Astronomers are considering the possibility that life exists on Venus, after discovering phosphine in its atmosphere.

On Earth, phosphine is a biosignature, a molecule produced by life. It’s a waste product made by anaerobic bacteria living in oxygen-poor environments, such as swamps and inside the guts of penguins. Apart from industrially-produced phosphine, it is not found anywhere else on Earth.

Why then, has phosphine been detected in the clouds 50km above Venus’s surface? Professor Jane Greaves, from Cardiff University, and colleagues are asking this very question.

 

What is the environment like on Venus?

Venus has never been the first place to look for life elsewhere in our Solar System. The atmospheric pressure is 93 times greater than on Earth, and with a 96% carbon dioxide atmosphere, Venus has endured a “runaway greenhouse” effect resulting in surface temperatures exceeding 400°C.

This was problematic for early spacecraft attempting to land on the planet. In 1966, the Venera 3 Soviet space probe crash-landed on Venus, making it the first spacecraft to reach the surface of another planetary body. Over the years that followed, Venera 4, 5, and 6 all failed to make it to the surface. In 1970, Venera 7 was the first spacecraft to land successfully on Venus and recorded surface temperatures of a searing 455-475°C.

Despite this, the temperature and pressure in the clouds just 50km up are similar to Earth’s surface. At a comfortable 20-30°C, if life were to exist on Venus, the clouds are where we’d find it.

How was the phosphine detected?

When searching for life beyond Earth, astronomers look for gases that could indicate life. For example, methane on Mars could be produced by life. But methane is also made by natural chemistry in the environment. Phosphine, however, is more difficult to produce through chemical means, making it an ideal biosignature. The team behind the discovery hypothesised that if life exists in the clouds of Venus, it would produce phosphine.

Greaves and colleagues used the James Clerk Maxwell Telescope (JCMT) in Hawaii to look for phosphine on Venus. JCMT is a large radio telescope, which means it measures radio waves, part of the electromagnetic spectrum with longer wavelengths than visible light.

James Clerk Maxwell Telescope, used for the initial detection of phosphine on Venus. Credit: Denys via Wikimedia Commons (CC BY 3.0).

When light from the Sun hits Venus, it is reflected. Molecules in the planet’s atmosphere absorb some of the light, creating gaps in the reflected light spectrum called absorption lines. Phosphine absorbs light at a specific wavelength of around 1mm, so if any phosphine was present, the emission spectrum would show a dip at 1mm. The team tuned JCMT to that specific frequency and looked for the dip in the signal.

But it wasn’t as simple as looking at a clean spectrum. JCMT is a bit telescope, and Venus is very bright, so the light bounced around the telescope creating noise in the spectrum. Initially, the data were too noisy to discern a meaningful signal.

After receiving funding from the University of Cambridge, Greaves was able to spend more time looking at her data. Finally, 18 months after the data were collected, she found the dip in signal she was looking for.

But, as per the scientific method, the results must be repeatable.

Greaves presented her data to the director of the Atacama Large Millimeter/submillimeter Array (ALMA), Chile, another radio telescope which could measure the light reflected from Venus. Despite three earlier rejections from ALMA, the team were allowed 1.5 hours of time with the telescope to confirm the discovery of phosphine.

Atacama Large Millimeter/submillimeter Array used to confirm the phosphine measurements. Credit: ESO/C. Malin via Flickr (CC BY-NC-ND 2.0).

Three months later, Greaves received the data and the spectrum showed the dip characteristic of phosphine. Now Greaves had a robust result: two independent telescopes had detected phosphine on Venus.

 

What if this isn’t life?

Astrobiologist Dr William Bains, from MIT, was tasked with ruling out alternative explanations for the phosphine. He asked the question of whether any natural chemical reactions in Venus’s atmosphere could produce phosphine.

Bains identified around 75 combinations of different atmospheric gases that could react to produce phosphine, but all of them required the input of energy and were therefore not spontaneous reactions. Adding an energy source, such as UV light which splits molecules into highly reactive components, yielded a further set of phosphine-producing reactions. But these were still 10,000 times too slow to account for the level phosphine observed by Greaves.

Furthermore, the phosphine in Venus’s clouds is being replenished. Phosphine lasts for mere hours before it breaks down, so the fact that we can observe it in this quantity means it is being produced all the time.

No known reaction on Venus produces enough phosphine, so life could well be the answer. But even in the clouds, the environment is not exactly pleasant.

Life, but not as we know it

High levels of sulphuric acid pose a problem for any potential life. Life requires a solvent, and the only solvent available in the clouds is sulphuric acid, a highly corrosive acid, and one that’s incompatible with known biochemistry.

This means that there are only two ways that life could plausibly exist. First, Venusian biochemistry could be radically different to that on Earth. Secondly, life could be similar to that on Earth, using water as a solvent, but encapsulated in a “protective shell”, inside droplets of sulphuric acid. This shell would need to be tougher than Teflon to withstand the extreme acidity, while still letting the bacteria exchange gases for energy.

Are we alone?

A long-standing question in humanity is: are we alone; is there life elsewhere? If there is life on Venus, and it evolved independently to life on Earth, the implications are huge. Life would be everywhere in the Universe. It would increase the chances of finding life on Mars, on moons such as Europa and Enceladus, and on other planets in distant solar systems.

The next step is sending a space probe to fly through Venus’s atmosphere to collect and analyse cloud samples, looking for life.

We do not yet know enough about the chemistry of Venus’s atmosphere to say whether or not there is life in the clouds of our twin planet. But this could absolutely be the first step towards discovering life beyond Earth.

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