Scientists have proposed a new theory which suggests that possible lifeforms in the clouds of Venus could be setting off a cascade of chemical reactions that is making the environment much more habitable.
This self-sustaining chain of events could also explain many of the strange anomalies present in the planet's upper atmosphere that have been puzzling scientists for decades.
The new hypothesis has been published today in Proceedings of the National Academy of Sciences by a team of researchers from Cardiff University, MIT and Cambridge University.
Scientists have long been puzzled by the presence of ammonia in the atmosphere of Venus, 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.
In their new study, the researchers modeled a set of chemical processes to show that if ammonia is indeed present, the gas would set off a cascade of chemical reactions that would neutralize surrounding droplets of sulfuric acid.
This would, in turn, increase the pH of the clouds from roughly -11 to 0. While still very acidic, this would be within the range of acidity that life could tolerate.
The team note that there are lifeforms on Earth that produce ammonia to neutralize and make livable an otherwise highly acidic environment.
"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 co-author of the study Dr William Bains, from Cardiff University's School of Physics and Astronomy.
As for the source of ammonia itself, the authors propose the most plausible explanation is of biological origin, rather than any non-biological source such as lightning or volcanic eruptions.
"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."
Combing through data from past missions to Venus, the team identified anomalies, or chemical signatures, in the clouds that had gone unexplained for decades.
In addition to the presence of oxygen and certain non-spherical particles, anomalies included unexpected levels of water vapor and sulfur dioxide.
The team tested whether the anomalies might be explained by dust, arguing that minerals, swept up from Venus' surface and into the clouds, could interact with sulfuric acid to produce some, but not all, of the observed anomalies.
Although the chemistry checked out, the physical requirements were unfeasible: a massive amount of dust would have to loft into the clouds to produce the observed anomalies.
The team then wondered if the anomalies could be explained by ammonia and set about modelling a series of chemical processes in search of an answer.
They found that if life were producing ammonia, the associated chemical reactions would naturally yield oxygen.
Once present in the clouds, ammonia would dissolve in droplets of sulfuric acid, effectively neutralizing the acid to make the droplets relatively habitable.
The introduction of ammonia into the droplets would transform their formerly round, liquid shape into more of a non-spherical, salt-like slurry and once ammonia dissolved in sulfuric acid, the reaction would trigger any surrounding sulfur dioxide to dissolve as well.
The presence of ammonia then could indeed explain most of the major anomalies seen in Venus' clouds. The researchers also show that sources such as lightning, volcanic eruptions, and even a meteorite strike could not chemically produce the amount of ammonia required to explain the anomalies.
"There are many other challenges for life to overcome if it is to live in the clouds of Venus," said 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, a set of proposed privately funded missions, of which Seager is principal investigator, that plan to send spacecraft to Venus to measure its clouds for ammonia and other signatures of life.
