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I don't think we can answer this at this stage in any definitive way. It's asking us to predict future science. You can't know what direction it will go and what we will learn about Mars. So it can't be timetabled. Carl Sagan's 60 rover missions and 30 orbiters searching for life, mentioned above, was just for the sake of having a number to work with for his probability estimates.
Let's take it step by step, and send humans to the Moon and asteroids and Mars orbit first, and make the next decision based on what we find out from that first phase. The main thing right now should be not to close off future possibilities. If we make that decision in the future, it will be an informed decision.
We wouldn't know Mars completely, as that would take for ever. We don't understand Earth completely yet. But we'd know a lot more about Mars than we do today. We need to leave it to our future selves or descendants to evaluate what they know then, and to decide whether they know enough yet to make this decision. I think that with the rapid pace of science, you can only foresee the future perhaps ten or twenty years ahead in detail, and even on that timescale, surprises are likely.
I see two possible futures here. One, where we find that the Mars surface is uninhabitable and humans land there just as they did on the Moon. That would be fun and exciting just as it was for the Moon. The young space geek, science fiction loving kid in me would just love that!
In this future, humans could perhaps land early on - though there are still issues to think over about introducing earth microbial spores that could interfere with future plans to transform Mars. It is after all, an entire planet, connected through the dust storms, and the dust able to protect spores from UV light. Also, after the initial excitement, it might well turn into a place like any other with humans cramped in habitats, complaining about food and the difficulties of being cut off from Earth, sending video clips talking about how they never get to see a blue sky or sunlight or trees or grass any more, saying how homesick they are, and how they can't get out of their habitat - I'm sure there'd be some grumbling from people who didn't realize quite what they are getting into. It might not be as glamorous as it seems in advance.
The other future is one where the Mars surface turns out to be habitable. In the most exciting future here, perhaps nearly all those suggested habitats turn out to be habitable. Nilton Renno's salt / ice interfaces, the seeps of briny water, the pure water at 0 C in Richardson crater due to solid state greenhouse effect through pure ice, and on the surface using the 100% night time humidity, all of those inhabitable.
Almost anything that could happen in this future is exciting. Even Zubrin's picture of a Mars with the same lifeforms as Earth would be exciting, how could that possibly happen? How did they all get there, and when, and why didn't they evolve in some different direction from their cousins on Earth? The future of uninhabited habitats would be interesting too. We'd learn a lot from both these futures.
The most thrilling future of all here, though, would be the discovery of indigenous life or early life precursors. Like the possible life forms in ALH84001, with the debate going both ways about whether it is life or not. Mars has such a different past from Earth, at its most habitable for just a few hundred million years, with oceans, early life could have evolved there and still be there. Those early life forms might never have gone extinct on Mars. That would be the most exciting of all, life that was made extinct on Earth, and they could be extremely vulnerable to Earth life. That would fill in a huge gap in our understanding of life.
Either an early form of life on Mars, or some form of life that's followed a totally different direction of evolution from Earth life. Even perhaps more than one form of biology, different directions explored and perhaps none of them have yet taken over as the only form of biology on Mars.
That would be like discovering an exoplanet, complete with its own extraterrestrial life, in our own solar system. Ok, humans can't land on the planet, not early on anyway. But to compensate, they can explore it by telepresence, and we can all participate, looking at the streaming feeds from Mars and walking via virtual reality through the landscapes they uncover in their explorations from orbit. It would be exciting to follow the expeditions of the telenauts exploring Mars from orbit. And there's something also fascinating about a place you can't go to in person, for whatever reason. It would add to the interest and mystique of Mars.
For me, that's by far the most exciting future here. So I'm rooting for that future where the habitats turn out to be inhabited - and most exciting of all, a future with some form of indigenous life, early or life precursors or alternative biology. This is a scientific possibility at present, something that could turn out to be true. I hope this is what we'll discover on Mars in our near future.
There might be other possibilities that we can't see yet. I hope we don't end up in a future where we accidentally introduce Earth life to Mars however. That would be so sad, to do that by mistake, if that's not what we want to do.
These planetary protection issues arise so acutely for us because there is only one Mars within reach.
If Mars has early pre-DNA life on it, then for all we know, it may be the only place in our entire solar system with early life on it.
Or if for instance Europa or Enceladus also have early pre-DNA life, Mars could be the only terrestrial planet with this form of life in our solar system.
If there are habitats there, but no life, again it is the only terrestrial planet in our solar system which is habitable at all on its surface and has no life on it, a situation that could teach us a lot about the role of life in planetary processes as well as help us distinguish exoplanets with and without life.
If it has complex life precursors but no life, again it is the only terrestrial planet in our solar system like that.
And so on for all the other various possibilities for what we may find there. Mars is our only opportunity to study a terrestrial planet of that type. And because we can't travel outside of our solar system easily then this means it is the only terrestrial planet of its type in the habitable zone of its star in the entire universe that is accessible to us to study close up.
If we could travel at warp speed to distant star systems within days or week, maybe we might know of thousands of Mars like planets with pre-DNA life, or whatever it is that makes Mars unique in our solar system. Then, it might perhaps be a matter of less consequence what we do to Mars. We could try experiments then with some of those many planets.
Even then I think we have a fair bit of responsibility for those planets. Even though we would have many planets at our disposal, it would be an irreversible change, still for any of them. We'd have responsibility for the future beings that might evolve on those planets which we transform, maybe millions of years into the future. We'd have to think through whether they would be able to keep their planet habitable in the long term future, especially future intelligent lifeforms that may arise there which we may not even be able to predict at present. For instance, to start a new habitable planet that would unterraform a few thousand or a few million years into the future might well be seen as irresponsible even if we have thousands of planets at our disposal to experiment with. We might still have some kind of a "prime directive" that applies even to planets with only primitive life.
But - it would be a different situation. We would still have ethical dilemmas and responsibilities, but we wouldn't have so much to lose by transforming the planet. As it is though, we have only the one Mars in our solar system, just as we have only the one Earth.
There is no immediate prospect of developing a realistic warp drive as far as we know, ideas yes but nothing concrete. There are issues with warp drives also as if it is possible, it may permit travel backwards in time which is an idea that challenges causality. Faster than light travel is common in science fiction, but though we do have theoretical possibilities for it in our own universe already, such as the Alcubierre drive, this requires exotic matter with negative energy which we don't know how to make. It's not yet at all clear that we will ever be able to do it in practice. Certainly we have not got any spacecraft able to do this yet, and I don't think many would say we should plan on the assumption that we will be able to do this in the near future.
So we need to proceed carefully with actions that may change Mars irreversibly. We may never be able to access another planet like this, within a travel distance less than decades even traveling at a tenth of the speed of light.
Even in future if we ever do develop a warp drive, it may still be unique. It might be that only Mars has early life that's related in some way to Earth life, to give some example (the hypothesis that Earth life originated on Mars first). There may be many other such connections, including just that it is a planet of exactly the same age (to within a few million years) around the same sun and with a shared history.
Other planets may have other forms of early life in other solar systems but that close connection to Earth could make Mars of especial interest to us. That's just one example. It's not impossible that Mars, and Earth are unique in various ways in the entire galaxy. Also, on the knowledge we have so far, it's not impossible that our solar system is the only solar system that has life in it in the entire galaxy.
So one way or another, Mars could be unique and very precious to us, as the only planet of its type accessible to us, right now, and quite possibly also for many or all future civilizations that arise on Earth in the future.
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