Despite a barrage of recent claims, the case for life on Earth's sister planet remains alive and well.

Image: Craiyon

Introduction

In September of 2020, a team of researchers formally announced that they had detected traces of phosphine gas – a compound whose only known origin is through life – in the atmosphere of Venus. Hype around the prospect of Venusian life quickly swelled. Many were quick to imagine colonies of floating microbes, feeding off the abundant light from the sun. However, almost as quickly as it appeared, the excitement vanished. Reanalysis of the data revealed an error in processing, sparking a highly-controversial debate calling into question the legitimacy of the results.



Since then, many have been quick to dismiss the possibility of life on Venus. A multitude of findings arguing against the possibility of Venusian life have appeared, seeking to squash any previously-held optimism. With boiling temperatures, crushing pressures, and clouds of acid, it’s easy to imagine why.

However, floating on the 50km-high cloud-tops is a hidden gem – the most earth-like environment in the solar system, furnished with sea-level pressures, and pleasant, tropical, 75ºF temperatures. A far cry from the scorched, pressure-cooker surface of Venus that is given the most attention. We’ve known about Venus’ habitable zone for quite some time. In 1985, the Soviet Union deployed two balloons in the atmosphere of Venus, sampling pressure, temperature, and wind speeds – as part of the VEGA program.

The probes further cemented our understanding of Venus’ habitable zone – and its potential for life.

And although recent findings have cast a shadow of doubt, looming in these clouds is still the possibility for life. This scientific paper illustrates that, despite these findings, there’s still plenty of wiggle-room for life to exist.

This article will explain, in-depth, the arguments made against the possibility of life on Venus – and the counterpoints to each made by the paper.

This paper, titled “Venus' Atmospheric Chemistry and Cloud Characteristics Are Compatible with Venusian Life” made its debut in the scientific journal Astrobiology, in June 2023. Its authors are William Bains, Janusz J. Petkowski, and Sara Seager. This article, explaining the paper, was written as a project for the Open Avenues Micro-Internship program.

Life on Earth

On Earth, life exists thanks to a convenient list of coincidences. To name a few,

    ✦   Earth’s distance from the sun allowing liquid water to exist
    ✦   A thick atmosphere to protect us from harmful solar & cosmic radiation
    ✦   A magnetic field
    ✦   A relatively stable climate

These factors allowed life to evolve to where it is today – and to recover from some pretty inconvenient coincidences, including a few mass-extinction events.

Many of Earth’s neighboring planets simply weren’t lucky enough to have these conditions. While it’s widely believed that Mars had liquid surface water (and a thick atmosphere) 3-4 billion years ago, those conditions have since fleeted. In roughly the same timeframe, Venus is also thought to have had liquid surface water, milder temperatures, and many other life-supporting conditions.

And while the old Venus is gone, the prospect for life on the planet still remains – albeit in a new set of circumstances, and in a different place. This proposed form of life would have to adapt to a few challenges – which, while hard, are not impossible to overcome.

Energy

On Earth, life is blessed with an abundance of free chemical energy, distributed all over the place. It’s part of the reason why life arose on Earth in the first place.

On Venus, however, there’s almost no free chemical energy by comparison. Any life would have to work much harder to extract energy from its surroundings.

However, the authors of the paper argue that while this would be a hurdle for Venusian life, it’s not a barrier. Life would have to work harder, but it could still acquire energy nonetheless. Additionally, Venus has plenty of light energy – more than Earth – due in part to its closer proximity to the sun, and the reflective albedo of the white clouds in its atmosphere.

Photosynthetic microbial life in the Venusian clouds would thrive – and perhaps, it already is. Scientists have long been puzzled by the presence of an “unknown UV absorber” – aerosolized particles floating ~75km in the atmosphere, absorbing the plentiful UV light.

Additionally, recent research has explored sulfur-based metabolisms as a source of energy, finding limited support due to the scarcity of reduced compounds. Despite this, alternative mechanisms like heterotrophic oxidation may explain observed phenomena like SO2 depletion and generate energy. These findings suggest that while chemical energy may be scarce on Venus, the abundance of light energy and potential metabolic pathways could still support microbial life in its clouds.

Lack of resources

Unlike life on Earth, Venusian life would be bound to the clouds – separated from the ground by immense heat and pressures. Venusian life would have to extract resources from the clouds – and it turns out, there’s plenty of the building blocks necessary for life.

Water Scarcity

Venus is often seen as a dry planet with little water, a crucial element for life. The atmosphere lacks chemically available water, posing a challenge for life as we know it. However, the paper suggests that water may exist in the higher atmosphere of Venus, challenging the idea of complete aridity and opening up possibilities for life in the clouds.

The existence of upper-atmospheric water, coupled with the fact that life could thrive in that same area of the atmosphere, could add merit to the concept of life on Venus.



Although hypothetical Venusian life may not rely on water, all currently-known known life does. Thus, while water-reliant life is an assumption made by many scientists, it doesn’t have to be.

Hydrogen Scarcity

The absence of free hydrogen in Venus' atmosphere is another obstacle to habitability. Hydrogen is essential for biological processes – like metabolism, photosynthesis, and nitrogen fixation – and exists in plenty on Earth. However, on Venus, it’s a different story. Almost all free hydrogen has escaped into space, being displaced by heavier molecules like CO2.

However, organisms may be able to extract it from compounds in the atmosphere. While at a higher energy cost, life could extract hydrogen from the water in the atmosphere. The paper outlines how Venusian life could do this, using the 3 most abundant compounds in the atmosphere – carbon dioxide, sulfur dioxide, and nitrogen.



Additionally, it’s entirely possible that hydrogen could be extracted from sulfuric acid in the Venusian atmosphere. Like Earth, Venus has white clouds – but instead of being composed of ice crystals, they’re composed of sulphuric acid. It’s entirely possible that life could use sulfuric acid as a source of hydrogen.

Metal Scarcity

The availability of metals for biological functions on Venus is a concern. The planet's surface may seem devoid of essential metals, but upwelling surface dust and meteorites could provide a source of these metals in the atmosphere. Additionally, known biochemistry can adapt to the absence of certain metals – such as cobalt or nickel. Even if these metals are not readily accessible in Venus’ atmosphere, life could very much still exist.

Other hostilities

High Acidity

It’s no secret that the Venusian atmosphere is filled with clouds of sulphuric acid. While the placement and distribution of the clouds in the atmosphere is still mostly unknown, one thing is clear; any life residing in the clouds would have to adapt to it. Researchers suggest life could adapt by neutralizing the acidic environment or embracing it entirely as a solvent. Despite the obstacle of acidity, these adaptations offer hope for life in Venus' clouds.

High Radiation

Venus' thick atmosphere shields against harmful solar radiation, creating potentially habitable zones within the cloud layers. The presence of an "unknown UV absorber" suggests Venus' atmosphere can mitigate radiation effects. Recent calculations indicate radiation may not be a significant barrier to life in Venus' clouds.

Conclusion

Venus – our closest planetary neighbor, and sister planet by many metrics – poses its fair share of challenges for life to thrive. These challenges, while harsher and ultimately more hospitable than those on Earth, still do not bar the possibility of life completely on Venus.

While both academics and laypeople alike are keen to jump on Venus’ hostilities, little is done to consider otherwise. Despite recent findings, the doors are still wide open for the prospect of life on our sister planet – or so claims this paper.

Astrobiology – which is also the name of the paper’s publication – is still a purely theoretical and speculative branch of biology. So far, there is no conclusive evidence of life beyond Earth. The only datapoint we have to work with is life on Earth itself.

Because of this, the assumptions we make are entirely driven by our understanding of biology on Earth. In an environment as foreign as Venus, our Earth-centric assumptions might break down entirely. The paper mentions – very briefly – the possibility of life using sulfuric acid as a solvent instead of water – which no life on Earth does.

In order to remain a scientifically-grounded field of study, astrobiology must be speculative in nature. If we wish to theorize life outside of this world, we need to make some out-of-this-world assumptions. By its very nature, astrobiology sits on the periphery of what we consider possible. Therefore, and because of this, astrobiology is never “real” – until it finds conclusive proof of life outside of Earth.

Venus’ suitability extends far beyond the promise for life – and, for many of the same reasons, remains a better first candidate for colonization than Mars. Since the 1960s, scientists and science-fiction authors alike have envisioned floating, air-filled habitats situated in the temperate zone of Venus’ atmosphere. Breathable air – which would be buoyant in an atmosphere of CO2 – could be produced by plants in an onboard greenhouse, which breathe in the abundant CO2 in Venus’ atmosphere.

Furthermore, the habitats would be powered by the abundant solar power provided by Venus’ closer proximity to the sun. Its thick atmosphere would take the place of Earth’s magnetic field, shielding colonists from harmful solar & cosmic rays. Additionally, the near-Earth-like gravity would be far less taxing on the human body than the 0.38Gs on Mars.

Venus is also closer to Earth than Mars. Because it orbits closer to the sun, it orbits faster – and therefore, has more close-approach windows. Venus reaches its minimum distance to Earth every 19 months, while Mars does every 26 months.

The only real problem – the highly acidic environment – is much more easy to tackle than the deadly radiation, low gravity, thin atmosphere, and wildly-varying temperatures on Mars.

Our interest in Venus, therefore, is twofold – to investigate the possibility of extraterrestrial life, and to use Venus as humanity’s first planetary stepping-stone in a wider effort to extend into space.

Both require challenging pre-existing notions, and stepping out of what’s familiar and comfortable – so that humanity can do the same in the future.