namelessfly wrote:I agree that simply using thermalisis to separate CO2 in an atmosphere in situ is not going to work unless you have something else to bond with the free Oxygen and Carbon. Perhaps hydrogen extracted from Methane from comet could be injected to bond with the free Oxygen to form water that would precipitate out might be a solution. However; it is more plausible to just boil off a CO2 atmosphere from a planet then replace the atmosphere with the proper mix of Nitrogen and Oxygen as well as water if needed for a hydrosphere. Yes, the volumes seem daunting but given the energy budget that an interstellar colony ship should have it should not be that big of a problem. Pick a comet, grab it and bag it, then apply controlled heat to distillation separate the gases that you want, then use your colony ship's fusion rocket as a tug to boost it to the proper orbit. Alternatively; you might thermolisis to extract Oxygen from an orbiting moon to drop on planet.
We can scream about how huge the volumes and masses are but if humans have the technology to build a fusion rocket powered colony ship then this terraforming is within their energy budget. Perhaps no one has realized this because I am the first to crunch the numbers?
Think about it from a philosophical perspective. We worry about our current industrial civilization's 1eex13 Watt energy budget screwing up Earth's environment but our interstellar colony ship will have an energy budget of 1eex15 to 1eex18 Watts. Finding the energy to process and move the relevant masses would probably be less problematic than properely understanding the Macro chemistry to create a viable ecosystem.
Why do you keep harping on the energy budget?
No one has suggested that energy budget is a problem!
How long do you think it would take to "boil off" the existing atmosphere? This is called Jeans Escape. Unless you raise the temperature high enough, it will take millennia. At the temperatures we will be dealing with, all the gasses will be dissociated and probably ionized. If the
mean velocity of atomic oxygen is equal to escape velocity, the temperature would be around 64,000 degrees Celsius. Let's say we raise the temperature of the atmosphere to 100,000 C to get rid of it. (Note--at this temperature, a lot of heavier elements on the surface will be volatizing into the atmosphere. But presumably they will condense again once it cools off.)
The heat capacity of the atmosphere is on the order of 1 J/(g K). For simplicity, let's assume that stays true at all temperatures and pressures, even though we know it is more than an order of magnitude more at higher temperatures. The mass of Earth's atmosphere is 5e18 kilograms. So to raise the temperature of the atmosphere to 100,000 C, we need to apply 5e26 Joules. You suggest that a slowship can generate 1e18 Watts. If you had 1,000,000 slowships, it would take 100 years to raise the temperature of the atmosphere to that temperature.
Of course, it would actually take much longer, because the planet would be radiating heat at a tremendous rate. In fact, that is 20 times the surface temperature of the sun. At 100,000 C, the planet will be radiating 3e27 Watts! Even if we go with 64,000 C, it will be radiating 5e26 Watts. In 1 second, the atmosphere would radiate away its entire latent heat, unless it were refreshed constantly!
Conclusion--you can't boil the atmosphere away in less than geological time. At the temperatures that will be practical (far less than stellar temperature), Jeans Escape will give an atmospheric half-life of millennia.
