World-Building exercise: Martian temperature

Let’s say you wanted to give Mars an Earthlike climate, with the same global average temperature as modern-day Earth and an atmosphere composed primarily, but not entirely, of oxygen and nitrogen.

How much CO2 would you have to put into the Marian atmosphere, before it could trap enough heat to raise Mars’s average temperature to be equal to Earth’s? What partial pressure of CO2, should I say?

First of all, Mars’s atmosphere is only 1% as dense as Earth’s (its much smaller gravity and no magnetosphere doesnt help in retaining what it has/had). Second, it happens to already be comprised mainly of CO2, 95% with adjust a hint of nitrogen and traces of argon, oxygen and methane.

So first, it’d need about 7,900% more nitrogen and 2,100% oxygen than what’s present in the atmosphere.

I suppose finding, or bio-engineering plants that can thrive in its soil to produce a more oxygen rich atmosphere would be a start, yet irrigation would be a severe problem, for one. I’m not even sure how much nitrogen is even available on Mars to account for adding that much into the atmosphere.

Another thing that would help. A very dark surface that absorbs most of the sunlight in the first place.

Also, while of course it doesn’t have the same climate, or atmospheric pressure, there are already local climates at the surface that are relatively warm enough for human exposure; some Viking measurements came in at around 80°F IIRC. I’d actually turn on the A/C if I were living there.

A lot of CO2 would be required, probably more than the lethal dose for an unmodified human.

Here’s Robert Zubrin’s analysis, which looks at a one-bar atmosphere of pure CO2; this would raise the temperature of the planet to 300 K.
a terraformed Mars would have oxygen and nitrogen as well, and more importantly water vapour, which would be another important greenhouse gas.

Perhaps some non-toxic artificial greenhouse gas would be found; but I think that by the time we get anywhere near terraforming Mars, we will have biotechnology sophisticated enough to modify humans to tolerate high CO2 levels.

You wouldn’t actually need that much CO2; a large part of the greenhouse effect on Earth is due to water vapor, although if you removed all of the CO2, feedbacks would cause a much larger drop in temperature than from the CO2 alone.

As for a non-toxic alternative greenhouse gas, sulfur hexafluoride would work very well, being over 22,000 times more effective than CO2 and a lifetime of up to 3,200 years. The density would also be a plus (assuming it stays mixed in the atmosphere), helping to reduce losses into space, but probably not a concern on timescales of its atmospheric lifetime (what does it degrade into though, since many fluoride and sulfur compounds are toxic?). CFCs could also work, but you want an ozone layer to stop UV (or use some non-toxic long-lived gaseous chemical that blocks it).

I would guess we’d be machine intelligences by the time we could realistically terraform Mars, and wouldn’t need to terraform other planets.

Hmmm … okay, let’s say you give Mars an oxygen-nitrogen atmosphere equal to sea level air pressure on Earth, and you don’t want to add more CO2 to it than a modern human could safely breathe. And you don’t want to muck around with adding methane or sulfur hexafluoride or anything else exotic.

How much water vapor would need to be in the atmosphere to raise Mars’s global average temperature to match the Earth’s current global average temperature?

Bit of a problem, there; the water vapour would form clouds in Mars’ cool atmosphere, increasing the albedo and cooling the planet. Water vapour alone is probably not enough to create a warm Mars. I think orbiting mirrors would help a lot;

and maybe something a bit more speculative; J Storrs Hall’s Weather Machines, transparent balloons with adjustable mirrors that could reflect outgoing heat back to the surface.

Another problem with water vapor is that if you try to add more, it will condense out; while a strong greenhouse gas, it can’t work by itself due to this very fact, only act as a feedback; thus the amount is related to the levels of non-condensing greenhouse gasses like CO2. As for clouds, their effect depends on their altitude; thin high clouds have a warming effect while low thick clouds have a cooling effect, but the latter also have a warming effect at night (of note, observations on Earth indicate that clouds likely reinforce a warming trend, although probably only up to a point; Venus has a very high albedo and is only so warm because of the ridiculously dense CO2 atmosphere; it’d be far hotter without clouds).

Advocates of Martian “terraforming” talk about sending hardy green plants to Mars-the idea is that these plants will convert the poisonous carbon dioxide martian atmosphere to oxygen. My question: is there enough solar radiation on Mars to allow photosynthesis? Mars is far away-it receives only a small percentage of the solar radiation on earth. Would green plants be able to survive there?

A bigger problem is that the atmospheric pressure is very low (0.6% of Earth’s). As for plants being able to survive on the sunlight that reaches Mars (about a third to half of Earth), plants that grow in the shade shouldn’t have any problem; plus, think of plants that grow indoors under artificial lights, which supply a very small percentage of the daytime solar insolation (1,366 watts per square meter, equivalent to around 30 kW of incandescent bulbs; it only seems bright to us because the human eye readily adapts to various lighting conditions).

Well, while we’re discussing such things, maybe we should start by moving Mars to one of Earth’s trojan points. Mars will always be the same distance from Earth, and it will get a heck of a lot more sunlight. Then we start seeding it with kudzu.

It would require several km/s of delta-V just to put Mars onto the transfer orbit, and a nearly-equal amount of delta-V to circularize the orbit at its new perihelion.

I’m trying to imagine how you’d impart that much delta-V to an entire planet.

Well, in theory it could be done, but it would be a long, slow and inefficient process. You’d have to collect as many of the the asteroids in the solar system as you could, and attach little rocket motors to them. Then you send them on fly-by trajectories past Mars, stealing a tiny bit of delta-vee from the planet on every pass. Eventually you could alter the planet’s orbit to match that of Earth.

This is an oversimplification, of course; Paul Birch of the British Interplanetary Society worked all this out in some detail, and you have to involve the Sun and Jupiter too, as end points for the asteroid’s orbits; suffice it to say that the energy requirement for moving Mars is much, much bigger than that for terraforming it (and that is a fantastically large requirement itself).

Simple. You suspend a massive rocket engine in the upper atmosphere of Uranus, and use the gases as fuel. Then you redirect Uranus’ orbit so as to bring it into the inner solar system, and let its massive gravity allow it to act as a tugboat, perturbing Mars into the proper orbit.

The main reason the earth has an atmosphere has to do with its innards. The core is fairly liquid, mostly heated by radioactive decay (yes, the planet is a huge nuclear reactor). Two things at play here. First, the structure of the the upper layers help move core material to the surface. The earth is nearly 50% Oxygen, so that helps explain where the air comes from in the first place.

Mars seems to have a much thicker crust and a mantle that is less conducive to upwelling, so the Martian atmosphere simply does not get refreshed internally, which really is critical to maintaining ecosystems such as are here. There is a lot of oxygen on Mars, which in theory might be combined with hydrogen from the solar wind to drum up some good seas, except, here we encounter the second problem.

Mars’ core is warm-ish. A big sphere being good for retaining heat because it has the lowest ratio of surface area to volume, all its internal heat may just be residual. Whatever the case, the core is below the temperature it needs to be to form a stable magnetic field. Earth’s magnetic field helps shield the atmosphere from the solar wind, on Mars the solar wind is slowly drawing it off. The Martian air is going away and it is not getting replaced at an adequate rate.

A stable, habitable climate is not infeasible, I do not believe Mars is quite outside the “goldilocks zone”, but these other issues need to be addressed before attempting to establish one. Probably the most logical approach would be to go up in between Mars and Jupiter and start altering the course of some of those rocks out there. Just lob a bunch of asteroids at the red planet, from various angles so its orbit is not disturbed by too much, until it gets hot enough again for vulcanism and magnetism. That is half the battle right there.

I was at a space exploration conference a few months ago where Steve Squyres, having just given a keynote lecture, was asked about the possibility of terraforming Mars. He said, roughly,

Mars’ low surface gravity is a main reason that its atmosphere is so thin, and its surface so cool. We’re becoming very convinced that the planet had large quantities of volatiles in is past (signs point to large amounts of water flowing on the surface, for example), but these appear to have been lost in the long run.

If were were to introduce enough volatiles to thicken the atmosphere up again, it’s not clear the planet would retain them.

Keep in mind though that it bled off those volatiles over the course of eon’s. We’re humans, we only need it to hold the atmo for a few tens of thousands of years. By that point, it will be time to have either left the system, or better tech will have been developed.

I’ve long been a fan of the plausible but horrifically expensive idea of a large orbital mirror focusing more sunlight on the surface of Mars. This alone may be enough to start the process.
Hypothetical: Assuming we develop a “cheap” way to move planets around, what is to stop us from, say, smashing Io into Mars to give it the additional mass? What would the effects be of a slow impact of Io into Mars?

If you’re going to smash something into Mars (assuming that was even feasible), why not a bunch of water-ice asteroids? Heat up Mars, add water vapor & clouds, make rain & oceans?
As long as we’re going madly hypothetical…