Also having handled aerogel in the past, it has a very rough surface texture. I suspect it would quickly build up a layer of sand, negating the greenhouse effect.
(They also ran tests on aerogel pebbles, but don't talk about sealing questions for that form.)
https://www.nature.com/articles/s41550-019-0813-0
"The higher carbon dioxide partial pressure on Mars versus Earth is favourable for plant growth, but the low total atmospheric pressure means that at temperatures of 273 K or higher, the undersides of silica aerogel greenhouse shields would need to remain slightly pressurized relative to the atmosphere to avoid loss of water vapour either vertically or laterally. This would place light demands on their structural properties, which could plausibly be met by interspersing the silica aerogel with thin layers of solid transparent material or via organic polymer reinforcement""
The surface area of aerogel foam is huge and its ability to absorb heavy toxic metals from water is also huge.
I wonder if aerogel desalinization filters could be a thing.
I would have liked a discussion over how this stuff could be produced on Mars, because no self-respecting rocket engineer would allow transplanetary shipping of a material largely composed of air.
But yes, some kind of fission or fusion reactor would be on a colonization plan, if it is easier/cheaper than solar. One problem though would be getting fissile material. You can probably find some on Mars or its moons or some asteroids, but it would require lots of heavy industry to get enough of it for reactors.
Bringing significant amounts of nuclear fuel from Earth would be really unpopular, what with the risk of rockets blowing up. That risk or fear is also what prohibits the use of nuclear propulsion for space travel...
The bulk mass of a (normal) reactor is shielding and cooling plumbing. You don't need any shielding on Mars, and cooling could maybe (probably?) be some kind of radiator/convector to the surrounding air. Maybe you could even crack the CO2 with the heat directly?
Fusion needs a lot plumbing and containment. Fission is just basically a pile of fissile material in proximity. Not very hard if you can skip on all the protection.
You're right that, if you could find some useful endothermic reaction like vaporizing CO2, you could use it as a heat sink. The problem is that distributing heat to arbitrary places in the martian ground (remember, this CO2 is not magically flowing to you) requires an even more complex heat exchanger than that used by a simple earth-based reactor.
Not to mention that having no shielding would be hell for your robots; radiation hardening of automated systems remains non-trivial. But the fundamental thermodynamics strike me as harder.
Maybe there are niches where this would make sense, but the thermodynamics is the elephant in the room.
That's all for a typical high-power reactor setup. You can, of course, have a low-power setup with nuclear power. RTGs [1] use this approach, though they mainly rely on decay heat rather than self-sustaining reactions, which is just as well because they are also thermodynamically limited by the rate at which they can radiate heat away.
One addendum I should have noted: the efficiency of a thermal engine on mars would actually be perfectly reasonable for a small engine (it's nice and cold there, perfect for high efficiencies). The issue is that you can't dump the heat fast enough, so your heat sink will get hotter, trashing your efficiency and reducing your cooling capacity (both of which exert downward pressure on your final power output in a serious way).
[1] https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...
Shielding on Mars could just be made from surface rock. or you bury the reactor in the first place. Radioactive waste (like the contaminated shielding) is probably a non-issue on Mars because there already is too much uninhabitable space anyway.
Anyway, such things are decades or centuries in the future. In the next decades, photovoltaic or other solar energy generation will rule on Mars.
[0]https://en.wikipedia.org/wiki/Depleted_uranium#Civilian_appl...
The main trick in making aerogel is drying it out, in a way when the liquid doesn't collapse the porous structure by its surface tension. Peeps on youtube usually do this through a "supercritical CO₂" process -- which basically avoids having any liquid surface.
That said, manufacturing aerogel on Mars seems way more feasible than transporting it.