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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.