In an earlier post, I described the difficulty a small planet, like Mars, faces when people are trying to terraform it: the planet simply doesn’t have enough gravity to hang onto the gases we need to survive.
Some people suggested that maybe instead of Mars, we might look at Venus, since it has the right size.
In this post, I look at the most important deciding factors for the success of a terraforming operation.
Distance from the sun:
Venus is at 0.7 AU from the Sun, which is beyond the inner margin of the habitable zone at roughly 0.9 AU – yeah, we’re very close to that. I hope it doesn’t give you nightmares.
Mars is at 1.5 AU from the Sun, on the very outer edge of the habitable zone. However, the width of the habitable zone depends not just on the sun and its strength, but also on the composition of the planet’s atmosphere and on the presence f clouds. With the addition of clouds, the inner boundary of the habitable zone could be made to stretch inwards. However, water vapour and carbon dioxide (the most likely gases to form clouds) are both greenhouse gases, so the effect likely cuts both ways. There is a lot more leeway on the outer side of the habitable zone. With the addition of carbon dioxide clouds, it can be made to stretch outwards, to as much as 2 AU or even more.
Atmosphere composition and density
The atmosphere of Venus is so dense that we can’t see its surface. It consists of mainly carbon dioxide (96%), some nitrogen and sulphur dioxide. Nasty, nasty stuff
The atmosphere of Mars also consists of mainly carbon dioxide, but it is extremely thin. Both atmospheres lack oxygen and nitrogen, but because of its high temperature, Venus also lacks water, of which Mars has quite a bit, even though it’s frozen. Mars also has frozen carbon dioxide. When thawed, both carbon dioxide and water vapour are greenhouse gases, and both would further increase the rate of warming (this is called positive feedback). At Venus, you’d need to get rid of massive quantities of carbon dioxide. Again, this is possible in theory (see Terraforming by Martin Beech), but even if you got it to work, it would be a process taking millennia, while a warming of Mars could be achieved in a few hundred years.
Mars, at only 15% of the Earth’s mass, with 32% of Earth’s gravity, is below the mass required to hold onto a breathable atmosphere.
Venus, at 81% of Earth’s mass, and 90% Earth gravity, is not.
Basically, providing you can create an atmosphere resembling Earth’s, Venus would retain it without the need to replenish it, whereas Mars would not.
Mars has a nice 24-and-a-bit hour day. At Venus, however, the planet’s day (243 Earth days) is longer than its year (225 Earth days). This, in combination with stronger sunlight, would make plant growth extremely hard, unless you could somehow speed up the planet’s rotation. I need not say that this would be hard to achieve, although theoretically possible.
Required change in temperature
In order to harbour life, the plant’s average temperature must be high enough for liquid water to exist on its surface. Every degree of change a planet requires to bring its temperature within this range is going to cost effort and time, and a lot of resources.
The freezing point of water is 273K; the average surface temperature of the Earth is 288K.
The temperature of Mars is 210K
The average temperature of Venus is 735K, and is the hottest object in the solar system apart from the sun, hotter even than Mercury.
Add to this that an increase in temperature is probably easier to achieve than a decrease, the clear winner is Mars, by many, many miles.
So yes, while people may have concluded from a previous post that Mars is too hard to terraform, and maybe we should look at Venus, maybe these people need to think again. It seems that its size is just about the only thing it has going for it.
The conclusion from this should probably be that terraforming is never going to be easy, and nor should it be in your fiction. Lots of things to go wrong, lots of things taking longer than expected, lots of unexpected stuff happening.