scale changes to the moon's surface, such as the formation of large basins.

Because the moon had not entirely cooled on the inside, magma began to seep through cracks caused by impacts. Around one billion years ago, it's thought that volcanic activity ended on the near side of the moon as the last of the large impacts made their mark on the surface. The moon continued to be battered by smaller impacts. Some of the best-known impacts from this period include the Tycho, Copernicus, and Aristarchus craters. So, while the moon today may seem to be an unchanging world, its appearance is the result of billions of years of violent activity.

Video credit: NASA