I think the background to this is a kind of mythological version of the Gaia hypothesis - the idea that if you introduce life to any planet that has a capability for sustaining life, that it will automatically turn into a second Earth, almost like Earth. But a bit of reflection will show that this can’t be the case. Earth itself is going to become uninhabitable half a billion years from now, when it becomes too hot for anything except the most heat tolerant thermophiles

Grand Prismatic Spring, Yellowstone National Park, home to heat tolerant thermophiles. Eventually as the sun gets hotter, only heat tolerant thermophiles will survive on Earth - though that’s half a billion years into the future, long enough for humans to evolve a second time from the simplest of multicellular microscopic lifeforms.

Say we find some exoplanet that hot, then adding life to it will not turn it into a second Earth. It may have life already, but though habitable, it’s not very habitable. It’s even possible that Venus has life in its upper atmosphere and if so, this life has just lingered as the planet got hotter, and has not kept it habitable for less acid and heat tolerant lifeforms.

In the case of Mars, it probably started off nearly as habitable as Earth, but lost much of its atmosphere, and also got colder. Being further from the Sun, it never was warm enough for trees to grow, unless it had powerful greenhouse gases such as methane making up much of its atmosphere.

So, it may have habitats for psychrophiles - cold tolerant microbes. They’d also be salt tolerant, able to withstand high levels of perchlorates and use them as food. But that doesn’t mean that if we seed it with extremophile photosynthetic life like Chroococcidiopsis that it will become habitable to Earth life. Indeed the first effect would be to cool it down even further as the microbes convert CO2 into oxygen so removing what little warming effect its atmosphere has. But probably there isn’t enough liquid water for it to have a significant effect on its atmosphere. It may be there already indeed, being one of the few microbes that might possibly both survive a trip to Mars on a meteorite lasting for a century or more and to find a habitat it can survive in once it is there.

If you could magically transfer Earth’s atmosphere and ecosystems to Mars, they wouldn’t survive long as it would be far too cold, with not even much by way of CO2 to warm up the planet. Longer term there’s no continental drift and so though the volcanoes are still active on geological timescales, there is so little activity there we haven’t yet spotted anything on the entire planet, same land area as Earth. This means there is almost no CO2 returned to the atmosphere so long term it will lose carbon into carbonate rocks, Perhaps this has happened already. There’s very little by way of CO2 in the form of dry ice on Mars. Possibly enough to double its atmospheric pressure, not enough to get to ten times its current pressure to start a runaway greenhouse effect.

And then CO2 is poisonous to humans. You couldn’t even survive in a CO2 atmosphere at Earth pressure with an oxygen supply, but would need a full closed system with both oxygen and nitrogen supplied to you, or you’d die of CO2 poisoning.

Then you need nitrogen too.

And if you sort all that out, you still have the temperature issue. Even with an Earth pressure atmosphere of CO2, Mars would be too cold for trees. Converting all that CO2 to organics (covering Mars with a thick layer of the organics taken out of the atmosphere) might take as long as 100,000 years by natural photosynthesis, and it would cool the atmosphere down even further.

You need to make artificial super powerful greenhouse gases continuously, converting many cubic kilometers of fluorite ore into greenhouse gases every century, using a network of hundreds of nuclear power stations - or reflect extra sunlight to Mars with planet sized thin film mirrors. Both are examples of mega-engineering.

Anyway - it’s a case of ecoengineering, and so far we have no experience of that at all. We are bound to make mistakes. It was tricky enough to get the small closed system habitat biosphere II working - closest approximation we have in experiments to a complete ecosystem set up in miniature. They gave up because of an unexpected interaction with the concrete. That could be fixed in future closed system habitats, and we can already achieve artificial closed systems consisting of just humans, hydroponics, and plants to create nearly all their food and oxygen - but we are nowhere near the situation where we can confidently set up a new planet ecosystem, and predict everything that could go wrong with an entire planet.

This idea, perhaps based on Lovelock’s Gaia, this popular mythology is that life will automatically make Mars into a second Earth. The idea, in science fiction at least, is that it will just happen by itself with a bit of a helping hand from humans.

But even for Earth, probably there was a large element of luck as well. I think the weak Gaia hypothesis is generally accepted that there are many cycles that help keep the Earth’s climate more or less in balance at present. But what about the evolution of photosynthetic life at just the right time to remove CO2 from the atmosphere when the Sun was warming up, to cool down Earth enough to compensate? How can that be due to Gaia? Surely it is just coincidence that this happened at just the right time, and if it happened later, the Earth would get far too hot, and if sooner, it would have got too cold for a while in the past. Either way, the Earth might easily have been uninhabitable for anything except microbial life for billions of years, and we would probably not have evolved.

But instead, we were lucky, it continued at more or less the right temperature, with a few hiccups, for billions of years. David Waltham in his “Lucky Planet” argues that this is to a large part coincidence, and I think there’s something in that myself.

The Gaia hypothesis explains many things but I don’t think it explains why oxygen producing CO2 removing life evolved at just the right time to cool down the Earth.

Then what would all this do to the search for life on Mars? Even if Mars is sterile, it’s a wonderful opportunity to find out what happens on a planet with an atmosphere, liquid water (probably) even in small amounts on the surface, organics, and no life. There’s also the possibility that we find life precursors there, replicating chemicals without a cell wall, say, or autopoetic cells, metabolism without exact replication, or other life precursors which the theorists have suggested. Something like that would be very vulnerable to introduction of Earth life, and of exceptional interest, helping to fill in huge gaps in our understanding of evolution, of the possibilities for exobiologies and what we might find on exoplanets.

I think the thing to do is to start with small scale closed systems in space habitats, like the Stanford Torus or habitats in lunar caves. Also study exoplanets. Look at ideas for terraforming Mars, but it’s nowhere near time to start on them, if it ever will be. There’s enough material in the asteroid belt to build habitats with a total land area a thousand times that of Earth, so we can create the equivalent of a thousand planets just using materials from the asteroid belts. Building a single large habitat for 10,000 people is something you could complete in a decade or two. Terraforming a planet, if it is possible at all, would take at least a thousand years according to the most optimistic estimates.

Meanwhile the Moon has many advantages over Mars. It actually beats Mars on many comparisons of resource utilization too. I think the Moon is the place to go at least until we have tens of thousands, maybe millions in space. Study Mars robotically, also possibly from orbit telerobotically and through flybys. But send humans to the Moon for now, and to Mars orbit, not to the surface. And I think it is far far too soon to think about trying to transform Mars. After all any attempt to transform a planet is likely to take at least a thousand years with present day technology. How can we know what our descendants a thousand years from now will want? I think it is very likely that they would prefer a pristine Mars as it is now, to study especially if it has unusual and unique biology or life precursors. I think it’s unlikely that if we do clumsy attempts to try to terraform it, bound to fail, that they will look back at those attempts and feel we left the planet to them in a better shape than we found it, and far more likely to wish they could somehow undo what we had done to the planet. Leaving it “as is”, studying it, exploring it from orbit via telepresence (which gives a much more immersive experience also), that all leaves our futures open, and we can make decisions about what to do next later on once we know a lot more about Mars.

See also my

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?

Case For Moon First