Phosphine on Venus: a sign of life?

Scientists have detected a gas in Venus' atmosphere that they can't explain.....
22 September 2020

Interview with 

Ben McAllister, University of Western Australia

VENUS

global view of the surface of Venus

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Venus shares a lot in common with the Earth in terms of its size, proximity to the Sun, and possibly its past history. As a result, scientists have wondered for a long time if life could have got started there. But those theories were dealt what seemed like a knockout blow a few decades back when space probes revealed conditions sufficiently extreme to melt metal on the planet’s roasting hot surface. But could life nevertheless eek out an existence in the planet’s upper atmosphere, where the conditions are more mild? This week that theory became a lot more plausible when scientists announced they’d picked up the signature of the chemical phosphine, regarded as a sign of life, coming from clouds above Venus. Ben McAllister…

Ben - As far as planets in our solar system where you wouldn’t want to live are concerned, Venus, our nearest neighbour, might just take the cake.

It’s famously inhospitable, with surface temperatures hot enough to melt lead, thick clouds of sulphuric acid, corrosive rain, and crushing surface pressures. For life as we know it, it’s hell.

And yet, despite all of that, earlier this week an international group of astronomers reported something truly remarkable. In Venus’ upper atmosphere, they detected the presence of a chemical compound known as phosphine - a phosphorus atom joined to three hydrogen atoms, which is widely regarded by scientists as a signature of life.

If the hypothesis is correct, it’s difficult to overstate how big of a deal this is. It would be the first ever detection of extraterrestrial life; proof that we truly aren’t alone in the Universe.

Now, don’t get too carried away - nobody is theorising little green men just yet. If we are looking at life here, it is very likely some kind of extreme microbial life, tiny organisms capable of surviving in the harsh, acidic environment of Venus’ upper cloud decks.

So how did the team discover this?

They used a technique discovered originally by Robert Bunsen - of “burner” fame - back in the 1800s. His breakthrough was to realise that different chemicals absorb and emit specific colours - or wavelengths - of light, and a particular combination of colours are unique to a given chemical compound. So by measuring the colours of light that the atmosphere of Venus, or another planet, absorbs and emits, we can identify the cocktail of chemicals that are present without having to actually go there. 

The team turned the Hawaii-based James Clerk Maxwell Telescope and the South American ALMA telescope to stare at Venus and record the light coming from the planet’s atmosphere.

What jumped out from the data was a signal consistent with phosphine at a concentration of about 20 parts per billion, indicating possible life in the upper atmosphere.

But phosphine isn’t just produced by life. There are other natural chemical processes that can produce it. And it’s also been found elsewhere in the solar system before - for example, around gas giants like Jupiter.

However, on a rocky planet like Venus, with the kinds of conditions we know exist there, we aren’t aware of any mechanisms for making phosphine other than as a by-product of living organisms.

So, it is possible there is some new chemistry, or some other as-yet-undiscovered mechanism which is responsible for the presence of phosphine in the atmosphere of Venus, and it definitely requires further study to confirm - but for now, it’s arguably the strongest signature of extraterrestrial life we have ever detected.

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