Life in the clouds sounds romantic, except when those clouds are made of sulphuric acid.
Yet scientists are pondering whether life could exist around Venus after a rare chemical was found in its atmosphere.
The discovery, published today in the journal Nature Astronomy, not only has implications for Venus, but the search for life beyond our solar system.
Phosphine, a short-lived toxic gas, is produced on Earth either by microbes that thrive in the absence of oxygen, or by industrial processes.
« [The discovery] suggests either some exotic chemical process occurs we haven’t got or thought of on Earth — or maybe that some kind of very robust organism survived the runaway greenhouse effect, and evolved up to live in the clouds, » said Jane Greaves of Cardiff University.
The idea that life could exist in the clouds of Venus is not new.
It was considered a possibility right up until the 1960s when spacecraft sent back the first details of our closest planetary neighbour.
Those craft revealed thick, acrid clouds that let through only 3 per cent of sunlight and a surface temperature that exceeds 470 degrees C. It was the epitome of hell.
Despite high levels of acidity, the clouds 50 to 60 kilometres above the surface are a balmy 30 degrees C. And that’s where phosphine has now been found.
« This brings back into play the longstanding — but not much tested — idea of habitable cloud decks on Venus, » said Professor Greaves, who led the international research team.
Professor Greaves and her team first detected the tell-tale wavelength signature emitted by the molecule using the James Clerk Maxwell Telescope in Hawaii.
« When Jane sent me the spectrum I sat in front of my computer blinking for about half an hour. I didn’t believe she’d actually found it, » said the telescope’s deputy director, Jessica Dempsey.
Further observations at the more sensitive Atacama Large Millimeter/submillimeter Array (ALMA) in Chile confirmed the result.
« We aren’t saying it’s a 100 per cent robust detection of life, but what we can say is that we’ve opened it up to the possibility that it is that, » Dr Dempsey said.
Professor Greave’s colleagues have modelled a number of different scenarios — including chemical reactions in the clouds, minerals being blown up from the surface, volcanoes, lightning or delivery by meteors — and none could explain the amount of phosphine seen the atmosphere.
And while the gas has been detected in the atmosphere of Jupiter and Saturn, Professor Greaves said the same processes do not exist on Venus.
« [On these planets] there is abundant hydrogen to form it at high pressure, deep down, and then it wells up. [But] free hydrogen isn’t present on Venus or Earth, » she said.
Is it life, but not life as we know it?
If life did exist in the clouds of Venus it would be very different to anything on Earth, Dr Dempsey said.
« We know that extremophiles on Earth can exist in acidic environment, but that acidic environment might only be 5 per cent, and it’s still very much water based.
« Whereas the clouds in question we’re talking about are more like 90 per cent acid. »
Venus was probably habitable up until about a billion years ago, said planetary astrophysicist Stephen Kane of the University of California, Riverside.
While life could no longer exist on the blistering surface, Dr Kane said it would be extraordinary to know whether any last traces of life from that time could still survive in the clouds.
But he believes the signal is more likely to come from a geological or chemical process we don’t understand.
« We do see phosphine produced on Earth by biological processes, much more so than geological, but that’s not to say it would happen that way everywhere, » he said.
Lucyna Kedziora-Chudczer, a planetary astrophysicist at Astronomy Australia who studies the atmosphere of Jupiter, agreed
« There are too many difficulties to justify the similarities of life that we have on Earth happening on Venus, » Dr Kedziora-Chudzer said.
But, she said, the discovery of the gas — a difficult feat to have achieved — adds to our toolbox for looking for life on other planets.
The presence of phosphine has been proposed as a potential marker of life around alien rocky planets like ours.
The discovery of the molecule around Venus still needs to be verified by other studies, said Frank Mills, a planetary scientist at the Australian National University.
But if phosphine really is present and is a so-called « biosignature », it would expand the scope of feasible extraterrestrial life being considered, said Dr Mills, who worked on Europe’s Venus Express spacecraft.
Venus close enough to find out if it is real
The beauty is that Venus is close enough for us to explore that question, said Dr Kane, who is working on one of the two discovery projects greenlit by NASA to go to the planet in the next decade.
And the discovery renews focus on Venus.
The last NASA mission specifically tasked with exploring Venus was in the 1990s when the Magellan spacecraft mapped the planet’s surface before plunging through its atmosphere.
Since then, orbiters from the European Space Agency (Venus Express) and Japan (Akatsuki) have studied the planet’s atmosphere and weather from afar.
« Venus is a very mysterious planet in many ways … but we have so much to learn, » Dr Kane said.
Dr Dempsey agreed.
« We are really glad there is a new focus coming back on our next door neighbour, » she said.
« We’ve been looking in the other direction for life on planets, » she said of the focus on Mars, and moons of Jupiter and Saturn.
« Maybe we were looking in the wrong direction. »
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