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adamginsburg t1_je1r14n wrote

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 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.


icansmellcolors t1_je2jd6b wrote

There is something so special about this kind of interaction that gives me hope for the internet.

Thanks for the insights.


low_altitude_sherpa t1_je3q35h wrote

Now we need some armchair scientist to tell him he is wrong, citing his own paper.


AllHailCapitalism t1_je3unss wrote

Reminds me of the story where a woman has a discussion with some guy at a conference. He mansplains to her that her theory is wrong, and she should really read a recent publication on the subject by X, who is considered a top authority in the field. She then tells the guy to take a look at her badge, because she's X.


I_eat_staplers t1_je1zuhx wrote

> very warm and dense, [...] NaCl in the gas phase

  • Sodium chloride/Boiling point: 2,669°F (1,465°C)

What kind of scale are you used to that this qualifies as "warm"?!

Fantastic discovery and response! This is incredibly interesting and certainly opens up a lot more questions.


adamginsburg t1_je20qy1 wrote

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 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.


Metaphoricalsimile t1_je69847 wrote

I wonder if someone could set up an experiment to expose NaCl to an alpha flux (so simulated solar wind) and see how it changes the sublimation rate even at lower temperatures.


scutiger- t1_je20ryr wrote

When talking about stars, the cold end is in the low thousands of degrees.


Welpe t1_je3mau9 wrote

Scales get weird in physics. A week or two ago I was explaining just how cold helium needs to be to display superfluidity and ended up describing something like 25K as “balmy”, which it is compared to 2.1K or whatever it was for helium.


adamginsburg t1_je20zq7 wrote

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.


InterestedListener t1_je3n20j wrote

I just want to say you are incredibly smart and I really enjoy reading your explanations even though a lot of it is over my head. Thank you for sharing so much!


seriousnotshirley t1_je20uni wrote

For some reason I thought HEXOS had identified NaCL in Orion about 15 years ago but it looks like it's not coming up.

I'm curious what the temps were for NaCl close to stars and what the spectra looks like. I can't imagine identifying anything at higher temps.


adamginsburg t1_je21h3n wrote

Huh, I hadn't heard of that, but that's super interesting if so. I don't see any mention of NaCl in 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)


seriousnotshirley t1_je2h9rb wrote

It was 15 years ago so I probably mis-remembered it. My chem professor did a lot of rotational spectroscopy and had invited someone from HEXOS to give a presentation.

I know the stuff I looked at at room temp looked like a bunch of noise. I tried writing some algorithms to help fix parameters of the molecule to match observed spectra and it went badly above something like 50 K. I'm surprised you were able to pick out transitions near a star! Nice work.


Gutsy_Bottle t1_je2wt7t wrote

You mean to tell me y’all can see molecules in space?


platoprime t1_je3oiud wrote

It is absolutely insane the amount of information that can be extracted from the "color" of incoming light. They're talking about trying to see the light from a distant star but not just any light. Specifically they are looking for light from that distant star when it passes through the atmosphere of a planet orbiting that star. The difference between that light and the light of the star can tell you about the chemical composition of the planet's atmosphere.


Hoihe t1_je49fpt wrote

Look up Dr. György Tarczay or Astrochemistry in general!

There is a whole field of science about studying molecules in space and proving they exist by replicating astronomic conditions in a lab.

I got to tour Dr. Tarczay's lab and it was super impressice. Look up Tarczay VIZSLA for an article about his latest piece of equipment.


wildfyr t1_je2gcpb wrote

Amazing stuff, thank you for your analysis!


ULMmmMMMm t1_je2r3i8 wrote

What’s the salt content like on mars (if you know)?


notoriousbsr t1_je3201e wrote

I love people like you who do things like this. Faith in humanity somewhat restored


montyy123 t1_je39jum wrote

For the not astrophysicist: most of the time NaCl is solid of a liquid in our realm. It’s so interesting that most of the time, most of the places it’s in gas (I assume actually plasma?) form.


adamginsburg t1_je3aypv wrote

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


MurkyPerspective767 t1_je3vfmy wrote

Are all salts NaCl? I was under the assumption that salts were a family of compounds, of which NaCl is merely the most common?


Strawbuddy t1_je41kcb wrote

Nah metallic salts are the thing. Sodium chloride, potassium chloride, lithium bromide not all good on fries though


ScienceIsSexy420 t1_je4ng2c wrote

You are correct. A salt is an ionic compound (and technically not a molecule at all), and there are many salts other that sodium chloride: potassium chloride, potassium hydroxide, sodium hydroxide, calcium carbonate, etc.


adamginsburg t1_je4q4fn wrote

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


robirahman t1_je3gp6f wrote

What is the structure of NaCl gas? If you heat a crystal of solid NaCl at low pressure, does it sublimate into pairs of atoms?


adamginsburg t1_je4o8a8 wrote

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.


imyourzer0 t1_je3zxzb wrote

I recall reading elsewhere that the elements themselves (not compounds, per se) are distributed throughout the observable universe according to Zipf’s law (or something like it), so that they get less common as the atomic number increases. So, would it be a reasonable extrapolation to estimate compounds’ prevalence, then, by the chemical reactivities of their forming elements?


adamginsburg t1_je4qfp5 wrote

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


imyourzer0 t1_je4sktv wrote

Ah right! I think I mushed two youtube videos from one channel together in my memory. The second part of my question was really what I was interested in, though. Given some distribution of the elements across the universe, can we estimate the prevalence of the compounds they form, based on the elements' reactivities? For instance, this would predict that hydrocarbons should be common, since hydrogen is extremely prevalent and carbon is extremely chemically reactive?


adamginsburg t1_je53yx7 wrote

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!


imyourzer0 t1_je61740 wrote

I certainly don't know this bit, but I would assume that more complex molecules (which from what you're saying we know less about) are exponentially less likely. I say this mostly because the probability of finding element 1 and element 2 at some point in the universe is certainly less than the probability of finding just 1 or just 2. So, once you've dealt with all the combinations of two or three, whatever's left is unlikely to severely tilt the scales, unless that numbers game really reverses under some conditions. But, I take your point that if we can't describe larger molecules well, it's hard to say whether something more has its finger on the scale. Thanks for the answers!


adamginsburg t1_je66y45 wrote

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.


imyourzer0 t1_je68is5 wrote

That’s pretty wild! Like we’ve all heard from Sagan that we’re made of star dust, but I never really thought I ought to be emphasizing the dust part much!

I’ve got a whole bunch more questions, but I’ll spare you—you’ve been more than patient enough.



Frooper t1_je4f12i wrote

Absolutely fascinating, thanks for commenting. One thing: NaCl is not a molecule, right? There is not a single NaCl molecule on earth, just the ionic compound. Or is this different in space?


adamginsburg t1_je4rmd8 wrote

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.


Admetus t1_je4iasg wrote

Is salt indicated by absorption spectra in the dust?


adamginsburg t1_je4rrxp wrote

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


Lucretius t1_je5155n wrote


I recognize that this is not NA+ and Cl- related, but it does have to do with the relative abundance of low atomic number elements, so I was wondering if you would be willing to weigh in on it.

I friend of mine and I are trying to guess what the long-term potential for various forms of space settlement and colonization are across all conceivable intelligent species... a sort of: These are the universal ground rules kind of list. For that reason, we've been focusing on energy sources on the thinking that regardless of the exact nature or needs of the intelligent species, they will need energy sources to engage in whatever their civilization does.

To that end, we have a disagreement on the viability of proton-boron fusion as a sustainable form of energy with particular emphasis on small icy bodies on the outskirts of solar systems (Kuiper belt and Oort cloud bodies). The disagreement is concerning the relative abundance of Boron. As you know, Boron is, like Beryllium, mostly NOT formed in stars or left over from the big bang, but rather formed from Lithium and cosmic rays. I've been arguing that because stellar magnetic fields partially protect objects inside them from cosmic rays, we should, if anything, see MORE Boron in small icy bodies that spend all or most of their time outside stellar magnetic fields, and that therefore there should be more than enough boron to sustain a proton-boron-fusion based civilization in the outskirts of a solar system without ever needing to actually approach a star.

Am I right? Do we have any way of knowing how much boron is in such small icy bodies?


adamginsburg t1_je53c6u wrote

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 (, 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.


Aggravating_Paint_44 t1_je3nw38 wrote

Would it be fair to say that you see it where you can and at rates you’d expect but don’t want to extrapolate because of the lack of direct evidence.


adamginsburg t1_je4p90w wrote

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 ( - 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.