The moon has been exposed to the vacuum of space for billions of years. It has no atmosphere and has been bombarded by meteors since its creation. We’ve sent astronauts there, and they’ve smelled the moon.
So what does it smell like?
Astronauts describe the smell as spent gunpowder. Though, they didn’t intentionally try to ‘sniff’ the moon. Moondust is extremely fine and dry. The astronauts complained that it sticks to everything. As a result, they tracked a lot of moondust and particles in the lander with them upon returning from a moonwalk.
This resulted in the first ever recorded case of extraterrestrial hay fever. “It’s come on pretty fast,” Apollo 17 astronaut Jack Schmitt radioed to Houston. Years later he recalls, “When I took my helmet off after the first EVA, I had a significant reaction to the dust. My sinuses and nasal cavity became swollen.”
After a few hours, the symptoms eased up. “It was there again after the second and third EVAs, but at much lower levels. I think I was developing some immunity to it.”
Moondust and gunpowder are obviously not the same thing. Gunpowder contains chemicals and organic molecules not found in lunar soil. So what is moondust made of?
Nearly half of what makes up the lunar soil is silicon dioxide glass created by meteors hitting the moon. This fuses the topsoil into glass and shatters the same into tiny pieces. Moondust is rich in calcium, iron and magnesium tied up in minerals such as olivine and pyroxene.
So why the smell?
No one knows exactly. There are several theories however. ISS astronaut Don Pettit offers one possibility:
“Picture yourself in a desert on Earth,” he says. “What do you smell? Nothing, until it rains. The air is suddenly filled with sweet, peaty odors.” Evaporating water from the ground carries molecules and the smells to your nose that have been trapped in dry soil.
“The moon is like a 4-billion-year-old desert,” he says. “It’s incredibly dry. When moon dust comes in contact with moist air in a lunar module, you get a ‘desert rain’ effect as well as some lovely odors.”
Gary Lofgren has a similar theory. “The gases ‘evaporating’ from the moon dust might come from solar winds.” Unlike Earth, he explains, “the moon is exposed to the hot wind of hydrogen, helium and other ions blowing away from the sun. These ions hit the moon’s surface and get caught in the dust.”
Yet another possibility is that moondust ‘burns’ in the lunar lander’s oxygen atmosphere. “Oxygen is very reactive,” says Lofgren, “and would readily combine with the dangling chemical bonds of the moondust.” This process, called oxidation, is similar to burning. Though it happens too slowly for smoke or flames, the oxidation of moondust may produce an odor like burnt or spent gunpowder.
Back on Earth, moondust does not have much of a smell. There is plenty of moondust stored at the Lunar Sample Lab in Houston. Lofgren has held dusty moon rocks with his own hands. “It does not smell like gunpowder,” he says.
The samples brought back by the astronauts have been in contact with moist, oxygen-rich air. Any chemical reactions ended long ago.
This wasn’t supposed to happen though. The Apollo astronauts took special containers to the moon to hold the samples in vacuum. The sharp edges of the dust cut into the seals of the containers, allowing water vapor and oxygen to leak in during the trip back to Earth. No one knows how much the dust was altered by that exposure.
Bonus Factoid: Did you know it’s illegal to own moon rocks or dust? Since all moon rocks were brought back by the United States government, they are the property of the government. Occasionally, the US government has gifted the governments of countries samples.
Photo courtesy of JAXA.