Phosphine On Venus — The Tantalising Possibility Of Chemistry Of Life On Earth’s Twin

In neighbouring Venus, scientists have detected a rare gas. Phosphine — think of it like phosphorus in wedlock with three hydrogens — was discovered in a section of the Venusian atmosphere some 50 kilometres above the planet’s surface.

This has sparked the unlikely possibility of life floating in the clouds of Venus, since phosphine is considered a marker of life. But it may also be possible that some unknown chemistry is driving the production of the molecule.

The search for phosphine was launched by a team of scientists led by Professor Jane Greaves of Cardiff University in 2016. The idea was to look for it as evidence for the presence of microorganisms on Earth’s twin.

Two sophisticated machines were used for the purpose — the James Clerk Maxwell Telescope in Hawaii and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Upon studying the data emerging from each observatory, the Hawaii telescope provided the detection of phosphine while the Chile one, much later, confirmed the earlier detection.

“The fact that they were able to get the signal, though it was weak, twice means that they are on to something,” says Dr Sanjay Limaye, an expert in the atmospheres of planets who had earlier hypothesised the presence of microbial life in the clouds of Venus.

“Confirmation in other spectral lines should follow and if, after that, phosphine is detected, it boosts the possibility of life on Venus because phosphorus is an ingredient of life,” he says.

At 20 parts per billion, researchers detected a relatively large amount of phosphine in the clouds. In comparison, phosphine levels on Earth are counted in parts per trillion.

Modelling the chemistry for Venusian conditions, from photochemical processes to chemical processes calculated from thermodynamics and rock chemistry, has not been able to account for the large quantities detected. Not even close.

The implication is that either chemistry does not explain the production of this phosphine or that some other unknown chemistry is at work and waiting to be uncovered through further investigation.

The possibility that has caught the attention of scientists and public alike since the announcement was made 14 September night, is that biology, or life, may be creating the phosphine on Venus.

This speculation has gathered steam because of how phosphine is produced on Earth. On our planet, phosphine comes about in a couple of ways — one associated with life, another with nature. The life association comes from microorganisms called anaerobic bacteria that live in oxygen-free environments. Plus, humans can make phosphine in a laboratory.

In nature, processes such as lightning or volcanic activity are known to produce the substance in small amounts.

Laboratory concoction can be safely ruled out on Venus, leaving us with the chance that microbes are behind the phosphine production — more a highly educated guess than science.

Life On Venus, A Living Idea

The possibility of life on Venus has been discussed for decades.

Published in 1967, a paper authored by noted biophysicist Harold Morowitz and astronomer and science communicator Carl Sagan had said: “While the surface conditions of Venus make the hypothesis of life there implausible, the clouds of Venus are a different story altogether.”

Morowitz and Sagan speculated that if a small amount of minerals could drift upwards from the surface to the clouds, where conditions were relatively more favourable, it would not be “difficult to imagine an indigenous biology in the clouds of Venus”.

Astrobiologist David Grinspoon made his case in the late 1990s. He suggested that the chemical disequilibrium in the trace-gas constituents on Venus could be attributed to the presence of microbes.

In 2002, Dirk Schulze-Makuch and Louis N Irwin proposed a space mission with the aim of flying through the potentially habitable cloud layer on Venus so that a sample could be picked up and brought back for study on Earth.

They went by models that said Venus harboured favourable conditions for life in the early solar system when the Sun was fainter. As per that understanding, life may have subsequently relocated to more habitable zones like the cloud cover as surface conditions turned hostile.

More recently, Dr Limaye wrote about the potential for life in the clouds of Venus.

The senior scientist at the University of Wisconsin-Madison told Swarajya that the inspiration for his idea came in Ladakh. He was there to check the acidity levels of thermal springs. His ‘pH’ measurement exercise was motivated by two factors — a) life is believed to have originated on Earth in the hydrothermal springs, and b) Venus has hyperacidic conditions.

If he could find acidic conditions in these springs, he thought, he would be on to something. Unfortunately, he said with a laugh, “all of them were alkaline, none of them acidic”.

Later, when he was in Tso Kar, a salt lake in Ladakh, he witnessed the flight of salt dust. “And that was the eureka moment for me, to see that the salt dust could be lifted up and rise in the air.”

This sparked the possibility that microbes may have risen up to the clouds from the surface of Venus for survival, suggested earlier by Morowitz and Sagan, and Schulze-Makuch and Irwin.

On Earth, this upward drift of life forms is confirmed. Specially equipped balloons have detected microorganisms at heights of about 40 km in the atmosphere.

Naturally, Dr Limaye and team suggested this transport mechanism while discussing the possibility of life on Venus in their 2018 paper.

The broader hypothesis came about from a chance encounter — a meeting with Grzegorz Słowik at a workshop for teachers. Slowik had noted that bacteria on Earth exhibited light-absorbing properties that could account for the dark patches on Venus, a century-old mystery since they were first observed through ground-based telescopes.

The paper by Limaye, Slowik and others proposed that microorganisms in the Venusian atmosphere could be behind the absorption of light.

In the background of this decades-long chain of fiddling with the possibility of life on Venus has come the latest speculation — inferred from the presence of phosphine in the clouds.

Atmospheric Slice Of Possibility

Venus is dissimilar to Earth in important ways from the prism of life.

The surface temperature is at over 450 degrees Celsius compared to Earth’s average of around 15 degrees Celsius. The atmospheres are vastly different, with nitrogen and oxygen making up most of Earth’s air while carbon dioxide envelopes Venus, with a little sprinkling of nitrogen.

The topography is different too, as also are atmospheric pressure at the surface and even the direction in which the Sun rises on each planet (the Sun rises in the west on Venus).

But at the elevation that phosphine has been detected in the latest study, Earth and Venus experience a sort of meeting point in temperature and pressure.

“If you walked out of the door now, it would be like Venus in that zone. Except that you would be in the middle of a sulphuric acid mist and there would be 15-20 km of cloud still above you,” Dr Limaye tells Swarajya.

It is here that life may be possible on Venus. And that’s a big may.

The scientists behind the research are exercising restraint in their pronouncements too. “We are not claiming we have found life on Venus,” said professor Sara Seagar at the virtual press conference of the Royal Astronomical Society. “We are claiming a confident detection of phosphine gas whose existence is a mystery.”

It’s The Chemistry, Stupid?

Keeping the biology question aside, phosphine detection in the clouds may be coming from unfamiliar chemical processes. As the researchers themselves have noted, some “unknown photochemistry or geochemistry” may be playing a role.

Dr Sudha Rajamani, associate professor and deputy chair, biology, at the Indian Institute of Science Education and Research (IISER), Pune, says, “I am going to be cautiously optimistic till it is proven beyond doubt that the detection is really a by-product of a life-driven activity. Phosphine could also be produced by other processes like the kind that is considered to occur in the core of Jupiter and Saturn.”

Phosphine is cooked up in the cores of Jupiter and Saturn, where the temperature and pressure are extreme. Here it is driven by geological and chemical processes. It helps that unlike the rocky planets of Earth and Venus, gas giants Jupiter and Saturn are mostly made of hydrogen and helium. Phosphine gets the hydrogen required to come into existence in these planets before it is pushed up by convection currents through the atmosphere.

This is not expected to happen on Venus because of vastly different conditions. Still, some unknown chemical pathway may still explain the phosphine in the atmosphere of Venus. It will need more investigation.

In a separate paper, going over a hundred pages, a team led by Dr William Bains of the Massachusetts Institute of Technology have painstakingly considered a wide range of chemical sources for phosphine on Venus but without luck. Some of the sources considered, such as volcanic activity, meteoritic impacts, and lightning strikes, can produce phosphine but in extremely tiny amounts compared to the levels detected.

The question of chemistry is an important one because chemistry and biology aren’t separate like how they are structured in schools and colleges. On Earth, chemistry gave way to life.

“The chemical origin of life on Earth is supposed to have been facilitated in certain niches that are rich in chemical and thermal fluxes,” says Dr Rajamani, who is the principal investigator of the COoL lab at IISER Pune that looks into how life would have emerged on the early Earth from chemistry.

“Among the niches that are hypothesised to have facilitated the transition from chemistry to biology on the early Earth are hot springs and terrestrial geothermal pools, like the ones near the edge of volcanoes,” she says.

So, Dr Limaye was, in fact, right to look at the geothermal pools in Ladakh for his pH measurement exercise.

Dr Rajamani explains that these places are rich in “chemical inventory”, hosting small molecules that may have come to Earth on the back of meteorites or made in chemical reactions occurring between water and the rocks sitting beside these hot spots.

“Driven by day-night cycles, tidal activity or seasonal changes, these smaller molecules would have transitioned to more complex molecules. Eventually, the genetic system (molecules that encode information) is thought to have gotten encapsulated in primitive membranes, leading to the emergence of the earliest cells,” she says.

That sets us on course to life.

Looking for these chemical signatures that could give way to a life form, not just in our neighbourhood but in exoplanets far away, could tell us whether or not a planet is capable of hosting life as we know it.

For this reason too, the investigation into the chemistry of phosphine production on Venus is critical. It could inform our search for potential habitability elsewhere too.

So, What Next For Venus?

In many ways, work has just begun.

The international team behind the phosphine detection on Earth’s sister planet have to confirm the presence of the molecule by pursuing its signature at multiple frequencies in the electromagnetic spectrum — essentially, use other types of telescopes to track it down.

Even as that work goes on, computer modelling and simulations will be necessary to check for possible chemical and biological routes to phosphine.

In the larger scheme of things, there’s only so much investigation that can happen from Earth. A mission to Venus is necessary.

“Future space missions to Venus should involve both orbiters and balloon experiments that will enable us to study Venus’ acidic clouds in greater detail, and, for example, map the extent of phosphine present in its atmosphere,” says Dr Rajamani.

Several Venus missions are in the concept stage, but India could be the first to reach the planet since the surprise detection of phosphine.

“The Indian Space Research Organisation has a wonderful chance to detect phosphine first,” says Dr Limaye.

India’s Shukrayaan-1 mission was earlier scheduled to launch in 2023, but delays on account of the pandemic may set it back by a year or two.

With the latest discovery, Venus is set to quickly rise up the ranks of celestial targets for potential life, alongside Mars, Jupiter’s moon Europa, Saturn’s moons Titan and Enceladus.

“We are in a similar moment for Venus like Mars, except Venus is much easier to search for life than Mars,” says Dr Limaye.

Perhaps, as NASA administrator Jim Bridenstine said in a tweet, “It’s time to prioritise Venus.”