Is there life on Venus? A multinational team of astronomers using high-altitude infrared telescopes in Hawaii and Chile have identified the gas phosphine in the upper atmosphere of the planet. So what? The significance of this discovery is that phosphine is regarded as a biosignature, possible evidence of microbial life, as Duncan Steel explains. 

Venus stinks. I’d say that is meant literally, except that no astronaut is ever likely to stand on the planet’s surface and breath deeply. The temperature there is near 500°C, and the pressure crushing, about 90 times that on Earth.

But it stinks, regardless. Clouds of sulphuric acid droplets shroud the planet, with some hydrogen sulphide (the smell of rotten eggs) mixed in too. It’s a truly inhospitable place, to humans at least. Thing is, though, it might be just the sort of place where some form of microbial life might thrive.

Scientists trying to understand the possibility of life elsewhere – nowadays termed astrobiologists – have a variety of ways of working. One is looking at terrestrial life in seemingly- impossible environments, like within solid rock, or deep within caves where no sunlight ever penetrates, or even the hot and acidic lakes in New Zealand’s geothermal areas. Such life forms are termed “extremophiles”: bugs that like extremes.

Life needs water

Anywhere we find liquid water on Earth, we find life. Naturally, we consider water elsewhere to be the most likely place we might find some type of life, perhaps under the arid surface of Mars, or beneath the icy covering of Jupiter’s large moon Europa.

Venus may be too hot on its surface for water, but we know that mixed in with its sulphuric acid clouds there is some water. It’s not a new suggestion that there might be some sort of life there – it’s just that before now we had no evidence for it.

Extraterrestrial life in your compost?

A place in your back garden that may stink is the compost heap. Strangely enough such mounds of decaying greenery can be of interest to astrobiologists.

If you keep your compost well-churned and so aerated, it can smell almost sweet: oxygen-loving microbes – aerobic bacteria – are chewing up the old vegetation and releasing the nutrients, ready for raising next year’s flowers and veggies.

If the compost heap is not kept forked- or turned-over, however, no oxygen gets in, and then a different type of microbe will flourish: anaerobic bacteria. These are baddies, and can be dangerous to us. That’s why such a compost heap smells really bad. The stench is a warning, telling you to avoid ingesting the gunk that forms. Isn’t evolution wonderful?

Swamp gas

A natural type of compost heap is a swamp: decaying botanical (and indeed zoological) matter rots under stagnant water, and one may see bubbles rising to the surface. That’s marsh or swamp gas. To a chemist, it’s methane: a carbon atom with four attached hydrogen atoms.

Methane, along with ethane and larger hydrocarbon molecules, can be drilled for, and collected as “natural gas”, often along with oil and petroleum. These molecules were made by life in the distant past, and they will not persist in an atmosphere containing oxygen: they are soon oxidised (perhaps in your car’s engine).

Finding methane and oxygen together is a sign of life, because something must be making the methane. That’s why we look for methane on Mars, and some astronomers search for its spectral signs in the planets we now know to orbit distant stars.

Phosphine: toxic but tantalising

Another gas that could be a sign of life is phosphine, and that is what has just been found in the upper atmosphere of Venus. Phosphine is somewhat like methane: it’s a phosphorus atom with three hydrogen atoms attached. Also like methane, it can be produced in an anaerobic environment, such as the unturned compost pile.

Paradoxically, phosphine is highly toxic to animals. A form of it is commonly used in rodent poisons, and a level as low as one part per million in the air could be fatal for humans. (In the first episode of the Breaking Bad TV series, it was phosphine that Walter White generated rather explosively in his caravan, taking out the two criminals he trapped inside.)

Leaving all that aside, while we might be justifiably cautious about phosphine on our own planet, finding it elsewhere in the cosmos is a cause for some excitement among those looking for extraterrestrial life. It is considered a “biosignature”: an indicator of possible biological activity, because non-biological mechanisms do not seem feasible suppliers of the sorts of amounts that have now been detected on Venus.

The clouds of Venus

Although the Venusian surface is too hot for life – at the hundreds of degrees Celsius there, complex molecules are split apart, making life impossible – up in the clouds, about 50 kilometres above the rocky planet below, the temperature is a far more life-friendly 30°C. The problem for any life theory is that the amount of acid there makes it difficult to imagine how any microbes could survive.

Regardless, finding phosphine in Venus’s atmosphere has already sparked huge interest in disparate areas of science. The original discovery team of astronomers was led by Professor Jane Greaves of Cardiff University. They teased the fingerprint of phosphine out of the infrared spectra they had recorded with the mountain-top telescopes: to observe in the infrared one needs high-altitude observatories so as to be above the water vapour in our own atmosphere, because it absorbs infrared radiation (hence global warming).

Understanding what the phospine detection means, though, requires researchers with other expertise, not least in atmospheric physics and chemistry. The consensus so far is that no known natural mechanism (such as lightning, or perhaps upwelling from the surface) could be producing the amount of phosphine measured in the peculiar Venusian clouds. At the very least, the discovery will be subject to intense scrutiny, and there will be lots of follow-up observations.

I could say “watch this space” – but that is precisely what astronomers do.

Dr Duncan Steel, an astronomer based in Nelson, once worked on Nasa’s Pioneer Venus Orbiter spacecraft.