The Moon smells like gunpowder. Nobody knows why.
Twelve people have walked on the Moon. Every crew came back indoors with dust on their boots, took off their helmets, and smelled something nobody had planned for.
Only 12 people have ever smelled the Moon.
All twelve were American, and all took part in NASA's Apollo program between 1969 and 1972. Six landings, two people walking on the surface each time.
Gene Cernan was the last to leave, on 14 December 1972. That was more than 53 years ago.
Counted here: people who walked on the surface. Apollo 13 never landed, and astronauts who only orbited are not counted.
Here is the odd part. None of them smelled it outside.
United States · 12 moonwalkers, one square each
Names are in the table at the end. Source: Associated Press, Space.com.
They couldn't smell it outside. The dust came in with them.
On the Moon, the astronauts wore sealed suits and helmets. Nothing could reach their noses. Then they climbed back into the lunar module, and moondust came in on their boots and gloves.
The dust was clingy, and brushing did not get it all off. When the helmets came off, they could feel it, smell it and even taste it. Apollo 16's John Young said it tasted fine.
The feel, they said, was soft like snow but a little rough. Aldrin was quoted a few ways: gunpowder, firecracker smoke, burnt charcoal. All of them mean burnt.
Why would a rock smell like a gun?
Different crews, one smell: something burnt
Mostly paraphrased. Sources: Universe Today, Space.com, Nautilus.
Moondust and gunpowder share nothing.
Modern gunpowder is built from two chemicals, nitrocellulose and nitroglycerin. Both burn. NASA's Gary Lofgren says neither is found in lunar soil.
Hold a match to moondust and nothing explosive happens. So the crews were not smelling gunpowder. They were smelling something that smells like it.
Jack Schmitt says every Apollo astronaut was used to handling guns, so they knew the smell well. Before the first landing, the crew was told the dust might, very unlikely, catch fire.
So what is the dust made of?
Two lists of ingredients with no match
Gunpowder
- Nitrocellulose
- Nitroglycerin
Moondust
- Glass from meteorite hits
- Iron
- Calcium
- Magnesium
Shared chemicals: none
Source: Universe Today, quoting NASA's Lunar Sample Laboratory.
Almost half of moondust is glass, made by space rocks.
For billions of years, meteoroids have slammed into the Moon. Each hit melts the top layer of soil into glass and shatters it into tiny, jagged bits.
The glass is silicon dioxide, the main ingredient of sand. The rest holds iron, calcium and magnesium, locked inside minerals called olivine and pyroxene.
"Almost half" is NASA's Lofgren, as reported in 2006. Other studies find the glass share changes with where the soil is from and how old it is. Treat it as a good estimate, not a fixed number.
Glass and metal. Why would that smell like a fired gun?
What moondust is made of, roughly
Soft ends mean an estimate. Source: Universe Today; NASA, The Lunar Regolith.
Nobody knows why it smells. There are three guesses.
The smell only showed up inside the cabin. So every idea starts with what changes when dust meets cabin air.
The ideas are below. None has been proven.
All three could be partly true at once. None has been tested on the Moon itself, which is why Schmitt says the dust must be studied there.
There is one big clue. It is back on Earth.
Three ideas, none proven
Desert rain
Dry dust meets damp cabin air. Like the smell when rain hits a desert, old trapped odors are set free. The Moon is a 4-billion-year-old desert.
Don Pettit, space station astronautSolar wind
Gas from the Sun, like hydrogen and helium, gets stuck in the dust. Footsteps loosen it, and warm cabin air lets it out.
Gary Lofgren, NASA lunar sample labLoose bonds
Smashed rock leaves loose chemical bonds. Oxygen in the cabin grabs them in a very slow burn, much like rusting.
Jack Schmitt and Gary LofgrenSource: Universe Today.
Back on Earth, the smell is gone.
NASA's lab in Houston keeps hundreds of pounds of moondust. Lofgren has held it, sniffed it and sniffed his own hands. It does not smell like gunpowder.
The astronauts were not imagining things. The leading idea is that the samples met moist, oxygen-rich air, and the smelly reactions ended long ago.
Sample containers were meant to keep out air. But the jagged dust cut their seals, so air got in during the three-day trip home. No one can say how much the dust changed.
NASA treats smell as a safety tool, too. It pays people to sniff.
Where the smell could have been lost
About 842 pounds came back from six landings. Sources: Universe Today, KUOW, from Charles Fishman's One Giant Leap.
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NASA pays a small team to sniff things before they fly.
Volunteer sniffers test materials headed for the living areas of the International Space Station, and earlier, the Space Shuttle. Five of them smell each item. They score it from 0, no smell, to 4, offensive. A score above 2.5 fails.
A bad smell can make astronauts queasy, and that can put a mission at risk. Station air is clean and warm, so odors linger. After the 1967 Apollo 1 fire killed three astronauts, NASA added more testing of materials, sniff tests included.
Not literally every object: the tests cover items going into the habitable areas. Sniffers are screened with a 10-bottle test of seven odors and three blanks. Chief sniffer George Aldrich reports more than 800 "smell missions".
One last smell is a joke. It is five centuries old.
NASA's smell score: pass up to 2.5, fail above it
Sources: ScienceAlert, BBC Science Focus, NASASpaceflight.
Long before Apollo, people joked the Moon was cheese.
The line "the moon is made of green cheese" is printed in 1546, in a book of sayings by the English writer John Heywood. Older folk tales already told of a fox tricking a wolf with the Moon's reflection as a block of cheese.
"Green" here means young, as in fresh cheese that has not aged. It has nothing to do with color. And nobody serious believed it. It was a way of calling someone gullible.
A popular retelling says young cheese looks mottled, like the cratered Moon. I found no source for that, so treat it as a story, not a fact.
So which Moon story is stranger?
A joke from 1546, then the real Moon
Sources: Wikipedia, culture magazine, WYTV.
The joke was cheese. The truth is stranger.
In 1546, the Moon was a punchline about cheese. In 1969, twelve people proved it was a dry, glassy, gray desert. And then it smelled like a fired gun.
We know what the dust is made of. We know it lost its smell on the way home. We still don't know why it smelled at all.
Schmitt's view is that only someone standing on the Moon can finish the job. The next crews to land may get to sniff the answer.
What we know, and what we don't
Known
- Crews reported a burnt-gunpowder smell
- Dust is glass plus iron-rich minerals
- Samples on Earth don't smell
Unknown
- Which chemical makes the smell
- Rain, solar wind or loose bonds
- How the dust changed on the trip home
Based on the sources listed at the end.
Five numbers from a smelly little mystery
people have walked on the Moon.
Space.comyears and counting since the last one left, in December 1972.
Space.compounds of Moon rock and dust brought home from six landings.
KUOW, Fishmanout of 4 is the smell score where a space material fails.
ScienceAlertthe year Heywood printed the green cheese line.
Wikipedia