Less likely, yes. Exponentially less, no. You'd be roughly right if molecule formation was just a matter of random chance associations, which is true in the diffuse ISM, but it is not true in dense clouds where molecules form. A large fraction of all molecules form in clouds that get cold enough that the molecules stick to the surfaces of solid (dust) particles. Once they're there, they're in rich company: there's hydrogen, carbon, oxygen, etc. in abundance - and then more "normal" chemical processes (i.e., things you might find happening on Earth) start to take over. So yes, the numbers game starts to reverse pretty hard!

Purely from gas-phase processes, though, you're basically right; we expect that most molecules with >5 atoms rarely form in the gas phase. We usually draw the line at methanol, CH_3OH, which is a bottleneck in the formation of more complex molecules.

Yes, but. Reactivities depend greatly on physical conditions. In a cloud that's 100K, the molecular composition will be totally different than a cloud that's 1000K or one that's 10K. To predict which molecules form, you need a good census of how much gas is in each phase. We can actually tell that pretty well in galaxies by looking at various molecular and atomic emission lines.

The limit is actually in our knowledge of the chemistry, though. While we have good models to predict simple molecules, like CO, CO2, H2O, etc., we have a hard time with more complex molecules because the chemical reaction networks get very complicated, and in many many cases, the reaction coefficients are unknown. For example, our knowledge of Sulfur-containing molecular chemistry is very poor - there are too many reactions that haven't been measured in the lab, so we don't know what to predict. There is a lot of work left to be done in astrochemistry!

In short, I don't know - it's beyond my expertise. I'm not sure we have any way to measure boron; it's not (afaik) commonly detected in stellar atmospheres. I haven't checked the molecule lists (https://www.reddit.com/r/askscience/comments/124xb33/is_nacl_relatively_common_in_the_galaxyuniverse/je2x7n8/), but I'm not aware of any boron-containing molecules either. Your arguments sound plausible, but I'm afraid I can't weigh in on the argument.

I'm not sure salt absorption lines have ever been detected. We detected emission lines from gas-phase salt.

NaCl, as a single pairing of one Na atom and one Cl atom, is a molecule. But I think you're right, we consider crystalline ionic compounds to be ionic compounds, not molecules, when they're solids. Probably there are some isolated NaCl molecules on Earth, but you're right that when we encounter salt, it's mostly in crystals.

However, in gas phase, it floats around as NaCl. If you heated NaCl hot enough in a lab at atmospheric pressure (~1500 K according to another poster), you would have a bunch of NaCl gas floating around.

Zipf's law is a continuous power law distribution; while it's a good approximation when we don't know much, and therefore describes a ton of nature to the accuracy that we can measure it, it's not the best we can do with elemental abundances. Elements do something funnier; see the figures on https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements

Yeah, as others have said, there are other salts. We detected a couple of the most common and familiar ones: NaCl and KCl.

Well, it's a bit worse than that. We don't really know what to expect. We can estimate how much NaCl there is based on how much Na and how much Cl there is - we can measure those directly from stars, or specifically the sun (https://ui.adsabs.harvard.edu/abs/2009ARA%26A..47..481A/abstract) - but then we have to guess at how much of each of those atoms is in NaCl. Some Na is in other molecules (e.g., NaOH), and some Cl is in other molecules (like HCl). It might even be integrated into more complex molecules or integrated into crystalline structures (I don't know much about solid state materials; this is someone else's domain).

But, generally, you're right: we have no direct evidence as to where NaCl is, so I wouldn't claim to know. It is possible that there's a ton of NaCl sitting on dust grains, undetectable, but it is also possible that there's virtually no NaCl in dust, and it only exists where we see it. Our best bet, based on what we know of chemistry from lab work, is that Na and Cl are in NaCl on dust grains, but we have never measured that, as far as I'm aware. It's possible there are measurements from, say, the stardust mission, but I haven't seen those results.

It has to stay stuck together as a molecule, as NaCl bonded together, to be NaCl gas, otherwise it's a mix of atomic Na+ gas and atomic Cl-. That's probably how it comes out of the dying AGB stars. Gas doesn't have structure, though. It just fills whatever vessel it's in. If that's the ISM, it just spreads out until it's pressed on by something else.

we actually only encounter salt as a solid most of the time. when salt dissolves in water, it is part of the liquid, but it's not liquid salt exactly - that would be molten salt, and i think it requires much higher temperatures than we see on earth.

the gas phase nacl we detected in orion is just gas, not plasma - the nacl is not ionized. when we see nacl, it is as a gas, but we think that most nacl in space is solid. it's integrated into the dust particles that pervade space, and on those particles, it is solid.

we do detect na and cl on their own in elemental form in gas too. when there's enough ultraviolet radiation around, the nacl gets dissociated (split) into its constituent atoms. we see this in the diffuse interstellar medium, ie, not close to any particular stars

Yep. There are lists of the molecules we've detected: https://en.wikipedia.org/wiki/List_of_interstellar_and_circumstellar_molecules, https://www.astrochymist.org/, https://arxiv.org/abs/2109.13848 (last is a regularly updated, curated, peer reviewed list maintained by Brett McGuire)

I don't know, sorry. My strategy to learn about that would be to search NASA's ADS, though: https://ui.adsabs.harvard.edu/search/filter_database_fq_database=AND&filter_database_fq_database=database%3A%22astronomy%22&filter_property_fq_property=AND&filter_property_fq_property=property%3A%22refereed%22&fq=%7B!type%3Daqp%20v%3D%24fq_database%7D&fq=%7B!type%3Daqp%20v%3D%24fq_property%7D&fq_database=(database%3A%22astronomy%22)&fq_property=(property%3A%22refereed%22)&p_=0&q=martian%20salt&sort=date%20desc%2C%20bibcode%20desc (you don't need all those filters, but they help a bit)

I think you're on the right track that L/T/Y dwarves (brown dwarves) should have cool enough atmospheres to have NaCl in them. I don't know what references to go to say for sure, though.

One of the problems isn't just that the salt molecules need to be warm to emit (that's true), but that the wavelengths at which we see their radiation are tough to observe in stars and planets. The detection we reported was in a disk - which is very, very big compared to a star or planet, and so we could see it at radio/millimeter wavelengths. We generally can only detect stars themselves at optical and infrared wavelengths, and it turns out that NaCl and KCl don't have many transitions at wavelengths we usually observe (e.g., https://ui.adsabs.harvard.edu/abs/2014MNRAS.442.1821B/abstract). Most of their strong emission/absorption lines are at >=26 microns, which is just at the edge of what JWST is capable of observing with its MIRI instrument. No other telescope has observed at these wavelengths with enough sensitivity to pick up salt molecules. I think there's some possibility JWST will detect salts in either hot jupiters or brown dwarves, though; there are weaker salt lines covering JWST's whole range. The trick is, there are lots of other molecules that could obscure the salts in an atmosphere - I'm not sure whether we'll be able to identify the molecules cleanly. It's a much easier job at radio wavelengths.

Evidently yes - it spontaneously forms in gas just after that gas is expelled from stellar atmospheres (e.g., https://ui.adsabs.harvard.edu/abs/2023arXiv230206221C/abstract).

Huh, I hadn't heard of that, but that's super interesting if so. I don't see any mention of NaCl in https://ui.adsabs.harvard.edu/abs/2014ApJ...787..112C/abstract or the other HEXOS papers, but I'm just doing a ctrl-f, so it's possible I missed it (if they specified the isotopologue, for example)

Someone had asked a question about "Don't we detect salt in the Orion Nebula with microwave radiation", then deleted it - I had already written an answer, so I'll share:

Sort of. The article OP linked is talking about NaCl detected with millimeter-wave spectroscopy in the disk around a star that is immediately behind M42 (the Orion Nebula). Since they're along the line of sight, we often say that this object (Orion Source I) is "in" the nebula, but we have pretty good evidence that it's actually behind the nebula. The nebula itself is made of ionized (very hot, ~10,000 K) gas; Source I sits in the Orion Molecular Cloud, which is much cooler (~few hundred K; still warm by molecular standards).

I'm not aware of any microwave detections of NaCl toward the Orion Nebula itself, but I have an observing program ongoing that should pick it up if it's there. Maybe.

That's the boiling point at atmospheric pressure. The NaCl we observed is likely not that hot - probably only ~100K but maybe 1000K (fig 6 of https://ui.adsabs.harvard.edu/abs/2019ApJ...872...54G/abstract shows that there's some ambiguity - we measure two temperatures and are not sure how to reconcile them!). We observe NaCl in an effective vacuum, so the boiling point (more likely sublimation point) is much lower. That said, it's possible that non-thermal mechanisms are responsible for releasing the NaCl into the gas phase - in other words, the gas isn't at the boiling point, but something knocked the NaCl molecules off the dust grains. Another possibility is that individual dust grains got very hot briefly, hot enough to vaporize, but again the gas isn't all that hot. We don't know for sure; we haven't yet come up with a consistent model to explain all the observations.

Just to give you a sense of boiling points: water transitions to gas at 373 K at atmospheric pressure. In the interstellar medium, it sublimates at closer to 100-150K.

There actually is a decent amount expelled in gigantic jets, but the jets from quasars are relativistic (i.e., travel at a significant fraction of the speed of light) and escape the galaxy. Google "radio galaxies" and look at those images: they show jets shooting to megaparsec size scales (i.e., 10-100x bigger than galaxies), so that material totally escapes the galaxy.

That said, there is probably some material from quasars that gets mixed back into the galaxy - I think not that much, but honestly there's a lot unknown about gas cycling in the vicinity of rapidly accreting black holes. Nevertheless, even if all the accretion disk material got fed back into the galaxy, it would represent a truly tiny fraction of the galaxy's mass, much less than the material made by supernovae (our black hole is 10^6 solar masses, our galaxy is ~10^12 solar masses, of which ~10^11 is baryonic - so the black hole is a tiny fraction of the galaxy, and the accretion disk is a tiny fraction of that. my numbers here are super rough)

Just a quick two cents here: supernovae, yes, but not quasars. Quasars are accreting black holes, and while there might be some production of heavy elements in their accretion disks, those elements likely do not get returned to the surrounding galaxy to form new stars. Besides supernovae, neutron star mergers (which another poster already noted) may also produce significant heavy elements, and AGB stars also produce some of the moderately-heavy elements - but with quite a different distribution. Cartoons like this one https://svs.gsfc.nasa.gov/13873 give a good summary of which routes are responsible for making each.

As the author of the referenced paper: I actually still don't know how common salt is in the universe. Another poster noted the relative abundance of Na and Cl - we have a pretty good sense of how much of each of these elements are out there. But we can only see NaCl, the molecule, in special locations: the disks around high-mass stars (see also https://ui.adsabs.harvard.edu/#abs/2023ApJ...942...66G/abstract) and the dissipating envelopes of dying medium-mass stars (Asymptotic Giant Branch, AGB, stars). Otherwise, we think NaCl is present, but it is probably in the solid phase and doesn't produce any easily-observable radiation. When it's in the solid phase, it is part of dust grains, and I don't think we know exactly what it does in the dust (e.g., is it mixed with water in crystals? or stuck in some silicates? or something else?).

High-mass young star disks and AGB stars are unique in being very warm and dense, which are the conditions needed to have NaCl in the gas phase and able to produce observable millimeter-wavelength radiation. We might see it in one other place, in a hot molecular cloud, but that detection is not confirmed.

There are some other cool features of molecular salt: there might be salt clouds in hot jupiters, since salt can form at higher temperatures.