In the rather epic video that accompanies the recent SpaceX vision for interplanetary travel and colonization, it's the last few seconds that are perhaps the most intriguing. As the planet Mars spins on its axis we're given a peek into a future where this dry, cold, dusty world starts to transform into a place of a different hue, with standing water and perhaps a hint of green.

The idea of re-engineering a planet - terraforming it - is not new, and not for the faint-of-heart. But given the audacity and success of many of Elon Musk's projects, the notion that we might alter Mars to better suit us is at least worth some reconsideration. 

Terraforming is one item in a long list of ways in which humans might persist on a world that, in all other respects, appears to exist for the sole purpose of killing us. If you don't die en-route, on landing, or from sheer psychological stress, you'll die from exposure to low pressure, extreme temperatures, UV radiation, cosmic rays, corrosive martian regolith (laced with perchlorates), starvation, long-term low gravity, or stuff we just haven't thought of yet. 

There is also the sticky question of what you do to protect any indigenous life on Mars. Do rock-eating microbes have rights? They would certainly have a scientific value that is hard to overstate, and plopping thousands of filthy apes on the surface of an otherwise pristine natural environment would probably be the worst possible thing to do. I won't tackle this here - but it's a key question.

Before we jump all the way to terraforming, there are potential intermediate steps to mitigate the hazards of life on Mars for large numbers of people. You can certainly build habitable structures - perhaps buried in regolith to help with insulation and radiation protection. You might even start to build 'biodome' type facilities where you effectively make a little Earth in a bubble. These are sometimes termed 'para-terraforming' approaches. But you probably want to start figuring out how to exploit the land itself.

One idea is canyon-tenting. Mars has landforms like canyons, craters, or lava-tubes that might only need a 'lid' structure to create a vast volume of environment where we could introduce atmosphere, moisture, and have control. 

The ultimate living off the land though is partial to full terraforming. So how do you do that?

The first step would be to try to add density to the martian atmosphere - currently a lung-emptying 1/1000th of an Earth atmosphere in pressure. A thicker atmosphere and higher temperature could allow liquid water on the surface - sourced from large martian frozen water deposits.

Mars does have huge deposits of frozen carbon dioxide towards its polar regions, releasing that as gas could have a twofold benefit: boosting atmospheric pressure and boosting the global retention of infrared radiation - the 'greenhouse' effect. At present the martian atmosphere is significantly less opaque to infrared (IR) radiation than the Earth's - an IR 'optical depth' (a statistical measure of how efficiently light gets stopped) for modern Mars is about 0.2 compared to Earth's 0.83 - Venus is about 60. 

If we could release most of the frozen carbon dioxide on Mars we'd be able to raise atmospheric pressure to about 1/3rd that of the Earth and get to a runaway point where the polar temperature is permanently over the sublimation point - preventing CO2 ice from forming again. To do this we have options like:

  • lots and lots of thermonuclear explosions above the poles (an approach Elon Musk has mentioned, at least informally)
  • giant mirrors in space to focus solar power onto the polar regions (a 125 km radius mirror could help raise polar temperatures by around 5 Kelvin).
  • dumping ammonia into atmosphere as a greenhouse gas by dropping cometary (icy) bodies. Has advantage of also adding a 'buffer' of nitrogen gas to the martian atmosphere.
  • pumping CFC's into the atmosphere to boost the greenhouse effect - needs about 40 million tons (about 3 times the total CFC production ever manufactured by humans to date).
  • spray paint the martian surface to be less reflective (would take a long time to have an effect)

...and these are just the first steps.

The list of potential showstoppers is equally long:

  • climate is complex. Would we just make Mars a world of storms and uselessly extreme climate zones?
  • you might not explode in the low pressure anymore, but what about oxygen? It could take centuries to build a breathable atmosphere.
  • we probably don't understand well enough how Mars would lose elements to space - once water vapor is abundant it's going to start breaking up via photodissociation in the upper atmosphere where the hydrogen can be lost permanently. Would we actually dry up Mars?
  • Mars is far less geophysically active than Earth, has no plate tectonics. For longer term climate stability this could be key. In other words, we might make Mars habitable for a few thousand years and no longer.
  • Earth is literally alive with microbes of vast genetic diversity, permeating the rocks, interleaved with geochemistry. Can we seed Mars in a way that reproduces the systems that help keep Earth the way it was when it produced us?

It's clear that the answers are not going to be simple. The SpaceX vision is a wonderfully provocative and stimulating counterpoint to the last few decades of limited human spaceflight. It also resonates with the concerns any rational person has about the existential crises that face any species living on a planet (not just self-inflicted crises either). Like any vision it's a starting point. 

There is another intriguing side to all of this. We've undertaken an uncontrolled experiment on Earth since the industrial revolution. Would a controlled experiment on Mars offer us insight to all worlds and all possible human futures?