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In an article which has just been published in the journal Astrobiology, Sara Seager of MIT and colleagues suggest a way in which microbial life could permanently reside in the lower atmosphere of Venus. The idea that microbes could exist in Venusian clouds, at an altitude of 50 to 65 kilometers above the ground, was proposed more than 50 years ago by the late Carl Sagan, and has since been put forward by many other authors, including myself.
The possibility of living in such an exotic place may seem strange at first. But keep in mind that Venus was once in the habitable zone of our solar system, and “habitability” must take into account the entire lifespan of a planet. In fact, one hypothesis holds that three of the four inner planets – Venus, Earth, and Mars – harbored surface life early in the history of the solar system. Venus got too hot afterwards, so that life can only stay in the atmosphere. Mars has become too cold, so life can only exist underground today. Only Earth is hospitable enough to maintain a diverse and buzzing biosphere.
Given that this idea has been around for years, what is new in Seager’s work? On the one hand, previous articles have not clarified what life in clouds means and how it might interact with the atmosphere. The only exception was a 2004 article by myself and colleagues, in which we pointed out that sulfur (specifically a compound called cyclooctasulfur) can be used by microbes as UV sunscreen and as a means of converting ultraviolet light into other wavelengths of light that could be used for photosynthesis. We hypothesized that this could be the basis of an ecosystem in Venus, where certain chemotrophic organisms complete the nutrient cycle.
One problem with this scenario was that many of these microbes would fall through the clouds into the lower haze layer and meet a fiery end. So we estimated that they would only reside in clouds for about a month, and that their rate of reproduction in the atmosphere would balance out the loss of microbes falling from the clouds. Seager and his colleagues came up with a much more elegant solution. They suggest that the droplet habitat in which the microbes reside would inexorably develop and be forced by gravity to settle into the warmer, uninhabitable layer beneath the Venusian clouds. As the droplets evaporate during settling, the microbes dry out and the bottom haze layer becomes a deposit for a parched, dormant life. But ascending air currents would regularly bring dormant microbes back to the clouds, where they would be rehydrated and become active again.
Gravity waves (not to be confused with gravitational waves) are an atmospheric phenomenon that can form ripples of air on bumpy surfaces. The waves then propagate vertically upward, increasing more and more in amplitude until they break just below the cloud tops. That way, the desiccated microbes that fell to the surface could cause it to return to the clouds.
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Some of these microbes would still be lost. But this kind of life cycle would increase the chances that an aerial biosphere could last for millions of years, and possibly still here today.
Certainly, the idea of life in the Venusian atmosphere remains very speculative. Seager herself emphasizes just how extreme this environment really is, more than any other known environment on Earth. But the door of possibilities has opened a little wider. And if there is any chance of living on our twin planet, we should investigate. There are some interesting ideas on how to do this, including with airships. Hopefully a space agency will get there.
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