Are clouds alive

Is there life on Venus? Researchers puzzle over discovery

Astronomers have detected a molecule in the atmosphere of our neighboring planet that is considered a promising biosignature. Even if this does not prove anything, interest in Venus is likely to increase rapidly.

Mars is very popular with astrobiologists. With the exception of the earth, the prospects are nowhere so good that you will find traces of primitive life. That is why we regularly visit the red planet. Earth's other neighboring planet, Venus, is much less in the spotlight. The "evening star", like the earth and Mars, lies in the habitable zone of the sun. A strong greenhouse effect turned the surface of Venus into a hostile glowing hell a long time ago. It is all the more astonishing what astronomers are now reporting in the journal “Nature Astronomy”. Using two telescopes, they detected small amounts of a molecule in the cloud cover of Venus that is considered a biomarker on Earth.

How you can recognize life on another planet is currently a major research topic. Gases such as oxygen, methane or ammonia are considered possible indicators. However, since these can also arise abiotic, their detection does not necessarily mean that a planet is animate. A year ago, Sara Seager's group at the Massachusetts Institute of Technology brought another biosignature into play: monophosphane (PH3). This phosphorus-containing molecule is produced on earth by microorganisms that live in low-oxygen conditions. In contrast, no geochemical processes are known on rocky planets that produce this molecule in significant quantities. This makes monophosphine a better biosignature than oxygen or methane.

Surprising discovery

In 2017, Jane Greaves' group from the University of Cardiff and the James Clerk Maxwell Telescope in Hawaii discovered a spectral fingerprint of monophosphane in the atmosphere of Venus. From the shape of the absorption line, the researchers concluded that the gas occurs in the middle and upper cloud layers of Venus and is present here in a concentration of 20 ppb (parts per billion). Greaves says she didn't really expect evidence of this molecule. The experiment was actually an exercise for future studies of extrasolar planets. She was all the more surprised by the result.

To be on the safe side, Greaves' group repeated the measurement two years later with the Alma telescope in Chile. The result was almost the same. The spectra were analyzed independently by several people, says Greaves. In addition, they have examined very carefully whether there could be a mix-up with another molecule. Then you are more than 90 percent sure that the measured signal comes from monophosphane.

Kevin Heng from the Center for Space and Habitability at the University of Bern, who was not involved in the investigation, can understand Greaves' astonishment. The atmosphere of Venus consists largely of CO2so be rich in oxygen. If so, then phosphorus should be found here in oxidized form. A reducing gas like PH3However, which contains a lot of hydrogen, should be rapidly degraded in the Venusian atmosphere. It is therefore necessary to explain where the monophosphane is constantly flowing in from.

Search for the source

To answer this central question, Seager and Greaves' groups have come together. In their joint publication in “Nature Astronomy” they left no stone unturned to explain the origin of the gas abiotic. They took into account every conceivable geochemical process, they modeled chemical processes in the Venusian atmosphere that are triggered by the UV light of the sun, and they investigated the influence of lightning and volcanoes. But none of these processes provide enough monophosphine to even come close to explaining the measured concentration. The conclusion of the researchers is therefore: Either chemical processes must take place on Venus that are not known on Earth. Or one has to consider the unlikely sounding possibility that the monophosphine is biological in origin.

In fact, the animated Venus is a popular subject in science fiction literature. The astronomer Carl Sagan made this idea socially acceptable. In the 1960s, in the journal Nature, he investigated the question of whether there could be life on the surface or in the cloud cover of Venus. Seager has also dealt with this possibility. As far as we know today, it is far too hot on the surface of Venus for complex molecules to exist, she says. It looks different in the atmosphere. At an altitude of 50 to 60 kilometers there is a “sweet spot” where temperatures are between 0 and 100 degrees Celsius. This is roughly the height at which the monophosphane was localized.

Hypothetical life cycle

Microorganisms can also be found in earthly clouds. However, this is a temporary phenomenon. As soon as the clouds dissolve, the bacteria rain back to earth. It's different on Venus. Here life should only take place in the clouds. Seager recently outlined how this would be theoretically possible in the journal “Astrobiology”. The microorganisms therefore thrive in the liquid droplets of the clouds. As these drops collide with each other, they get bigger and heavier over time and sink into deeper layers of the Venusian atmosphere. This is potentially fatal for the microorganisms as their liquid environment evaporates. Seager speculates, however, that the microorganisms survive as spores until they rise again into cooler cloud layers through diffusion processes and there initiate the formation of new droplets. This is how the next life cycle begins.

Seager makes no secret of the fact that this is highly speculative. She points out that one can hardly imagine a more hostile environment than the clouds of Venus. These consist for the most part of corrosive sulfuric acid. The elixir of life water, on the other hand, is in short supply.

Heng also warns against jumping to conclusions. Just because all conventional explanations have failed does not mean that the monophosphine must be of biological origin. There could be mechanisms of origin that were previously unknown. As long as you don't know them, you can't rule them out.

All researchers agree that only further measurements and modeling can solve the puzzle. Ideally, a probe would be sent to Venus to examine the atmosphere on site and possibly even bring samples back to Earth. For a long time, Venus was Earth's ignored twin sister, says Seager. Hopefully that will change now.

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