Since the beginning of planetary science, the planet Venus, or “Earth’s evil twin,” has attracted fascination. First, it was considered a potentially Earth-like world with swamps or oceans in the early 20th century and now as a potentially cautionary tale for what can happen to a planet with a runaway greenhouse effect. Venus has gone through a dramatic transformation. It was probably more Earth-like billions of years ago, possibly even with liquid water oceans. In the last billion years, however, Venus may have experienced a catastrophic runaway greenhouse effect making it the dead, hothouse world it is today, closest known example to the Medieval conception of Hell with a temperature of 467 degrees Celsius (872 degrees Fahrenheit). The fate of Venus also has implications for the future of our own planet.
Habitable conditions on Venus may not be entirely impossible, however. Although its surface would be unable to support life as we know it, the upper atmosphere of Venus, about 50 km above the surface has Earth-like temperatures and could host extremophilic organisms. As humans and our technological creations move out into space, Venus may also serve as a subject of terraformation. Venus may once again become an Earth-like world with oceans if it is successfully terraformed. Like with Mars though, confirming the existence or non-existence of life in Venus’s clouds may be ethically required before any terraformation attempt can be made. Otherwise, maybe Earth is the evil twin after all.
Introduction
Venus is simultaneously one of the most Earth-like and one of the most alien planets in our solar system. It is comparable to Earth in terms of its size (diameter ~ 12,100 km) and has geologic regions that appear to be analogous to the cores of continents on Earth. On the other hand, Venus is also incredibly alien. It has extreme surface temperatures (~500 C or ~900 F) and a surface atmospheric pressure of 93 bar, or 93 times Earth atmospheric pressure at the surface, comparable to the pressure 1 km or half a mile deep in Earth’s oceans. Understandable, Venus is generally considered profoundly inhospitable to life on Earth.
Because of the very bleak prospects of Venus’s habitability, the planet Venus has been ignored relative to her brother planet Mars for the last 30 years. This began to change in 2020, however, when a paper came out suggesting the presence of phosphine in Venus’s upper atmosphere. Phosphine is an organic compound that is found either in the atmospheres of the giant planets or in penguin guano on Earth. Confirmation of phosphine in the clouds of Venus would definitely be a very compelling indication of life in Venus’s atmosphere since it is not clear what geological process would produce it on Venus.
The presence of phosphine is highly contested and probably will continue to be until we send another spacecraft to Earth’s evil twin. The more significant outcome of the discovery is a revived interest in the planet Venus, as two missions are planned, VERITAS and DAVINCI to explore Venus, the first NASA missions to Venus since the Magellan mission ended in 1994.
There is also another reason Venus has become more significant. Earth sized exoplanets are of significant interest because of their potential to be Earth-like. Earth-sized, however, still does not mean Earth-like since Venus is also roughly Earth-sized and definitely not Earth-like. We still do not know enough about planetary processes to know which is the weird case. Do most Earth-sized planets end up like Earth while a few become like Venus or is Venus the typical case? Understanding what happened to Venus may also help us to predict whether exo-Earths or exo-Venuses are more common in the universe.
Geological Timescale of Venus
One way that Venus is more like Earth is that most of its geology is young, whereas the other terrestrial planets (Mars and Mercury) and the Moon all have mostly geologically ancient surfaces dating back 3-4 billion years in some cases. The current surface of Venus is estimated to only be 300-800 million years old based on crater counts, which assume that craters will accumulate over time on surfaces without an erasing mechanism so that surfaces with more craters are generally older than surfaces with fewer craters. There are differences of opinion on how to frame the geologic timescale of Venus, but a generally well-supported version of the Venusian timescale is one divided into four periods, the pre-Fortunian, Fortunian, Guineverian, and Atlian.
The Pre-Fortunian represents the period of Venus’s history before the catastrophic resurfacing event that made Venus into a sort of hell. No current geology on Venus likely survives from that time. The Fortunian period is characterized by intense tectonic activity creating large plateaus that have been compared to Earth continents. The Guineverian is a period of excessive volcanic eruptions where large volcanic plains were created by hardening lava flows. The cratered plains later cooled and resulting in wrinkle-ridges. The Atlian period is defined by a mixture of volcanic and tectonic resurfacing and the emergence of the present-day state of Venus.
Pre-Fortunian
For 80-90% of its history, Venus may have been a very different planet. Studies of deuterium/hydrogen ratio within the atmosphere of Venus show that the planet has lost significant amounts of water over the course of its history, enough water to warrant an ocean. We will not really know until we send a spacecraft to collect more data, but Venus may have been an Earth-like world with oceans and least disconnected seas for most of its history.
The cataclysm
Although it is debated, some think that Venus ha always been the way it is and that there was no cataclysm, a common hypothesis is that Venus experienced as catastrophic event about 800 million to 1 billion years ago which involved a runaway greenhouse effect and to the overturning of the crust. Crater counts of the surface of Venus suggest that the current crust of Venus is less than 1 billion years old and that the pre-catastrophe geology has not survived. If there was a catastrophe, it is not clear what caused the event. One possibility is the emplacement of a large igneous province or flood basalt led to the release of enough carbon dioxide and other greenhouse gases that triggered the runaway greenhouse effect and evaporation of any primordial ocean. While speculative, the evaporation of a primordial ocean may have shut down or significantly slowed any pre-existing plate tectonics, which would have meant that greenhouse gases could not be recycled the way that are on Earth but would continue to build up in the atmosphere. This could easily make a hypothetical ancient Earth-like Venus into modern Venus.
Fortunian Period
Venus after the cataclysm
As far as planetary scientists can tell, no geology from the pre-Fortunian survives to the present-day. Although it is possible that geologic fragments like rocks and minerals have been preserved in later rock layers. The oldest identifiable units of Venus are rugged highlands called tessarae (singular = tessera). The Fortunian period gets its name from Fortuna Tessera, one of the major highland regions. The tessarae probably formed through series compression and extension similar to how mountains form on Earth when two continental plates collide leaving to the crust in the middle becoming folded or wrinkled. Unlike mountain building on Earth though, The tessarae of Venus did not involve the collision of tectonic plates, but rather a single planet-wide plate that was in tension in some areas and under compression in other areas, possibly driven by convective processes within Venus’s mantle. The tesserae may be the Venusian analogue to the low density crust that makes up the cores of Earth’s continents. This period probably only lasted about 100 million years based on crater counts and stratigraphic relations to the rest of Venus.
Guineverian Period
The period of tessera formation was followed by a long period of volcanic eruptions. During this period, named for the volcanic plain, Guinevere Planitia, vast lava flows filled the low lands between the tesserae. These lava lakes hardened into volcanic plains. Over time as these plains cooled, the rock contracted forming wrinkle ridges. The Guineverian period lasted perhaps 200 million years (very roughly based on crater counts). It is not clear what led to this transition from tectonic deformation being dominant to volcanism being dominant. One possibility is the role of rifting. If there was a period where Venus was a lava world of seas of molten rock (perhaps other than right after the catastrophe), it would have been this period.
Atlian Period
The volcanism-dominant Guiniverian was succeeded by the Atlian period, named for the shield-volcano studded plain Atla Regio. The Atlian represents perhaps the last 500 million years of the history of Venus and is characterized by both volcanic activity and tectonic activity. It is during this period that some of the most interest feature on Venus likely formed. These features include large shield volcanoes which led to the formation of new lava flows that lack the wrinkle ridges of the earlier Guiniverian plains.
Another feature associated with this geologic time period are the Venusian coronae. The coronae (singular = corona) are roughly circular concentric regions about 200 km across. The only other planetary body with features comparable to the coronae on Venus is Uranus’s icy moon Miranda.
The transition from a volcanism-dominant regime to a mixed regime of volcanic and tectonic processes during the Atlian and suggests an increasing geological complexity, and geodiversity, on Venus over time. If compared to the evolution of biological complexity, and biodiversity, the catastrophe at the end of pre-Fortunian time could be considered a geological mass extinction event. The increase in the diversity of tectonic and volcanic features could represent a gradual restoration of an original complexity over time. This is analogous to the gradual restoration of biodiversity as new organisms and ecosystems evolve and new species fill empty niches in the aftermath of a mass extinction event. Geological and biological complexity also may not be completely unrelated. Coincidentally, coronae have been suggested to be evidence of a more Earth-like plate tectonics on a regional scale on Venus. Could an Earth-like system of plate tectonics be emerging on Venus? Could it be a restoration of something that previously existed before the pre-Fortunian catastrophe?
This is pure speculation, but if Earth-like plate tectonics (meaning where rock is being recycled at subductions zones between separate tectonic plates) is being slowly re-established on Venus, this may present hope for the future of the habitability of Venus. It was largely plate tectonics on Earth that allowed for the development of stable temperature regimes and biogeochemical cycles that could allow for the flourishing on life on Earth. Perhaps in another 500 million years, Venus could be Earth-like again. One challenge to this is the fact that Venus has entirely lost its water and it was probably the presence of water oceans that enabled plate tectonics by lubricating the rock on Earth. Kick-starting plate tectonics on Venus may therefore require some sort of intervention.
Life on Venus?
Current life (as we know it at least) on Venus at the surface is unlikely given its extremely hostile conditions. One the other hand, the increasing geological complexity of Venus and also the possibility of a re-ignition of Earth-like plate tectonics does at least make it plausible that the emergence of biosphere could exist in Venus’s future even without human intervention.
Such life on Venus, however, re-emergence of Earth-like plate tectonics and re-introduction of large volumes of water notwithstanding, is still unlikely to be Earth-like life and will more likely be some sort of weird-life, such as silicon-based life.
Astrobiology researchers, such as Sarah Johnson at Georgetown University, have made a case for agnostic biosignatures based on the premise that life may be something that might emerges out of any complex chemistry or chemical disequilibrium and not just carbon-based chemistry. Increasing complexity in Venus’s geology could also mean increasing complexity in its geochemistry, which may lead to a form of life even if it is not as we know it.
What about the atmosphere?
A present day biosphere on Venus is very unlikely but if there is life on Venus in the present epoch, it would have to be in the upper atmosphere. About Fifty kilometers or thirty miles above the torrid ~500 °C (~900 °F) surface is a region that is about 60 °C (140 °F) and the atmospheric pressure is only 1 atm, same as at sea level on Earth. While still hotter than Earth’s hottest deserts and in an atmospheric zone containing significant levels of sulfuric acid, conditions in Venus’s upper atmosphere at 50 km attitude are still drastically more favorable to life compared to Venus’s surface since organisms on Earth are known to live in comparable temperature and PH conditions. Earth has an aerobiosphere made up of microbes that live at high altitudes in the atmosphere. Could remnants of a Venusian aerobiosphere from the pre-Fortunian have survived surface paroxysms in the upper reaches of the atmosphere?
Venus in the Anthropocene
On March 1, 1966, a space object crashed onto the surface of Venus. This object was the Soviet Venera 3 lander. The intention of the object was to extend the awareness of the Earth system to the planet Venus. The first space probe to successfully land on Venus and return was the Venera 4 lander which made touchdown over year later on October 18, 1967.
Although the probe didn’t last long on the surface (only 93 minutes), this represents the beginning of the Anthropocene on Venus. However tenuously, humans had now entered picture through their robot avatars and had in some way changed the chemistry of Venus. The unfolding the of the Anthropocene in the solar system has been slow, however, and is likely to be even slower on Venus because of its harsh conditions.
Unlike Mars, which was mostly visited by U.S. spacecraft until fairly recently with Indian and Chinese spacecraft joining NASA on the red planet, Venus was mostly explored in the 20th century by the Soviet Union. As the Moon and Mars are about to be absorbed by space capitalism, Venus remains stubbornly communist with only Soviet hardware actually having reached the surface. Perhaps the same way that communist countries in the cold war considered themselves to be resisting American imperialism, Venus is joining the revolution and resisting Earth bio-imperialism by destroying the spaceships of alien invaders with its corrosive atmosphere and surface conditions.
Speaking of Earth bio-imperialism—Terraforming Venus?
Terraforming Venus may seem far fetched, but Venus may be more promising as a terraformation target than Mars. It is easier to take away atmosphere than add atmosphere. Where as Mars’s atmosphere is too thin, Venus’s atmosphere is too thick. Furthermore, Venus is closer to Earth in size, meaning that it may be overall easier to establish Earth-like conditions on Venus with its comparable gravity. One Early suggestion for terraforming Venus included using sunshades to block out the sun so that the atmosphere freezes to the ground. In this scenario, once the atmosphere had been frozen out as dry ice, it would be buried leaving only the CO2 needed to reproduce an Earth-like atmospheric pressure. Water and oxygen would also still need to be added in this scenario.
If terraformation of Venus actually happens, it is likely to be less dramatic and brute force. For example, microbes could be used to process the atmosphere to reduce sulfuric acid and carbon dioxide and increase atmospheric oxygen. This was suggested in an early paper by Carl Sagan. Furthermore, there isn’t a need to settle the surface. We could just live in sky cities like Cloud City on the fictional planet Bespin from Star Wars.
Before terraforming Venus, however, the most significant issue might be the ethical one. While it is unlikely that there is life in the clouds of Venus, the discovery of life there could mean that terraforming Venus would require us wiping out a native biosphere. Is it our place to decide which biosphere continues and which one does not? Like with Mars, it could be argued that in that case Venus belongs to the Venusians. Terraforming Venus may end up being the right decision, but we must take into account planetary protection and confirm whether there is indigenous life on Venus if we want to be more than just Earth bio-imperialists. Otherwise, maybe Earth is the evil twin after all.