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The lunar astronauts universally described the smell of moon dust as similar to spent gunpowder.

The exhaust gases from the LM's descent propulsion system (which combusts Aerozine-50 with N2O4) didn't instantly vanish from the landing site; it's estimated that the mass of the moon's atmosphere was briefly doubled by LM exhaust during each landing. (The RCS thrusters on the LM use the same propellant combination.) In addition, the remaining contents of the descent stage oxidizer tanks were vented shortly after landing (102:47 in this transcript).

Moon dust doesn't have a lot of nitrogen in it. Hydrazines, N2O4, and nitrocellulose/nitroglycerin gunpowders all do.

N2O4 has "a sharp, unpleasant chemical odor"; it vaporizes rapidly to NO2 which has "a characteristic sharp, biting odor". Combined with moisture in the astronauts' lungs and nasal passages, it could convert to nitric acid, with an "acrid, suffocating odor".

Is it possible the Apollo astronauts were smelling traces of their LM rocket exhaust, or the vented oxidizer? Call it the "who smelt it, dealt it" hypothesis.

Note that to an Apollo astronaut -- i.e. a 20th-century American with military experience -- "gunpowder" would mean smokeless powder, not black powder.

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No, I don't think so. Combusted hydrazine smells like ammonia, because it is ammonia, plus nitrogen and hydrogen. Unburnt hydrazine also smells like ammonia, but has a sharp sting to it that, if you smell it, you should leave that building.

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  • $\begingroup$ According to the last post in this thread, no ammonia in simulation of hydrazine combustion: yarchive.net/space/rocket/fuels/hydrazine.html -- is the real thing that different? $\endgroup$
    – Russell Borogove
    Commented Jul 28, 2015 at 22:16
  • $\begingroup$ Your question title just says hydrazine, which is usually burnt as a monopropellant and generates ammonia. With an oxidizer the products are different, but still don't smell like gunpowder. $\endgroup$
    – Mark Adler
    Commented Jul 28, 2015 at 22:22
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    $\begingroup$ Archaic black powder gives a sulfurous smell; gunpowder as encountered by mid-20th-century Americans (i.e. Apollo astronauts, military men) would be smokeless nitrocellulose with no sulfur in it. en.wikipedia.org/wiki/Smokeless_powder $\endgroup$
    – Russell Borogove
    Commented Jul 28, 2015 at 23:19
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    $\begingroup$ For what it's worth, apparently some gun enthusiasts say that gunpowder smells like ammonia: quora.com/What-does-gunpowder-smell-like $\endgroup$
    – called2voyage ♦
    Commented Jul 5, 2018 at 19:29

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