Monday, 14 November 2016

Terraforming Venus

I thought I would look at some of the figures involved in terraforming Venus, discussions of which can be found all over the place e.g. here. The numbers from this page come from here. Venus is the planet next to earth as you go towards the Sun. It has a year of 224.7 Earth days, and a day length of 243 Earth days (longer than the year!). It also rotates in the opposite direction to most other planets, likely due to some large collision early in its life.

I'll quote a little bit from wikipedia: Venus has an extremely dense atmosphere composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases, most notably sulfur dioxide. The mass of its atmosphere is 93 times that of Earth's, whereas the pressure at its surface is about 92 times that at Earth's—a pressure equivalent to that at a depth of nearly 1 kilometre under Earth's oceans. The density at the surface is 65 kg/m3, 6.5% that of water or 50 times as dense as Earth's atmosphere at 20 °C at sea level. The CO2-rich atmosphere generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least 735 K (462 °C).

Terraforming Venus would involve removing most of the sulfuric acid and carbon dioxide, needless to say this is a monumental task that we don't currently have the technology for. Some of the truly staggering figures:

Total Mass of Venus Atmosphere = 4.80E+20 kg
Percent Atmosphere CO2 = 96.50%
Total Mass of CO2 = 4.63E+20 kg
For comparison:
Total Mass of Earth's Atmosphere = 5.10E+18 kg
Mass Ratio Venus to Earth = 94.118

Utilizing the Bosch reaction (CO2 + 2H2 -> C + 2H2O) , combining hydrogen with carbon dioxide to make carbon graphite and water could be used to remove Venus' carbon dioxide. This reaction requires the introduction of iron, cobalt or nickel as a catalyst and requires a temperature level of 530-730 degrees Celsius. Since the reaction releases some heat, it could be self sustaining if you could supply enough hydrogen. A problem with this is that that the production of elemental carbon tends to foul the catalyst's surface, which is detrimental to the reaction's efficiency [ref].

Molecular Weight of CO2 = 44
Molecular Weight of 2*H2 = 4
Total Initial Molecular Weight = 48
Molecular Weight of C (graphite) = 12
Molecular Weight of 2*H2O = 36
Total Final Molecular Weight = 48

You could use water from e.g. comets to get hydrogen for the reaction. You would have to use Electrolysis to split the water and initiate the reaction. Alternatively you could reuse the water output from the Bosch reaction , recover the Hydrogen and continue. But this way you wouldn't have oceans on Venus at the end.

If you were to provide hydrogen for the reaction (4E19 kg), this massive reaction would create an ocean nearly as large as 1/4 of the Earth’s ocean:

Ratio of 2*H2O to CO2 (36 / 44) = 0.818
Resultant Mass of H2O = 3.79E+20 kg
Density of H2O = 1.000 g / cm^3
= 1,000,000.000 g / M^3
= 1,000.000 kg / M^3
Volume of Resultant H2O = 3.79E+17 M^3
= 3.79E+08 kM^3
Area of Venus Surface = 4.602E+08 kM^2
Average Depth of H2O = 0.824 kM
Total Volume of Earth's Oceans = 1.300E+09 kM^3
Average Depth of Earth's Oceans = 3.682 kM

It would also result in the deposition of a layer of graphite with an average thickness over the entire surface of Venus roughly equal to a 40 story building:

Ratio of graphite to CO2 (12 / 44) = 0.273
Resultant Mass of graphite = 1.26E+20 kg
Density of C (graphite) = 2.230 g / cm^3
= 2,230,000.000 g / M^3
= 2,230.000 kg / M^3
Volume of Resultant C (graphite) = 5.66E+16 M^3
= 5.66E+07 kM^3
Area of Venus Surface = 4.602E+08 kM^2
Average Depth of C (graphite) = 0.123 kM

So after converting all the carbon dioxide to graphite, you would be left with ~120 meter thick blanket of carbon over the planet. Though this may not be terrible, carbon makes a pretty good base for soils. There would still be a great deal of atmospheric pressure due to all the oxygen though, and a spark would result in a mighty conflagration that would undo all your work. A better idea may be to trap the carbon dioxide as carbonate rocks, but for this you would need large quantities of calcium and/or magnesium (About 8×10^20 kg of calcium or 5×10^20 kg of magnesium would be required). This would have the advantage of removing a lot of the oxygen, which would lower the atmospheric pressure. Unfortunately Magnesium carbonate begins to decompose at 350 degrees C, so Calcium might be a better bet. You would also need to remove all the sulphuric acid since it would dissolve all your carbonate rocks, using up the sulphuric acid but producing more carbon dioxide.

Of course you have to power all this as well, electrolysis doesn't come cheap. Solar panels can work well on Venus because the sunlight is 4 times stronger. Perhaps some genetically engineered plant...

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