Terraforming Venus would be much harder than building floating cities, which I explored previously. It would require vast resources, and as such is probably not feasible for at least several centuries. However, if we did this, we would have another habitable Earthlike planet in the Solar System, with similar gravity, a larger land surface area, and a vast amount of exploitable minerals. Despite this, a terraformed Venus would still face many challenges, chiefly amongst them a very long day-night cycle, 117 days in total. This would lead to vast temperature differences across the planet, and potentially make even a terraformed Venus effectively uninhabitable.
Venus currently is not habitable, due to the extremely dense carbon dioxide atmosphere both exerting a very high pressure on the surface and heating it to 465°C by the greenhouse effect. Any successful terraforming proposal would need to remove almost all of the atmosphere. Luckily, almost all of the problem surrounding terraforming are interlinked. The 3.5% of the atmosphere that is nitrogen actually provides more gas than is needed to create a mostly nitrogen atmosphere when the carbon dioxide is removed, such that extreme flammability is not a problem, as would be the case with an oxygen-rich atmosphere. By removing the carbon dioxide, both the problems of pressure and temperature would be solved, and by converting some of the carbon dioxide to carbon and oxygen, we would be able to create a breathable atmosphere, by using the oxygen.
Removing the carbon dioxide from Venus' atmosphere would be an exceptionally difficult task, in part because many of the forms which the carbon dioxide could be converted to are not heat stable, and would merely decompose under the extreme heat of Venus, releasing the carbon dioxide back into the atmosphere. This prevented Carl Sagan's original idea from working, where genetically engineered bacteria were to be introduced into the atmosphere, converting carbon dioxide to graphite. Under such conditions, the graphite would spontaneously combust as the oxygen content rose, preventing the carbon dioxide from ever being removed from the atmosphere.
One proposal suggests taking advantage of the presence of large quantities of calcium oxide and magnesium oxide in Venus' crust. These react with carbon dioxide to form carbonates; sequestering the carbon dioxide. If this was taken advantage of, the pressure on the surface could be significantly reduced, but this would require large sections of the crust to be exposed. In addition, the reaction reverses at high temperatures, such as those on Venus' surface, so the carbonates wouldn't trap the carbon for long, making it virtually impossible to terraform Venus using this method alone.
Another proposal suggests introducing a vast amount of hydrogen into Venus' atmosphere, to react with the carbon dioxide, forming graphite. This would serve two purposes: it would remove the carbon dioxide from the atmosphere, while adding water to form oceans. In addition, the graphite produced would remain stable, as the oxygen would not be released into the atmosphere in diatomic form, but would instead be present as part of the water molecules. There are two main problems with this design, both of which are very restrictive. Firstly, the water would not remain on Venus as a liquid, but would be present in the atmosphere as water vapour, acting as a greenhouse gas and not leading to a noticeable decrease in surface temperatures. Secondly, the hydrogen would need to be transported, at great cost, from Jupiter. This wouldn't be possible with current technology. In addition, a large quantity of iron aerosol would need to be added to the atmosphere to facilitate the reaction, again increasing the cost of the project. For these reasons, this project would only work in combination with solar shades, a system of mirrors designed to reduce the insolation received by Venus. A solar shade positioned at the L1 Lagrange point between Venus and the Sun roughly 4 times the diameter of Venus would be capable of significantly cooling Venus. Ignoring the immense cost of such an array, this would be an effective method of terraforming, especially when combined with either of the above methods for carbon capture.
After a low-density, habitable atmosphere has been created, a magnetic shield will need to be created, to protect Venus from the solar wind. Venus lacks a magnetic field itself, as its core has cooled. Therefore, a magnetic shield placed at the L1 Lagrange point between Venus and the Sun, as would be needed for a terraformed Mars, is essential to prevent the atmosphere from being stripped away, leading to a situation similar to Mars, where the atmosphere is almost non-existent, and radiation levels on the surface are dangerous.
Another problem faced after the terraforming of Venus would be a possible reversion to the current state of Venus, triggered by volcanic eruptions, or by a runaway greenhouse effect. This is believed to have happened once before on Venus, 700 million years ago, when an unknown event, probably a massive series of volcanic eruptions, released vast quantities of carbon dioxide into the atmosphere, leading to the evaporation of Venus' oceans, and causing a positive feedback loop where all of the carbon stored on Venus was released into the atmosphere, creating the hellish environment that characterises Venus today. If another similar event occurred, the stores of carbon would be released into the environment in a feedback loop, essentially recreating today's Venus. These sorts of events seem to be very rare, so hopefully this shouldn't happen
At first, the extremely long day-night cycle seems like it might pose a problem. Currently, this doesn't lead to a significant temperature difference across the surface of Venus, as the pressure and temperature are so high that the carbon dioxide exists as a supercritical fluid, allowing heat to quickly transfer across the surface. In addition, the sulfuric acid clouds reflect almost all of the light from the Sun, reducing the difference in luminosity between the two sides. The new low-density atmosphere would not transfer heat so easily, possibly leading to large temperature differences between the two sides of the planet. However, it appears that this wouldn't be the case. The Sun would heat up the side facing it, but this, in turn, would lead to evaporation, producing clouds, reflecting the sunlight and preventing further heating. Overall, the climate of a terraformed Venus would resemble that of a high-latitude region on Earth, with the day and night morphing into seasons. Venus would therefore have two seasons, a warm humid summer, and a cold dark winter.
Overall, terraforming Venus would be very useful, potentially providing a home for billions of people. However, the resources needed to do so are so great, and some of the technology so advanced, that this doesn't seem like a feasible project, at least for several centuries.
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