The last piece I wrote here, “Death on Mars”, sure got a conversation going. In a vigorous flurry of commentary on social media I was cast as either a voice of reason or a total pariah for daring to suggest that certain bold ideas for Mars exploration might face some rather nasty roadblocks. Why roadblocks? Because physics. Because biology.

As I explained in that piece, I am actually very enthusiastic about the remarkable advances being made in the space-launch industry. But if we’re going to take ideas like the large-scale human settlement of Mars seriously (putting aside reasonable concerns about our priorities while Earth is undergoing changes that challenge humanity) we need to have a longer conversation and to take some care over our enthusiasm. If only to ensure that when failures do occur we don’t give up because our expectations were too high and too unrealistic.

In that spirit (not to be taken as a rigorous analysis, but as some points to think about) here are some further questions about a human presence on Mars:

It may be bad but the surface of Mars is more hospitable than the Moon or Venus, right?

In many respects this is true. There is a modest atmosphere, so you’re not contending with absolute vacuum. Or such extreme temperature variations as on the Moon. Or the crushing pressures on Venus. Although, at around 0.7% of Earth’s sea-level pressure, any on-foot exploration of Mars would require a full pressure suit of some form. That’s to both stop your body from swelling and hemorrhaging in nasty ways, but to also make sure that enough oxygen is driven into your bloodstream. 

Even if pressurized to about 1/3rd of Earth’s surface pressure (like current NASA ‘soft’ EMU suits in low-Earth orbit) that means you need a close-to-100% oxygen atmosphere in the suit, and will contend with a ‘pre-breathe’ protocol to prevent decompression sickness where nitrogen bubbles form in your body. On the ISS, NASA's pre-breathe sessions can run from 4-12 hours depending on the protocols followed for a spacewalk. In other words, you’re not going to be quickly popping your suit on for an after-dinner stroll on Mars, unless you stay in a low pressure, high oxygen environment all the time (with increased hazards for fires) or have high pressure suits ---which are certainly under development. 

The low pressure, very dry martian atmosphere also means that volatile substances tend to boil or sublimate very quickly. Solid or liquid water exposed at low latitudes on Mars (where temperatures are more moderate) will turn to vapor quite fast. There’s no hauling chunks of polar water ice back to camp unless you seal them in an airtight container. There will also be constraints on any kind of adhesives, sealants, and other materials that you allow to be exposed to the raw environment (also because of UV damage from sunlight, which is significant).

A third of Earth’s surface gravitational acceleration sounds pretty good though?

On the face of it perhaps. You’d feel buoyant, lighter, with less effort to move your body. But we really don’t know what 0.37g does to a human body over time. We have good data on the extremes: 1g and micro (or zero) g. We know that micro-g is challenging and requires a lot of effort to stave off the worst of the physiological effects: from dramatic bone density loss (and kidney stress as all that calcium tries exit your system), to cardiovascular changes, immediate and prolonged epigenetic changes, and vision impairment possibly related to increased cerebrospinal fluid in the brain’s ventricles (which may also have cognitive impacts). Medication regimes could help, but they might be regimes for life if you stay on Mars. And that means either reliance on Earth’s pharmaceutical supply or extensive in-house production.

There have also been experiments simulating the simple effect of lower gravity on human walking speed. Because of the nature of our anatomy and gait it appears that on Mars we might weigh less but we would only be able to comfortably walk at about half our normal pace. Martian Olympics might not be so very exciting.

But Mars has great natural resources for living off the land right?

It does. Mars has a huge amount of water in total (in subsurface ice and bonded in ordinary regolith), and it’s got a surface mineral composition that’s pretty familiar in many ways. It receives a decent amount of solar electromagnetic radiation---roughly 60% of the power per square meter on Mars compared to a similar site on Earth. And towards the polar regions are vast deposits of frozen carbon dioxide. The problem is getting your hands on any of this in the first place. 

Take the regolith (soil). Data from the Curiosity rover suggests that, by mass, 1-3% of regolith is water. So, in principle, cooking up regolith to release that water and then condensing it could provide a practical supply. There are caveats though. 

For one thing, if you wanted to use this water for growing your food au naturel (like potatoes, for instance), martian soil sucks. That’s because it is actually toxic due to chlorine-containing compounds called perchlorates. Earth-based experiments on plants exposed to the level of perchlorates on Mars indicate that most species suffer. Those that are resistant end up with high perchlorate concentrations in their structures. You don't want to eat that.

There are terrestrial microbes that actually consume perchlorates (since they're energy-rich oxidants) and are already used to help clean contaminated water on Earth. So farming on Mars would have to involve a lot of careful preparatory work to get rid of toxins in the water and soil, and to ensure a supply of the right nutrients (even in hydroponic growing). The larger the scale of food production the bigger the challenge.

And there’s the rub for most of Mars’s plentiful resources. They’re going to need extensive and careful refining and purification, with the possible exception of brute force applications (like electrolysis of water to make oxygen and hydrogen). In all instances that requires a substantial energy budget. And before you forget: Having oxygen is not enough to keep humans happy, you also have to scrub the carbon dioxide from the air that we exhale or we die---involving more energy use and materials, and possibly a critical role for plant life on Mars.

We’ll just end up terraforming Mars, that’ll be the real solution?

It could be, if we really knew how to accomplish such a thing beyond back-of-the-envelope theorizing. I’ve written about that before on these pages. It would require engineering on a scale exceeding anything that we've attempted before. It would even exceed the scale of our unintentional influence on the carbon dioxide concentration of Earth’s atmosphere over the past couple of centuries. 

We could heat up the martian polar zones (perhaps using orbital mirrors), releasing the frozen carbon dioxide to thicken the atmosphere. We could bring in comets and ammonia to boost the greenhouse effect and build a nitrogen rich chemical buffer in the atmosphere. We could try to engineer microbes capable of transforming the soil chemistry and adding molecules to the atmosphere. 

But we might not be able to predict all of the outcomes. If any of this engineering really worked we’d have a new planet, with new climate zones and atmospheric circulation (cyclones, tornadoes, precipitation after billions of years of drought) that would, by the very nature of these things, be unpredictable and tricky. What if terraforming simply makes Mars into a maelstrom-ridden hell?

What next then?

This list could go on. The point is not to reject any of our ideas about putting people on Mars, or even establishing the kind of existential hedge-fund that folk like Elon Musk are talking about, or to prematurely dismiss the kind of expansionist (and rather anachronistic) view of what our species should do with itself in the future. All are worth discussing and perhaps doing something about. But don’t kid yourself into thinking that we either know how to really do any of this, or that it won’t kill a lot of people in the process. Figuring out the details is key. The best kind of journey is often one that is well prepared for.