Cloudy desert: Whether the clouds of Venus could harbor life is hotly debated. Now researchers are providing another argument against it. According to your calculations, the Venus clouds with a relative humidity of 0.4 percent are much too dry even for extremophile microorganisms. The clouds of the gas giant Jupiter, on the other hand, could offer better opportunities, because in its upper regions the relative humidity is 58 percent, as the scientists report in the specialist magazine “Nature Astronomy”.
In its early days, Venus was just as friendly to life as our earth, until a galloping greenhouse effect and the growing sun put an end to it. If life ever developed on our neighboring planet, it either perished in the glowing hell – or it has found a refuge in the cooler Venus clouds. In fact, some researchers evaluated the detection of phosphine gas in the Venusian atmosphere as an indication of possible biological activity. Since then, however, more recent measurement data have raised doubts about this thesis.
Water activity and the limit to active life
Now a team led by John Hallsworth from Queen’s University Belfast is providing further arguments against a sanctuary in the clouds of Venus. They investigated how much water there is for potential life forms in these atmospheric layers of the planet. To do this, they determined the water activity – an equivalent of the relative humidity. Like these, the water activity depends primarily on temperature and pressure, but can also be influenced by chemical factors such as the presence of strong acids.
From studies with microbes from extreme locations on earth and laboratory tests we know that the limit of survival is at a water activity of around 0.58 – this corresponds to a humidity of 58 percent. Up to this value, microorganisms can maintain metabolism, growth and cell division in the long term. Although many microbes survive significantly drier conditions, this is the minimum for active reproduction, as the researchers explain.
Venus: Not enough water available
But what does it look like in the clouds of Venus? Although the temperature and pressure are favorable there, the high concentration of sulfuric acid leads to reactions that drastically reduce the availability of water, as the research team has determined. “Our research has shown that the sulfuric acid clouds of Venus have too little water for active life to exist,” says Hallsworth.
So far, heights between 40 and 70 kilometers above the surface of Venus have been considered potentially life-friendly because the temperatures there are between 130 and minus 40 degrees. “The relative humidity in this zone varies, but remains consistently below 0.4 percent,” reports the team. With this, the water activity of 0.004 is around a hundred times below the limit value for active life. Even fine droplets in the clouds of Venus would probably not be more life-friendly because of their high acid content, according to the researchers.
Jupiter: Colder, but humid enough
But what about other planets in the solar system? For Jupiter, the scientists evaluated data from NASA’s Galileo and Juno missions. The latter had only recently provided indications that the water content of the Jupiter clouds could, at least locally, be higher than previously assumed. According to calculations by Hallsworth and his team, the water activity in the cloud zone, which is between plus 10 degrees and minus 40 degrees, is 0.585 – this corresponds to a relative humidity of 58 percent.
“According to these parameters, the atmosphere of Jupiter would even be better suited for life on earthly models than that of Venus,” the researchers explain. However, this assumes that there are also the necessary nutrients in the Jupiter clouds. “Whether the Jupiter clouds are a suitable place for an initial emergence of life is also another question,” the team said.
Mars: Too cold and too thin
However, it looks much more unfavorable for our neighboring planet Mars. Its extremely thin atmosphere, which is very cold at less than minus 73 degrees Celsius, only contains water in the form of ice crystals – and very little of it. “So there are ice clouds at best,” said Hallsworth and his colleagues. “Microbial cells cannot absorb water under such cold conditions.” At 0.537, the water activity is also below the limit for active life.
“With regard to water activity and temperatures, we can therefore determine that the Jupiter clouds have the most favorable conditions for life in the solar system – apart from Earth,” state Hallsworth and his team. For the remaining gas planets Saturn, Uranus and Neptune, however, the corresponding data are still missing to be able to calculate the water activity.
Also suitable for exoplanets
According to the team, their approach could also be used in the future to assess the livability of exoplanet atmospheres. “The James Webb Space Telescope will be able to determine the temperature, pressure conditions and the water content of the atmospheres of foreign planets,” say the scientists. “These data then make it possible to estimate the water activity in the gas envelopes of these planets.” (Nature Astronomy, 2021; doi: 10.1038 / s41550-021-01391-3)
Quelle: Queen’s University Belfast