Phosphine Gas in the Cloud Decks of Venus

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A beautiful paper co-authored by @Paul.B.Rimmer just came out in Nature Astronomy. This article may be a landmark in the field, perhaps becoming the first validated indication of life on another planet.

https://www.nature.com/articles/s41550-020-1174-4

The paper’s conclusions are appropriately measured:

Even if confirmed, we emphasize that the detection of PH3 is not robust evidence for life, only for anomalous and unexplained chemistry. There are substantial conceptual problems for the idea of life in Venus’s clouds—the environment is extremely dehydrating as well as hyperacidic. However, we have ruled out many chemical routes to PH3, with the most likely ones falling short by four to eight orders of magnitude (Extended Data Fig. 10). To further discriminate between unknown photochemical and/or geological processes as the source of Venusian PH3, or to determine whether there is life in the clouds of Venus, substantial modelling and experimentation will be important. Ultimately, a solution could come from revisiting Venus for in situ measurements or aerosol return.

It is worth reading in depth. An excellent summary of the finding and its meaning can be found here:

https://www.nationalgeographic.com/science/2020/09/possible-sign-of-life-found-on-venus-phosphine-gas/

See my coverage, in A Debut Article on Panda's Thumb :

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I’ll watch this post in case people have questions.

My main contribution to the paper was the atmospheric chemistry.

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I have some questions for you @Paul.B.Rimmer.

  1. In your view, how should this finding impact the larger conversation about origins?

  2. What are they key points you think are most likely to be overlooked in the reporting of this paper?

  3. What findings or component of the paper is the most exciting?

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Congratulations @Paul.B.Rimmer!

I think this is both incredibly interesting and seriously overblown in the media at the same time. But then, if this turns out to be life, then it’s the story of the century!

XKCD has a good commentary on the situation.

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Not at all, at present. There’s too much we just don’t know.

The first step is to make sure the molecule we see is phosphine and not some unknown molecule that is producing the feature, how much there really is, how it’s distributed through the clouds, and what implications that has for where to look for its source.

We then need to find out if there’s actually life in the clouds of Venus.

Even when that’s done, there’s still a real possibility that life from Earth hitched a ride to Venus early on, so this may not imply more than one origin.

If on Venus, Enceladus, Mars, Europa, any life is found that has a separate origin from life on Earth, that has enormous implications for origins of life and the prevalence of life in the universe. It suggests it’s not all that hard to make life from non-life after all. This is why exoplanets are so compelling. Unless you appeal to panspermia, finding life in another stellar system, close enough for us to see evidence of it, strongly implies that origins of life is relatively easy.

Two things come to mind:

(1) The uncertainty inherent in science. So far, no known explanation except for life can account for the detection, but we don’t know a lot. A lot more investigation needs to be done, especially involving abiotic phosphorus chemistry in sulfuric acid droplets. There’s also huge problems (that you will know more about) with life thriving in 90% sulfuric acid.

(2) That lots of people since Carl Sagan have thought about life in the clouds of Venus and have worked on this question way before our group started looking into this detection and its possible implications. I’d especially highlight David Grinspoon’s research into possible life on Venus.

The detection of phosphine itself. Whatever its cause, by all rights it just shouldn’t be there, and the mystery is compelling.

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David Grinspoon has written a popular level book on Venus. It is well written and very informative: Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet: Amazon.co.uk: Grinspoon, David Harry: 9780201406559: Books

Papers (I cite, but I do not necessarily agree):

Schulze-Makuch, D., Grinspoon, D.H., Abbas, O., Irwin, L.N. and Bullock, M.A., 2004. A sulfur-based survival strategy for putative phototrophic life in the Venusian atmosphere. Astrobiology , 4 (1), pp.11-18.

Schulze-Makuch, D. and Irwin, L.N., 2002. Reassessing the possibility of life on Venus: proposal for an astrobiology mission. Astrobiology , 2 (2), pp.197-202.

Izenberg, N.R., Gentry, D.M., Smith, D.J., Gilmore, M.S., Grinspoon, D., Bullock, M.A., Boston, P.J. and Slowik, G.P., 2020. The Venus Life Equation. arXiv preprint arXiv:2007.00105 .

Limaye, S.S., Mogul, R., Smith, D.J., Ansari, A.H., Słowik, G.P. and Vaishampayan, P., 2018. Venus’ spectral signatures and the potential for life in the clouds. Astrobiology , 18 (9), pp.1181-1198.

And of course:

Morowitz, H. and Sagan, C., 1967. Life in the clouds of Venus?. Nature , 215 (5107), pp.1259-1260.

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This one seems to be a potential huge pitfall. The fact that you bring it up implies there’s still a non-neglible probability that it isn’t phosphine. What could in principle be causing the signal, but which isn’t phosphine?

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We considered all known molecules that have features in the ballpark of the phosphine 1-0 transition. No known molecule except for phosphine can explain the feature.

It is possible that the feature is caused by an unknown molecule.

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This seems to be the largest weak point in the study that I found. @Paul.B.Rimmer how did you account for the fact that temperature can shift absorbance spectra?

Given that there is such a wide range of temperatures on Venus, shifting spectra for all species, how do you filter out something that absorbs that band in a temperature range outside what you considered? I imagine that the sharpness of the peak does give you some support. Even if the absorption peak is relatively stable to temperature for PH3, the absorption of a confounder chemical may not be temperature stable.

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This would be a question for Hideo Sagawa. I suspect that because the feature is seen in a temperate region this is not a serious issue, but I’m not competent to comment further.

From the paper:

As the continuum against which we see absorption28 arises at altitudes of ~53–61 km (Extended Data Fig. 2), in the middle/upper cloud deck layers17, the PH3 molecules observed must be at least this high up. Here the clouds are ‘temperate’, at up to ~30 °C, and with pressures up to ~0.5 bar (ref. 29).

EDIT: There is one more thing I can add: We also detected HDO in the data where expected.

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So it seems you can localize the signal to specific altitude zones, and these zones have well-understood temperature? The temperature is sufficiently homogenous in these zones? And you can screen out signals from other zones? And the other zones don’t show a stronger peak in that wavelength?

Answers to these questions might reduce concerns about a temperature-based confounder.

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Here’s my (quite limited) understanding. The continuum kicks in when the temperature is at 30 deg C. The temperature profile is reasonably well-constrained. We can’t see below the continuum near the feature, so anything deeper is screened out.

As for your other questions, I don’t know. I’ll ask Hideo and get back to you.

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In the article, it is stated:

“Trace PH3 in Earth’s atmosphere (parts per trillion abundance globally11) is uniquely associated with anthropogenic activity or microbial presence—life produces this highly reducing gas even in an overall oxidizing environment.”

Are there known biochemical mechanisms for the microbial production of PH3 from chemicals that are not anthropogenic?

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Even given a Goldilocks zone in the atmosphere of Venus, the convection currents are a conveyor straight into the crematorium. Of course, life may have evolved a solution, but I would wager on an inorganic source.

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There will be a Reddit AMA with many of the authors. There are some excellent questions here (e.g. by @Art an @swamidass) that I am not able to answer, and you can ask them there:

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For some time, researchers have looked into how life could be sustained in clouds that rain out to a depth where the sulfuric acid evaporates. One recent paper that considers a possible life cycle in the clouds of Venus:

Seager, S., Petkowski, J.J., Gao, P., Bains, W., Bryan, N.C., Ranjan, S. and Greaves, J., 2020. The Venusian lower atmosphere haze as a depot for desiccated microbial life: A proposed life cycle for persistence of the Venusian aerial biosphere. Astrobiology

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I did some digging around and have found a body of literature that deals with microbial phosphine generation under anaerobic conditions. The biochemistry still seems to be up in the air, but it would seem as if phosphine can be generated aside from anthropogenic sources.

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From the article cited in the OP:

Not long after, Greaves got in touch with Sousa-Silva, who spent her years in graduate school working out whether phosphine could be a viable extraterrestrial biosignature. She had concluded that phosphine could be one of life’s beacons, even though paradoxically, it’s lethal to everything on Earth that requires oxygen to survive.

“I was really fascinated by the macabre nature of phosphine on Earth,” she says. “It’s a killing machine … and almost a romantic biosignature because it was a sign of death.”

And the abstract of the linked article:

A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O2, only a handful of gases have been considered in detail. In this study, we evaluate phosphine (PH3). On Earth, PH3 is associated with anaerobic ecosystems, and as such, it is a potential biosignature gas in anoxic exoplanets. We simulate the atmospheres of habitable terrestrial planets with CO2- and H2-dominated atmospheres and find that PH3 can accumulate to detectable concentrations on planets with surface production fluxes of 1010 to 1014 cm−2 s−1 (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and ultraviolet (UV) irradiation. While high, the surface flux values are comparable to the global terrestrial production rate of methane or CH4 (1011 cm−2 s−1) and below the maximum local terrestrial PH3 production rate (1014 cm−2 s−1). As with other gases, PH3 can more readily accumulate on low-UV planets, for example, planets orbiting quiet M dwarfs or with a photochemically generated UV shield. PH3 has three strong spectral features such that in any atmosphere scenario one of the three will be unique compared with other dominant spectroscopic molecules. Phosphine’s weakness as a biosignature gas is its high reactivity, requiring high outgassing rates for detectability. We calculate that tens of hours of JWST (James Webb Space Telescope) time are required for a potential detection of PH3. Yet, because PH3 is spectrally active in the same wavelength regions as other atmospherically important molecules (such as H2O and CH4), searches for PH3 can be carried out at no additional observational cost to searches for other molecular species relevant to characterizing exoplanet habitability. Phosphine is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets from any source that could generate the high fluxes required for detection.

https://www.liebertpub.com/doi/abs/10.1089/ast.2018.1954?journalCode=ast&

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The AMA starts soon, so vote up my question here to be answered: (Reddit - Dive into anything)