Phosphine detection highlights unknowns of Venus’s atmosphere

The observations of a spectral signature associated with phosphine (PH₃) in the clouds of Venus , reported 14 September in Nature Astronomy by Cardiff University’s Jane Greaves and her colleagues, have prompted a flurry of interest. Here on Earth, phosphine is associated with microbial life: The presence of the odorless, colorless gas in the atmosphere likely results from the reduction of phosphate in decaying organic matter. Simulations of atmospheres on habitable rocky planets with CO₂– and H₂-rich atmospheres suggest that PH₃ can accumulate to detectable concentrations , and astrobiologists consider it to be a potential indicator of life on other rocky planets.

Microbial life in the clouds of Venus, although not impossible, would require extraordinary proof. Yet models of Venus’s atmosphere do not support the presence of phosphine at the 20 ppb levels that Greaves and her team reported. One step in resolving the mystery is to confirm the detection of PH₃, which has a spectral signature that is notoriously difficult to detect using Earth-based telescopes. Additionally, researchers want to better understand the atmospheric chemistry on our neighboring cloud-blanketed planet.

A standout absorption line

Observing at millimeter and submillimeter wavelengths, which straddle the boundary between radio and IR, offers astronomers a unique tool for probing the chemical composition of galaxies, gas clouds, and other targets. Absorption lines at those wavelengths indicate transitions between rotational quantum states of gas-phase molecules, which depend on the molecule’s shape and thus its chemical identity.

In 2017 Greaves peered at Venus’s atmosphere through the James Clerk Maxwell Telescope at Mauna Kea Observatory in Hawaii. Those observations revealed the 1.12 mm absorption line associated with PH₃. “To detect phosphine from the surface of Earth, we needed to find transitions excited on Venus that are relatively strong and spectrally distinct,” says the University of Manchester’s Anita Richards, who coauthored the Nature Astronomy paper. The 1.12 mm line was a good compromise between weaker lower frequencies and higher frequencies that would be more likely to be obscured by spectrally similar molecules in Earth’s atmosphere.

To verify the result, the team conducted follow-up observations in 2019 using 43 radio telescopes of the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile’s Atacama Desert, one of the driest places on Earth. The expected strength of the 1.12 mm absorption line was well within the capabilities of ALMA. But the real challenge came from detecting a single line against the optically thick layers of Venus’s atmosphere.

Could other gases absorb at wavelengths near the wavelength associated with phosphine? One possibility is sulfur dioxide, which Greaves and her team expected to produce a weak line in Venus’s cloud decks. That was easily ruled out as a contaminant because the spectral signature of SO₂ could mimic that of PH₃ only at temperatures much hotter than those measured in the upper clouds. Current databases do not show other chemical species that can explain the observed feature.

Both the authors and outside scientists call for further work to confirm the presence of phosphine on Venus, and they note that PH₃ has other potentially observable absorption lines. However, those higher-frequency spectral features could require a future large airborne or space-borne telescope to overcome obscuration by water vapor in Earth’s atmosphere. Another possibility for future confirmation is to make use of ground-based IR telescopes, which have previously been used to identify PH₃ in the hydrogen-rich, reducing atmospheres of Jupiter and Saturn. Deep within those giant planets’ atmospheres, temperatures and pressures are high enough for phosphorus to exist as phosphine. Convection currents then transport the molecule to the upper atmosphere.

Unknown chemistry

The most provocative question is whether the phosphine has a biotic origin. On Earth, humans create PH₃ as a pesticide in industrial settings, and the molecule has been found both in anoxic habitats and in laboratory mixtures of bacteria. But no known enzymes create phosphine. The molecule may be a decay product produced in anaerobic environments where living things have died.

Life on a planet with acidic clouds would not look like life on Earth. Acid chemically extracts the water from sugars, such as those in cell walls or in RNA, and leaves behind charcoal. To test for life in a place where any known life should not exist, one needs to consider another reducing gas or look for complicated macromolecules that are not necessarily proteins or RNA. “It’s easier to prove it’s not life than get evidence that it is life,” says paper coauthor William Bains of MIT.

Venus’s atmosphere is hot and dense, is composed mainly of carbon dioxide, and supports opaque clouds of sulfuric acid. Beyond that, “there’s a lot that we don’t really know,” Bains says. The team’s calculations of chemical products resulting from possible Venusian geologic and atmospheric activity, including lightning and meteorite strikes, could not explain the 20 ppb of phosphine in Venus’s acidic environment. There is too much oxygen to promote formation of phosphine, and any phosphorus should exist as phosphate in the highly oxidizing atmosphere. Instead, it is likely that the actual reaction pathway simply has not yet been considered.

One major unknown is the photochemistry of the cloud droplets themselves. If there are phosphorous species in the atmosphere of Venus—and the Vega descent probe in 1985 indicated that there are—then some would dissolve in and react with the sulfuric acid and other chemicals in the cloud droplets. But precisely what happens to the droplets when irradiated by solar UV light is still a mystery. A reaction of sulfuric acid and phosphoric acid, for example, won’t directly yield PH₃. UV light could alter that reaction and lead to other pathways.

Further, the observations do not provide information about the temporal evolution or lifetime of phosphine. Uncovering new chemical pathways could help explain not only the formation of PH₃ but also the rate at which it is ultimately destroyed—and why that rate is slower than expected in the oxidizing atmosphere. “Exploring the importance of potential loss pathways will help to define what processes must be overcome for the phosphine to be detectable,” says Kandi Jessup, a senior research scientist at the Southwest Research Institute who was not involved in the phosphine work.

Chemists are beginning to explore cloud droplet photochemistry in laboratories on Earth. “Cook up something like the clouds of Venus, add phosphorus, get a UV source, and see what happens,” Bains says. “But make sure you have a good fume hood.”

To unambiguously confirm the relevance of laboratory studies, researchers need to directly measure the chemical species and their reactivity in the Venusian atmosphere. Balloon missions could do that, as could satellites that monitor the atmosphere. “The take-home message is that we cannot really answer the question from Earth,” says Paul Byrne, a planetary scientist at North Carolina State University.

In February NASA announced its selection of four missions to consider developing for flight; two of them are focused on Venus. VERITAS would fly a high-resolution radar and carry a surface mapper, though that might not help resolve the phosphine question. DAVINCI+ is a descent probe that would traverse the atmosphere and might be able to look for PH₃. Whether those missions go ahead is expected to be announced in 2021.

Both VERITAS and DAVINCI+ would provide important information about Venus, but the missions were designed with different science questions in mind. Ultimately, Byrne points out, NASA should direct a mission with the appropriate instrumentation to look for phosphine from orbit or to sample the cloud droplets.