In late 2020, scientists studying Venus’s atmosphere announced the surprising – and controversial – discovery of phosphine, a chemical produced primarily by living organisms on Earth. At the time, Jane Greaves of Cardiff University in Wales and her colleagues wondered if the phosphine was a sign of microorganisms in Venus’ atmosphere. Other scientists agreed, but phosphine alone would not be proof of life, and subsequent studies called into question whether the phosphine was ever present at all. Then, in March 2021, a study led by Rakesh Mogul of Cal Poly Pomona confirmed and expanded on the original discovery of phosphine. It suggested that other “biologically relevant chemicals” in Venus’ atmosphere appear to be out of balance, which is another sign of life.
The new research focused on re-analyzing data from the old Pioneer Venus mission, which launched four probes into Venus’ atmosphere in 1978, collecting data as they plummeted toward the surface. The Pioneer Venus Multiprobe segment of the mission included three small probes and one larger probe. The data from the largest probe was analyzed by the scientists. On March 10, 2021, the tantalizing peer-reviewed findings were published in Geophysical Research Letters.
From the paper:
We re-examined archived data obtained by the Pioneer Venus Large Probe Neutral Mass Spectrometer. Our results reveal the presence of several minor chemical species in Venus’ clouds including phosphine, hydrogen sulfide, nitrous acid (nitrite), nitric acid (nitrate), hydrogen cyanide, and possibly ammonia.
The presence of these chemicals suggest that Venus’ clouds are not at equilibrium; thereby, illuminating the potential for [possibly life-related] chemistries yet to be discovered.
Mogul and his colleagues discovered that the original 1978 analysis focused only on the most common chemicals expected to be found in Venus’ atmosphere. In a March 25 article for The Planetary Society, he told space journalist Nancy Atkinson:
The focus on the minor and trace [chemical] species was minimal. That’s what we realized after looking at the archival data and the associated publications. We immediately found signals in data that other publications hadn’t discussed or mentioned. That was all we needed for motivation to keep going.
In addition to phosphine, the new analysis suggested the presence of hydrogen sulfide, nitrous acid, nitric acid, hydrogen cyanide, carbon monoxide, ethane, and potentially ammonia and chlorous acid.
These chemicals, according to Mogul and his colleagues, could be evidence for redox disequilibria, or processes suggestive of life. On Earth, for example, microbes use the redox disequilibrium found in natural environments such as water to generate energy. Could something similar be happening in Venus’s atmosphere? Are parts of the atmosphere potentially habitable for microorganisms?
The data for this study came from the Large Probe Neutral Mass Spectrometer (LNMS), which was on the largest of the four probes that descended to Venus’ surface in 1978. Several times during the descent, the atmosphere’s composition was measured. LNMS targeted neutrally charged gas molecules in the atmosphere. Phosphine is one of those gases.
The Pioneer Venus data are significant because they were obtained in situ, in the atmosphere itself, rather than remotely by Earth-based telescopes, as the other data from last year were.
The disequilibrium in the Earth’s atmosphere is caused by life, but it is unknown whether the same is true for Venus. This latest study supports that possibility, but more data, most likely from a return mission, is required to be certain. Astronomers believe that this type of disequilibrium could be used to look for signs of life on exoplanets. Wouldn’t it be amazing if the first evidence came from somewhere much closer to home? The following is an excerpt from the linked paper in Science Advances (2018):
Chemical disequilibrium in planetary atmospheres has been proposed as a generalized method for detecting life on exoplanets through remote spectroscopy. Among solar system planets with substantial atmospheres, the modern Earth has the largest thermodynamic chemical disequilibrium due to the presence of life.
Today, the surface of Venus is utterly uninhabitable, with temperatures of 840 degrees Fahrenheit (450 degrees Celsius) – hot enough to melt lead – and crushing atmospheric pressure. However, the middle layers of the atmosphere are temperate and Earth-like in temperature and pressure, despite the fact that the clouds contain a lot of sulphuric acid. However, there is growing evidence that the planet was much more Earth-like earlier in its history, with rain, lakes, and oceans. However, something happened less than a billion years ago that caused a catastrophic greenhouse effect, transforming Venus into the hellish world we see today.
Could there have been some kind of microscopic life that sought refuge in the clouds, away from the burning surface? Perhaps.
It will be very interesting to see what other follow-up studies say about this latest chapter in the enigma of phosphine on Venus, as well as the possible disequilibrium. As Mogul put it:
There are always mysteries to be solved and I think what we just showed that sometimes old data can reveal new stories. This is all a process, and moving forward is what science is all about.
Bottom line: A new analysis of data from the 1978 Pioneer Venus mission uncovers evidence of not only phosphine, but also possible chemical disequilibrium in Venus’ atmosphere, which could be another sign of biological activity.
READ MORE: What Would It Be Like To Stand On Venus?