Venus has long been considered Earth’s twin, but since the late 1950s we’ve realised she is Earth’s “Evil Twin”, with a 92 bar mostly CO2 atmosphere, very little water and a 116 day ‘sol’. How do we make her a more pleasant place?
The first proposal came from the 1930s, when spectroscopes were first turned towards Venus, and no oxygen was drected, and CO2 seemed to be the main component of the atmosphere. The nature of the clouds was unknown, but water seemed likely and the planet was imagined to be a global ocean. Naturally the solution was to generate free oxygen via electrolysis, expelling the excess hydrogen to space. Unfortunately the planet was inhabited and horrendous conflict ensued (Olaf Stapledon’s “Last and First Men”.)
Large amounts of carbon dioxide are incompatible with water, as it dissolves too readily. Thus the next suggestion, from the late 1930s: Venus’s clouds are polymerised formaldehyde dust and the planet is dry due to water being bound chemically in the dust and rocks. The greenhouse effect created surface temperature near 80-100 C could be managed by suitable insulation and cooling systems, but the colonists could employ a catalyst to destroy the formaldehyde and liberate oxygen and water [Poul Anderson's "The Big Rain", Cyril Kornbluth & Frederik Pohl's "The Space Merchants", early 1950s.]
In the mid 1950s the global ocean returned as a possibility, but so did a hot dusty desert or a hot ocean of crude oil. No particular model seemed more likely than the others, though oxygen was still unobserved (contra many bad SF stories).
Then in the late 1950s the temperature of Venus was observed to be ~300 C in radio frequencies. This suggested that the planet was hot from a massive greenhouse effect and all the planet’s water was in vapour form. Carl Sagan suggested, in 1961, that the planet could be made more Earth-like via seeding the atmosphere with blue green algae, converting the carbon dioxide into oxygen, the algae eventually falling into the hot depths to be reduced to carbon and their water returning to the atmosphere to be cycled again. Eventually a rich oxygen atmosphere would result, with a surface coated in carbon and the clouds condensing as rain… or so it would if the atmosphere was mostly nitrogen as most assumed in the early 1960s. For example, the 300 C Venus with 4 bars surface pressure and atmosphere of 80% nitrogen, would result in a breathable N2/O2 atmosphere after the algae had finished their job.
Unfortunately for Sagan’s scenario, and the 1960s & 70s SF based on it, the Russian and US probes to Venus revealed, by 1970, that the atmosphere was 96.5% carbon dioxide, with a surface pressure of 92 bars and a temperature of 462 C. Another complication was the bright clouds. These had so invitingly looked like water, but proved to be only partly water and mostly sulfuric acid. Yet not much sulfuric acid and not very much water at all, in fact. Venus is fantastically dry.
What can be done to make Venus more Earth-like? Firstly, contra the apparent evidence, Earth has about as much carbon dioxide as Venus – but on Earth it’s bound up in the rocks as carbonate minerals. On Earth the exhalations of the mantle, in the form of volcanic gases, have mostly dissolved in the oceans and have largely been locked up chemically. Venus, in a sense, is Earth absent water and unable to bury her atmosphere.
A common suggestion is to remove the atmosphere of Venus via blowing it or throwing it into space. Venus’s atmosphere masses 478,000 trillion tonnes and to launch it all into space requires a minimum of 54 MJ/kg – the equivalent of 6.1 trillion megatonnes of TNT or the total fusion of 73 billion tonnes of deuterium. However applying all that energy efficiently is a herculean challenge. Using asteroid collisions adds additional heat to the planet, which merely adds to the problem we meant to solve.
However the 127,400 trillion tonnes of carbon locked up in the atmosphere is a resource quite unlike any other. The entire asteroid belt contains a fraction of the total carbon available in the atmosphere of Venus. Eventually the carbon could be exported off world, thus disposed of in time, but how do we get it out of the air and cool the planet down?
One suggestion is to dispose of it the same way Earth did naturally, by turning it into carbonate rock. To do so we could cycle atmosphere through the crust, but there might not be enough lime (calcium and magnesium oxides) to react it with. Alternatively we mine magnesium (from Mercury, for example) and send it to Venus to burn in the atmosphere, then combine more carbon oxide with the resulting magnesium oxide to make magnesium carbonate:
Mg + CO2 => 2 MgO + C
2 MgO + 2 CO2 => 2 MgCO3
Thus for every atom of magnesium, 3 molecules of carbon dioxide are disposed of. Magnesium will also react with the sulfuric acid in the clouds and quickly reduce the greenhouse effect significantly. While the clouds reflect ~75% of the light, they also prevent a significant amount of heat from escaping. Without the clouds the temperature would fall by ~140 degrees to about ~600 K.
Just how much magnesium is required? To combine chemically with ALL the carbon dioxide would require a mass equivalent to 1/3 the mass of carbon dioxide – about 160,000 trillion tonnes. Solar-powered mass-drivers could easily fling it towards Venus, into the orbital path of the planet, to produce brightly burning fireballs in the upper atmosphere as the magnesium burns. The energy to throw from Mercury towards Venus is about 31 MJ/kg, which means the total energy needed to remove Venus’s atmosphere via burning Mercurian magnesium is ~20% of the energy needed to remove the atmosphere kinetically. Plus the carbon resource stays on Venus. The surface will be covered in hundreds of metres of magnesium carbonate and plain carbon powder. And Venus will still be dry.
Venus has plenty of oxygen for making water – what it lacks is hydrogen. If we made water from all the oxygen in the carbon dioxide atmosphere of Venus, then an ocean about 830 metres deep would result. Or about 340,000 trillion tonnes. The nearest source is the Sun, in the form of the solar wind, 90% of which is protons (hydrogen nucleii). However the total mass ejected by the Sun, per year, is about ~22 trillion tonnes. If it could ALL be gathered, which is unlikely, then it’d take ~1,500 years to gather enough. Perhaps ~100 metres equivalent of water would be enough, which would be ~180 years of collecting all the Solar Wind’s output. Creating a magnetic field vast enough to collect a significant fraction seems a larger task than terraforming Venus, but I’ll leave that computation as a task for the Reader…
The only other major sources of free hydrogen is the atmospheres of the Gas Giants, of which Uranus has the most accessible gravity-well. Fortunately Venus is at the bottom of the Sun’s gravity-well, relative to Uranus, which means a net energy gain, if the energy can be recovered from infalling payloads via regenerative braking – albeit on a vast scale. Reacting hydrogen with carbon dioxide, via the Bosch Reaction goes as follows:
CO2 + 2 H2 => 2 H2O + C
…thus adding to the carbon powder created via burning magnesium. Exposed to the present day sun, while the surface is still incredibly hot, and the water vapour will merely add to the greenhouse effect. Some kind of cooling is required. A large soletta reflecting away some of the sunlight seems to be a given, but is it a permanent necessity?
Kim Stanley Robinson’s novels, “Blue Mars” and “2312” makes the Venus Shade Soletta’s presence a major vulnerability of the Venusian terraforming effort. Stan (as he prefers to be called) uses Paul Birch’s approach to Venus’s carbon dioxide – freeze it out by shading the planet 100%. Birch’s scenario then requires burying the dry ice – it’ll build up to ~700 metres thickness over the whole planet – underneath insulated blocks or in carefully chosen “pits” around the planet. Eventually it’s exported off-world or used locally in other forms. Until that burial process is complete, the planet must be kept in the shade. And once it’s buried, the soletta is retained to give Venus an Earth-like 24 hour day and reduce the insolation. Or at least that was the plan until cyber-terrorists almost succeed in destroying it and undoing the whole carbon freeze-out process.
However if we begin by combining the carbon dioxide chemically, then that dramatic scenario is less likely. Once the ocean condenses and percolates into the regolith, with some spectacular geothermal activity as the crust cools, then I wonder if a permanent shade soletta is absolutely required. Recent modelling of cloud formation on slow rotating planets – like Venus – suggest that a vast, stable cloud system will form at the sub-solar point, increasing the average albedo of the whole planet, and suppressing a runaway Greenhouse effect. Once the carbon dioxide is removed, Venus will have an atmosphere of nitrogen giving a surface pressure of ~2.07 bars. Some of the carbon dioxide will be need to make oxygen, via photosynthesis presumably, but the whole planet’s ecology will need to be tweaked to handle the 116 day ‘sol’ (time from sunrise to sunrise, different to the sidereal day, which is 243 days long.) A terrestrial analogy would be arctic vegetation which has a short growing season and a long dormant phase. Perhaps tubers and root plants as well as cold-tolerant species? There would be, effectively, two ‘seasons’ – the day-light Growing Season, and the night-time Dormant Season. Hibernation/torpor/estivation might need to be tweaked into all the animals. No doubt humans will have their own light, but it would be a shame to impose a terrestrial Day/Night artificially, when we could create a planetary-scale experiment on adaptation to such exotic conditions. Migration, following the most congenial temperature and light-level as the planet turns, might also be a reasonable adaptation.
What can we do with carbon dioxide? One possibility, suggested by Stephen Gillett in the late 1990s, is to turn it into the carbon dioxide analogue of silicon dioxide (silica or glass). When Gillett made this suggestion the stuff was purely theoretical. Since then it was first made in a high-pressure laboratory from carbon dioxide in 2006. Called ‘amorphous carbonia’ it really is a glassy solid, just like silica. At present it hasn’t been successfully ‘quenched’ from high pressure, though it has been cooled to room temperature. One day we might discover the trick of making the stuff stable under more reasonable conditions and use it as a construction material. Conceivably, and this was its allure for Gillett, we can imagine a quasi-biological self-replicating ‘organism’ making the stuff out of the air of Venus, and eventually ‘glassing out’ the excess carbon dioxide as the carbonia ‘shells’ of the quasi-organisms. If we can discover how to do this trick via our burgeoning abilities at making artificial lifeforms, then it’ll solve Earth’s excess carbon dioxide problem too.