# What are the prospects for follow-up observations of phosphine on Venus?

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.

• 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. Sep 15 '20 at 6:28
• "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. Sep 16 '20 at 10:41
• @Cornelisinspace Yeah, the first one is the transition that was observed by Greaves et al.; I wasn't aware of the second one. Sep 16 '20 at 15:13
• Relevant paper placing an upper limit on phosphine abundance based other spectral wavelengths (doi.org/10.1051/0004-6361/202039559), and relevant preprint stating that the detection by Greaves et al. (2020) is a false positive (arxiv.org/abs/2010.09761). Oct 26 '20 at 16:12