The recent suggestion of phosphine gas in the atmosphere of Venus has regenerated interest in the idea of life in clouds; however, such analyses usually neglect the role of water activity, which is a measure of the relative availability of water, in habitability. In a new study in the journal Nature Astronomy, planetary researchers computed the water activity within the clouds of Venus and other solar system planets from observations of temperature and water-vapor abundance; they found that the Venusian water activity was more than a hundred times below the lower limit at which life can exist on Earth; however, Jupiter’s clouds had a high enough concentration of water, as well as the correct temperature, for Earth-like life to exist there.
This composite image, taken by JAXA’s Akatsuki spacecraft, shows Venus. Image credit: JAXA / ISAS / DARTS / Damia Bouic.
“The recent suggestions of biogenic substances, such as phosphine in the Venusian clouds, continuing a history of speculation about life on Venus,” said senior author Professor Christopher McKay, a planetary scientist in the Space Science Division at NASA’s Ames Research Center, and his colleagues.
“The surface of Venus is considered too hot for organic life forms but the lower cloud layer, at an altitude of 40 to 70 km (25-43.5 miles), has a temperature range that makes it potentially habitable based on our knowledge of terrestrial-type life.”
“Earlier studies have suggested the lack of liquid water, or at least the low availability of water, as a potential barrier to life.”
“Although several recent analyses have queried whether adequate water is available, they also propose active cellular metabolism in the sulfuric acid-rich droplets of the Venusian atmosphere.”
Water activity, measured on a scale of 0 to 1, is equivalent to the relative humidity, or availability of water, in a planet’s atmosphere.
The water activity of an environment can considerably affect organisms, including those that are capable of living in extreme environments, known as extremophiles.
Earth-like life requires a water activity of at least 0.585 for metabolism and reproduction to take place.
“We derive water activity of atmospheres without a model of any sort, based only on direct observations of pressure, temperature, and water concentration,” Professor McKay said.
In the study, the researchers calculated the limit for life due to water activity within the clouds of Venus and other planets in the Solar System.
They found that droplets of sulfuric acid reduce the water activity of Venus’s clouds to below 0.004, more than 100 times less than the limit for life.
Comparatively, the representative water activity in the Martian clouds is 0.537, which is slightly below the habitable range for life and similar to that of the second layer, or stratosphere, of the Earth’s atmosphere.
The lowest atmospheric layer of Earth, the troposphere, is, however, permissible for life.
Jupiter’s atmosphere has a biologically permissive water activity of greater than 0.585 for temperatures between minus 10 degrees Celsius and 40 degrees Celsius (14-104 degrees Fahrenheit), although factors such as cloud composition may limit habitability.
“Our research shows that the sulfuric acid clouds in Venus have too little water for active life to exist, based on what we know of life on Earth,” said lead author Dr. John Hallsworth, a researcher in the Institute for Global Food Security at Queen’s University Belfast.
“We have also found that the conditions of water and temperature within Jupiter’s clouds could allow microbial-type life to subsist, assuming that other requirements such as nutrients are present.”
“The search for extraterrestrial life has sometimes been a bit simplistic in its attitude to water,” said co-author Dr. Philip Ball, a researcher based in London, UK.
“As our work shows, it’s not enough to say that liquid water equates with habitability.”
“We’ve got to think too about how Earth-like organisms actually use it — which shows us that we then have to ask how much of the water is actually available for those biological uses.”
“We have also performed calculations for Mars and Earth and show that these calculations can be done for planets outside our Solar System,” Dr. Hallsworth said.
“While our research doesn’t claim that alien (microbial-type) life does exist on other planets in our Solar System, it shows that if the water activity and other conditions are right, then such life could exist in places where we haven’t previously been looking.”
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J.E. Hallsworth et al. Water activity in Venus’s uninhabitable clouds and other planetary atmospheres. Nat Astron, published online June 28, 2021; doi: 10.1038/s41550-021-01391-3
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2021-06-29 20:51:04Z
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