Twenty-four hours ago, at the time of writing, we were alone in the Universe, as best we knew. Over the course of Monday 14th September 2020, that fact may have changed forever.

Research, led by Professor Jane Greaves, published in Nature journal on Monday morning and honoured with a special episode of The Sky At Night, has strongly suggested the presence of the biosignature phosphine (PH₃) in the atmosphere of Venus. In this article, I’ll explain how this discovery was made, what it might mean, and why it may have shattered the scientific paradigm.

What were the observations?

Using the 15-meter James Clerk Maxwell Telescope (JCMT) on Mauna Kea, Hawaii, and inspired by Carl Sagan’s idea of life in Venusian clouds, Professor Greaves’ team have become the first researchers in history to look for a particular spectroscopic sign of life on another planet and to find exactly what they were looking for.

Spectroscopy relies on quantum theory, which predicts that every molecule vibrates in very particular ways. Molecules, as collections of atomic nuclei ensconced in “clouds” of electrons, have particular and very exact energy values corresponding to these precise vibrations. When a molecule is hit by waves of light that match its precise energy requirement, it absorbs that light and performs a vibratory jig.

Physicists and chemists take advantage of this behaviour of molecules because it means each molecule absorbs a unique array of wavelengths of light. By measuring exactly which wavelengths get absorbed on passing through a sample of gas and computer-analysing the spectrum produced, we can deduce the presence of certain compounds.

“The presence of phosphine on Venus is totally inexplicable by current science.”

That’s why, when Professor Greaves performed her 18-month analysis of the radio wave data from Venus and discovered the sharp dip of radio wave intensity at exactly 1.123mm in wavelength (the characteristic value for phosphine), she knew phosphine must be present.

To check that it was no fault of the instruments used, Professor Greaves did two things: first, she booked out another telescope — this time, the huge ALMA array of 66 radio telescopes in Chile, intended to observe star formation in the early Universe. Second, she looked for deuterated water (HDO), well-known on Venus and similar in vibrational energy to phosphine. ALMA confirmed the sightings of the JCMT, and deuterated water was found exactly as it should be: the equipment was not faulty.

Why is phosphine important?

Phosphine (PH₃) is not, in and of itself, an incredibly exciting compound. It’s composed of just one phosphorus atom and three atoms of hydrogen, arranged in a triangular-based pyramid with the phosphorus at the central vertex. On Earth, it is produced only from industrial processes in factories or by certain microbes decaying organic matter, giving phosphine the characteristic smell of dead fish. In fact, phosphine-producing bacteria live in the guts of penguins, so PH₃ is found in abundance in penguin faeces.

However, the presence of phosphine on Venus is totally inexplicable by current science. There are no penguins, for one, and no factories either. Venus also has an atmosphere consisting of so much sulfuric acid (being around a billion times more acidic than Earth’s atmosphere and having clouds of pure acid) that the highly-reduced phosphorus of phosphine molecules would be oxidised and the molecule would break apart in a matter of hours.

While a phosphine concentration of 20 parts per billion doesn’t sound too impressive, given the rapid breakdown of phosphine by sulfuric acid, we should see approximately 0 phosphine molecules on Venus. Unless, of course, the phosphine on our sister-planet is being produced continuously.

Does phosphine mean life?

No, not necessarily. As I mentioned, we know how to make phosphine inorganically — we do it in factories all the time. In fact, there are dozens of reactions we can use to make phosphine, many involving the hydration of metal phosphides releasing PH₃ gas. If these are possible in labs on Earth, why can’t they be invoked to explain the presence of phosphine on Venus?

As Dr William Bains explained in Monday night’s special episode of The Sky At Night, around 75 different possible reaction pathways for phosphine were independently considered, and none were satisfactory.

“If there is life on Venus, then Professor Greaves hasn’t just discovered life on Venus, she has discovered life everywhere.”

There is an important concept in thermodynamics known as the “Gibbs free energy”. This Gibbs energy is defined as the maximum amount of energy you can extract from a substance without expanding or compressing it — in essence, the energy you are “free” to have.

In nature, reactions will always tend, from start to finish, to decrease the amount of free energy hanging around. That’s known as the Second Law of Thermodynamics: the change in Gibbs free energy for any reaction must be negative.

For every single pathway tested by Professor Greaves and Dr Bains, the change in Gibbs energy was positive. They were thermodynamically impossible reactions under Venusian temperature and pressure — there must be some other explanation. Life is the obvious remaining conclusion, though we should be cautious to claim it as the only possible explanation.

Other possibilities include production via complex volcanic geochemistry, lightning strikes, and meteorite impacts, but none of these are satisfactory either. Venus would have to be at minimum 200 times more volcanic than Earth to produce a consistent and large enough supply of phosphine. Lightning is actually less frequent on Venus than on Earth and produces around 10,000,000 times too little phosphine. Meteorites could only deliver a few tonnes of the stuff to Venus’s atmosphere per year — a negligible amount. Even simple photolysis (splitting apart molecules using UV light) produces reaction rates between 10,000-100,000 times too slow. This is all explained by Dr Bains et al. in a paper entitled “Phosphine on Venus cannot be explained by conventional processes.”

What would life be like on Venus?

Unpleasant, to say the least. Venus has an incredibly acidic atmosphere which is 500°C at the surface and has a pressure akin to being 3000ft underwater. In some ways, as confessed by Professor Greaves herself, Venus is the last place you’d ever look for life in our Solar System.

However, as Carl Sagan pointed out in the 1960s, around 50km above the surface of Venus, the atmosphere becomes rather temperate, even pleasant (if you ignore the lethal acidity). Temperatures and pressures are comparable to Earth’s. There are even plans in the NASA archives for “aerostats” — giant, blimp-like human habitats floating at these ambient levels above the Venusian surface. This was the level at which phosphine was discovered by Professor Greaves.

Interestingly, exactly as you’d expect if some phosphinogenic microbial life is circulating in the atmosphere, there is no phosphine at the poles. This is because cold, sinking gas carries them down, smothering their biosignatures. All of the phosphine is concentrated in mid-latitudes, on rising atmospheric currents at suitable temperatures.

Implications of life on Venus

One term that kept being thrown around in Monday’s episode, even by Professor Greaves, was “bacteria”. A loose usage of this word implies that life must be something like we have here on Earth, when, in fact, nothing could be further from the truth. If there truly is life on Venus, entirely unrelated from that on Earth, then it is totally unconstrained by the terrestrial domains of life: Bacteria, Archaea, and Eukarya. It wouldn’t even necessarily consist of DNA — many other forms of chemically encrypted information are possible, and the acidity of Venus may rapidly corrode the sugars in the backbone of DNA anyway.

From 1859, when Darwin and Wallace proposed descent by natural selection, to the present day, evolutionary biology has left no stone unturned in the tree of life — quite literally. However, the actual origins of all life on Earth are rather more difficult to explain, invoking well-known theories such as the “primordial soup” as well as reams of others.

The odds that life only evolved twice in the entire spatiotemporal expanse of the Universe, and each time was on a neighbouring planet in the same Solar System, are — if you pardon the pun — astronomical. If there is life on Venus, then Professor Greaves hasn’t just discovered life on Venus, she has discovered life everywhere.