Submitted by PHealthy t3_124xb33 in askscience
Seems like almost all instances of water in the galaxy, it is likely salt water but I really ask because I came across this article:
https://scitechdaily.com/alma-discovers-ordinary-table-salt-in-disk-surrounding-massive-star/
that's a lot of salt, yes?
Can't ask for a better reference! Thanks for your insight.
There is something so special about this kind of interaction that gives me hope for the internet.
Thanks for the insights.
"I am the source!"
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Now we need some armchair scientist to tell him he is wrong, citing his own paper.
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.
Sounds like fun, source?
Professor Stanton was the woman I was thinking of! Thanks for posting!
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Thank you for responding!
> very warm and dense, [...] NaCl in the gas phase
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.
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.
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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.
When talking about stars, the cold end is in the low thousands of degrees.
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.
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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.
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!
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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.
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)
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.
You mean to tell me y’all can see molecules in space?
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)
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.
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.
Amazing stuff, thank you for your analysis!
What’s the salt content like on mars (if you know)?
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)
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I love people like you who do things like this. Faith in humanity somewhat restored
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.
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
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Are all salts NaCl? I was under the assumption that salts were a family of compounds, of which NaCl is merely the most common?
Nah metallic salts are the thing. Sodium chloride, potassium chloride, lithium bromide not all good on fries though
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.
Yeah, as others have said, there are other salts. We detected a couple of the most common and familiar ones: NaCl and KCl.
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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?
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.
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?
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
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?
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!
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!
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.
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.
Cheers
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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?
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.
Cool, thanks alot!
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Is salt indicated by absorption spectra in the dust?
I'm not sure salt absorption lines have ever been detected. We detected emission lines from gas-phase salt.
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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?
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.
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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.
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.
So the most common element is hydrogen, followed by helium and so on. Stars are basically fusion reactors that fuse element up untill iron on the periodic table. The Wikipedia page of " Abundance of the chemical elements " will show you how little of the universe is not hydrogen helium. So by mass I'd say it's quite rare for sure, but compared to things like gold or uranium it's far less rare. Most we can do are estimates since it's really hard to find direct evidence on far away planets.
IIRC quasars and supernovae are where you get the heavier elements, right?
Yes! The creation of those elements take energy instead of releasing it.
Thank you! It's fascinating that we have any kind of nontrivial amounts of those elements at all in our grasp, considering their sources.
Somebody may correct me if I’m wrong, but it helps that the stars that go supernovae have comparatively short life spans, so there have been several more “generations” of them. If I recall, the life span of those stars is measured in the millions of years, versus our sun which is projected to have a 10 billion total life span (and is about 4.5 billion years into it).
Edit: and versus the estimated ~14 billion year age of the universe.
You're somewhat correct — there are basically two known generations of stars, and a third hypothesized one.
The very first generation of stars would have lasted millions to tens of millions of years, were very metal-poor (being composed almost exclusively of hydrogen and helium left over from the big bang) and would almost all have gone supernova early on. None are still around today, and there is only scant evidence that they existed at all. Obtaining better evidence for this first generation of stars is one of the primary missions of the James Webb Space Telescope.
The second generation of stars that formed had a middling metallicity, as they formed from material that included the higher-mass elements formed from the first generation of stars. These were lower in mass on average and lasted much longer, hundreds of millions to billions of years.
Our Sun is a third generation star, which was likely formed from the compression of gas by second-generation stars going supernova. Third-generation stars like our Sun are much lower mass and higher metallicity, and have much longer lives on average.
All that being said, we would have obtained a mix of many elements because our Sun (and most second- and third-generation stars) and solar system were almost certainly formed out of gas clouds that had materials from numerous other exploded stars from both the current and previous generation. The second generation of stars was a lot more diverse than the first generation, and the third generation even moreso, so the diversity of elements that we seen in our solar system today comes from many different kinds of exploded stars in the two most recent generations.
Hope that helps!
> The second generation of stars that formed had a middling metallicity, as they formed from material that included the higher-mass elements formed from the first generation of stars.
I’d like to point out for any chemistry enthusiasts not well versed in astronomy. In astronomy, it’s hydrogen, helium, or metal.
Astronomy, the field where Oxygen is a metal, and four orders of magnitude can be a rounding error. Love it.
Out of curiosity, why this divide? Is it just because hydrogen and helium constitute such large part of all matter that it makes no sense to divide the tiny remaining part further?
That's the most important part I think, hydrogen and helium make up 98% of the universe as they were produced immediately after the big bang, all other elements matter much less.
There's also the fact that the chemical behaviour of an element does not matter much at the temperatures encountered in stars - the properties we expect of a metal, for example, actually depend on the atoms being cool enough to stick together. If you heat up iron to 3000°C it stops being a metal and just behaves like any other dense, hot gas. But since hydrogen and helium are so much lighter than other elements they will still have different behaviour at such temperatures (for example, they rise to the surface of a star)
> the properties we expect of a metal, for example, actually depend on the atoms being cool enough to stick together.[...] But since hydrogen and helium are so much lighter than other elements they will still have different behaviour at such temperatures
Hey, that makes sense, thanks for the explanation. I've kinda wondered why they use the terminology myself since I learned it. My specialty is organic chemistry.
Yes, the luminousity of a star (which is a direct consequence of "units of matter fused per second") goes up as greater than the cube of mass, about M^(3.5). That means that even though they contain a lot more fuel, they burn through it far more quickly. So for example, a star with two solar masses has roughly twice as much fuel* as the sun, but it burns around 13 times as fast, so its lifespan is less than one sixth of the sun's, or maybe around 1.5 billion years**
So if you plug in a star with, say, 20 solar masses, all of a sudden, you're looking at a lifespan of a small fraction of a billion years.
* It gets a bit more complicated in that large and medium stars have a radiative zone at the core (high pressure supressing convection) underneath a convective zone at the surface. Small stars, smaller than the sun, are entirely convective, meaning that they can use the fuel from the entire stellar mass. Large stars have smaller convective zones which don't interface with the core, meaning that they can run out of fuel even if there's a substantial amount of hydrogen in the upper layers of the star. This is why using mass to calculate star lifetimes isn't as simple as using the entire star's mass to look at how much fuel will be fused. This is also why red dwarf stars have exceedingly long lifespans.
**These are highly handwavey numbers, don't check me on it, but you get the gist.
This is only true for Type II supernova. Type Ia supernova occur when a white dwarf (created in the death of a star like our sun) siphons enough mass of a companion star.
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> It's fascinating that we have any kind of nontrivial amounts of those elements at all in our grasp, considering their sources.
It's easier to think of it as an extremely large number (number of stars) multiplied by an extremely small number (probability of producing those elements) which rounds out to a reasonably-sized number.
The takeaway is not that the amounts available are nontrivial; rather, it's that we are trivial.
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There are actually some exothermic reactions that produce elements more massive than iron.
However, these are usually very short lived in the time immediately before supernovae and are limited by photodisintegration. They don't meaningfully contribute to the amount of heavier elements (Which are primarily produced during nova).
That's interesting! Thanks for the information.
Mergers of neutron stars are the probable primary source of heavier elements according to recent research.
This is fascinating, and it's remarkable that we have all that many heavier elements, considering all that
Yes it is! I absolutely love this stuff.
Estimates put the current count of neutron stars at one billion in our galaxy and a total of one hundred billion stars total. So around one percent of stars in our galaxy are neutron stars. Most stars are in binary orbits so taken all together it lines up with the distribution quite nicely I think. Plus remember it's by mass so one gold atom counts for as much as 79 hydrogen atoms. If we viewed it by atomic count instead of total mass heavy elements are even rarer than the graph implies.
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.
This is really helpful thank you! So there's not much significant matter expelled from accretion disks?
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)
Oh just saw that you're the author of the referenced paper! Gosh oops that I missed that!
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Congratulations on finding the salty disk!
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What matters is how stars fuse them. Iron, nickel, neon, are more common than chlorine.
Similar to how oxygen is actually the 3rd most common element in the universe.
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I was looking for more than just a Google answer, for relative abundance it would seem there are massive deposits where it is found but yes, absolutely, there is very little NaCl in the universe.
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Isn't "it's somewhere between the most common in the universe and the most rare in the universe" not particularly precise?
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We're not even close to depleting earth's supply. It's just that years ago, it stopped being economical to bother capturing it as it comes up with natural gas. Uranium decay creates the helium deep underground. So when it gets expensive enough, they'll rebuild the capture and separation equipment.
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The important point doesn't change though. We can tap tons of wells of helium that we didn't tap for natural gas because there was too much helium. The price just has to go up to make that profitable. We're not going to run out of helium any time soon.
Qatar is currently producing about 1/3 of the total helium production from its natural gas production. It could produce more if the price was higher.
Helium in natural gas isn't that rare, it's just that the very low prices we've had for years don't make it economical to extract.
Interesting. He's not shitting about the cost of doing by air separation, though, he may actually be underselling it. I used to work in air separation, and various companies have tried going after the rare gasses, like xenon and helium. You're dealing with so few molecules that keeping the in/out flow in the columns stable is nearly impossible.
The news you hear about helium supply is not (or shouldn't be) about extractable amounts on the planet — it's about what's commercially available.
Extraordinary amounts of helium are just discarded during things like natural gas extraction because helium isn't profitable enough for those companies to extract, store, and sell. That means commercial supply goes down and, with constant or increasing demand, price goes up. At some point, it becomes profitable to separate and sell it again, at which point supply increases, price goes down, and the cycle repeats.
This process is being intensified, especially in creating lower price floors, by long-standing selloffs of nationalized helium reserves that were created when we thought dirigibles were the future of warfare.
Helium is effectively a non-renewable resource (decay products are created very slowly and need to accumulate over millions of years to be harvestable, besides) and we will run out of it someday, but that day is still very far off and unlikely to happen in the lifetime of anyone alive barring major life extension advancements (yes, please!).
What we will see is continued boom/bust cycles as reserves are depleted and markets stabilize on current real extraction costs. And it'll likely be a steady increase over time as the long-term depressive effect of stockpile release dwindles.
Helium can be synthesized via nuclear reactions and, in a hypothetical situation where the Earth really "ran out," that's what we'd likely end up doing. It'd just be many orders of magnitude more expensive than today and probably make asteroid capture look very appealing. But that hypothetical day is very far off past much more prominent existential threats.
Well, it's not a uniform distribution. The fact that it's the second most abundant element in the universe doesn't mean it was the second most abundant element on Earth.
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Interesting!
As an aside, where are the other elements beyond Iron coming from if not stars?
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(Have read through the first few chapters of Stars and Their Spectra by James Kaler, so most of what I will say is explained in more detail there.)
On the topic of chemical composition of stars, only cool stars, like M dwarfs or L and T type ultra-cool brown dwarfs can have complex molecules survive in their cores. If the star is any hotter (which is the result of a lot of other variables) only pure elements or hardy molecules with strong bonds can survive that chaotic environment without being ripped apart. For example L-T stars can be identified if their chronosphere contains TiO, which is a) a molecule and b) has titanium, which is heavier than sodium or chloride, so somewhere out there NaCl should exist.
But as far as we know L-T stars along with white dwarves and type 0 stars make up about 5% of stars in the universe, so not a lot of NaCl should exist in stars both in quantity and spatial spread.
Like you said, detecting chemicals on exoplanets is much more challenging, where NaCl can exist in greater quantities, without being disturbed by immense heat and pressure stellar cores have. We just cannot know for sure, but it is fun to think about! (Which is a very non-sciency way of saying we should be looking into this.)
EDIT: After an experienced scientist chimed in I realized my answer didn't take into account the fact that the molecule itself needs to radiate some energy for us to detect it, or it should absorb some energy from the envelope to create noticeable dips in the spectrum. In either case, it will heat up, not so much to break the molecule, but not as cold as L-T stars would require (about a few hundred kelvin at most). I also didn't base my speculation on any real detection, just wanted to chime in since what I learned about seemed to coincide with this topic. Still leaving this up in case anyone wants to take this info and go on their own rabbit hole. Also, just saw JWST detected some silicate dust in a what looks to be a hot jupiter (VHS 1256 b), so exoplanets being challenging to study might already be changing with JWST's observations.
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.
Why is iron more common than lighter elements?
Those are two separate things so we can handle them separately:
NaCl in the galaxy as a molecule.
Nope, by mass and density it's actually going to be pretty far down.
There's a table of the elements by how frequently they get created by stars and you'll find that while their not uncommon they are pretty far down and pretty far apart.
So finding both together is even lower than that.
Now the second question:
Water being salt water.
That one is a bit more tricky to detail but in short:
Salt is really really easy to dissolve. In water.
What that means is that any body with liquid water in it that also contains masses of salt will dissolve on in the other unless luck keeps them separate.
And once it's in it's very very unlikely that chance will separate them again.
If you have elemental chlorine and sodium will it form into NaCl with out needing to be dissolved in water first?
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).
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itz zpectral linez have alzo been found in variouz aztronomical objectz
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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 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.