Last year, a study made waves by suggesting a chemical that had been proposed as a potential indicator of life was present in the atmosphere of Venus. While the hellish conditions on the planet's surface preclude the existence of any sort of life there, it remained possible that a milder environment existed in the planet's clouds, high above its surface. So the prospect that the chemical was indicating life couldn't be immediately discounted.
In the months following, other researchers cast doubt on the claim that the chemical was present at all. And today, a paper is being released the suggests that the conditions in Venus' clouds are in no way compatible with life even remotely similar to that on Earth. Although the temperatures in the clouds are indeed milder, there's nowhere near enough water to support life, and most of what's present is in droplets that are mostly composed of sulfuric acid.
Setting limits
In a press conference announcing the results, John Hallsworth of Queen's University Belfast said that the new work was inspired by the apparent detection of phosphine in Venus' atmosphere. He and his collaborators realized that two areas of research had combined to create other ways to examine the prospects of life on Venus. One was a study of life in extreme conditions on Earth, driven in part by a NASA effort to determine how best to protect Mars from contamination by the probes we were sending there.
The second was also NASA-driven: we'd sent probes into the atmospheres of some planets and imaged others. While these probes didn't specifically look for life, they provided direct measurements of things like temperature and pressure, which set limits on things like the amount of water present in the atmosphere, and the form that it will adopt.
Relevant to Venus, people have identified organisms that can maintain a metabolism within various limits: temperature, acidity, and water content. Since temperature changes with altitude, the former sets limits on what altitudes can be considered. The latter two are relevant because Venus is thought to be a very dry planet, and its clouds are generated not by condensed water but rather by the presence of sulfuric acid droplets that would contain some water.
The world record holder for surviving dry conditions is currently a salt-tolerant fungus, which can run a metabolism and undergo cell divisions with very little water present. Scientists quantify the amount of available water through a measure called the water activity. In simple conditions like a humid atmosphere, this is the same as the relative humidity—the amount of water present versus the maximum amount at that temperature and pressure. But it can also be measured in a way that takes into account things like dissolved salts or ice formation.
For the acid extreme, there is a microbe that survives down to a pH of -0.06, which is the equivalent of having sulfuric acid account for a bit over 10 percent of the weight of its solution (with the rest being water).
Not all clouds bring rain
Applying that information to the conditions on Venus yields grim results. Based on measurements from its atmosphere, the researchers calculate that Venus' relative humidity would be below 0.4 percent—over 100 times lower than the record low tolerated by an organism on Earth.
If you assume that Venusian life could have evolved methods of pulling water out of the sparse atmosphere, then the sulfuric acid becomes a big problem. The researchers calculate that the droplets that form around sulfuric acid would have so little water that concentration by weight of sulfuric acid would be 78 percent—at a minimum. The droplets would max out as nearly pure sulfuric acid with a tiny bit of water.
At that point, the acidity of the sulfuric acid is less of an issue than its ability to chemically degrade molecules in order to form new water molecules to dissolve in. A graphic demonstration of this process is available in this video, which shows sugar converting into pure carbon when the water is stripped out of it. The authors of the paper list all of the problems it creates: "Sulfuric acid dehydrates the cellular systems, removes water from biomacromolecules, reduces hydrophobic interactions, and damages plasma-membrane integrity."
With Venus ruled out, the researchers turn their attention to elsewhere in the Solar System. Mars' clouds are at temperatures well below the point where metabolisms shut down entirely on Earth, based on measurements made by probes that have gone through its atmosphere. Any water present is ice that, for good measure, is bombarded with enough UV radiation to sterilize it. So Mars' clouds are ruled out as well.
What about Earth and Jupiter?
Earth's upper atmosphere is also likely to be too dry to support life, but the relative humidity of its lower atmosphere (the troposphere) can vary anywhere from zero percent to 100 percent. Most clouds in the troposphere, however, will have a water activity that's compatible with life, which is consistent with findings that a variety of microbes probably survive the trips through the clouds that some of them end up taking.
Finally, the oddest finding comes from a look at Jupiter, which had been visited by a probe that was dropped off during the Galileo mission. The probe happened to fall through a dry region of the giant planet's atmosphere, but we know that different cloud bands can differ in their composition, and some of them are likely quite wet. Ammonia is a complicating presence but is mostly present at altitudes above those where temperatures are within the range compatible with life.
While there are a lot of uncertainties, the overall conclusion is that there's likely to be enough water around to support life at altitudes where the temperatures would range from -30º to 10º C.
That’s life
The researchers note that this same approach should help us look into high-altitude life as we begin to get details on the atmospheres of exoplanets. It won't tell us anything about surface conditions, however (although some of those could probably be inferred from other data). "To be able to pinpoint potential habitability based on that personally excites me," Hallsworth said.
The other notable thing here is that this applies to life as we know it: water-based, with extensive use of hydrocarbons and both hydrophilic and hydrophobic interactions. Other liquids have very different boiling and freezing points and would favor very different chemistries. To date, we have no indication that life could form within them, but it's still an exciting possibility. As Chris McKay of NASA's Ames Research Center said during the press conference, "Part of me hopes that, when we do find life elsewhere, it's really, really different."
Nature Astronomy, 2021. DOI: 10.1038/s41550-021-01391-3 (About DOIs).
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