I think the main issue here is that it is irreversible. Not so much the microbes as such. If you could “unterraform” Mars if you make a mistake, or if you could arrange so that half of Mars retains its native life in a wildlife habitat of some sort, then that would be different.

But Mars is one closely connected system with its global dust storms. If there is life there, or even if there are just habitats for life there, then introducing Earth microbes would eventually spread to them all.

This introduces a new geological era to Mars. For all future time, we can’t study Mars in its current form. This closes off our options for the future. From now on we can only proceed into a future with the Earth microbes on Mars. It’s a bit like our recent EU referendum, a decision by the UK people to leave the EU. There’s no going back on it once Article 50 is triggered.

Well, once the first microbes are introduced to Mars able to reproduce there and spread to its habitats, if we do that, and if there are habitats there, there is no going back at all. Not even decades or centuries, or millions of years later. We then have to continue forward into that future with Earth microbes on Mars.

So, I wouldn’t actually go so far as to say we can never terraform Mars. Rather I’d say that as a young species, we don’t know enough to make such decisions. We should keepour future options open until we have more wisdom than we do today. Right now we haven’t even discovered a single example of biology evolved independently of Earth.

If we are lucky, then Mars could be as interesting as an exoplanet in our own solar system. An exoplanet with life evolved independently - or an exoplanet with no life but habitats for life - both are just immensely interesting for biology and science, and we can’t know what we’d learn from it.

The nearest other planet resembling Mars is surely at the least light years away. If we had Star Trek like abilities to travel at warp speed through our galaxy and if we found that planets like Mars are common, that would be a very different situation. We might make different decisions in that case. But as it is now, I think we have to treat Mars as immensely precious until we know otherwise. It would be so tragic to mess things up there.

As for terraforming, I think there’s much to go wrong, and we shouldn’t treat Mars like a giant petri dish. There are plenty of other places to do these experiments. We can try doing it in the lunar caves, or in large Stanford Torus type habitats. They also may well go wrong too, I’d expect many problems. But you can build such a habitat on timescales of decades instead of thousands of years. If something goes wrong with the atmosphere, you can purge the atmosphere and start again, or scrub out problematical chemicals or microbes. With technology we have today there is no way we can do that with a planet.

So, I don’t think planets are the place to do our early experiments in large scale closed systems. We have to start smaller scale than that and learn in smaller places. Plus we can study exoplanets and learn from those too.

And we can study Mars, and we can do theoretical analysiss of the effects of changing the planet in various ways. Eventually maybe we decide to do something. But just as with our recent UK exit from the EU - that’s something that affects the entire world. Not just us but all future civilizations that arise on Earht for billions of years into the future. How do you know that your attempt at terraforming will be welcomed when it reaches completion thousands of years from now?

Even if there are no habitats for life on Mars at all right now, I still think we shouldn’t attempt to terraform it, but should start smaller first. There may be many ways it could go wrong, end up in states that are problematical for us and for all future generations.

So, the main thing here is that it’s an irreversible change that doesn’t just affect us, but all future civilizations that may arise on Earth for all future time. For now, I think we need to proceed in a way that keeps all future options open, for as long as we can.

Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?

I think we should start with the Moon anyway and as we explore the Moon we can simultaneously explore Mars robotically, and make decisions later on once we have a much better understanding of the planet.

There’s so much focus on Mars also, but what about Mercury, the cloud tops of Venus, Jupiter’s Callisto, the asteroid belt etc? Why rush to Mars the planet most vulnerable to Earth microbes? If you introduce Earth microbes to the Moon, no problem, only contaminates the immediate area around your habitat. A few kilometers away the lunar surface will still be pristine. I think we need to start with places like that, where our impact on the celestial bodies we explore and study is more limited and especially, avoid large scale irreversible changes to planets, until we learn more.

See my Case For Moon First