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Sufficient phosphorous acid to be decomposed in the measured quantity of phosphine ?

From Phosphine gas in the cloud decks of Venus:

We also rule out the formation of phosphorous acid ($\ce{H3PO3}$). While phosphorous acid can disproportionate to $\ce{PH3}$ on heating, its formation under Venus temperatures and pressures would require quite unrealistic conditions, such as an atmosphere composed almost entirely of hydrogen.

But according to Vega mission results and chemical composition of Venusian clouds there is $\ce{P4O6}$ in the atmosphere which according to Wikipedia would react with water to form phosphorous acid.

However there is supplementary information in the chapter "Equilibrium thermodynamics in the atmosphere and surface" at the end of the article "Phosphine gas in the cloud decks of Venus":

As an example of our approach, we present a calculation for phosphorous acid ($\ce{H3PO3}$). This compound will spontaneously decompose on heating to form phosphoric acid and phosphine; this is a standard laboratory method for making phosphine. Phosphorous acid is not stable in gas phase, but could in principle be formed in cloud droplets by reduction of phosphoric acid.

Edit for some additional informatiom.

Also from the supplementary information (page 13):

Reactions of $\ce{P4O6}$, $\ce{P4O10}$, $\ce{H3PO4}$ and $\ce{H3PO3}$ were considered (the last of these only in solution phase in the clouds),..

So not considered was the possible occurence of $\ce{H3PO3}$ below the clouds.

The Recent Evolution of Climate on Venus (page 23) states:

The evaporation of $\ce{H2SO4}$ occurs at about 48 km, the average cloud base.The vapor phase continues to exist down to 432 K (38 km), where it is thermally decomposed.

So below 38 km, and above 160 ⁰ C there would be water in the gas phase, free from sulfuric acid, that could react with $\ce{P4O6}$.

Could have that possibility been ruled out for some reason ?

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    $\begingroup$ You bring up an interesting point. Unfortunately this matter is heavily reliant on quantitative modelling of reaction kinetics in a very unusual environment, so I'm not sure any of us here have the capacity and resources to answer this satisfactorily. As Maurice mentioned, you'd pretty much have to ask the authors directly. $\endgroup$
    – Nicolau Saker Neto
    Commented Sep 17, 2020 at 1:09
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    $\begingroup$ Even if there is $\ce{P4O6}$ in Venusian atmosphere, there is always no water vapor(only 20 ppm). This is not enough to make even a significant of phosphorous acid. $\endgroup$
    – Nilay Ghosh
    Commented Sep 17, 2020 at 3:58
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    $\begingroup$ This is a follow up question based on my above comment. $\endgroup$
    – Nilay Ghosh
    Commented Sep 17, 2020 at 5:14
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    $\begingroup$ Speaking of asking the authors directly, you can try this thread on Reddit's AskScience. $\endgroup$
    – Nicolau Saker Neto
    Commented Sep 17, 2020 at 5:46
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    $\begingroup$ @BuckThorn That is an intriguing question "Since there isn't enough water, how is phosphorous acid formation possible?" $\endgroup$
    – Nilay Ghosh
    Commented Sep 17, 2020 at 8:48

1 Answer 1

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The issue seems to be not the reaction between $\ce{P4O6}$ and water, but whether $\ce{P(III)}$ can form at all. The OP's first reference says no whereas the second reference says yes. Basically the two references used different thermodynamic assumptions. The first one assumes an atmosphere in equilibrium while the second assumes only a partial equilibrium; in the latter case $\ce{CO}$ is allowed to form from nonequilibrium (radiation-driven) dissociation of $\ce{CO2}$, and minor species are then equilibrated with the $\ce{CO/CO2}$ couple. In the case of phosphorous oxides this favors $\ce{P4O6}$ over $\ce{P4O_{10}}$, although the former could be oxidized by other species such as sulfuric acid.

The issue has become important because the recent discovery of $\ce{PH3}$ among the clouds of Venus has led to a hypothesis that this compound is generated by Venusian microbes, but if $\ce{P(III)}$ is available for a disproportionation reaction it could provide an alternative abiotic (non-life) source. As noted above, the references disagree on this, and the question of whether this alternative to a biological source of $\ce{PH3}$ is viable awaits resolution.

Relevant to the question of whether $\ce{PH3}$ is a true biosignature is this question about organic compounds on Venus, which apparently spawned the current one.

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    $\begingroup$ Your answer proves that all the comments of the "nobody here can answer this" variety under the question are "premature" in the Mark Twain sense :-) $\endgroup$
    – uhoh
    Commented Sep 17, 2020 at 6:07
  • $\begingroup$ "..this favors P4O6 over P4O10, although the former could be oxidized by other species such as sulfuric acid." How does this correspond to the forming of phosphorous acid in cloud droplets by reduction of phosphoric acid ? $\endgroup$
    – Cornelis
    Commented Sep 17, 2020 at 14:58
  • $\begingroup$ We have a three-way competition between reduction of the phosphorus from +5 to +3 (by CO), oxidation back to +5, and disproportionation of the +3 state to give phosphine and maybe other low oxidation state prodtcts. We need more complex modeling and better experimental knowledge to sort this all out, hence the question "awaits resolution". $\endgroup$
    – Oscar Lanzi
    Commented Sep 17, 2020 at 15:38
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    $\begingroup$ Made an edit to make it more clear that there is steam below the clouds , free from sulfuric acid that could react with P4O6. $\endgroup$
    – Cornelis
    Commented Sep 19, 2020 at 17:35
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    $\begingroup$ I would like to note that the formation of CO by photochemical reaction of CO2 occurs near the cloud top. And because of the Hadley circulation on Venus near the equator SO2 is transported upwards from below the cloud deck that needs O for SO3 and could favor P4O6 over P4O10. $\endgroup$
    – Cornelis
    Commented Sep 21, 2020 at 9:18

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