When Neil Armstrong and Buzz Aldrin left the surface
of the Moon and climbed back into
the Lunar Lander, they brought a bunch of Moon dust in with them.
Half of this dust is super fine, razor sharp glass,
and it clings to everything.
It also — according to almost all
of the 12 astronauts who set foot on the moon, smells
like something about to catch fire… or wet ashes…
or mostly… spent gunpowder.
Question is: why?
All these smells are related to combustion reactions,
which are pretty common here on Earth.
This is a combustion reaction, specifically this one.
This is also a combustion reaction,
but it’s muuuuch faster,
because instead of getting the oxygen from the atmosphere,
explosives have an oxidizer
built right into the chemical structure of the fuel,
or they’re paired with a strong oxidizer like potassium nitrate.
Let’s look at nitroglycerin,
an explosive used in gunpowder.
Here’s what happens — in theory — when it explodes.
The thing is… none of the gases produced
smell like spent gunpowder, or anything else.
You’re breathing all of them in right now.
Do you smell anything?
But combustion reactions
— including explosive ones — never happen “perfectly.”
Lots of other products,
some of them not smelly, and some of them quite smelly, get formed.
These molecules wander up to the back of your nose, where you have a small patch of neurons
— brain cells — each of which has one type of smell receptor.
That receptor is a protein
that is partly inside the cell,
and partly sticking out into the empty space of your nose.
When a small molecule binds to a receptor,
its neuron sends a signal up to the brain,
which then weaves all of these signals together
into something you recognize as a smell.
One molecule can activate a variety of receptors;
and most familiar smells are a combination of molecules,
so humans can smell a wide variety of things.
Dogs still have us beat, though.
We’ve got about 400 different types of smell receptor
s but dogs have over 1000.
So back to the Moon.
As you may have noticed, it’s not on fire,
and as far as we know there was no recent civil war reenactment there.
So what’s causing these combustion or explosion-like smells?
Most Moon astronauts agreed that the smell was explosive or combustion related,
which suggests it’s not just a random fluke.
So there must be some chemical process that caused the same —
or similar — small molecules
to wander into the astronauts’ noses.
This is not a crazy idea.
There’s actually a fair bit of chemistry happening on the Moon.
The Moon is constantly getting pummeled by ions from the Sun,
cosmic radiation, and tiny meteorites crashing into its surface.
It’s possible that all this abuse creates some molecules
that could become smelly, if
they were to react with something else — say,
the air in the lunar lander.
Because there’s no atmosphere on the Moon,
these hypothetical molecules just sorta hang
You’ll notice that Buzz here is wearing a spacesuit,
so right at this very moment,
he’s not smelling the Moon.
All the smelling happened after the moonwalks,
inside the lunar lander, in an atmosphere
made up of nitrogen, water vapor, carbon dioxide, and… oxygen. Also,
空气里充满了氮气 水蒸气 二氧化碳和氧气
remember down here on Earth,
combustion smells are caused by carbon-based molecules
reacting with oxygen.
OK, now this is where we get into educated guesswork.
We don’t know exactly what happened up there,
but there are a few theories floating around on the Internet,
also known as the theory machine.
Possibly the simplest explanation for what was going on is the “ desert rain effect. ”
We made a video about this. Basically,
dry Moon dust hitched a ride into in the lander,
and then water in the lander’s
atmosphere liberated small smelly molecules trapped in the dust.
Those molecules then wafted into Buzz’s nose.
Another similar explanation is
that larger carbon-based molecules trapped in Moon dust
were slowly oxidized by the oxygen in the lander’s atmosphere,
breaking down into small smelly molecules
that wafted into Buzz’s nose.
This is sort of like a super-slow-motion combustion or explosion reaction.
But — as a lunar dust expert we talked to pointed out —
we haven’t yet found any small smelly molecules trapped in Moon dust,
nor have we found any larger carbon-based molecules that might break down into smallersmelly ones.
Like sand here on Earth,
Moon dust contains lots of mineral fragments and glassy shards,
but they don’t smell.
Some scientists think that free radicals might have something to do with the smell.
Free radicals are formed when cosmic or solar radiation
knocks an electron off a molecule in the dust,
forming highly reactive species
that are missing their full set of 8 electrons.
Other scientists think ions from the sun might be involved.
And then there are micrometeorites, which are exactly what they sound like
and can create free radicals and ions.
The idea here is that the free radicals or ions could be formed in Moon dust
, and then react with the atmosphere in the lander,
producing the spent gunpowder smell.
Or, in a related twist:
the free radicals themselves react directly with cell membranes
in your nose, “ tricking ” your brain
into thinking that you were smelling something.
The problem with that last theory,
as our smell and gunpowder experts pointed out, is
that if there really were lots of free radicals getting
into your nose and reacting with everything,
you wouldn’t smell much —
you would just feel a lot of pain.
For this episode, We talked to two experts
in lunar dust, an explosives expert, and a smell expert.
Each scientist had their own take on what was most plausible.
Some pointed out that it could be a combination of things.
Most of our experts seemed to think that
either carbon-based molecules, free radicals, or
ions in Moon dust reacted with the lander’s atmosphere, producing the smell.
But all of them agreed that the only way to find out
is to actually get our hands on some fresh Moon dust and run some real experiments.
Which means going back to the Moon.
What theory do you guys think is most plausible?
Let us know in the comments.
Links to all sources in the description.
Have fun going down this rabbit hole, and see you next week!
PBS is bringing you the universe with SUMMER OF SPACE,
PBS的SUMMER OF SPACE频道 将宇宙展示在你的眼前
which includes six incredible new science and history shows streaming on PBS.org and the PBS Video app,
along with lots of space-y episodes from PBS Digital Studios creators.
Follow me over to EONS to check out their Summer of Space episode on CELESTIAL CYCLES AND EARTH’S CLIMATE.
在EONS上关注我 观看“天体秩序和地球气候”栏目的 宇宙之夏剧集
When Neil Armstrong and Buzz Aldrin left the surface