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For the purposes of this question, let's assume that the team really have detected phosphine. My question is how do we know that phosphine is on Venus, and not closer to home?

I've just found out a bit more about this detection. It was detected on 2 separate ground-based telescopes at different times, and was detected via the absorption spectra.

There is a lot of atmosphere between those telescopes and the initial light source, both on Venus and here on Earth. So how do we know that the absorption took place in Venus' atmosphere and not our own (or perhaps even somewhere in between, although this seems less likely)?

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    $\begingroup$ And is the phosphine detected when you look at a star? $\endgroup$
    – ProfRob
    Commented Sep 15, 2020 at 20:24
  • $\begingroup$ Note that the findings are being questioned. arxiv.org/abs/2010.09761 $\endgroup$
    – ProfRob
    Commented Oct 27, 2020 at 19:19

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Essentially what they did was assume that normally when observing with their telescope the spectral absorptions they see are due to the Earth's atmosphere. Which is a pretty good assumption. They then normalize the data to those absorption, so if there was any phosphine gas within the package of atmosphere they are looking through, it will be taken into account. Thus, any phosphine signature detected after this normalization is done is because it is present in the Venusian atmosphere.

Here is also a pretty good article from 2010 about how astronomers use the same technique to characterize exoplanet atmospheres, which is a lot more difficult than looking at Venus.

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On the sky at night programme on bbc 4 yesterday, the scientists explained how they were able to detect different levels of phosphine at the equator vs the poles of Venus.

For me this indicates that the gas isn’t being detected locally

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