Acid-loving microbes may have evolved in the thick, highly-acidic clouds swirling round Venus, and might even still be there.
Prospects for Venusian life have generally been dismissed because of the harsh conditions on the planet鈥檚 surface. But discoveries of ever more microbes on Earth that live in highly acidic conditions is strengthening the case that life may be able to thrive in Venus鈥檚 dense cloud layer.听About 50 kilometres above the planet鈥檚 hellish surface, sulphuric acid clouds have both milder temperatures and pressures than the surface.
The case to re-think the possibility of life on Venus was made in Vienna this week at the annual meeting of the European Geosciences Union. at the Science and Life Foundation in Santiago, Chile, presented a poster on the abundance of acidophile microbes throughout the tree of life on Earth.
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鈥淓very single branch of life has acidophiles,鈥� says Holmes. In his Vienna poster, Holmes showed that around 20 acidophiles appeared across all three of the major forms of life on Earth: eukaryotes, which include everything from yeast to humans; bacteria, which include the first acidophiles found; and archaea, primeval organisms with no cell nucleus.
Branching out
There are examples of acidophiles among cyanobacteria, themselves very ancient, and among types of bacteria common in the human gut. Holmes says that so many species of acid-loving creatures have now been discovered on Earth that they can no longer be regarded as freakish curiosities, and so could equally have evolved and thrived in acid-rich environments elsewhere in the solar system.
鈥淭he organisms we work with love sulphuric acid,鈥� he says. 鈥淭hey won鈥檛 grow unless you give it to them.鈥� They also consume the exotic sulphur compounds that have been identified in Venus鈥檚 clouds, including polymers such as sulphane sulphur.
The arguments against the possibility for life on Venus remain strong, however. At the planet鈥檚 surface, there鈥檚 no water, temperatures reach 477掳C, and the atmospheric pressure is 92 times that of Earth鈥檚 surface.
Ancient waters
Though Venus doesn鈥檛 have water on its surface now, it may have hosted liquid water on its surface for at least 2 billion years of its existence, according to simulations by of the NASA Goddard Institute for Space Studies in New York and his colleagues. Back then, these mid-altitude sulphur clouds may have been cool and water-rich enough to meet conditions for life 鈥� and they may still be.
The clouds also contain ferric chloride, a compound Holmes says could be key for recycling electrons vital for metabolism in microbes that survive on acidic sulphur-containing substances.
鈥淭he major new contribution of Holmes鈥檚 team is to elucidate how widespread in the tree of Earth life this acid-loving quality is,鈥� says of the Planetary Science Institute in Tucson, Arizona, and Way鈥檚 collaborator on the simulation work two years ago. 鈥淚t鈥檚 a mainstream feature of Earth life, and similar capabilities on Venus would make life very well-adapted to such a cloud niche,鈥� he says.
Life in the clouds
鈥淚 think what Holmes proposes is possible assuming my models are correct in estimating the possibility of liquid water on the surface of Venus for 2 billion years,鈥� adds Way.
One other challenge to microbes surviving in the clouds is gravity. They would have to multiply fast enough in the droplets to keep the colony going in the relatively cool clouds, before some fall and perish in the extreme heat and pressure lower down.
With at least three other major international missions teed up to re-visit Venus over the next decade 鈥� planned by India, the European Space Agency and NASA 鈥� backers of the case for life say they should be primed to seek signs of it. 鈥淚t would be a wasted opportunity otherwise,鈥� says Holmes.
Read more: Solar system mysteries: What happened to Venus?
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