Today, it was announced that phosphine has been found in Venus' atmosphere, rekindling the discussion about the possibility of extraterrestrial life. Venus has been visited by space probes for about 50 years now; how come none of these detected the presence of this gas? Much data was gathered about the composition of Venus' atmosphere, but not the presence of this molecule. Were we simply not looking for it, or were the instruments back then not precise enough to detect the amount of phosphine?

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    $\begingroup$ I think you might be overestimating the number of Venusian missions & underestimating the density of the atmosphere... $\endgroup$ Commented Sep 14, 2020 at 18:29
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    $\begingroup$ And who would think to look for phosphine there anyway? Although a case may be made for it (phosphorus compounds are known in the Venusian atmosphere and so is the similarly reduced hydrogen sulfide), it would not really he intuituve. $\endgroup$ Commented Sep 14, 2020 at 19:48

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From the recently published paper Phosphine gas in the cloud decks of Venus.

The presence of PH3 implies an atmospheric, surface or subsurface source of phosphorus, or delivery from interplanetary space. The only measured values of atmospheric phosphorus on Venus come from Vega descent probes, which were only sensitive to phosphorus as an element, so its chemical speciation is not known. No phosphorus species have been reported at the planetary surface.

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    $\begingroup$ Your linked paper says "PH3 is found elsewhere in the Solar System only in the reducing atmospheres of giant planets (12, 13)" and the first reference discusses FTIR around a wavelength of 4–5 μm. You don't need to drop a probe to sample the atmosphere to detect phosphine as an element, just do spectroscopy. So the question is really why spectroscopic results were possible for Jupiter and Saturn but not for Venus, until now. $\endgroup$
    – uhoh
    Commented Sep 15, 2020 at 0:42
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    $\begingroup$ @uhoh there could be the issue with interference. I have read they still have to verify phosphine is not really SO2. Would not be a problem on the giant planets! $\endgroup$ Commented Sep 15, 2020 at 1:48
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    $\begingroup$ @OscarLanzi I heard from the YouTube channel Cool Worlds that they detected PH3 with a sigma of 15, which would indicate a very strong likelihood it was detected correctly. $\endgroup$
    – Drake P
    Commented Sep 15, 2020 at 6:28
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    $\begingroup$ @Cornelisinspace oh that's interesting! Those are millimeter waves, basically radio astronomy. The JCMT is a 15 meter radio dish, and 1.123 mm corresponds to about 267 GHz. See Spectroscopic parameters of phosphine, PH3, in its ground vibrational state for some review of studies of several planets for PH3 using several different techniques. Band 6 of ALMA covers 1.1 to 1.4 mm so there might even be higher resolution images of Venus, but I don't know. $\endgroup$
    – uhoh
    Commented Sep 15, 2020 at 14:26
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    $\begingroup$ @Cornelisinspace I just saw this: ericweisstein.com/research/thesis It's an online version of Eric Weisstein's thesis!!! Here are the chapters related to Jupiter and Saturn specific to PH3: ericweisstein.com/research/thesis/node66.html $\endgroup$
    – uhoh
    Commented Sep 15, 2020 at 14:39

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