Life on Venus? A DLR FAQ about the trace gas phosphine
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Life on Venus? A DLR FAQ about the trace gas phosphine
Artist’s impression of Venus, where astronomers may have first detected phosphine in 2020. Data acquired by the James Clerk Maxwell Telescope on Mauna Kea (Hawaii) and the Atacama Large Millimetre/Submillimetre Array (Chile) were analysed for this. Phosphine could be present in the upper layer of the cloud cover. However, the observation is controversial among experts.
Credit:
ESO/M. Kornmesser/L. Calçada & NASA/JPL/Caltech (CC BY 2.0)
In 2020, the planetary research community and the interested public turned their attention to Venus. A research team from the University of Cardiff had detected the gas phosphine in the high clouds of Earth’s inner neighbouring planet for the first time. Phosphine (PH3) is produced on Earth, either naturally by organic weathering processes or artificially – for example for use as fertiliser. So, were traces of life on Earth’s neighbour indirectly discovered in 2020 by detecting phosphine? That would have been a sensation. Or was it much ado about nothing?
After an extensive response in the media and on the internet, the first disillusionment came rather quickly. Criticism and even opposition soon arose. The evidence was not statistically significant, some said; even the presence of phosphine did not necessarily mean that it was of biological origin, others emphasised. Subsequently, additional observations and measurements using telescopes were carried out. It must be emphasised that even the authors of the Cardiff study never claimed to have found traces of life in Venus’ atmosphere. Further studies were mostly unable to detect phosphine in the venusian atmosphere. Then, in the summer of 2023, the team from Cardiff that made the first announcement published an additional statement. They had been able to detect phosphine again using the James Clark Maxwell Telescope in Hawaii.
So, what is the current state of affairs? Can we conclude that there is life on Venus or not? And can a team from the DLR Institute of Planetary Research in Berlin shed more light on the darkness? After a lot of discussions in the almost three years since the excitement about the detection of phosphine on Venus, we have compiled a set of Frequently Asked Questions (FAQs).
Why was there so much excitement about phosphine on Venus?
Phosphine can be a biomarker which, as already mentioned, is either of organic origin or artificially produced on Earth. Phosphorus – the element that together with hydrogen makes up phosphine – is essential for life on Earth because all important building blocks of life contain phosphorus. This includes deoxyribonucleic acid (DNA), the carrier of genetic information. Whether phosphine is present in Venus’ atmosphere at all, and if so, in what quantity, is therefore a major topic in planetary research. Proof of the existence of extraterrestrial life would be one of the greatest sensations in the history of scientific research.
Venus is globally and permanently enveloped by clouds of sulphuric acid, which make any view of the hot, solid surface impossible at optical wavelengths. Observations of the molecule phosphine published in 2020 suggest that it might exist at an altitude of between 53 and 61 kilometres. This ultraviolet-light image was acquired in 2016 by the Japanese orbiter Akatsuki.
Credit:
Planet-C Project Team
How did the controversy arise?
The observations and evaluations published since 2020 are very contradictory. On the one hand, very different concentrations of PH3 were discovered; on the other hand, it was even ruled out that phosphine is a component in the atmosphere of Venus at all. The first published concentration by the Cardiff team in 2020 was 20 ± 10 ppb in the cloud cover of Venus (Greaves et al., 2020) and excited the community. ‘ppb’ stands for ‘parts per billion’, which means that for every molecule of phosphine there are a billion other atmospheric molecules. The evaluation of the same data by another research group could not confirm the value (Snellen et al., 2020). There were further measurements by various groups that shifted the value for the PH3 concentration downwards.
In 2021, the atmosphere of Venus was also searched for spectral PH3 signatures by an instrument on board the Stratospheric Observatory for Infrared Astronomy (SOFIA), the joint research aircraft formerly operated by NASA and DLR. The evaluation of these data revealed an upper limit at a likewise very small PH3 value (Cordiner et al., 2022).
Early, and at that time sensational, images of Venus at close range were transmitted by NASA’s Mariner 10 spacecraft in February 1978 on its way to Mercury. The image data has recently been reprocessed and (right) contrast enhanced. These are false-colour images that can be used to better depict the dynamic processes in Venus’ atmosphere. Clouds of sulphuric acid envelop the planet globally at an altitude of 50 to 60 kilometres. There, the atmospheric pressure is comparable to that on Earth. At this height, the clouds race around the planet at speeds of up to 250 kilometres per hour, 50 times faster than the planet rotates.
Credit:
NASA/JPL-Caltech (Kevin M. Gill)
Why is phosphine not necessarily an indicator of the existence of life??
Detection of PH3 would not yet be proof of life, because there could also be abiotic – that is, physical rather than biological – processes that produce this trace gas. So, it is important to find out whether the amount of phosphine that might have been detected is really an indication of life. To do this, one has to understand the abiotic processes. Only if all abiotic sources of PH3 can be excluded, or if they are too weak to be able to produce the measured amount of phosphine on Venus, could it have been produced by organisms.
The processes that could have produced PH3 abiotically – that is, without life – on Venus were recently presented by Fabian Wunderlich and a team from the DLR Institute of Planetary Research in a paper in Astronomy & Astrophysics. To do this, they determined the abiotic reaction chains that give rise to phosphine in the form of model calculations.
It is only possible to observe the geological structures on the surface of Venus with radar – optical cameras cannot see through the cloud cover. In the 1990s, NASA’s Magellan spacecraft revealed a Venusian surface almost entirely formed by volcanic processes, which raised many scientific questions.
Credit:
NASA/JPL
What do the DLR model calculations suggest and what are the implications?
Fabian Wunderlich, John Lee Grenfell and Heike Rauer from the DLR Institute of Planetary Research, who were responsible for the study, used more details and more recent data in a photochemical 1D model (Wunderlich et al., 2023). Among other things, they have extended the existing model by 79 reactions for a total of 13 species that contain phosphorus. Their work shows that yes, a small amount of PH3 could be produced at altitudes between 50 and 60 kilometres (in the cloud cover of Venus) by purely abiotic reactions.
However, the error margins in the model calculations are very large. The amount of phosphine could differ – depending on the scenario – by six orders of magnitude – that is, by a factor of one million. So, improved knowledge about the atmospheric processes that involve phosphorus is needed. More precise observations of life forms on Earth that produce phosphorus compounds or in which phosphorus-related processes take place are also required. To answer the question as to whether PH3 is a biosignature, the detection of other phosphorus compounds such as phosphorus monoxide would be helpful. This is formed abiotically in the lower atmosphere of Venus – probably through the decay of larger observed molecules containing phosphorus – and is then transported to the upper layers.
For future observation and modelling of the Venusian atmosphere, the DLR work provides important information and poses critical questions – both for observational and modelling research and for the community that determines reaction rates in the laboratory.
After more than two decades of evaluating the measurements performed by NASA’s Magellan mission during the 1990s, a great many questions have arisen about the formation and geological development of Venus. They are to be answered with new space missions. Why, for example, did Venus – which is almost the same size as Earth, with almost the same mass and a very similar geochemical ‘inventory’ – develop so differently? And was there once water, perhaps even life, on Venus too? ESA’s EnVision mission will address these questions in the next decade. DLR is developing part of the spectrometer for this mission.
Credit:
ESA/VR2Planets/Damia Bouic
Which space missions could provide clarification?
ESA’s Jupiter Icy Moons Explorer (JUICE) mission, in which DLR is involved, will visit Venus on its way to the Jupiter system. A close fly-by to accelerate and modify its elliptical trajectory is planned for August 2025. The JUICE Submillimetre Wave Instrument (SWI), which was developed and built under the leadership of the Max Planck Institute for Solar System Research, will be able to detect very low phosphine concentrations.
DLR’s contributions to the JUICE mission will be deployed after the spacecraft’s arrival in the Jupiter system. Our planetary research is involved in the mission with the GALA instrument and the JANUS camera, as well as through other scientific team memberships, some of which are funded by the German Space Agency at DLR.
You can read more about the JUICE mission here on the blog under the tag ‘JUICE \ GALA’ and on our mission page on DLR.de.
