Venus: A Runaway Greenhouse in a Neighbouring Hellscape

Venus is the second planet from the Sun. Named after the Roman Goddess of love, it is shrouded in a veil of swirling clouds, reflecting most of the incoming solar radiation, making it the second brightest non-luminous object in the night sky, after the Moon. Often referred to as Earth’s ‘sister planet’, it has a diameter of about 80% and a mass of about 90% that of the Earth, making it the second largest terrestrial planet of our solar system. Whirling along the inner radius of the Sun’s habitable zone 1, it could well have been an Earth-like paradise in the past. Yet, Venus’ charm is deceptive, for beneath its thick atmosphere lies one of the most hellish places in the solar system. What could have possibly gone wrong with Earth’s evil twin?

Venus is a boiling hot rock looming in space, with an average surface temperature of over 460°C, hot enough to melt lead. It is the hottest planet in the solar system, hotter than even Mercury which is the closest planet to the Sun. Composed primarily of carbon dioxide (96-97%), a little bit of nitrogen (2-3%), and some trace elements, the Venusian atmosphere is so dense that the atmospheric pressure at its surface is over 90 times higher than on our planet. For perspective, such enormous pressures, enough to crush the human body, are attained at depths greater than 900 m in the oceans. The planet has a menacing weather system: Thick sulphur clouds rage across the planet and rain sulphuric acid, but the intense heat vaporises the acid droplets before they reach the surface, a phenomenon known as virga 2, creating a precipitation cycle that never reaches the ground. All of these likely happened due to a phenomenon called the runaway greenhouse effect that the planet might have been inflicted with in the past.

The temperature of any terrestrial planet in a solar system is dictated by the fine balance of the starlight it receives and the radiation it sends back into space. Any imbalance would leave the planet in a dynamic state where its temperature changes until radiative equilibrium is restored. This energy trade-off is governed by the Stefan-Boltzmann law, which states that the energy radiated by a black body 3 per unit surface area per unit time is proportional to the fourth power of its absolute temperature. Lurking alone in space, planets receive energy primarily from solar radiation, or starlight in the case of exoplanets, which is mainly UV or visible light. As a planet heats up, it emits radiation, primarily in the infrared region of the electromagnetic spectrum, known as outgoing longwave radiation (OLR) to maintain the energy balance. If greenhouse gases are present in the atmosphere, the greenhouse effect 4 would render the average surface temperature higher than without. This happens because greenhouse gases block the OLR, disrupting the exchange of energy by the planet. The greenhouse effect on the Earth renders its surface temperature about 32°C higher than it would be otherwise, proving to be vital for life. In this way, a planet’s atmosphere has a significant say in its final temperature. An extreme greenhouse effect, however, could turn a habitable paradise into a desolate landscape. Venus could have been one such place.

The temperature on Venus, or any planet, is maintained by the combined effects of various feedback cycles. On Earth, for example, evaporation of seawater increases the concentration of water vapour, a greenhouse gas, in the atmosphere, thereby increasing the intensity of the greenhouse effect. Consequently, elevated temperatures speed up the evaporation of the oceans, further increasing the vapour content in the atmosphere. This is a positive feedback cycle. On the other hand, elevated temperatures result in increased energy release by OLR (Stefan-Boltzmann law), thereby cooling the planet; a negative feedback loop. A runaway greenhouse effect happens when the concentration of water vapour in the atmosphere reaches a critical threshold beyond which OLR can no longer cool the planet. Although the detailed mechanism is complicated, it can be understood as a combination of radiative properties and the nature of the atmosphere (troposphere). Intense solar radiation, like in the case of Venus, would cause liquid water to evaporate quickly and result in higher atmospheric vapour concentrations that lead to elevated temperatures due to enhanced greenhouse effects. However, there is a limit to the amount of solar flux a planet can receive without starting a runaway greenhouse effect. Beyond a certain concentration of greenhouse gases, the Stefan-Boltzmann feedback breaks down as the temperature required to maintain this limit results in a water vapour optical depth 5 that does not allow sufficient OLR required to cool the tropopause to leave the atmosphere. This leaves the planet in a precarious state of indefinite temperature increase until radiation is possible outside the absorption bands of water vapour; hence the name ‘runaway’. Such a scenario renders the planet devoid of any surface water. A runaway greenhouse effect is expected to take place on Earth after 1 – 2 billion years as the Sun’s luminosity increases due to nuclear fusion processes in the core.

A runaway greenhouse effect involving carbon dioxide and water vapour likely occurred on Venus in the past. There could have been oceans of liquid water on the planet, much like on Earth, until the brightness of the Sun increased to the point that the incoming solar flux crossed the maximum limit6, evaporating all ocean water in the process. If this happened, Venus’ stratosphere would have been hydrated before hydrodynamic escape 7 caused all the water to leak into space. This explains why the Venusian atmosphere is almost entirely devoid of water vapour. A piece of strong evidence for this scenario is the ratio of deuterium to hydrogen in its atmosphere which is several times higher than Earth’s: Light hydrogen would escape the atmosphere more readily than its heavier isotope Deuterium.

The sunlight reaching Venus is strong enough that water vapour would rise high into the atmosphere and be photolysed into hydrogen and oxygen by UV light. While hydrogen would escape into space being lighter, oxygen would bond with the surface material to form oxides (iron oxides). This would have collapsed the water cycle on Venus. Water vapour is also a good absorbent of carbon dioxide. The loss of water vapour partly explains the elevated concentration of carbon dioxide in the atmosphere. Water also aids plate tectonics as it acts as a lubricant, reducing friction between moving tectonic plates. The absence of water also explains the lack of plate tectonics on Venus, rendering it a stagnant lid 8 planet. This lack of plate tectonics is a major contributor to the large concentration of carbon dioxide in the atmosphere. On Earth, carbon dioxide released through volcanic activity is efficiently subducted 9 through the carbonate-silicate geochemical cycle 10, with the aid of precipitation. Venus is also known to have intense volcanic activity. All these factors, along with the runaway greenhouse effect, have given Venus a thick gas blanket almost entirely composed of carbon dioxide.

Despite Venus’ extreme conditions, humans have managed to send space probes to the desecrated planet. At approximately 127 minutes, the Soviet Venera 13 mission 11 boasts the record for the longest-surviving lander on Venus. The Venusian surface is young, only a few hundred million years old. It is heavily cratered, most of which still have their distinct features intact, suggesting that they were formed not too long ago. Interestingly, all the craters have suffered similar degrees of erosion, meaning they were formed at a similar time too. This suggests that there could have been a catastrophic event in the recent geological past that erased all former traces from the planet’s surface. This is one explanation for the lazy retrograde rotation of Venus, a rare occurrence in our solar system. Unlike most other planets, Venus rotates clockwise, taking around 243 Earth days to complete one day-night cycle. Strangely, a year on Venus is around 225 Earth days long, making a day on Venus longer than a year. The event was likely a mega-collision with an asteroid that brought the planet to a rotational stand-still or entirely flipped it around to reverse its rotation. The reversal of rotation could, otherwise, have been the result of powerful Solar tides in Venus’ thick atmosphere over a few billion years, or a mix of both. This is still a subject of study.

Venus is known to have active volcanic activity. Several volcanoes, many of which are over a kilometre in diameter, have been detected, many of which are presently active. There is a hypothesis, albeit speculative, that Venus could be a giant super volcano, with large amounts of magma that erupt in giant plumes throughout the planet every few hundred million years. One such global event may have occurred a few hundred million years ago, rewriting the crustal features of the planet. The lack of tectonic movement on Venus allows slowed leaks of lava to continue at one spot for extended periods to give it unique volcanic features known as pancake domes 12, which could be several kilometres wide but less than a kilometre tall. These are huge lava domes, made of highly viscous material, that could spread due to the planet’s scorching heat.

At an equatorial rotational speed of about 6.5 km/h, Venus rotates so slowly that you could jog around its equator faster than the planet rotated. This also makes Venus the roundest of all planets in the solar system as the centrifugal bulge is weak. Another result of Venus’ slow rotation is its feeble magnetic field. This has led to increased atmospheric stripping by the solar wind, leading to the loss of lighter gases such as hydrogen and helium from its upper atmosphere 13. The reduced shielding from solar radiation is also a cause of its high temperature.

Slow as the planet’s rotation may be, Venusian winds can reach speeds of 100 m/s, circling the planet once in about four Earth days. That’s about 60 times its rotational speed. This phenomenon, where the atmosphere rotates much faster than the planet itself, is called super-rotation 14. Such extreme winds are likely a result of the high atmospheric pressure and the absence of surface obstacles. This smoothness of the Venusian surface is largely due to the lack of water and associated erosion processes. Some features of the Venusian atmosphere remain unexplained. Near its poles are double-eyed anticyclonic structures 15 called polar vortices 16, that show a characteristic S-shaped pattern of clouds. They could be a result of the interaction of the Sun’s electric field with the atmosphere. The average wind speeds of the planet have been found to increase recently. 17

The surface conditions on Venus are quite uniform, with the poles being nearly as hot as the equator. This is because its thick atmosphere blocks most of the sunlight, with less than 10% reaching the surface, so solar flux does not greatly influence local temperatures. If you were to land on Venus, you would always find it dark, much like twilight on Earth on a cloudy day. Visibility on the planet is extremely poor. Moreover, despite its proximity to the Sun, the solar energy received by the planet’s surface is less than on the Earth. The decks of opaque sulphur clouds that cover the planet make optical observations of the surface from outside the atmosphere impossible. All information obtained about its topography is purely through radar imaging 18. Despite the harsh surface conditions, its upper atmosphere has similar temperature and pressure conditions to the Earth’s. This, coupled with the fact that breathable air is a lifting gas 19 on Venus, makes its upper atmosphere a candidate for future colonisation.

Venus has no known moons. This is likely because even if it had one in the past, its proximity to the Sun would cause the latter’s gravitational influence to drift the moon out of planetary orbit. Like our moon, Venus also exhibits phases in the night sky as it orbits around the Sun. When it is at the farthest from the Earth, directly behind the Sun, its full face is visible. At its closest point, it forms a crescent. Despite only a part of it being visible, it appears the brightest during this time due to its proximity to the Earth. Venus has an axial tilt of approximately 177.3° (effectively 2.3°), due to which the seasonal variations are minimal, leading to a uniform climate. A rare, but interesting event involving Venus is its solar transit. A solar transit of Venus takes place when it passes directly between the Sun and the Earth, becoming visible against the solar disk. During a transit, it appears as a black dot moving across the face of the Sun. This rare event occurs in pairs, separated by 8 years, followed by a gap of over a century. The last pair of transits occurred in 2004 and 2012, and the next won’t occur until 2117! Transits are of great interest to astronomers as they provide opportunities for the study of Venus’ atmosphere, and also serve as an analogue to the transit methods used to detect exoplanets orbiting distant stars. Lastly, there is an international agreement that all surface features on Venus be named after women or Goddesses of various cultures. Cool!

Venus: A Runaway Greenhouse in a Neighbouring Hellscape

(Recommended reference: Venus: Crash Course Astronomy #14 (youtube.com))

1* The habitable zone, also called the Goldilocks zone, is the range of orbits around a star within which a planet can contain liquid water on its surface given sufficient atmospheric pressure. It is considered a key factor in determining the potential habitability of a planet.

2* A virga is a streak or shaft of precipitation that evaporates or sublimates before reaching the ground, often resembling curtains descending from the base of a cloud. It can have varying effects on the atmosphere including evaporative cooling and heat bursts.

3* A black body is an idealised physical object that absorbs all electromagnetic radiation incident upon it, regardless of the angle or frequency. It also emits radiation with maximum efficiency, with a spectrum that solely depends on temperature. Stars, like our Sun, closely resemble black bodies.

4* The greenhouse effect is a natural process where certain gases in the atmosphere, known as greenhouse gases, trap heat from the Sun, warming the planet. Some common greenhouse gases are water vapour, carbon dioxide and methane. The phenomenon got its name from greenhouses, which are glass chambers in which certain plants are cultivated especially in cold climates as glass lets sunlight in but traps heat inside, creating a microclimate for plants. It is the same reason that your car gets so hot when left in the Sun.

5* Optical depth is a measure of the extent to which a medium absorbs (attenuates) light as it passes through.

6* See Komabayashi-Ingersoll limit: https://en.wikipedia.org/wiki/Komabayashi-Ingersoll_limit

7* Hydrodynamic escape is a thermal atmospheric escape mechanism by which heavier atoms/ molecules, such as noble gases and water vapour, escape the planetary atmosphere through collisions with lighter molecules. It often involves heating the upper atmosphere, increasing the molecular kinetic energy allowing escape from the planetary atmosphere.

8* In a stagnant lid planet, the lithosphere is largely immobile and behaves as a single, continuous plate, with no significant horizontal movement. Such a tectonic regime is characterised by a lack of plate tectonics, thick lithospheric lid, localised and sporadic volcanism, and limited crustal deformation.

See https://en.wikipedia.org/wiki/Lid_tectonics

9* Subduction is a geological process where one tectonic plate moves below another, sinking into the mantle. On Earth, the continental lithosphere is recycled by subduction as it sinks underneath the oceanic lithosphere. It is also directly involved in forming mountains, deep-sea trenches, and volcanic arcs.

10* The carbon-silicate geochemical cycle describes the transformation of silicates to carbonates by weathering and sedimentation, and the transformation of these carbonates back to silicates by metamorphism and volcanism. It regulates the long-term carbon dioxide levels in the Earth’s atmosphere over geological timescales. Carbon dioxide is removed from the atmosphere by weathering and sequestration in sedimentary rocks and returned through volcanic outgassing.

11* Venera 13 was part of the Soviet Venera Programme meant to explore Venus.

See https://en.wikipedia.org/wiki/Venera

12* A pancake dome is an unusual type of lava dome found on Venus. They have broad, flat profiles, similar to shield volcanoes on Earth, that form from a single large, slow eruption of silica-rich lava. Pancake domes on Venus are up to 100 times bigger than lava domes on the Earth.

See https://en.wikipedia.org/wiki/Shield_volcano

13* A part of a planetary atmosphere can be lost to outer space by atmospheric escape, gradually changing the atmospheric composition over time. Different gases have different escape rates based on their molecular masses, interaction with solar radiation, and the presence of magnetic fields.

See https://en.wikipedia.org/wiki/Atmospheric_escape (Jeans escape)

For a planetary body to retain a particular gas in its atmosphere for periods comparable to the age of the solar system, the average velocity of the gas molecules should be less than a sixth of the planet’s escape velocity. Lighter gases like hydrogen and helium have higher molecular speeds and escape more easily than heavier gases like nitrogen and oxygen, which have lower molecular speeds. This is why gas giants retain hydrogen in their atmospheres, but terrestrial planets do not.

See https://www.mathscinotes.com/2016/08/planetary-atmosphere-leakage

14* Atmospheric super-rotation refers to a phenomenon in which a planet's atmosphere rotates much faster than its surface. It has been in the atmospheres of Venus, Titan, Jupiter, Saturn, and various exoplanets. It is thought to be driven by atmospheric waves such as Rossby waves and Kelvin waves, which play a vital role in the El Nino-Southern Oscillation on Earth.

See Superrotation in Planetary Atmospheres | Space Science Reviews (springer.com)

15* An anticyclone is a large-scale weather system characterized by high atmospheric pressure at its centre, around which air circulates in a clockwise direction in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, opposite to cyclones. Anticyclones are typically associated with calm and clear weather conditions, as opposed to cyclones, which are associated with low-pressure and stormy weather. They typically don’t bring rain.

16* Polar vortices are large areas of low-pressure and cold rotating air surrounding the Earth's poles. These vortices are always present near the poles but strengthen in the winter and weaken in the summer. They also exist on other rotating, low-obliquity planets, such as Venus.

17* See ESA - The fast winds of Venus are getting faster

18* Radar imaging is a technology used to create images of objects or landscapes using radar waves. These waves are transmitted from a radar antenna, bounce off the target object, and then return to the radar system. By analysing the time it takes for the waves to return and their intensity, radar imaging systems can create detailed radio images. The radar moves along a flight path and continuously illuminates the surface with radar waves, building an image of the area being scanned. Applications of radar imaging include meteorology, aviation, geology, and military applications

19* A lifting gas, or lighter-than-air gas, has a lower density than gases in its surroundings and rises above them as a result. Lifting gases are used in high-altitude ballooning, weather balloons, and buoyant space exploration vehicles, among many other applications.

Venus has a carbon dioxide atmosphere, which is about 50% denser than Earth's air, so ordinary Earth air could be a lifting gas over there. Proposals have been made for floating human habitats in the Venusian atmosphere at altitudes where the pressure and the temperature would be Earth-like.

See High Altitude Venus Operational Concept - Wikipedia

Venus, shrouded in thick clouds of sulfuric acid, showcases the power of a runaway greenhouse effect, turning its surface into one of the hottest and most hostile environments in the solar system.