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Can the density of molecular hydrogen in the solar system be estimated remotely or would a space craft have to be sent up to measure it directly?

I understand that there is such a thing as the 21-cm hydrogen line emitted by neutral hydrogen atoms. Would hydrogen molecules H$_2$ emit 21-cm radiation?

I ask this question as I am wondering if some fraction of the dark matter is actually just H$_2$.

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  • $\begingroup$ Solar Lyman $\alpha$ scattering has played a big role in measuring H$_2$ concentrations throughout the solar system, but I've seen quite a range of results for molecular neutral hydrogen density. There's certainly a difference in density as you move away from the Sun, but just how big that change is seems to vary. Be careful; the density of neutral hydrogen in the interstellar medium is quite different from that in the interplanetary medium. I'm pretty certain, though, that neutral hydrogen has been ruled out as a potential dark matter candidate. $\endgroup$
    – HDE 226868
    Commented Sep 14, 2016 at 17:54
  • $\begingroup$ I understand that Voyager 1 has just recently reached the edge of the heliosphere. I don't know if it can now measure the concentrations of H$_2$ in the interstellar medium. $\endgroup$
    – John Eastmond
    Commented Sep 14, 2016 at 19:29
  • $\begingroup$ @JohnEastmond I am sure you have heard all about the pioneer 10 "anolomies"......ok, they got that sorted out, but that same idea might put (maybe an unreasonably high value upper bound), depending on the velocity measuring capabilities, on the amount of molecular hydrogen in that region. Or if it's plasma, then possibly on the signal strength from the probe, if anybody is still listening to it..... $\endgroup$
    – user108787
    Commented Sep 14, 2016 at 20:09

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Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is carbon monoxide(CO). The ratio between CO luminosity and H2 mass is thought to be constant.

My guess it's very low, because it's either been vacuumed up by the giant planets or blown away by the solar wind.

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$H_2$ plasma emissions, so it may not be what you are looking for.

If $H_2$ was part of dark matter, it would be easily spotted by now.

I hope you get a proper answer that I can learn from too.

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