The problem here is that Mars gets only half the sunlight Earth does. Even if you could somehow get an Earth atmosphere on Mars, it would not be warm enough for trees to grow at the equator. For that reason, terraforming ideas involve either space mirrors of similar scale to the planet in total area - or continual production of powerful greenhouse gases into the future for ever, or a mixture of both, to keep it warm.

Indeed, it’s a puzzle how Mars managed to be warm enough for liquid water in the past when it had oceans. An Earth like atmosphere wouldn’t make it warm enough for that to happen. Perhaps the early Mars gas had powerful greenhouse gases in it such as methane? Or perhaps it was just frozen over for most of the time except at times when Mars had very eccentric orbits taking it closer to the sun for part of each orbit (it’s eccentricity varies a lot and is sometimes far more eccentric than it is now).

Also on long timescales of millions of years, then Mars would lose all its water to space, through solar storms unless you found a way to prevent that. So again you need a way to resupply that water to Mars as it is lost, if you want it to be permanently terraformed.

As well as that, there’s lots more to think about. The many cycles, for instance on Earth CO2 gets brought back into the atmosphere long term through volcanoes - subducted as limestone through continental drift. On Mars long term it just turns into limestone and is gone for ever. You can turn it back into CO2 with powerful acids, or have microbes to digest the limestone, but you need a system very different from Earth’s.

Also it has to have three times the mass of gases in the atmosphere compared with Earth for the same atmospheric pressure. For an Earth pressure atmosphere the plants would have to produce three times as much oxygen to maintain breathable levels in the atmosphere.

Maybe this can all be sorted out. Super effiicent plants that produce lots of oxygen. Some kind of organism that produces greenhouse gases that are also human friendly, or space mirrors that are automatically self maintaining. Bots that mine water ice from the outer solar system and continually supply it to Mars without any need for supervision. Perhaps a thousand years from now, or even sooner, it might be a simple thing for us to do.

If it was like Star Trek that we could travel through the galaxy at “warp speed” and have millions of “Mars” planets at our disposal, maybe it wouldn’t be such a big deal to terraform Mars - or one of the many similar planets we know about. But as t is now, it’s the only planet we have like that.

And we have plenty of other ways to try similar experiments which don’t involve irreversibly transforming a planet. Such as using materials from the asteroids. The asteroids can be used to create habitats with living area equivalent to a thousand times the land area of Earth, an insight from the 1970s which is why they were so keen on such things as the O’Neil colonies and Stanford Torus.

We can complete such a habitat in decades, and if anything goes wrong we can scrub the atmosphere of harmful gases, or in worst case, scrap it and start again, build a new one from the materials of the old one. We can’t do that with planets..

For that reason I think we need to start with the Moon, asteroids etc, and leave our irreversible experiments with planets to a later date when we know much more than we do now.

You might be interested in my articles / kindle booklets on this topic:

• Case For Moon First
• Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths
• Trouble With Terraforming Mars
• Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
• To Terraform Mars with Present Technology - Far into Realms of Magical Thinking - Opinion Piece
• Why Nukes Can't Terraform Mars - Pack Less Punch Than A Comet Collision
• Our Ethical Responsibilities To Baby Terraformed Worlds - Like Parents
• How Valuable is Pristine Mars for Humanity - Opinion Piece?