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Today, it was officially announced that astronomers have detected phosphine on Venus via the $\text{PH}_3(0\to1)$ transition (Greaves et al 2020). While the line was found by both the James Clerk Maxwell Telescope and ALMA, and while the team is fairly confident that the detection is robust, follow-up observations would nevertheless be nice, particularly in other bands. Sousa-Silva et al 2020 note that phosphine has strong features in the 2.7-3.6, 4.0-4.8, and 7.8-11.5 micron bands, and while they're pessimistic about detecting phosphine in $\text{CO}_2$-dominated atmospheres around Sun-like stars using less than ~200 hours of observing time, those numbers are for exoplanets, where we'd expect substantially lower fluxes than we'd receive from Venus.

With all that in mind, what are the most promising bands to search for phosphine on Venus, in addition to to the $1\to0$ transition? Are they the three infrared bands discussed by Sousa-Silva et al, or could other trace components of the Venusian atmosphere block the signal at some wavelengths? I see that $\text{SO}_2$ was the only remotely feasible possibility for a source mimicking the observed phosphine transition, but that would require temperatures twice as high as observed.


As a side note, it's been announced that BepiColombo will use an onboard spectrometer to try to detect phosphine on Venus during two flybys of the planet en route to Mercury. The first will be on October 15, 2020, and the second will be on August 10, 2021. I haven't been able to find out more details on the planned observations, but the spectrometer (MERTIS) operates in the 7-14 micron band.

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  • $\begingroup$ Not good enough to be an answer, so a comment. I suspect this might change priorities at NASA and ESA with regard to Venus exploration. Might. Venus exploration has been downplayed for decades compared to Mars exploration, mainly due to the utter hostility of conditions at Venus's surface. $\endgroup$ – David Hammen Sep 15 at 6:28
  • $\begingroup$ "It is therfore fortunate that the PH3 molecule has two strong observable rotational transitions in the millimeter-submillimeter portion of the spectrum (J=1-0 at 267 Ghz and J=3-2 at 800 Ghz). From the Introduction of this thesis: ericweisstein.com/research/thesis/node67.html With thanks to @uhoh for revealing this to us. $\endgroup$ – Cornelisinspace Sep 16 at 10:41
  • $\begingroup$ @Cornelisinspace Yeah, the first one is the transition that was observed by Greaves et al.; I wasn't aware of the second one. $\endgroup$ – HDE 226868 Sep 16 at 15:13
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enter image description here

This figure is from the paper "Phosphine as a bio signature gas in exoplanet atmospheres". It shows the absorption cross section of Phosphine compared to other molecules. We can see that Phosphine has a distinct enough profile from the others molecules in the 7.8-11.5 microns range, with the exception of NH3. Probing from 2-11.5 microns should then allow to distinguish Phosphine from other molecules. Simulations seem to indicate that current state of the art nIR ground-based spectrometers could detect phosphine to a minimum of a few ppb, so expect a lot of follow-up observations from the ground in the coming months.

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  • $\begingroup$ I appreciate the answer, but this is from the second paper I mentioned, which I'd already discussed - and bear in mind that this is only a comparison with six other molecules, and not necessarily representative of the mixture represented on Venus. Plus, the detected amounts were of order 20 ppb, not near the ppm range. $\endgroup$ – HDE 226868 Oct 2 at 13:17
  • $\begingroup$ Sorry, I meant ppb. I edited my answer. The plot that I showed wasn't meant to be exhaustive, just to show that phosphine has a relatively distinct cross section. This isn't the case for say CO or CO2, which are hard to decouple in the nIR. If you simulate the atmosphere of Venus with its known composition and add in some phosphine, the resulting spectrum are distinct enough to be separated in cross-correlaton. $\endgroup$ – Shamaz Oct 2 at 14:02
  • $\begingroup$ Ah, thanks - ppb makes the outlook a lot brighter. $\endgroup$ – HDE 226868 Oct 2 at 17:21

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