To do anything resembling terraforming, you'd need a whole lot more atmosphere. Moss doing this:

    CO2 + sunlight => O2 + more moss

does nothing to change the "space suits mandatory" situation.

> To do anything resembling terraforming, you'd need a whole lot more atmosphere.

Perhaps not that much. Apparently plants fare much better than animals in low pressure, withstanding down to 10kPa atmospheric pressure. See https://ntrs.nasa.gov/api/citations/20050182975/downloads/20...

IIRC, there is a rule of thumb the atmospheric pressure drops by about 10% every 1100m one goes up close to earth surface. 10kPa would then be the pressure on top of a mountain rising ~24km above sea level.

But I bet the moss wouldn't fare well in perchlorate soaked soil anyway.

> Perhaps not that much [more].

From a quick search, the atmospheric density on Mars is ~20X lower than the top of Mt. Everest - the "dying zone". (And with the lower gravity, you'd need whole lot more than 20X, to get the same atmospheric pressure on Mars.)

20X'ing even a small planet's atmosphere is a big-ass ask. And if humans can only live inside pressure suits, or pressurized habitats on Mars - then "some plants can grow on the surface" is "that's nice, but...".

Hmm... Earth is much bigger than Mars, and has overheating problems. Gasses cool when they expand, per pV=nRT. In theory, if we exported a bunch of the Earth's atmosphere to Mars, we could kill 2 birds with one stone.

@xkcd, you want to take this?