Can Venus’ Atmosphere Support Life?

Amandeep Singh
Originally Written On: 23 Nov 2018

(Originally submitted as an assignment for the module ‘Astrobiology’) 

"Venus' surface today is pretty much like the Biblical vision of hell." - ...astronomers and their informative quotes.

Venus - the second planet in our solar system, with an atmosphere that crushes everything at it’s surface, with about 90 times more pressure than experienced on Earth (The Stardate team, 2006); it’s the hottest place in the Solar System after the Sun (so hot that even Lead will melt (Royal Society of Chemistry, 2014)) (NASA Science, 2018)(Seeker, 2016); it’s a planet where the greenhouse effect has gone into overdrive (Seeker, 2016). But it is theorised that Venus was not always like this. At one point of time, the physical conditions there might have been similar to what we have on Earth today - even oceans (Seeker, 2016). Today, due to complex physical and chemical phenomenon that transformed the planet over the course of hundreds of millions of years, Venusian surface is bone-dry- in case the opening quote wasn't clear enough. Then why the question mark in the title, one would ask. Well, the keyword here is atmosphere. The opening quote aptly describes the surface of Venus -- "hell-like", but that’s not the place where astronomers and astrobiologists think life can exist, or at the very least, survive.

Venus is often referred to as the "evil-twin" of Earth --"evil" because it’s literal hell there, and "twin" because it has very similar physical characteristics. To prove the physical similarity, here are some quick figures about Venus:

• > The diameter of planet Venus is 12,103.6 km - only 638.4 km less than Earth’s (NASA Science, 2018);
• > The mass of planet Venus is 81.5% of Earth’s (NASA Science, 2018).

Figure 1: Venus is a terrestrial planet and is sometimes called Earth’s ”sister planet” because of their similar size, mass, proximity to the Sun, and bulk composition. It is radically different from Earth in other respects.

But it’s also actually pretty different in other aspects:

• > It completes one rotation about it’s axis in about 8 Earth-months (NASA Science, 2018);
• > It has no moons (NASA Science, 2018);
• > It’s atmosphere is extremely dense, composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases, most notably sulfur dioxide (The Stardate team, 2006);
• > The mass Venusian atmosphere is 93 times that of Earth’s, whereas the pressure at its surface is about 92 times that at Earth’s -- a pressure equivalent to that at a depth of nearly 1 kilometre under Earth’s oceans (The Stardate team, 2006).

But the atmosphere is still the prime candidate to sustain life on Venus. The habitability of Venus’ clouds has been a subject of discussion and as a popular target in astrobiology research due to favourable chemical and physical conditions in the atmosphere: the presence of sulfur compounds, carbon dioxide (CO2), and water, moderate temperatures (30 - 60 deg Celcius) and pressures (0.4 - 2 atm) (Limaye et al., 2018).

This sounds hopeful, but it is a fact that there is much less known about the conditions on Venus than one would expect. The unique features of Venusian atmosphere were first observed almost a century ago by Earth-based telescopes, followed by subsequent ground-based polarimetry, satellite & entry missions and few landers (NASA, 2019). Together, these studies and observations indicated that the global cloud cover was composed of sulphuric acid droplets (1.1 mm equivalent radius) in a mixture consisting mostly of small particles (0.2–0.3 mm equivalent radius), with larger particles (2–8 mm diameter) present at lower altitudes (Limaye et al., 2018). To visualise what this means using an example: oil floats on water because oil is less dense than water. In other words, the lighter particles in Venus’ atmosphere float above the heavier particles, this in turn forms layers of clouds made of different compositions, like sedimentary rocks, but instead of minerals, the layers are made of gas and dust. The layer closest to the surface is the most dense, and although due to Venus’ extremely slow rotation there are no winds blowing except for the occasional breeze, the gas is so dense that it actually carries dust and minute rocks with it. This would make it next to impossible for potential astronauts or even equipment to stand or work closer to the Venusian surface, if the heat, pressure, and lack of oxygen aren’t a problem. The outermost layer of Venus consists of the lightest fluids, namely some amounts of Hydrogen and Helium, and other compounds. Since Venus has an extremely slow rotation period, the outermost layer of it’s atmosphere would be facing the full brunt of the Sun, facing all the heat, the components getting ionised bracing the solar storms, getting bombarded with radiation every second. So it is safe to say that the outermost layer of the atmosphere is as unstable and as extreme as the innermost layer.

So then where in the atmosphere are scientists hypothesising about the sustenance/existence of life? It appears to be that, like Goldilocks, life also prefers the "porridge" of fluids that is neither too hot, not too cold. Because the Venusian atmosphere is in layers, the outer layers filter all the radiation from the Sun. Even though Venus receives twice the amount of heat and radiation from the Sun than Earth (Crisp, 2007), only 20% of it ever reaches the surface (Crisp, 2007), the majority of it gets reflected from the outermost layers into outer-space (explains why Venus shines so bright, and is known as 'the Evening Star'). But the innermost layers are so dense that they don’t allow the planet’s heat to escape, hence creating such a super-heated environment. This creates a unique spot in the middle layers in the atmosphere - the middle layers are protected from the heat and radiation from the Sun, and also from the high surface heat and pressures. In fact, there can be an ideal spot that has almost the same pressure as that of Earth, not as hot as the surface, and still protected by the radiation. These hotspots may even host extremophilic life. As pointed out by Carl Sagan & Harold Morowitz (Morowitz and Sagan, 1967), human astronauts could also visit Venusian atmosphere, if not its surface - at least in theory. They could stay in airship-like vessels that would literally float in the safety of the middle layers of the atmosphere, over the dense innermost layers & covered by the outermost layers, protected by all harm. This scenario fuelled many science fiction articles and stories, but half-a-century after this idea was proposed, humans still are far behind the intended target.

Today other advanced techniques are used to see if any signs of life can be detected in the Venusian atmosphere. One such way is to look at the spectrum of radiation reflected by it. Since the atmosphere is composed of many layers, it reflects different wavelengths of radiation, providing more details about the different atmospheric compositions (Limaye et al., 2018). Using this technique, astronomers were able to successfully see what the Venusian surface looked like. With signs of dried lakes and riverbeds, it looks eerily similar to what, at some point, might have been our planet. By observing the radiation reflected from these atmospheric layers, scientists can detect the change in the spectra, which will help understand the compositions better, and may even provide some details in the extraterrestrial-life angle. 

Figure 2: As one of the brightest objects in the sky, Venus has been a major fixture in human culture for as long as records have existed. (a) Cloud structure in the Venusian atmosphere, 1979, observations in the ultraviolet band by Pioneer Venus Orbiter; (b)Global radar view of Venus (without the clouds) from Magellan between 1990 and 1994;

Nonetheless, it sure as hell will help understand how Earth didn’t end up like Venus and how different were the conditions that made Venus as inhospitable as it is today. Some even say that Venus shows us humans our future - a future in which we couldn’t control our greenhouse emissions, continued fighting amongst ourselves, and maybe nuked each other. This statement is debatable, but it is safe to say that even though Venus is the "evil" of the two siblings, Earth might be home to them.

References

Crisp, D. (2007). Venus Exploration Advisory Group Venus Exploration Advisory Group Greenhouse Effect and Radiative Balance on Earth and Venus. [online] Available at: https://www.lpi.usra.edu/vexag/nov_2007/presentations/crisp.pdf [Accessed 31 May 2020].

Limaye, S.S., Mogul, R., Smith, D.J., Ansari, A.H., Słowik, G.P. and Vaishampayan, P. (2018). Venus’ Spectral Signatures and the Potential for Life in the Clouds. Astrobiology, [online] 18(9), pp.1181–1198. Available at: https://www.liebertpub.com/doi/10.1089/ast.2017.1783 [Accessed 31 May 2020].

Morowitz, H. and Sagan, C. (1967). Life in the Clouds of Venus? Nature, [online] 215(5107), pp.1259–1260. Available at: https://www.nature.com/articles/2151259a0 [Accessed 27 Apr. 2019].

NASA (2019). Venus. [online] NASA Lunar and Planetary Science. Available at: https://nssdc.gsfc.nasa.gov/planetary/planets/venuspage.html [Accessed 31 May 2020].

NASA Science (2018). In Depth | Venus – Solar System Exploration: NASA Science. [online] Solar System Exploration: NASA Science. Available at: https://solarsystem.nasa.gov/planets/venus/in-depth/ [Accessed 31 May 2020].

Royal Society of Chemistry (2014). Lead - Element information, properties and uses | Periodic Table. [online] Rsc.org. Available at: https://www.rsc.org/periodic-table/element/82/lead [Accessed 31 May 2020].

Seeker (2016). There Could Have Been Life On Venus! YouTube. Available at: https://www.youtube.com/watch?v=DNILir-SvfM [Accessed 31 May 2020].

The Stardate team (2006). Venus’s atmosphere. [online] OpenLearn.edu. Available at: https://www.open.edu/openlearn/science-maths-technology/science/physics-and-astronomy/astronomy/venuss-atmosphere [Accessed 31 May 2020].