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Tuesday, 6 October 2020

Is phosphine a biomarker for life on Venus?

50 km up in the atmosphere of planet Venus, scientists found phosphine[1].

Cloud structure in the Venusian atmosphere by ISAS/JAXA, via Wikipedia Commons.
Sorry, what?

Phosphine.

You know, PH3. That commonly known… no?

It might not sound like much, but this innocuous molecule defies explanation.

Why should it?

It may not be well known, but this is not the first time we’ve heard of phosphine. The molecule itself is not mysterious. A simple combination of hydrogen and phosphorus, it’s a pungent, colourless gas, that’s both toxic and flammable. It’s highly reactive (that’ll be important later on) and belongs to the same family as ammonia. We can make it in lots of different ways – by exposing white phosphorus or calcium phosphide to water, by the disproportionation of phosphorous acid, or by reacting phosphonium iodide with bases. But it also occurs naturally on Earth.

 

Phosphine on Earth

 
Surely that means it’s not special? Well, yes and no.

The amount of phosphine in the Earth’s atmosphere is tiny – on the order of parts per trillion. And it varies quite a lot. This isn’t a surprise because the atmosphere is oxidising, and quickly breaks the phosphine down. The fact that there’s some there at all tells us it’s getting continuously formed and replaced.

We’re not sure exactly where atmospheric phosphine comes from on Earth, but we think it comes from reduced phosphorus in decaying organic matter. The environment isn’t reducing enough to do this, but anaerobic microbes are. We think volcanic action, lightning, or meteorites in the atmosphere create some phosphine too, but significantly less than the microbes. This makes phosphine a biomarker – an indication of life.

 

Phosphine on Jupiter and Saturn

 

Jupiter. NASA, ESA, Simon (Goddard Space Flight Center)
However, Venus isn’t the first planet we’ve found phosphine on. It’s on the gas giants Jupiter and Saturn too. And they have much more than Earth – 4.8 and 15.9 parts per million respectively[2].

Phosphine forms on the gas giants in what is known as “the deep atmosphere”. Here, high pressures and temperatures over 800 K can start the energy-intense reaction, making phosphine “thermodynamically favourable”. Convection currents stir phosphine throughout the atmosphere, replenishing the uppermost layers where otherwise, exposed to UV light, it breaks down rapidly. The convection currents are faster than the reactions, and so there is more phosphine found at the cloud tops than there “should” be[2]. This phosphine may even explain the reddish glint to Jupiter’s clouds (but scientists aren’t sure).

 

Phosphine on Venus

 
So why is phosphine on Venus so mysterious?

We know the phosphine didn’t get left there by humans messing around in the atmosphere. The phosphine is detected remotely, using spectroscopy[1]. Phosphine is identified when a specific frequency of microwave radiation gets detected: it’s “signature” frequency. Besides, there isn’t as much phosphine on Venus as on Jupiter and Saturn – around 20 parts per billion – but that’s still a lot more than there is on Earth[1]. Venus is hot, but it’s not hot enough to make phosphine the way Jupiter and Saturn do, and its atmosphere is highly oxidising, so it should break down and go away much faster. It is very volcanic, but scientists don’t think that can explain the amount of phosphine they’ve detected. So maybe, maybe, there’s life.

At first, this seems unlikely. With a surface temperature of about 698 K (400 degrees hotter than room temperature in the UK), Venus is considered a lifeless hell. But as you move further away from the planet, out through its atmosphere, it gets cooler. And this is where some scientists have postulated life could be, in what is known as the biosphere. It’s also where the phosphine was found.

It’s also important to remember that, back in Earth’s history, it was rather like Venus – volcanic. And we got life.

But it’s not the only explanation. There could be other photochemical or geochemical reactions we don’t know about going on. Whatever it is, there’s something new and exciting to be discovered...

 

References
why don't all references have links?

[1] Greaves, Jane S., et al. Phosphine gas in the cloud decks of Venus. Nature Astronomy (2020): 1-10. doi:10.1038/s41550-020-1174-4.
[2] Sousa-Silva, Clara, et al. Phosphine as a biosignature gas in exoplanet atmospheres. Astrobiology 20.2 (2020): 235-268.

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