Settling and Terraforming Venus: A Synthesis of Modern ApproachesSeth Pritchard

Key words: terraforming, direct liquefaction, sunshade, soletta, Venus, carbon dioxide, oceans, settlement

Venus is often overlooked as a target for settlement or terraforming because of its extreme environment, which is the result of a massive and dense super-greenhouse atmosphere. This paper presents a scenario in which Venus could be made habitable and ready for settlement in only a few centuries, based primarily on ideas put forward by Paul Birch in 1991. In addition, orbital habitats are constructed from additional materials used in the creation of the orbital infrastructure needed to begin and maintain the terraforming process. These habitats allow for the early settlement of Venus-space and can potentially provide a rapid return on investment. The massive CO2 atmosphere is frozen out using a sunshade and stored in large ice caps at the poles, where it is mined and processed into various materials such as artificial limestone and carbon nanotube composite structures. The sunshade is also a vast solar farm used to beam power to the orbiting habitats during the long 200-year occultation of the sun. Oceans are created by dismantling several moons of Saturn, including Epimethius and Janus. Finally, simple cyanobacteria and other organisms are imported and seeded in the oceans to begin fixing nitrogen and oxygenating the atmosphere. At this point, the surface is ready for human occupation.

1. Introduction

Venus is often called Earth's sister planet due to its similar mass and size. Of course, as planets go, Venus is in reality quite far from being Earth's twin. It is veiled in a thick blanket of sulfur dioxide clouds, which reflect almost 90% of all incoming visible light from the Sun, making Venus the brightest object in Earth's night sky, except for the Moon and the Sun. Because of this, early astronomers were barred even the smallest glimpse of the surface itself. Due to its similarity to Earth in terms of size and composition, many thought it was Earth's twin. Some speculated that the entire planet was drenched in a steamy, carbonaceous jungle underneath dense clouds of water vapor, while others envisioned a parched, desert planet. Much like Mars, Venus took on a mythical quality in the public imagination. Science fiction of the time imagined the planet to be a balmy, primaeval hothouse choked with vegetation and home to strange and fantastic creatures.

In reality, Venus' atmosphere is almost

entirely carbon dioxide, with a surface pressure of 92 bars. This results in a massive runaway greenhouse effect that is responsible for the incredibly high surface temperatures, which are hot enough to melt lead (8). In the upper atmosphere, about 55 kilometers above the surface, the temperatures are low enough to allow highly concentrated sulfuric acid to condense into thick clouds that enshroud the entire planet. Precipitation from these clouds falls down to about 25 kilometers above the surface before evaporating. The surface is wracked by ancient volcanoes and vast lava flows. Venus also lacks plate tectonics, making it unable to effectively transport heat from its mantle to the surface. As heat builds up inside the mantle, the crust begins to weaken until large portions begin to rapidly subduct over a period of 100 million years, completely re-working the surface in a spasm of global volcanism. The revelation of such a harsh environment lead to comparisons with the christian Hell. However, rather than torture, stepping out onto Venus would instead result in

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more or less instantaneous death from the crushing atmosphere, caustic clouds, and searing heat.

Table 1

Gas Concentration (2)

CO2 96.50%

N2 3.50%

SO2 150 ppm

Ar 70 ppm

H20 20 ppm

CO 17 ppm

He 12 ppm

Ne 7 ppm

The biggest challenge to making Venus habitable is to remove its significant

atmosphere. Table 1 displays the dominant constituents of the Venusian atmosphere. It contains some 1.2×1020 kg of CO2, almost all of which needs to be removed in some fashion. This is the first step to any attempt at terraforming Venus.

2. Removing the Super-Greenhouse Atmosphere

2.1 The Sunshade

The simplest and most brute-force solution to removing Venus' CO2 is to simply cool the planet down to the point where it all freezes out. To do that, a massive sunshade would need to be placed in the Venus L1 Lagrangian point (After the terraforming process is complete, the sunshade would then be used to control the solar flux striking the terraformed Venus). The construction of the sunshade would be by-far the most challenging part of the process, and a staggering feat of

engineering in its own right. In order to fully shade Venus from the Sun, the sunshade must be twice the diameter of Venus (if sitting at VL1)(1). Although this seems like an insurmountable construction project, the space environment offers unique benefits which make the construction of such a massive object easier than one might first expect. Firstly, materials are readily available with which to build the shade, in the form of asteroids. The estimated mass of the sunshade is around 7.6×1010 kg. Asteroids of similar mass include 101955 Bennu (the target of NASA's OSIRIS-REx sample return mission) at 1.4×1011 kg, and 25143 Itokawa (the target of Japan's Hayabusa sample return mission), with a mass of around 3.51×1010 kg. Others include the near-Earth object 2007 FT3 (5.5×1010 kg), and the infamous near-Earth asteroid 99942 Apophis (4×1010 kg) (5). Ideally, an asteroid with a high metal content would be selected. Itokawa, the only examples listed above whose composition is known, is a rocky asteroid with a low metal content. It is quite likely that a metallic asteroid in this mass range (or several smaller ones) can be identified and moved into the Venus L1 point, where they would be mined out and the sunshade constructed in-situ.

Secondly, the only real limit on the size of an object that can built in space is the amount of material available. By using asteroids, which are in near-unlimited supply in the asteroid belt as well as the space between Earth and Venus, this limit effectively becomes trivial, and construction is only limited by the strength of the materials and the forces the structure is subject to. Thus, for a civilization which already has a significant presence in space and has a mature interplanetary economy, the construction of the sunshade is not at all outside the practical realm, and indeed may not cost a great deal of effort and resources to build. While today such a mega-engineering project remains in science fiction, in a few centuries it may be perfectly reasonable to consider.

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With the sunshade in place, the atmosphere can begin cooling down via radiative energy transfer. The cooling of the atmosphere takes place in five stages. In the first stage, once the sunlight has been cut off by the sunshade, the temperature begins to drop to 304 K in about 58 years, the critical point of CO2. In the second stage, it then starts to rain liquid CO2 at a constant temperature until the pressure falls to CO2's critical pressure 27 years later. At this point, stage 3, the temperature and the pressure

begin to fall together. Once the CO2 rainstorm has finished, over 88 bars of CO2 will have rained out and drained into the basins, forming oceans. After 94 years of CO2 rain, stage 4 begins; the triple point of CO2 is reached (217 k), and the oceans begin to freeze over. It will take the oceans 17 years to completely freeze solid, while the remaining CO2 in the atmosphere snows out over a period of 9 years. In stage 5, after 200 years of cooling, the entire inventory of CO2 in Venus' atmosphere has now accumulated on the surface as vast CO2 glaciers, and a 3 bar atmosphere of nitrogen is left behind (6). During stage 4, the CO2 snow will accumulate over the entire surface of Venus.

Clearly, CO2 glaciers are not desirable on the Venusian highlands, so the CO2 must be moved to the basins. This can be done by selectively heating the highlands with reflected sunlight from orbiting mirrors, encouraging CO2 to deposit over the shadowed lowlands (6). The dry ice would cover the entire surface of Venus (assuming it was completely flat) to a depth of around 174 meters (11). In reality, most of it will accumulate in the lowlands and basins before freezing, forming thick glaciers many hundreds of meters deep.

Before the sunshade can be opened and the planet brought out of its artificial 200-year long night, the CO2 ice sheets must be insulated. The CO2 will eventually have to be dealt with in a more permanent fashion, either sequestered into carbonate minerals or exported off-planet, as eventually internal heat from the mantle will warm the crust to the point where the CO2 will begin to outgas. Since the ice sheet will eventually be buried under an ocean of water, Paul Birch suggested a covering of interlocking blocks of foamed rock with a layer of tough plastic sheeting, manufactured with sequestered carbon (1). It might be possible that such insulation may be enough to prevent significant loss, but even relatively small leaks have the potential to be locally disruptive.

However, there is another way that the dry ice can be maintained on the surface. Venus's axial tilt is very small. At 2.64°, there is little to no variation in seasons over the course of a Venusian year (2). This can be used to our advantage, as a way to keep the dry ice glaciers permanently solid. Instead of allowing the liquid CO2 to freeze into vast ice sheets in the basins, the dry ice could instead by moved to the poles and kept as vast dry ice caps many kilometers thick, covered in a strong protective polymer sheeting. Then, if the poles can be kept in permanent shadow, the CO2 ice caps should remain stable. Because Venus' axial tilt is so small, the positions of the poles will not shift greatly during its orbit, thus the ordeal of keeping them permanently shaded is made simpler. This may eliminate the need to

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Figure 1: Phase diagram of carbon dioxide. Rendered in Wolfram Alpha.

eventually export or sequester the CO2, by keeping it refrigerated at the poles under permanent darkness.

With the CO2 now out of the atmosphere and kept frozen as easily handled dry ice, it can then be used in various industrial processes. Large factories can be set up to separate the CO2

into O2 gas and solid carbon, with the O2 being vented straight into the atmosphere. The reclaimed carbon can then be used to synthesize ultra-strong composites made from nanotubes or fiber. Other facilities can take the raw CO2 and chemically sequester it into artificial limestone through various chemical pathways, as discussed in section 2.2. These materials can be used to build surface habitats and other components of infrastructure which will be necessary for the eventual settlement of the planet.

Because of the sunshade's vast surface area and proximity to the sun, if it was designed as an extremely thin flat disk, the light pressure on the illuminated side would impart a non-negligible thrust on the shade, effectively turning it into a giant solar sail (1). Paul Birch designed a modified combination-sunshade/mirror system which could actively stabilize itself against the incoming photon pressure from the sun (shown in figure 2). The sunshade would consist of a series of annular slats which are angled at 30°. Incoming sunlight is reflected to the next slat outwards where it is reflected back almost on its original path. Each set of slats is offset by 1° from the original 30°. The net result is that after bouncing around inside the sunshade, sunlight is deflected from striking Venus by about 4 degrees on either side. It would rely on a huge annular support mirror on the opposite side of Venus which would reflect sunlight onto the sunshade's inner cone, keeping it stable against photon pressure. This configuration is in fact dynamically stable; if the sunshade moves off-point, the combined effects of the annular mirror, sunlight, and Venus' gravity are enough to pull it back in place. The sunshade is also located closer to Venus than the L2 point. Thus, it only needs to be 1.27 times

the diameter of Venus. (6)

2.2 Carbonate Sequestration

A more permanent method of removing the excess CO2 is sequestering it in carbonate minerals. The most readily available materials for this would likely be calcium and magnesium mined from the crust of Mercury. Calcium and Magnesium both react with carbon dioxide to form calcium carbonate (CaCO3) and Magnesium Carbonate (MgCO3), both of which are stable under present Venusian conditions (i.e., they will not spontaneously decompose and outgas CO2). Two obvious challenges to this process are lifting the processed metals into orbit from the surface of Mercury (which requires a ΔV of 2,945 m/s), and then lifting them up the solar gravity well to drop them into Venus' atmosphere. Sequestering Venus' entire inventory of carbon dioxide would require a significant amount of either metal (8×1020 kg of calcium or 5×1020 kg of magnesium, respectively) (1). Note for comparison that the dwarf planet Ceres has a mass of around 9.43×1020 kg (3). Thus, capturing all of Venus'

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Figure 1: Birch's slatted sunshade system, from (1).

CO2 into carbonate rocks is not a practical solution, as the required amount of metal would literally be astronomical. However, chemical sequestration could be combined with other strategies, such as direct liquefaction/deposition via radiative cooling, to reduce time and expense.

Strip mining Mercury's surface can be done with automated mining robots. Because of the extreme temperatures on Mercury's dayside, mining operations could slowly work their way around the planet, following the terminator line in order to avoid direct sunlight. If need be, a small manned outpost can be stationed at the poles to oversee the robotic mining, supplied by deposits of water ice detected by MESSENGER (7). An equatorial mass driver system (specially insulated to deal with the extremes of temperature) can take advantage of the tremendous amounts of solar power to launch refined pellets of magnesium and calcium directly into orbit. Once there, the pellets would deploy a solar sail to place them on a Hohmann transfer to Venus. Launch windows for this transfer occur roughly every 4.7 months (10). Thus, launching as many pellets into orbit as possible during each launch window is desirable.

2.3 Sequestration via Bosch Reaction

Another chemical process available for removing CO2 is the Bosch reaction. Hydrogen is reacted with carbon dioxide to yield water and elemental carbon (graphite). The stoichiometry is thus:

CO2(g) + 2H2(g) → C(s) + 2H2O(g)

The easiest source of hydrogen would probably be in the form of near-Earth or near-Venus carbonaceous asteroids, which are about 25% water by mass, or perhaps so called main-belt comets, objects with fairly circular orbits within the main asteroid belt which appear to be the extinct nuclei of comets. These objects would be towed into Venus orbit, have their

water content extracted, and the hydrogen separated out. Once extracted, the hydrogen would be dropped in the Venusian atmosphere, where it would begin to react with the carbon dioxide. Again, the amount of hydrogen required to completely remove the entire inventory of CO2 is quite significant; 4×1019 kg (2). However, this approach is quite desirable, because it allows us to effectively kill two birds with one stone; it removes CO2 while also adding H2O. However, H2O is also a highly efficient greenhouse gas, meaning that if the temperature is above the boiling point, the water will simply remain in the atmosphere and slow the rate of cooling. Thus, this technique should only be applied late in the cooling phase, when the temperature has dropped below 373 K.

3. Orbital Settlement

While the orbital infrastructure required to begin the terraforming process is being constructed at Venus, large habitats could also be constructed at the same time to accommodate construction crew and project managers, as well as a means to partially fund the project by providing living space to future settlers. Large Stanford tori, O’Neill cylinders, and Bernal spheres can be built from the materials mined from asteroids tugged into Venus orbit, similar to the sunshade itself. Additionally, the sunshade can provide power to these orbital habitats by means of lasers. The sun-facing side of the sunshade can be converted into a vast solar farm. With a surface area of over 1.8×109 km2 (assuming the sunshade is Birch's slatted shade design), it would intercept over 4.84×1017 Watts (484.4 petawatts) of power, a truly enormous amount of energy. Thus, habitats can be constructed in low-Venus orbit, within the shadow of the sunshade, and still be able to be supplied with power.

It should also be pointed out that this is more than enough power to operate the laser station of a titanic interstellar light-sail. The construction of such sails, whose surface areas are considerably smaller than that of the

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sunshade (but still massive in their own right, being almost 1000 km in diameter), can be easily achieved given the ability to build the sunshade itself. Thus, the sunshade can also be used as the power source for interstellar propulsion systems.

The sunshade itself can also be used as a giant habitat. Living modules could be affixed to its frame to house construction workers while the shade is being assembled. Additionally, if the shade is set spinning for stabilization, the rate of spin could be set so that modules on the outer rim of the shade would experience a force similar to Earth or Venus gravity. Either way, the massive build-up of orbital infrastructure near Venus will provide another niche for settlement, and will open up Venus-space to industry and colonization centuries before the planet itself is ready to be settled.

4. Building an Ocean

Venus today has very little water in its volatile inventory; what little it has left exists as a very small fraction of its atmosphere. All of the water for Venus' ocean must be sourced from elsewhere. An obvious choice would be the nucleus of a main belt comet, or an ice moon from one of the outer gas giants, such as Saturn. it might be possible to bring large quantities of water ice to Venus by nudging one of Saturn's icy moons out of orbit and bringing it to Venus. Birch originally proposed a nuclear steam rocket fed by indigenous water ice to nudge the moons into momentum exchange trajectories to slingshot them into the inner solar system. However, a steam rocket has a very low specific impulse, and would require an obscene amount of propellant in order to achieve the required delta-V. Thus, a more modern suggestion would be a D+He3 burning fusion rocket for the initial burn. With a far higher exhaust velocity, a fusion rocket would not require nearly as much reaction mass as a steam rocket. In addition, the fusion fuel can be mined from the atmospheres of the planet the moon orbits (either Saturn or Uranus) as both deuterium and helium-3 are

present in small amounts in the atmospheres of both. Birch originally suggested Enceladus, as it's mass alone would create a layer of water over 100 meters deep (1). However, the Cassini mission has shown that Enceladus is a very interesting and dynamic world, with the potential for life in pockets of liquid water beneath its icy shell. Therefore, this paper will spare Enceladus and instead make do with several smaller ice-moons; Epimethius, Janus, Pandora, Prometheus, and Mimas. The ice-moons would fall towards Venus on a Hohmann trajectory, taking 5.85 years to arrive (10). The combined mass of all five moons is 4.02×1019 kg. Assuming they are made of pure water ice, this is equivalent to about 2.8% of the total water in Earth's oceans. The surface of the ice moons must be insulated from the Sun as it falls towards Venus to prevent any loss of water ice due to out-gassing, as it would effectively start to behave as a comet the closer it gets to the Sun.

Once towed into Venus orbit, the ice-moons would begin to be dismantled. Ice-impactors of uniform size and mass, made from water refined from the processed bodies of Epimethius and Janus, would be fashioned and de-orbited into the atmosphere. To avoid the possibility of a potentially catastrophic puncture of the cover over-top of the buried CO2 glaciers, the ice-impactors are small and fragile enough to ensure that they break apart as they enter the

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Figure 3: Five icy moons of Saturn. From top left to right; Janus, Epimethius, and Mimas. Bottom left to right; Pandora and Prometheus.

atmosphere. (Note that if the CO2 glaciers are instead concentrated in polar ice caps, this can be ignored. Indeed, the impactors may then in fact be deliberately aimed at the basins to improve the efficiency of delivery). This also has the effect of delivering their water content directly into the atmosphere as water vapor, where it can then condense and rain down to the surface. Care must be taken to ensure that the constant bombardment of ice shards does not begin to heat up the atmosphere to undesirable temperatures. Water is also a potent greenhouse gas, so large quantities of it in the atmosphere are also not desirable.

Compared to the Earth, Venus is a very flat planet. Most of its surface is made up of vast, smooth rolling plains of ancient lava. Over 80% of its surface elevation is within 1 km of the mean elevation (6). Thus, even though the total amount of water brought to Venus will be significantly lower than the water content in Earth's ocean, the Venusian oceans will be considerably wider, covering a greater portion of the planet's surface than Earth's. Again, assuming the surface of Venus was completely flat, the amount of mass deposited on the surface after ice-fall would amount to a layer of water about 87 meters deep across the entire surface (11). Accounting for the actual surface relief means that in the lowlands and basins it would likely be at least 100 meters deep, if not more. If necessary, additional ice moons can be imported, possibly from Uranus, until the oceans are a few hundred meters deep.

5. Establishing an Artificial Day/Night Cycle

Venus' slow rotation rate and retrograde rotation present challenges to the settlement of the post-terraformed planet. Venus completes one rotation around its axis every 2802 hours (116.75 Earth days) (2). Additionally, due to its retrograde rotation, the sun would appear to rise in the west and set in the east. Since spinning up Venus is out of the question (there have been methods proposed to induce a more Earth-like rotation, but these shall be ignored due to the absurd energy requirements and timescales needed to change the angular momentum of a large terrestrial planet. Additionally, some are unacceptably violent and would effectively resurface the entire planet), the best solution is to simulate a 24-hour day/night cycle using large orbital mirrors.

Paul Birch originally suggested using a set of polar-orbiting solettas to reflect sunlight down at the surface while the sunshade would permanently block out direct solar radiation (1). This would result in a truly strange day cycle. Because the solettas would be in a polar orbit, the image of the sun would appear to move from

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Figure 4: Topographic map of Venus, made with data from the Magellan orbiter. Note the two main "continents"; Ishtar Terra to the north and Aphrodite Terra near the equator. The lowest points are at -2.5 km below the datum and the highest point is Maxwell Montes, at 11 km above the datum. Map from http://laps.noaa.gov/albers/sos/sos.html

pole to pole (north-to-south or south-to-north, depending on the direction the solettas orbited), making the polar regions tropical. Additionally, due to Venus' extremely slow rotation, as the surface moves underneath the path of the sun image, over the course of a single sidereal day, the entire surface would have the sun image overhead at some point. The band around the equator would alternate between tropical and arctic, being tropical where the surface is under the path of the solettas and polar at the sunward and anti-sunward sides.

A simpler approach would involve the sunshade being “slatted”, opening partially to let in a measured amount of sunlight. The amount of sunlight allowed to pass would be equivalent to the solar flux which Earth receives at its distance from the Sun. A constellation of large solettas behind Venus then reflects additional sunlight down at the night side. In this way, the entire planet is illuminated. To simulate a “standard” 24-hour day, the sunshade would be opened for 12 hours to let in sunlight, and then shuttered for another 12 hours to simulate night.

6. Introducing Biology

Once the majority of the atmosphere has been removed and oceans of water have been imported, conditions on Venus would finally reach a point where simple organisms can be introduced into the environment. Because the atmosphere at this point is dominated by nitrogen and is essentially anoxic, the first organisms will need to be anaerobic in order to survive. Cyanobacteria are probably a very likely first choice for initial colonization. Certain species, mainly those within the order Nostocales (containing filimetnary and branching forms of cyanobacteria), are capable of forming heterocysts when in an anaerobic environemnt (4). Heterocysts are specialized cells which contain an enzyme called nitrogenase, which makes them capable of turning diatomic nitrogen in the atmosphere into bioavalable forms such as ammonium or nitrate, a process called nitrogen fixation. The

importance of this is outlined in more detail below. Additionally, they produce oxygen through photosyntheis, which over time will create an oxigenated atmosphere for more complex life to eventually breathe.

Nitrogen compounds, primarily NH3 (ammonia) and NO3 (nitrate) ions, are an essential nutrient for life, and play an important part in terrestrial agriculture, mainly as fertilizers. As such, nitrogen plays an important part in biological systems. However, nitrogen typically occurs in nature as N2, a diatomic molecule made up of two nitrogen atoms. N2 is quite stable and as such is fairly chemically inert. In order to be useful in biological processes, the nitrogen must be made available for chemical reactions. This process is called nitrogen fixation (9). On Earth, nitrogen in the atmosphere is "fixed" into ammonia by lightning. The amount produced this way is rather small compared to the amount produced by nitrogen-fixing organisms; on average, lightning is responsible for fixing about 1×1010 kg of N2 per year, while biological sources can fix over 1.75×1011 kg (9).

Nitrogen fixation will not only allow life to survive and spread throughout the oceans and the surface of Venus, but over time it would also allow for agriculture to be practiced on the surface. Nitrogen-fixing plants, such as legumes, could then be farmed and planted to increase the rate of nitrogen being fixed per year. Eventually, large forests could be planted which will in turn increase the amount of oxygen in the atmosphere. Oxygen build-up would likely take several centuries to several millennia before the atmosphere could be breathable by humans unassisted. At this stage, human occupation can begin, in sealed oxygenated environments on the surface and underground. Walking outside would require little more than a re-breather and a pressurized oxygen tank, as the pressure at sea level is only several times higher than on Earth, eliminating the need for a pressure suit as would be required on Mars. Additionally, the now-temperate climate would no longer be immediately lethal to humans and, in fact,

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would probably be quite pleasant.

7. Ethical Considerations and Summary

In addition to the energy considerations, terraforming Venus implies quite a lot of destruction on many bodies in the solar system. A great deal of Venus' surface will be eroded and destroyed by the accumulation and flow of condensed CO2, as well as the build-up of the oceans. The construction of the sunshade mandates the complete disassembly of large metallic asteroids. The oceans themselves would be made from the water ice of main-belt comets or even a small ice moon of Saturn. One might ask if such actions are justifiable from an ethical standpoint. Venus today is almost certainly uninhabitable. No known form of biology can survive there. Thus, terraforming does not pose any risk to a potential biosphere, as there is no biosphere to put at risk in the first place. However, it is likely that a great deal of geological history will be buried and destroyed during the terraforming process, especially in the vast basins and lowlands which will be filled with water. The huge amount of liquid CO2 that will wash across the surface during the great freeze will almost certainly cause heavy erosion, and the resultant dry ice glaciers will cover large areas of land, even if they are moved to the poles.

However, all this destruction would result in the creation of a new habitat for life. Certainly no-one will lament the loss of a few chunks of metal and ice from the solar system's countless reservoir of asteroids and comets, especially when their sacrifice results in the transformation of a hellish, brutal wasteland into a habitable world with oceans and life. At the end of this process, Venus is much changed from its original state. A ~3 bar atmosphere of nitrogen has replaced the original ~93 bars of CO2. The surface temperatures are akin to what are found on Earth, with similar diurnal variations. Much of its surface is covered in shallow oceans in which photosynthetic cyanobacteria fix nitrogen and slowly oxygenate

the atmosphere. Although the planet still rotates extremely slowly, an artificial 24 hour day-night cycle has been created with a special sunshade which selectively lets a portion of sunlight through for 12 hours, and a pair of solettas which reflect the sunlight on to the opposite side of the planet, thereby illuminating the majority of the surface simultaneously. The poles, however, are kept permanently shaded to store the now-frozen CO2, which can either be sequestered into carbonate rocks, used in various industrial processes (for instance, using the carbon as raw material for the manufacture of graphene or nanotubes), or simply kept frozen and tucked underneath protective polymer sheets.

Terraforming Venus will require planetary engineering on a massive scale. At the very least, an intensive stream of material from around the solar system is a mandatory requirement. This alone means that at best, such a project will require extensive orbital and interplanetary infrastructure, as well as a vast energy budget which will likely not exist for another few centuries. It should be stressed that this does not represent a near-term or even mid-term project. It is almost certain that before Venus is terraformed, there will be permanent stations the Moon, settlements on Mars, a mature asteroid mining infrastructure, and industrialization of the outer solar system to some extent, as the kind of infrastructure necessary to maintain these is a prerequisite to terraforming Venus, especially when it comes to moving very large objects the size of Janus. Unlike Mars, there is no convenient volatile inventory available to help build a habitable environment. All of them will have to be imported from elsewhere in the solar system, most important of them all, water. The amount of energy required for the project dwarfs current terrestrial energy use. And while there is essentially unlimited energy available in space in the form of sunlight, an extensive and well-established space-based civilization is required to fully take advantage of that resource. Ultimately, Venus might be a planet that can be

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made truly Earth-like, unlike Mars, which requires several compromises for a habitable terraformed environment. Despite the extreme difficulty and staggering engineering involved,

Venus may prove to be more valuable to terraform than Mars, in the long run.

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8. References

1. Birch, P. (1991). “Terraforming Venus Quickly”. Journal of the British Interplanetary Society, 44, 157-167. Retrieved from http://www.orionsarm.com/fm_store/TerraformingVenusQuickly.pdf

2. Williams, D. R. (2013, July 1). Venus fact sheet. Retrieved from http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html

3. Williams, D. R. (2004, Sept. 1). Asteroid fact sheet. Retrieved from http://nssdc.gsfc.nasa.gov/planetary/factsheet/asteroidfact.html

4. Meeks, J. C., & Campbell, E. L. (2002). Cellular differentiation in the cyanobacterium nostoc punctiforme. Archives of Microbiology, 178(6), 395-403. Retrieved from http://link.springer.com/article/10.1007/s00203-002-0476-5

5. Sentry risk table. (2013, November 30). Retrieved from http://neo.jpl.nasa.gov/risk/

6. Fogg, M. (1995). Terraforming: Engineering planetary environments. Society of Automotive Engineers Inc.

7. MESSENGER finds new evidence for water ice at Mercury’s poles. (2012, November 29). Retrieved from http://www.nasa.gov/mission_pages/messenger/media/PressConf20121129.html

8. Venus: Overview. (2013, October 25). Retrieved from http://solarsystem.nasa.gov/planets/profile.cfm?Object=Venus

9. Deacon, J. (n.d.). The microbial world: The nitrogen cycle and nitrogen fixation. Retrieved from http://archive.bio.ed.ac.uk/jdeacon/microbes/nitrogen.htm

10. Chung, W. (2013, August 02). Mission delta v and flight times. Retrieved from http://www.projectrho.com/public_html/rocket/appmissiontable.php

11. Calculations done by author, using the equation for the volume of a spherical shell;

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