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Living organisms may be transforming the clouds around Venus

Venus is sometimes referred to as Earth’s sister planet because of its similar mass and size, and scientists have speculated for decades about the potential presence of life on the planet. Local and remote observations of the physical and chemical attributes of the planet over the past 50 years have led to greater knowledge but have also identified anomalies that perplex scientists to this day. 

Unlike many planets, Venus has an atmosphere. However, this is composed of carbon dioxide (CO2) and the greenhouse effect of this gas leads to extremely high temperatures on the planet. The surface of Venus is approximately 700 K, which is way too hot to allow any life to exist there. 

Instead, scientists speculate that life may be able to exist at the Earth-like temperatures and pressures found in the clouds at altitudes of 48–60 km above the surface. Although these temperatures and pressures sound habitable for life forms, the prevailing consensus is that the clouds of Venus are made from droplets of concentrated sulfuric acid, which would make them totally inhospitable. 

However, in a study published in the Proceedings of the National Academy of Sciences, a team of scientists led by Cardiff University proposes that the clouds at these altitudes around Venus are not made entirely of sulfuric acid. Instead, their model suggests that the cloud droplets contain ammonium salts which may be the result of the biological production of ammonia gas (NH3).

Scientists have long been puzzled by the presence of NH3 in the atmosphere of Venus; this is a gas that was tentatively detected in the 1970s and that by all accounts should not be produced through any chemical process known on Venus. The scientists were guided by the in-situ observations of NH3 in the Venus cloud layers, both from Venera 8 chemical assay and from the Pioneer Venus probe mass spectrometry. 

The new model suggests that, if NH3 is present and forms ammonia salts within cloud droplets, it would have the effect of reducing the acidity of these droplets from a pH of -11 to a pH of between -1 and 1. This would be in the range that is tolerable to acidophile bacteria in certain environments on Earth. Furthermore, there are lifeforms on Earth that produce ammonia in order to neutralize and make livable otherwise highly acidic environments.

“We know that life can grow in acid environments on Earth, but nothing as acid as the clouds of Venus were believed to be. But if something is making ammonia in the clouds, then that will neutralize some of the droplets, making them potentially more habitable,” said study co-author Dr. William Bains from Cardiff University’s School of Physics and Astronomy.

This suggestion also helps to explain several long-standing mysterious features of the Venusian atmosphere, which have not been well explained by previous atmospheric chemistry models. For example, there is a uniform distribution of water vapor through the cloud layers, while the concentration of sulfur dioxide decreases as altitude increases. Unidentified large particles or unknown origin are present in the lower cloud layers. Another mystery is the presence of oxygen (O2) in the clouds – there is no known process for O2 formation in the cloud layers.

The scientists modeled the series of chemical processes that could take place in the clouds if NH3 gas were present and found that O2 would be produced naturally as a consequence of these reactions. Furthermore, once present in the clouds, NH3 would dissolve in the sulfuric acid in the cloud droplets, effectively neutralizing the acid and making them more habitable to living organisms. 

The introduction of ammonia into the droplets would transform their formerly round, liquid shape into more of a non-spherical, salt-like slurry. Once ammonia dissolved in sulfuric acid, the reaction would trigger any surrounding sulfur dioxide to dissolve as well, which may explain the change in concentration gradient of sulfur dioxide in the Venusian atmosphere.

The new model involving the presence of ammonia gas would thus explain most of the major anomalies seen in Venus’ clouds. 

But what is the origin of this ammonia gas? The researchers show that sources such as lightning, volcanic eruptions, and even a meteorite strike could not produce the amount of ammonia required to explain the anomalies. They therefore conclude that it might be produced by living organisms that are present in the outer clouds and that make the cloud droplets more habitable for themselves by producing ammonia.

“Ammonia shouldn’t be on Venus,” said study co-author Professor Sara Seager, from MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “It has hydrogen attached to it, and there’s very little hydrogen around. Any gas that doesn’t belong in the context of its environment is automatically suspicious for being made by life.”

“There are many other challenges for life to overcome if it is to live in the clouds of Venus,” said Dr. Bains. “There is almost no water there for a start, and all life that we know of needs water. But if life is there, then neutralizing the acid will make the clouds just a bit more habitable than we thought.”

Scientists may have a chance to check for the presence of ammonia, and signs of life, in the next several years with the Venus Life Finder Missions. These privately funded missions, of which Seager is principal investigator, plan to send spacecraft to Venus to measure its clouds for ammonia and other signatures of life.

By Alison Bosman, Staff Writer

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