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Are there any kind of contaminants or other reasons that might be dangerous? I'd assume that as far as microbes/bacteria goes, eating off the ground on the moon is much safer than it would be on earth.

Viglé
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Lunar regolith/soil/dust is probably rather nasty stuff to ingest.

Where the dust on earth has been whirled about in air currents and rounded off, lunar dust is quite sharp and abrasive. It's similar to the difference between pebbles that have been rounded by water in rivers and coastlines, and regular uneroded rocks. This is true even for the finest of particles.

If you scroll down to the 'harmful effects' section of the linked wiki page, it details some of the problems if you breathe it in (effects similar to silicosis). While your digestive tract is largely a little sturdier than your lungs when it comes to ingesting stuff you shouldn't, I doubt eating significant quantities of what amounts to glass dust would do you much good.

Edit: turns out it's even nastier!

As @TomášZato mentions in the comments, lunar dust also has a significant amount of unreacted molecules and compounds in it. On earth, these have usually reacted a long time ago and are now inert. Needless to say, ingesting particles of reactive matter is also not recommended.

Ynneadwraith
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    "Moon dust! The new craze is here: Internal scrub! Clean your system from the inside out!* **Warning: Effects might be permanent* – Martijn Mar 26 '19 at 13:15
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    @Martin Exfoliation...for your intestines! What's not to like? – Ynneadwraith Mar 26 '19 at 13:18
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    There is also likehood of all kids of unreacted matter. On earth, metals and salts have typically reacted with water if they do, but on moon there is no water. – Tomáš Zato Mar 26 '19 at 13:19
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    "The bean counters told me we literally could not afford to buy seven dollars worth of moon rocks, much less seventy million. Bought 'em anyway. Ground 'em up, mixed em into a gel." ref "And guess what? Ground up moon rocks are pure poison. I am deathly ill." ref – Baldrickk Mar 26 '19 at 13:30
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    @Baldrickk "Still, turns out they're a great portal conductor." – Ynneadwraith Mar 26 '19 at 13:50
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    @Ynneadwraith indeed. When life gives you lemons, don't make lemonade, make life take the lemons back! (especially prudent advice when covered in lunar regolith) – Baldrickk Mar 26 '19 at 13:53
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    The OP is asking about food dropped on the moon, not shoveling regolith into your mouth, so i'd guess any pieces large enough to be problematic because of exposed edges (a few cms) will be brushed off; Also, Active coal is also very reactive, but not problematic. – bukwyrm Mar 26 '19 at 14:39
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    @bukwyrm It's not the large bits that we're worried about. It's the dust. The fine abrasive particles that would coat the food and be very difficult to wholly remove (judging by the experience of cleaning astronauts EV suits). Not strictly deadly in tiny quantities, but still not something you want to be regularly ingesting (much like glass dust). Active coal may not be terribly problematic, but that doesn't mean ingesting things like elemental phosphorus, sodium, potassium and manganese are a great idea. – Ynneadwraith Mar 26 '19 at 14:46
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    Agree with @bukwyrm, we're not talking about ingesting "significant quantities" of regolith, we're talking about the amount stuck to a piece of food after a cursory brush off, which will be less than a gram. I'd expect the health consequences to be similar to eating a sandwich dropped in earth dirt - it's unlikely to kill you, but I wouldn't make a habit out of it. I wouldn't go back for a saltine on a full stomach, but if I dropped the last Thin Mint within 200k miles, you'd better believe I'm dusting it off and eating it. – Nuclear Hoagie Mar 26 '19 at 15:26
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    @NuclearWang What part of "glass dust" and "unreacted compounds" did you skip? We know Earth Dirty is not that toxic/dangerous (we ingest dirt all the time). You seem to be asserting 0 evidence that moon dust is equally non-toxic. – Yakk Mar 26 '19 at 20:00
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    @Yakk eating glass dust is harmless. it is a very old trope that it harms. emergency room doctors will tell you that even smallish bits will not cause lasting damage. it is all about the forces. imagine handling a slurry of glass particles. dont rub it between your fingers, thats nothing the digestive tract does. just massage it > no ill effects httpp://www.abc.net.au/science/articles/2007/01/18/1828923.htm – bukwyrm Mar 27 '19 at 05:56
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    @Baldrickk Your cunning plan of eating rock was about as cunning as a fox what graduated from Oxford university... – UKMonkey Mar 27 '19 at 15:51
  • I suspect that it's super safe to eat stuff dropped on the moon, this answer makes it sound too scary – minseong Mar 27 '19 at 16:45
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Brush it off, eat it.

Of course there are trace amounts of elements present that are potentially harmful, but the bulk is made up of benign stuff: 50% SiO2, 15% Al2O3, 10% CaO, 10% MgO, 5% TiO2 and 5-15% iron. There will be no microorganisms, no parasites, no viruses, no higher organic poisons. there will be mostly sand-like stuff.

About the trace amounts of less cheerful stuff: Consider how much regolith will remain on your dropped sand(ha!)wich - One gram would be much even if you rolled it around in the stuff. Say 1/1000th of that is Chromium (it is not), and pretend all of that then proceeds to villainly oxidise into hexavalent Chromium (the deadly stuff) instead of trivalent Chromium (the vitally important stuff). LD50 for hexavalent Chromium is 50-150 mg/kg ... And you just now ingested .01 mg/kg (if you weigh 100kg). You are safe (but don't make a habit out of it!)

The sharp edges of the stuff will not be an issue either: very large shards of freshly broken glass pose a hazard to your digestive tract, but anything on the scale of 'stuff clinging to your sandwich' does not. If you bite off a shard off a glass of water (as children sometimes do) many emergency personnel will only intervene by making them eat some bread afterwards. It all gets buffered by the slime. http://www.abc.net.au/science/articles/2007/01/18/1828923.htm

Of course it's not Best Practices to eat stuff from the ground, but a little regolith won't hurt.

bukwyrm
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  • Fine sharp fragments can be very distructive as anyone eating powered glass will know. – Slarty Mar 26 '19 at 16:14
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    It is my understanding that one of the Apollo 17 (?) astronauts was accidently exposed to a small amount of regolith and developed a significant rash from it. If I can find a reference, I'll add it, but even if not, I would not be so blythe as to just assume it cannot be harmful. – Paul Sinclair Mar 26 '19 at 16:18
  • Eating stuff that was not meant to be eaten used to be a sport, including powdered glass. @Slarty can you link a page that warns of eating powdered glass? Breathing it is a different case. – bukwyrm Mar 26 '19 at 16:20
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    @PaulSinclair i listed the bulk ingredients. If you have a Zn allergy and/or get the stuff in your breeches you might have a bad time. Point is OP asked about dropped food, not snuffing it, eating bulk amounts, rubbing yourself with it or any other regolith based abuse. If you can point to a compound that is critical in the amounts ingested (ppm in bolus of 1 g) and probable to exist in Regolith, go ahead. – bukwyrm Mar 26 '19 at 16:27
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    @bukwyrm - As I understand it (again, I'll have to do some searching to find the reference and confirm or deny my memory), the rash was not cause allergens or toxicity, but rather the skin irritation from the jagged dust. – Paul Sinclair Mar 26 '19 at 16:29
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    You'd have to have some pretty through cleaning methods for 'brushing it off' before eating that food. It's possible, but moon dust is difficult to get off of stuff. http://blogs.discovermagazine.com/d-brief/2018/03/02/lunar-dust/ – Zibbobz Mar 26 '19 at 16:29
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    @bukwyrm no you are right powdered glass does not appear to be as destructive as I thought. Hoever it does depend on the size of the particles https://www.fda.gov/downloads/Food/GuidanceRegulation/UCM252441.pdf – Slarty Mar 26 '19 at 16:39
  • Boil it, boiling water plus micro-silica particles quickly converts them into harmless clays. Considering the food would freeze dried from exposure anyway this is not that much of an issue.If your really scared add a little vinegar to speed up the process. – John Mar 26 '19 at 21:04
  • @PaulSinclair That reaction was (if it was), skin-glass-solid. we're talking mucous-slurry-mucous. as Zibbobz rightly states, regolith dust sticks to everything, so all EVA moon missions got dusted by it from handling the suits. i could not find the rash-story, but even if only one astronaut got rashes after everyone was dusted, that bolsters my hypothesis. alao, again, think about a slurry of glass dust - you couldpaddle around in it, just dont apply it high-pressure/high velocity – bukwyrm Mar 28 '19 at 06:38
  • @bukwyrm - I can't find it either, even though it is less than a month since I heard about it. But I don't remember where. It's too bad, because it had a number of good details (if it could be trusted). But without a reliable source, it is worthless. – Paul Sinclair Mar 28 '19 at 23:36
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It'll break your teeth. The moon is often very cold or very hot on the surface. The sandwich has potentially been flash-frozen or is now burnt depending on where it dropped.

lilHar
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    IIRC, day side gets to over 200° F. Might be more concerned about burning your tongue. – A C Mar 26 '19 at 20:20
  • Also true. I'll update the answer. – lilHar Mar 26 '19 at 20:40
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    How much heat capacity is in lunar regolith? Without atmosphere, the ground is going to cool from the sandwich and stop burning it. And especially if it's loose material, it's not going to conduct well. – John Dvorak Mar 27 '19 at 09:21
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    When I drop food onto a rock, I pick up the food not the rock. The rock is unable to break my teeth.

    When I drop food onto sand, I pick up the food with a little sand. The food tastes horrible, but it still doesn't break my teeth.

    As for flash frozen, this all depends on exactly what the food is; you're assuming that it's got a high water content; would be eaten at earth rtp and you're in the dark on the moon - but even if that's the case people eat ice cream all the time without breaking their teeth.

     – UKMonkey Mar 27 '19 at 15:59
  • @AC if you dropped cheese - maybe. If you dropped something with a low heat capacity, then it doesn't matter what temperature the moon is; by the time it touches your tongue it'll be a safe temperature again. – UKMonkey Mar 27 '19 at 16:01
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    Depends a lot on the scenario. Vacuum doesn't transmit heat, so it's going to be a lot closer to its last state. So we have a question of if you're inside a contained atmosphere somehow or not. Though, if you're outside any enclosure, dropping something into lunar regolith and immediately putting it on your tongue, you probably have a lot higher concerns than the food content... such as where did your helmet go, and where are you going to get your next breath. If you're in an enclosure, you're left with the question of why there's lunar regolith there and how it's getting indoors. – lilHar Mar 27 '19 at 17:52