Life on Venus? A DLR FAQ about the trace gas phosphine
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?##markend##
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.
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).
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.
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.
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.
Off-topic for all space and Jupiter enthusiasts
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.