It's believed that Mars' atmosphere was considerably more dense than it is today, but along the way, its core cooled, and it lost its protective magnetic shield, allowing the solar wind to strip away much of its atmosphere.
Off topic but I’d like it quite a lot if people could be informative without being eviscerating lol. I suppose this doesn’t usually happen, but it should happen more. That’d be so much better, so much more tolerable and happier. It’d be good for all of us probably lol.
What Would Feynman Do?
In this case, I'm imagining he'd say these are some of the things we think might explain bodies' keeping or losing atmosphere, but we have to remember that almost everything we think we know about planets and moons comes from looking at the light that bounces off of them and making guesses about the underlying rules, but we still don't know do much about the underlying rules of physics, so there's probably more that we don't know than what we do know about atmosphere retention.
PV=nRT. T is temperature.
Titan is not only protected by Saturn's magnetic field, but it is also very cold. Some of the molecules that are gaseous on Earth are liquids on Titan. Cold gases have lower pressure, and less likelihood of bouncing a light molecule high enough up in the atmosphere that the solar wind can grab it and blow it away.
Earth has a hard time holding on to light molecules like H2 and He, but its He is replenished somewhat by alpha decay, and it takes a long time to get from the inside of a rock to the upper reaches of the atmosphere. Most of the hydrogen is attached to heavier molecules. But it happens eventually, and even the Earth's magnetic field and gravity can't keep them. Venus is almost as massive as Earth, but it is hotter than Mercury and has no core-generated magnetic field. So most of its water has already thermally dissociated (which happens slowly starting at around 800 degC) and the H, H2, He, and monoatomic O bounces high up into the atmosphere, ionizes, and blows away. So now Venus has about 90 bar of CO2 and barely any water left.
One of the terraforming proposals for Venus is to transport a large quantity of hydrogen from Jupiter to Venus, and use Fe catalyst to react it with the CO2, to get graphite C, H2O, and O2. That would strip off much of the greenhouse blanket, but the planet would still have to be cooled off and protected from the solar wind to keep all that hydrogen around on a geologic time scale.
I don't know what you mean by "too far removed from current capabilities", but I doubt we'll even start working on the problem for two or three centuries.
No, it seems more likely to me that we'd use our growing knowledge of genetics and psychology to hack out the part of ourselves that needs to be outside, and opt for a purely enclosed existence on Mars.
However, the thought of a telescope-quality mirror 250x the size of the JWST is pretty amazing :)
Mars receives 593 W/m^2 flux, so each IKAROS-sized reflector could produce about 100 KW of energy. Given the expected difficulty of large scale terraforming and colonization efforts, it seems the cost of, say, the equivalent of 10,000 IKAROS-sized mirrors (~1 GW, comparable to a large nuclear plant) would be relatively minor.
Whether that would be more cost effective than shipping an equivalently powerful reactor or other generator is questionable - it will presumably depend on our lifting capacities.
So you also couldn't just vaporize the ice, you need to split it up. Solar heat could be sufficient, but the water will then be missed everywhere else on mars where life wants to grow.
And Mars is dry.
So I would first use the water in enclosed habitats. And then after, if there is plenty of water left, one could start to think about smoking that up ..
But there might be other options, once you have lot's and lot's of autonomous machines and rockets available and allmost unlimited fuel (sun?). But without that? Not a chance ...
The notable benefits from such an approach would be the ability to easily deliver asteroid-based water deposits (aim it at Mars, let reentry do the rest), as well as the significant simplification of ground based colonization technology - it's far easier to build resilient habitats for a non-breathable atmosphere than it is for a vacuum, and the risk of accidents and difficulty of venturing outside is much, much lessened (a face mask or filter and oxygen tank instead of a bulky spacesuit), not to mention the radiation protection afforded by a thick atmosphere.
But it has to be carefully considered, as it might hinder a longterm plan for a nice, breathable atmosphere.
But once we reach mars, we are probably busy first, with primitiv things such as life support ...
we could also bore into those tunnels and put caps on the hole afterward, have some of those channel light into those tunnels too.
Someday though we may be able to breathe on Mars