Terraforming the Solar System
 

If you parked a giant sail inbetween Venus and the Sun so that it Venus cooled down enough for humans to visit, what else would you need to do to make it habitable?
Obviously you'd have to leave it to cool down for a while, and your sail would let in just enough light for plants and other life. But what would be the next stages?

Re: Terraforming the Solar System
 

Well, you've still got an immense atmosphere, mostly composed of carbon dioxide, about 90 times the mass of Earth's atmosphere, along with some nitrogen and sulfur and the like. At the surface the pressure is roughly equivalent to the pressure in Earth's oceans half a mile down. The planet rotates east-to-west (opposite Earth), which almost certainly doesn't matter, and it takes 243 Eartg days for the planet to rotate once, which quite certainly matters a lot.

Note that there is some water vapor in that atmosphere, at about 20 parts per million or so, i.e. not very much at all.

As the planet cools, that CO2 is going to eventually start to rain out, but that's going to take a while. Each successive rain will vaporize a little deeper, cooling the atmosphere, but we're not talking a short time here.

You're going to have to add a lot of water from somewhere, or better yet, you're going to have to add a lot of hydrogen from somewhere. The latter is better because it means you can move less mass, and you can get rid of some of that CO2 by taking the oxygen atoms from it to combine with your hydrogen to make water, and you've got to have a lot of water.

(What do you do with the carbon left over? Good question...)

The classic solution to the CO2 problem is to use plant life of some kind (maybe heavily bioengineered) to break the CO2 up via photosynthesis, this lets you put the carbon into the biomass of your plants and frees the oxygen for breathing and to make more water with your imported hydrogen (you'll have to import some water to start the process, of course).

Now, photosynthesis takes energy. There is a very finite amount of energy available on Venus from the Sun, it's more than Earth receives (closer to the star), but still distinctly finite. This solar energy influx puts a hard limit on how fast your plants (whatever their nature) can turn CO2 into oxygen and carbon. If you want to go faster, you have to pump in more energy yourself from outside (and keep your eye on the greenhouse balances as you do this, you don't want to kick-start another runaway greenhouse!) Venus intercepts (close enough) 50% more solar energy than Earth.

Note too that no photosynthetic process is ever 100% efficient. So you can't get all the energy anyway.

Note too that as the water vapor content rises, you have to allow for the greenhouse factor from that water vapor, too. Water vapor is a very effective greenhouse gas.

Most photosynthetic processes are maybe 1% efficient, IIRC. If we assume advanced biotech, maybe we could make 5% efficient or 10% efficient plants. Maybe. Assuming 10%, that means we have 1.5 times Earth's insolation times .1, or about 2.5 * 10^13 kilowatts available from the Sun to manipulate the atmosphere.

How much water to do we need? Well, if we want a supply comparable to that of Earth it's at least 1 million trillion tons, give or take. That'll give us something close to an Earth-like ocean. Water is about 88% oxygen by mass, so we need to take .88 * 1 million trillion tons of oxygen from the atmosphere of Venus. Is there enough?

Well, CO2 is ~70% oxygen by mass. So, 90 times Earth's atmosphere of C02 is close enough to 5 * 10^17 metric tons. So 70% of that is 330,000 trillion tons of CO2.

Hmmm...unless I slipped a decimal somewhere, creating an Earth-like ocean would eat the entire Venusian atmosphere and then some, assuming we had the hydrogen to add. But we might be able to get by on less water, and we'll want to save some of that oxygen to breathe, too.

Earth's air is mostly nitrogen, and Venus' air is about 3.5% nitrogen. 3.5% of 90 atmospheres is still 3 times Earth's. So there's more than enough nitrogen on-site to make an Earth-like atmosphere, and in fact we'll probably want to get rid of some of that nitrogen. Some of it can go into the biomass of the new planetary biosphere, of course. But I doubt that could eat enough of it.

Re: Terraforming the Solar System
 

I hate to say it, but terraforming something that close
to the sun sounds crazy. Maybe it's an Ancients thing?
They're showing off.

Re: Terraforming the Solar System
 

Johnny1A.2 you are awesome! I knew I'd get a good answer on the Traveller forum but yours exceeded my expectations.
But damn you make it sound like hard work. I wonder what kind of world the Solomani have built there so many thousands of years into the future?

Re: Terraforming the Solar System
 

You're better off asking on the Transhuman Space board. This scenario was actually discussed for In The Well.

Re: Terraforming the Solar System
 

On Earth, a large part of the answer is carbonate minerals. We'd have something like 70+ atm of CO2 atmosphere if that carbon were released. The lack of water on Venus means it loses many of the weathering processes to make these minerals. And of course living creatures are a big source of calcium carbonate. Some of your terraforming lifeforms aren't going to be trying to turn carbon into plants. They'll be turning carbon into rocks.

Re: Terraforming the Solar System
 

Yeah, we had some discussion of the subject for Transhuman Space, and basically decided it was crazy. It's something like a thousand years for the atmosphere to just cool down to the point where you can try to do something more about it, and you'll probably have to import a mid-sized kuiper belt object to provide a usable quantity of water (with THS tech, that added additional time because you'd have to let the KBO slam into Venus, which would add some extra heat; that can be avoided with Traveller tech). We didn't really get into the topic of how to bind the CO2 in methods that don't make trouble, I think using blue-green algae to process it was many thousands of years.

Re: Terraforming the Solar System
 

I've never heard a plausible terraforming proposal that didn't involve time spans of at least centuries, to be honest, unless you posit technology so far ahead of ours that it's not quite clear why you need to terraform at all.

Even Mars would be a bitch.

Re: Terraforming the Solar System
 

It would be hard work! Notice that I didn't even address the rotation rate!

It's partly a scale issue, planets are enormous, the amounts of matter you have to manipulate to make gross-level changes to a planetary environment are staggering, the energy budgets necessary are fantastic by today's standards. For comparison, imagine how hard a task it would be to, say, move the Atlantic Ocean to a different place. That's the sort of scale terraforming involves.

Re: Terraforming the Solar System
 

Just want to use this topic to get this doubt out of the way ...

Is it actually possible to terraform mars by droping nukes on it at some point ? I heard it somewhere and thought it interesting