A study last year made waves by pointing out the presence of chemicals that have been proposed as possible indicators of life. Found in the atmosphere of Venus. Due to the hellish conditions on the planet’s surface, no life of any kind exists there, but a milder environment is possible in the clouds of the planet above the surface. Therefore, the possibility that the chemical represents life cannot be ruled out immediately.
Over the next month, other researchers cast doubt on the claim that the chemical exists. Today, a research paper is published stating that Venus’ cloud conditions are incompatible with life at distances comparable to Earth. The temperature in the clouds is actually much milder, but nowhere near enough water to support life, and most of what’s in the water droplets consists mostly of sulfuric acid.
At a press conference announcing the findings, John Hallsworth of Queen’s University Belfast said the new work was inspired by the apparent discovery of a phosphine in Venus’ atmosphere. He and his collaborators realized that the two areas of study were combined to create another way of examining the potential for life on Venus. One was a study of life in extreme conditions on Earth, sparked in part by NASA’s efforts to determine the best way to protect Mars from pollution by the probes we send there.
The second was led by NASA. We sent probes into the atmospheres of some planets and photographed others. Although the sensor didn’t specifically look for life, it measured things like temperature and pressure directly, setting limits on how much water is in the atmosphere and what shape it will take.
With respect to Venus, people have identified organisms that can sustain their metabolism within various limits such as temperature, acidity, and water content. Because the temperature changes with altitude, the former places a limit on the observable altitude. The latter two are important because Venus is believed to be a very dry planet whose clouds are created not by condensed water, but by the presence of some water-containing droplets of sulfuric acid.
The world record holder for survival in drought conditions is now a salt-tolerant mushroom, which can metabolize with very little water and undergo cell division. Scientists determine how much water is available through a measurement called water activity. In simple conditions, such as a humid atmosphere, this is equal to relative humidity, which is the amount of water for the maximum amount of temperature and pressure. However, it can also be measured taking into account dissolved salts or ice components, etc.
For severe acids, there are microorganisms that live up to a pH of -0.06, which corresponds to just over 10% sulfuric acid by weight of the solution (the remainder being water).
Not all clouds bring rain
Applying this information to the conditions of Venus produces dismal results. Based on measurements from the atmosphere, the researchers estimated that Venus’ relative humidity of less than 0.4% would be more than 100 times lower than the low record that organisms on Earth have tolerated.
Assuming that life on Venus evolved by drawing water from a lean atmosphere, sulfuric acid is a big problem. The researchers estimated that there would be so little water in the droplets that formed around the sulfuric acid that the sulfuric acid concentration would be at least 78%. The droplets will be maxed out like pure sulfuric acid with a little water.
At this point, the acidity of sulfuric acid is less of an issue than its ability to chemically break down molecules to form new water molecules to dissolve. A graphic demonstration of this process is shown in this video, which shows that removing water from sugar turns sugar into pure carbon. The authors of the research paper listed all the issues: “Sulfuric acid dehydrates cellular systems, removes water from biomolecules, reduces hydrophobic interactions, and impairs the integrity of the plasma membrane.”
With Venus excluded, researchers have turned their attention elsewhere in the solar system. Mars’ clouds are at a much lower temperature than the point at which metabolism on Earth completely ceases, according to measurements made by probes through the atmosphere. In a good way, all the water present is ice that has received enough UV light to sterilize. So the Martian clouds were also excluded.
What about Earth and Jupiter?
Although Earth’s upper atmosphere is too dry to support life, the relative humidity of the lower atmosphere (tropospheric) can vary from 0% to 100%. However, most clouds in the troposphere will have life-compatible pollinating activities, consistent with the finding that various microbes can survive their journey through the clouds ending up with each other.
Finally, the most unusual discovery comes from Jupiter, visited by a probe dropped during a Galileo mission. Although the probe has landed on a dry region of a giant planet’s atmosphere, we do know that other cloud bands may have different compositions, some of which are likely to be very wet. Ammonia is a complex entity, but most of the time it exists at higher altitudes than those where the temperature is within the range of biocompatibility.
Although there are many uncertainties, the general conclusion is that there is likely to be enough water to sustain life at altitudes with temperatures ranging from -30°C to 10°C.
life is like that
The researchers point out that this same approach will help us see life at higher altitudes as we begin to gain details about the atmospheres of exoplanets. However, it does not provide any information about the state of the surface (some of which can be inferred from other data). “It’s personally exciting to be able to determine potential habitability based on that,” Hallsworth said.
Another thing worth noting here is that this applies to life as we know it. It relies on water and makes extensive use of hydrophilic and hydrophobic interactions with hydrocarbons. Different liquids have very different boiling and freezing points and favor very different chemical uses. So far there are no indications that life could form within it, but it is still an interesting possibility. As Chris McKay of NASA’s Ames Research Center said at a press conference, “I hope that some of me will really be different when they find life elsewhere.”
Natural Astronomy, 2021. DOI: 10.1038 / s41550-021-01391-3 (DOI information).