Terraforming of Mars Ideas (and questions!)

[Paul451]

The thickness of glass needed to "float" a dome would be on the order of a meter thick

On the order of ten metres for 1atm. Which does give you some allowance for self-supporting structural strength.

There's breathable materials significantly denser than NitrOx

I'm amused by the idea that everyone on Mars -- men, women, children, even toddlers -- sounds like an angry Penn Jillette.

[edzieba]

Since it looks like the thread in the mars forum isnt returning...

The biggest problem with domes is that human-habitable pressure is so much higher than mars surface pressure, that "domes" cannot rely on surface anchors and instead  are effectively partially buried pressure spheres. The thickness of glass needed to "float" a dome would be on the order of a meter thick- and issac newton help you if a dome that heavy ever loses pressure.

But what if we could play with atmospheric scale height inside the dome?

There's breathable materials significantly denser than NitrOx, and the hellas basin is pretty deep, which also means that there's significant walls on all sides. Throw a zero-pressure-difference celophane barrier over the basin to prevent martian winds from dispercing our efforts, and fill the basin with those denser breathable mixtures, and how close to the armstrong limit can you reach?

I don't think denser mixes really gain you anything. You care about ppO2, and the highest ppO2 you can get with the lowest absolute gas pressure is a pure O2 atmosphere - because at the same pressure, any diluent gasses added are just displacing some O2 (and thus reducing ppO2) or adding to total pressure (because you're shoving extra molecules into the same volume).
For a 0.17 Bar ppO2 (nearly hypoxic but survivable) that's a density of 0.24g/l for a pure O2 environment. Mars' atmospheric density is 0.02g/l. That means regardless of the diluent added, you're going to need to be well above atmospheric pressure (~10x) to sustain life.

[rakaydos]

Since it looks like the thread in the mars forum isnt returning...

The biggest problem with domes is that human-habitable pressure is so much higher than mars surface pressure, that "domes" cannot rely on surface anchors and instead  are effectively partially buried pressure spheres. The thickness of glass needed to "float" a dome would be on the order of a meter thick- and issac newton help you if a dome that heavy ever loses pressure.

But what if we could play with atmospheric scale height inside the dome?

There's breathable materials significantly denser than NitrOx, and the hellas basin is pretty deep, which also means that there's significant walls on all sides. Throw a zero-pressure-difference celophane barrier over the basin to prevent martian winds from dispercing our efforts, and fill the basin with those denser breathable mixtures, and how close to the armstrong limit can you reach?

I don't think denser mixes really gain you anything. You care about ppO2, and the highest ppO2 you can get with the lowest absolute gas pressure is a pure O2 atmosphere - because at the same pressure, any diluent gasses added are just displacing some O2 (and thus reducing ppO2) or adding to total pressure (because you're shoving extra molecules into the same volume).
For a 0.17 Bar ppO2 (nearly hypoxic but survivable) that's a density of 0.24g/l for a pure O2 environment. Mars' atmospheric density is 0.02g/l. That means regardless of the diluent added, you're going to need to be well above atmospheric pressure (~10x) to sustain life.

It's not about the narrow breathable volume at the bottom of the basin. Its's about the composition of the uninhabitable-but-isolated bondary layer between the bottom on the basin and the rim of the basin. By changing the composition in this region to have a lower scale height, the top of this pillar of air can remain mars pressure while the bottom of the pillar of air can be... higher than it is currently.

High enough to be useful? I dont know enough to say.

[Paul451]

By changing the composition in this region to have a lower scale height, the top of this pillar of air can remain mars pressure while the bottom of the pillar of air can be... higher than it is currently.
High enough to be useful? I dont know enough to say.

Scale height is inversely proportional to molar mass of the gas. Hence, sulphur hexafluoride would have a scale-height 3.32x smaller than Mars' current 11.1km (so 3.34km.)

So picking a fairly arbitrary 10km between bottom of Hellas-deep and the membrane cap, that would give you 3 SH6 "scales", so 20 times (e^3) increase in pressure at the bottom compared to the top. Assuming Mars ambient pressure at the top (roughly 600pa), gives you around 12kpa at the bottom. So, 12% of Earth normal, around 1/8th SL. Equiv of 15km altitude. That's lower than the operating pressure of a pure-oxygen EVA suit, so you still need a pressure suit.

Sulphur hexafluoride is a potent (crazy potent) greenhouse gas, which might seem like a good idea, keeping temps above freezing and a much reduced day/night thermal cycle. But it might be too potent a GHG, making the basin too hot, even with the conductive/convective cooling through the membrane.

It's also an anaesthetic gas, so you can't just mix it with oxygen. But at that pressure, you'd still be living in pressurised habs and wearing pressure suits when outside, so it's not an issue.

However... Using Perfluorobutane as the fill gas, you nearly double the molar mass, which nearly halves the scale height again and increases bottom pressure (by e^6) to higher-that-Earth-SL. Which is starting to get interesting.

It's also still a stupidly potent greenhouse gas, but not as crazy as SF6, and given the high pressure, can be diluted with any arbitrary gas, like nitrogen, to lower the pressure, so might be more useful. It is, however, much harder to produce than SF6 AIUI. Same issue with SH6 in being an anaesthetic and nervous system depressant, so you still need a separate breather and good seals. However, being at high pressure means your habs can be unpressurised, and there's a greater risk of outside atmosphere flowing inwards through leaky seals, instead of outwards. It's still vastly safer than a pressure vessel in a near-vacuum, but now adds its own special risk. So you might aim to keep the outside pressure at 3/4 SL (via mix gases to dilute the perflurobutane) with the habs at a full 1atm, in order to ensure positive pressure. Likewise, when working outside, have your breathing mask or hood pressurised just slightly above that outside 3/4 SL, just enough positive pressure to give some protection against suffocation if your mask-seals are crappy. (No pre-breath required.)

Downside of all the fluorides is that when used around, say, arcing electrical equipment and high-temp industrial equipment, can break down into fluorine gas or hydrogen fluoride. Which is super unfun, yo. But I don't know how big a risk that is. Worst case, you put those systems in a nitrogen or CO2 bubble.

[Barley]

How flat is the floor of the basin?

If the atmosphere has a scale height of 2km then a 400m elevation gain is the equivalent of sea level to Denver.  Not a show stopper, but enough to get some noticeable effects.  Particularly if a settlement is large enough to be space limited, which seems likely if you get to this stage of terraforming.

[Lampyridae]

Since it looks like the thread in the mars forum isnt returning...

The biggest problem with domes is that human-habitable pressure is so much higher than mars surface pressure, that "domes" cannot rely on surface anchors and instead  are effectively partially buried pressure spheres. The thickness of glass needed to "float" a dome would be on the order of a meter thick- and issac newton help you if a dome that heavy ever loses pressure.

But what if we could play with atmospheric scale height inside the dome?

There's breathable materials significantly denser than NitrOx, and the hellas basin is pretty deep, which also means that there's significant walls on all sides. Throw a zero-pressure-difference celophane barrier over the basin to prevent martian winds from dispercing our efforts, and fill the basin with those denser breathable mixtures, and how close to the armstrong limit can you reach?

I don't think denser mixes really gain you anything. You care about ppO2, and the highest ppO2 you can get with the lowest absolute gas pressure is a pure O2 atmosphere - because at the same pressure, any diluent gasses added are just displacing some O2 (and thus reducing ppO2) or adding to total pressure (because you're shoving extra molecules into the same volume).
For a 0.17 Bar ppO2 (nearly hypoxic but survivable) that's a density of 0.24g/l for a pure O2 environment. Mars' atmospheric density is 0.02g/l. That means regardless of the diluent added, you're going to need to be well above atmospheric pressure (~10x) to sustain life.

It's not about the narrow breathable volume at the bottom of the basin. Its's about the composition of the uninhabitable-but-isolated bondary layer between the bottom on the basin and the rim of the basin. By changing the composition in this region to have a lower scale height, the top of this pillar of air can remain mars pressure while the bottom of the pillar of air can be... higher than it is currently.

High enough to be useful? I dont know enough to say.

Makes me think of the Shell World discussions a while ago: https://forum.nasaspaceflight.com/index.php?topic=49788.0

I'm sure there's a space for an intermediate application where you can have a massive air dam which is inward leaning, partly floating on the enclosed air, extending up in altitude to a hundred kilometres or so. At this scale, air behaves like weather systems, not necessarily a tyre filled with gas. The dam has a double wall, filled with SF6.

Scale height on Mars for warm breathable air is 22km. We fill it up with O2 to get 1/3 bar on the floor so that we can breathe and have Smokey the Bear warning signs everywhere. At the top of a 110km column (Hellas basin gives us 8km free), the air pressure is 0.007 bar, or ~200kg per square metre. We let it blast over the side, and it falls into the SF6 because there is a huge pressure differential (SF6 scale height is 8km; at the top of a 100km column the air pressure is basically vacuum. Process the lost oxygen from the SF6 moat and pump back into Hellas. The larger the area, the slower the leak rate into the moat. Opt for a higher leak rate and the air dam can be smaller.

Or you can forgo the SF6 and make the temperature in the moat just above liquid air temps; the moat still catches air and you pump it (or just let it flow) back into the reservoir. Adiabatic compression will heat the surrounding air somewhat but it has a big shade or something to keep it cool.

Somewhat further afield I note that imperfectly conducting gas disks around stars rotating through a magnetic field create a toroidal field that pinches it. I wonder if that could be applied to a toroidal basin/moat setup to squash the scale height.

[rakaydos]

Since it looks like the thread in the mars forum isnt returning...

The biggest problem with domes is that human-habitable pressure is so much higher than mars surface pressure, that "domes" cannot rely on surface anchors and instead  are effectively partially buried pressure spheres. The thickness of glass needed to "float" a dome would be on the order of a meter thick- and issac newton help you if a dome that heavy ever loses pressure.

But what if we could play with atmospheric scale height inside the dome?

There's breathable materials significantly denser than NitrOx, and the hellas basin is pretty deep, which also means that there's significant walls on all sides. Throw a zero-pressure-difference celophane barrier over the basin to prevent martian winds from dispercing our efforts, and fill the basin with those denser breathable mixtures, and how close to the armstrong limit can you reach?

I don't think denser mixes really gain you anything. You care about ppO2, and the highest ppO2 you can get with the lowest absolute gas pressure is a pure O2 atmosphere - because at the same pressure, any diluent gasses added are just displacing some O2 (and thus reducing ppO2) or adding to total pressure (because you're shoving extra molecules into the same volume).
For a 0.17 Bar ppO2 (nearly hypoxic but survivable) that's a density of 0.24g/l for a pure O2 environment. Mars' atmospheric density is 0.02g/l. That means regardless of the diluent added, you're going to need to be well above atmospheric pressure (~10x) to sustain life.

It's not about the narrow breathable volume at the bottom of the basin. Its's about the composition of the uninhabitable-but-isolated bondary layer between the bottom on the basin and the rim of the basin. By changing the composition in this region to have a lower scale height, the top of this pillar of air can remain mars pressure while the bottom of the pillar of air can be... higher than it is currently.

High enough to be useful? I dont know enough to say.

Makes me think of the Shell World discussions a while ago: https://forum.nasaspaceflight.com/index.php?topic=49788.0

I'm sure there's a space for an intermediate application where you can have a massive air dam which is inward leaning, partly floating on the enclosed air, extending up in altitude to a hundred kilometres or so. At this scale, air behaves like weather systems, not necessarily a tyre filled with gas. The dam has a double wall, filled with SF6.

Scale height on Mars for warm breathable air is 22km. We fill it up with O2 to get 1/3 bar on the floor so that we can breathe and have Smokey the Bear warning signs everywhere. At the top of a 110km column (Hellas basin gives us 8km free), the air pressure is 0.007 bar, or ~200kg per square metre. We let it blast over the side, and it falls into the SF6 because there is a huge pressure differential (SF6 scale height is 8km; at the top of a 100km column the air pressure is basically vacuum. Process the lost oxygen from the SF6 moat and pump back into Hellas. The larger the area, the slower the leak rate into the moat. Opt for a higher leak rate and the air dam can be smaller.

Or you can forgo the SF6 and make the temperature in the moat just above liquid air temps; the moat still catches air and you pump it (or just let it flow) back into the reservoir. Adiabatic compression will heat the surrounding air somewhat but it has a big shade or something to keep it cool.

Somewhat further afield I note that imperfectly conducting gas disks around stars rotating through a magnetic field create a toroidal field that pinches it. I wonder if that could be applied to a toroidal basin/moat setup to squash the scale height.

I'm pretty sure the problem is the sides of the air dam, where the armstrong-pressure breathing gas is trying to shear your anchor points off.

[Greg Hullender]

Hi! I'm not sure if this is the right place to post this, but I just need some scientific points of view.
I'm going to start drawing/writing a mini webcomic series based around space exploration in the Solar System in 2270 (just for fun).

So, after three years, does anyone know if the webcomic ever got done?

[rakaydos]

By changing the composition in this region to have a lower scale height, the top of this pillar of air can remain mars pressure while the bottom of the pillar of air can be... higher than it is currently.
High enough to be useful? I dont know enough to say.

Scale height is inversely proportional to molar mass of the gas. Hence, sulphur hexafluoride would have a scale-height 3.32x smaller than Mars' current 11.1km (so 3.34km.)

So picking a fairly arbitrary 10km between bottom of Hellas-deep and the membrane cap, that would give you 3 SH6 "scales", so 20 times (e^3) increase in pressure at the bottom compared to the top. Assuming Mars ambient pressure at the top (roughly 600pa), gives you around 12kpa at the bottom. So, 12% of Earth normal, around 1/8th SL. Equiv of 15km altitude. That's lower than the operating pressure of a pure-oxygen EVA suit, so you still need a pressure suit.

Sulphur hexafluoride is a potent (crazy potent) greenhouse gas, which might seem like a good idea, keeping temps above freezing and a much reduced day/night thermal cycle. But it might be too potent a GHG, making the basin too hot, even with the conductive/convective cooling through the membrane.

It's also an anaesthetic gas, so you can't just mix it with oxygen. But at that pressure, you'd still be living in pressurised habs and wearing pressure suits when outside, so it's not an issue.

However... Using Perfluorobutane as the fill gas, you nearly double the molar mass, which nearly halves the scale height again and increases bottom pressure (by e^6) to higher-that-Earth-SL. Which is starting to get interesting.

It's also still a stupidly potent greenhouse gas, but not as crazy as SF6, and given the high pressure, can be diluted with any arbitrary gas, like nitrogen, to lower the pressure, so might be more useful. It is, however, much harder to produce than SF6 AIUI. Same issue with SH6 in being an anaesthetic and nervous system depressant, so you still need a separate breather and good seals. However, being at high pressure means your habs can be unpressurised, and there's a greater risk of outside atmosphere flowing inwards through leaky seals, instead of outwards. It's still vastly safer than a pressure vessel in a near-vacuum, but now adds its own special risk. So you might aim to keep the outside pressure at 3/4 SL (via mix gases to dilute the perflurobutane) with the habs at a full 1atm, in order to ensure positive pressure. Likewise, when working outside, have your breathing mask or hood pressurised just slightly above that outside 3/4 SL, just enough positive pressure to give some protection against suffocation if your mask-seals are crappy. (No pre-breath required.)

Downside of all the fluorides is that when used around, say, arcing electrical equipment and high-temp industrial equipment, can break down into fluorine gas or hydrogen fluoride. Which is super unfun, yo. But I don't know how big a risk that is. Worst case, you put those systems in a nitrogen or CO2 bubble.

Concept: The Hellas Double-Dome.

A landing pad is established a safe distance from the rim of the hellas basin, and a train is set up to the basin. A (relatively) thin membrane is floated over the entire basin, using blown air to generate enough overpressure to suspend this outer membrane. A manufacturing site is established to manufacture [heavy gas mixture], which is pumped in under the dome. Overpressure at the top of the basin is filtered to extract [heavy gas mix] and release the Martian atmospheric gasses.

As the pressure from [heavy gas mixture] increases, Another dome is placed over the majority of the useful basin surface, pressurized with breathing gas to survivable levels, minimizing the effects of scale height on terrain WITHIN the inner dome. This inner dome is initially inflated to armstrong pressure, but pressure is allowed to increase to maintain a positive pressure differential with the bottom of the column of [heavy gas mixture] outside the inner dome, supplied by the train/tram through the uninhabitable intermediate layer.

The inner dome is then locally  para-terraformed at leisure.

[Paul451]

Just a reminder that Hellas has a diameter of 2300km (1400mi). This membrane could cover the most of western US, or western Europe.

The point of paraterraforming is to avoid megastructures. Each cell is supposed to be modest scale and immediately usable while you build the next...

[Slarty1080]

I think that paraterraforming is far more likely to provide a realistic means of making Mars more habitable than trying to implement some planet wide traditional terraforming project. It would also be far more incremental and produce results in a much shorter time frame.

With paraterraforming the idea is to progressively inclose the surface with interconnected pressurised structures. I don't know what the optimum size of structure might be, but it doesn't matter for the argument and will probably evolve over time. But it should eventually be possible to enclose many square kilometers at a time andd have the space available within a few months or years. No comet bombardment, no mass transportation of materials from Venus and no waiting for decades or centuries for the outcome of some giant planetary experiment.

[Robotbeat]

I think partial terraforming of mars to increase its surface pressure to Armstrong Limit is not ultimately that hard and would probably be easier than setting up a (largely) self-sustaining city of 1 million people. There’s a lot of misinformation about this, and it doesn’t require massive new technology or anything like that (massive solar sails launched on highly reusable rockets is good enough), although doubtless that will be developed. Wouldn’t take centuries, either.

Now if you’re talking creating a breathable atmosphere on Mars, I agree. Paraterraforming is easier.

[mlorrey]

Venus might be cooled with a combination of CO2 conversion and a huge, manufactured in space 'sunshield'. But the darn place rotates so terribly slowly this contributes to it's lackluster magnetic field - and who wants permanent daylight anyhow?! Mars is a better candidate for Terraforming. But I've always been very lukewarm for the idea. It would take a hell of a lot of money, energy and centuries of time to change it. Why bother? All the trillions spent on it and centuries to Terraform might instead be better used Terraforming Earth back to the 'ideal' state and building Starships to travel to other systems where Earth like planets had been discovered.

We already build gigantic sports stadiums and enclosed communities on Earth. Why not just use our Civil Engineering Super-know how to build pressurized Super-Domed cities on Mars? And if Mars does turn out to have a residual, albeit dying ecosystem left there - I don't think we have the ethical right to decimate it. Just my two cents worth of opinions...

Even with a 100% reflective sun shield it will take over a millennia for Venus to radiate all its stored heat, and that does nothing about getting rid of 91 of the 92 atmospheres of CO2 and SO2 that are the crux of the problem. You could make nanoballoon robots that replicate themselves from carbon and construct nanodiamonds they drop to the surface. This would take centuries still, and the surface would retain enough heat to cause the ignition of the many meters thick layer of nanodiamonds at some point (because by then you'll have raised the atmospheric levels of O2 to over 50% and you still have 92 atmospheres, now its mostly O2 rather than CO2 and SO2.
You really need a crapload of water on Venus so your robots can construct limestone bomblets, and thus build up a half mile thick layer of limestone sand, under a sea of water that absorbs the heat of the crust and radiates it to space via convection. This will still take millenia. Terraforming Mars is FAR simpler and faster.

[Robotbeat]

Venus might be cooled with a combination of CO2 conversion and a huge, manufactured in space 'sunshield'. But the darn place rotates so terribly slowly this contributes to it's lackluster magnetic field - and who wants permanent daylight anyhow?! Mars is a better candidate for Terraforming. But I've always been very lukewarm for the idea. It would take a hell of a lot of money, energy and centuries of time to change it. Why bother? All the trillions spent on it and centuries to Terraform might instead be better used Terraforming Earth back to the 'ideal' state and building Starships to travel to other systems where Earth like planets had been discovered.

We already build gigantic sports stadiums and enclosed communities on Earth. Why not just use our Civil Engineering Super-know how to build pressurized Super-Domed cities on Mars? And if Mars does turn out to have a residual, albeit dying ecosystem left there - I don't think we have the ethical right to decimate it. Just my two cents worth of opinions...

Of course it’s easier to geoengineer Earth than terraform Mars! That’s not an argument against terraforming Mars because the motivation has nothing to do with trashing Earth. (No matter how many times that strawman is repeated!!)

And terraforming Mars, in particular partial terraforming, is FAR easier than substantial crewed interstellar travel. Like you describe. Only someone who doesn’t really understand how hard interstellar travel is would say otherwise.

[Nomadd]

And terraforming Mars, in particular partial terraforming, is FAR easier than substantial crewed interstellar travel. Like you describe. Only someone who doesn’t really understand how hard interstellar travel is would say otherwise.

 I don't bookmark a lot of sites, but I really liked this one. It dos a great job of covering the details of various interstellar travel concepts.
 I know I'm not the first one to mention it here.

http://www.projectrho.com/public_html/rocket/slowerlight3.php#relcalc

[KelvinZero]

You could make nanoballoon robots that replicate themselves from carbon and construct nanodiamonds they drop to the surface.

I had wondered about nano-balloon robots quite recently, or more specifically a " lighter than air liquid".

Im not sure if my logic is correct, but I think instead of having an atmosphere that gradually thins to nothing you could fill a crater or canyon with a lighter than air liquid and trap an atmosphere beneath it. The compressed air beneath is denser than the liquid but above could be pure vacuum.

..or maybe it would just form a whirlpool hole and the air would gush through it.

[Paul451]

You could make nanoballoon robots that replicate themselves from carbon and construct nanodiamonds they drop to the surface.

I had wondered about nano-balloon robots quite recently, or more specifically a " lighter than air liquid".
Im not sure if my logic is correct, but I think instead of having an atmosphere that gradually thins to nothing you could fill a crater or canyon with a lighter than air liquid and trap an atmosphere beneath it. The compressed air beneath is denser than the liquid but above could be pure vacuum.
..or maybe it would just form a whirlpool hole and the air would gush through it.

If the balloon-layer is "lighter than air", then it's not significantly compressing the atmosphere. So the air beneath would expand up and out (pushing up the "liquid") until it matches the density of the balloon-layer. At that point, the balloons and top-most air would mix freely, and the air would be able to disperse, further lowering the pressure of the air beneath; rinse and repeat until there's nothing left but the nano-balloons sitting on the ground.

The only way to prevent it would be if the nano-bots secured the balloons together into a coherent, air-tight layer. Which is just a membrane again.

So the question just becomes "could we use nanobots to make a membrane of arbitrarily large size?"

[Vultur]

Venus might be cooled with a combination of CO2 conversion and a huge, manufactured in space 'sunshield'. But the darn place rotates so terribly slowly this contributes to it's lackluster magnetic field - and who wants permanent daylight anyhow?! Mars is a better candidate for Terraforming. But I've always been very lukewarm for the idea. It would take a hell of a lot of money, energy and centuries of time to change it. Why bother? All the trillions spent on it and centuries to Terraform might instead be better used Terraforming Earth back to the 'ideal' state and building Starships to travel to other systems where Earth like planets had been discovered.

We already build gigantic sports stadiums and enclosed communities on Earth. Why not just use our Civil Engineering Super-know how to build pressurized Super-Domed cities on Mars? And if Mars does turn out to have a residual, albeit dying ecosystem left there - I don't think we have the ethical right to decimate it. Just my two cents worth of opinions...

Of course it’s easier to geoengineer Earth than terraform Mars! That’s not an argument against terraforming Mars because the motivation has nothing to do with trashing Earth. (No matter how many times that strawman is repeated!!)

And terraforming Mars, in particular partial terraforming, is FAR easier than substantial crewed interstellar travel. Like you describe. Only someone who doesn’t really understand how hard interstellar travel is would say otherwise.

Yes... also, I doubt you'd find truly Earthlike (breathable) environments around nearby stars. "Earthlike" is a loose term... humans couldn't breathe Earth's atmosphere for most of its geological history. So you'll probably need terraforming technology anyway, if you want to live on off-Earth planetary surfaces.

(I personally think it would be easier to go from enclosed habitats on Mars to- once ISRU and asteroid mining is perfected - O'Neill style in-space habitats made from NEO or asteroid belt materials.)

[su27k]

How to terraform Mars for $10b in 10 years

The pitch is to mass produce small scale solar sails in terrestrial cell phone factories, launch them into Low(ish) Earth Orbit (LEO), and have them fly themselves to Mars where, hanging out near Sun-Mars L2, they would reflect additional sunlight onto the night side of the planet. I first wrote about this four years ago.

<snip>

Mars’ cross sectional area is about 36,000,000 km^2. 1000 sails add one additional square km to this tally. 150 million sails per year adds 150,000 km^2, which is an 0.4% increment in solar forcing. It is uncertain exactly how much extra heat is needed to heat Mars to the point where volatiles outgas from the regolith and trigger a positive feedback loop preventing radiative heat escape, but even if we brute force it, a decade of launches will increase the effective solar collection by 4% and, with 1.5 billion sails above Mars’ night sky, the view would be spectacular. As a rough estimate, a 4% increase in energy input would require 4% higher thermal radiation to achieve equilibrium, which depending on some geometric factors of order unity, would result in a 1% temperature increase, from 210 K to 212 K. This is twice what we’ve achieved with 250 years of industrial effort on Earth, burning a trillion tonnes of fossil fuels!

In fact, a solar sail placed near Sun-Mars L2 would have an apparent magnitude of about -0.75, similar to Polaris or Saturn, while billions of them filling a cloud with an angular extent of about 5 degrees would have thousands of sails per human eye pixel, creating a cloud of light like the Milky Way, only a substantially brighter echo of the reflected sun. While the surface brightness of this cloud would be only ~0.1% of the sun, the integrated brightness would be more than Earth’s Moon when full.

[sghill]

I've yet to read a single terraforming idea that is at the same time simpler, faster, cheaper, and more permanent than dropping large wet rocks onto the planet.