For decades, scientists engaged in the search for life in the Universe (aka. astrobiology) have focused on searching for life on other Earth-like planets. These included terrestrial (aka. rocky) planets beyond our Solar System (extrasolar planets) and ones here at home. Beyond Earth, Mars is considered to be the most habitable planet next to Earth, and scientists have also theorized that life could exist (in microbial form) in the cloud tops of Venus.
In all cases, a major focal point is whether or not planets have large bodies of water on their surfaces (or did in the past). However, a new study led by a research team from the UK and German (with support from NASA) has shown that the existence of life may have less to do with the quantity of water and more to with the presence of atmospheric water molecules. As a result, we may have better luck finding life on Jupiter’s turbulent cloud deck than Venus’.
The study that describes their findings, which was recently published in Nature Astronomy under the title “Water activity in Venus’s uninhabitable clouds and other planetary atmospheres,” was led by Dr. John E. Hallsworth of the School of Biological Sciences at Queen’s University Belfast. He was joined by colleagues from multiple universities in the UK and Germany, and the NASA Ames Research Center’s Space Science Division (SSD).
Venus has been the focal point of a lot of interest lately, ever since the announcement that phosphine gas had been detected in the planet’s dense atmosphere. These findings, according to a team of independent researchers, was a possible sign that microbial life might exist in Venus’ sulfuric acid clouds (aka. a potential biosignature). However, according to this latest study, Venus’ atmosphere doesn’t have enough water activity to support this claim.
This conclusion is based on a new method devised by Hallsworth and his colleagues to determine the leve of water activity in a planet’s atmospheres. They then applied this method on Venus’ atmosphere, where temperatures range from 30 to 80 °C (86 to 176 °F) at altitudes of 50 km (30 mi) above the surface and water vapor accounts for about 0.002% of the atmosphere by volume.
Ultimately, the researchers found that the water activity in Venus’ atmosphere was over one hundred times beneath the lower limit. When they applied this same method to Jupiter’s clouds, they found something else entirely. Above the stratosphere-thermosphere boundary (320 km above the troposhere) there is a “sweet spot” where temperatures are stable and the clouds have a high enough concentration of water vapor.
In short, the clouds of Venus do not have what it takes to support life, but Jupiter’s upper atmosphere does. This information is highly significant at a time when NASA and other space agencies are proposing various astrobiology missions for the near future. Before these missions can be sent out to search for life, it’s imperative that we prioritize destinations based on the likelihood of scientific returns.
As Dr. Hallsworth explained in a Queen’s University Belfast news release:
“Our research shows that the sulphuric acid clouds in Venus have too little water for active life to exist, based on what we know of life on Earth. 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.
“This is a timely finding given that NASA and the European Space Agency just announced three missions to Venus in the coming years. One of these will take measurements of Venus’s atmosphere that we will be able to compare with our finding.”
In addition, the results of this study present another possibility for widening the search for habitable exoplanets. As it stands, exoplanet characterization is focused on finding evidence of life on rocky planets that have surface water. However, the detection of sufficient water vapor in the atmosphere’s of gas giants – such as Exo-Jupiters and Exo-Neptunes – could also point the way towards life beyond our Solar System.
“We have also performed calculations for Mars and Earth and show that these calculations can be done for planets outside our solar system,” added Dr. Hallsworth. “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.”
Dr. Christopher McKay, a planetary scientists at NASA Ames and another co-author on the study, contributed his extensive expertise in planetary atmospheres and astrobiology to this research. “We derive water activity of atmospheres without a model of any sort, based only on direct observations of pressure, temperature, and water concentration,” he said.
In the near future, the James Webb Space Telescope (JWST) will finally be launched to space (currently scheduled for November of 2021). Using its advanced infrared imaging capabilities, the JWST will play a vital role in astrobiology and the characterization of exoplanet atmospheres. Along with missions like the Nancy Grace Roman Space Telescope (RST), the census of potentially-habitable exoplanets is anticipated to grow exponentially.
Said Dr. Philip Ball, an expert on physics and chemical biology of water and a co-author on the paper:
“The search for extraterrestrial life has sometimes been a bit simplistic in its attitude to water. 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.”