Elon Musk is proposing a lot of daring things to do to get to Mars before the 2020s are over. But perhaps the most striking feature of his plan is the simplest. He’s not just sending humans to Mars; he intends to bring them back.
At this point, every trip to Mars has been a one-way trip. NASA is only now planning a rover for a 2020 launch that will collect samples for return to Earth – how we’re going to get the small collection of samples back hasn’t been specified yet. Musk, on the other hand, intends to return everything: the people, the ship, and presumably any souvenirs cleared through customs. That intention requires a radical rethinking of the approach.
One of the main things that will have to change is what our hardware does once it’s there. So far, all of our equipment has been designed to test the chemistry present (although that will change on the 2020 rover – more on that in a future story). Musk’s plan envisions creating a chemical plant on the red planet, one that will make all the fuel needed to resurface and return a ship to Earth.
The basic chemistry of fuel making is simple. The challenges of doing it on Mars are quite profound.
A recipe for methane
The engines of the Interplanetary Transport System are being designed to burn a fuel – methane – with Mars in mind. While there is evidence of methane sources on the Red Planet, measurements of the atmosphere by the Curiosity rover indicate that whatever they produce is erratic and ephemeral. Everything indicates that the gas does not survive long in the atmosphere either, so harvesting the gas is out of the question. This means that fuel must be made from ingredients that are available and readily available. This was considered as part of the process of choosing methane in the first place.
Methane is simply a carbon atom bonded to four hydrogen atoms. The plan is to produce it using what’s called the Sabatier reaction, well-understood chemistry that’s more than a century old. It involves reacting carbon dioxide with hydrogen, breaking the carbon-oxygen bonds to form water and methane. The reaction is energetically favorable due to water as the endpoint, but requires energy, pressure and a catalyst to operate due to the stability of the carbon dioxide.
For the purposes of this article, we assume that Musk can get an essentially infinite supply of solar panels to Mars. No, owning Solar City doesn’t help; he’ll want to use high-efficiency gear that’s too expensive to store on a house, but will provide the most electrons per weight. Fortunately, these can be purchased from other sources. More panels would mean faster production, but they don’t fundamentally change the chemistry.
Insofar as Mars’ thin atmosphere has a lot of everything, it has a lot of carbon dioxide. There is also quite a bit of hydrogen available. It happens to be in the form of water, which would have to be split to release the hydrogen. Again, this is well-understood chemistry, and it just requires a catalyst and some energy input. Once the full production process is underway, at least some of the hydrogen from the Sabatier reaction ends up as part of water, which can be recycled to make more hydrogen.
The other notable thing is that the end products of burning methane are carbon dioxide and water, the same as the starting ingredients. So the processes used to make methane must first and foremost produce enough oxygen to power the engines. Of course, people will want some of this to breathe as well, so the water-splitting reaction may have to be done in excess. Or there are ways to extract oxygen from carbon dioxide, and there will also be a little bit in the atmosphere that is used to provide the CO2. In any case, the basic chemistry works out well.
Removing carbon dioxide from the atmosphere is a relatively simple process. In fact, it’s as simple as drawing in the atmosphere, which is more than 95 percent carbon dioxide. Water, on the other hand, is a bit more of a challenge.
There’s a lot of it in the polar ice caps. But it’s not clear that anyone would actually want to go to the poles, in part because of the incredibly harsh conditions there. The poles would also limit the amount of solar energy available, shrinking the production process. At least some of that water ice is carried by the wind to more temperate regions, enough to cause thin frosts and a weak water cycle. But it’s not clear if that’s enough to drive the levels of chemical production this plan requires.
There is an additional source, halfway between Mars’ equator and its polar ice caps: glaciers. “Numerous glacial forms have been identified in the mid-latitudes of Mars, and in recent years the acquisition of radar bearing data has revealed that the features are composed primarily of water ice,” says a recent paper making an inventory of Mars’ ice. It calculates there are more than 100,000 cubic miles of water ice in the mid-latitudes of Mars — that’s a lot of potential rocket travel. Musk’s presentation specifically referred to “water harvesting,” so it’s clear that’s what he’s aiming for (although he lists Mars’ total water supply, including the poles).
This still makes the landing sites geographically limited – you should drop near one of them. This excludes the equatorial regions, but the glaciers are widely distributed throughout the rest of the mid-latitudes. Of course, it seems that the landing sites would need a preliminary lander to survey the ice and ensure it is sufficiently accessible and of sufficient quality to risk an entire ship (and its occupants) by landing there. However, with the right site, this should work.
Chicken and egg
Really, the only place things get into trouble is in the form of a set of competing priorities. Musk’s presentation said, “The first ship will have a small propellant installation, which will be expanded over time.” However, a small propellant installation means it takes more time to produce enough fuel for the return journey. And more time either means more supplies to feed everyone while that propellant is being made, or it means fewer passengers.
The obvious solution would be to send a small crew and lots of hardware to a landing site ahead of major passenger journeys, which is likely what will be in the works. That would allow the factory to expand with the very first passenger transport journeys. It would also provide the chance to pre-produce a lot of fuel before the second ship lands.
While this makes sense, it means that more and more infrastructure is tied up in one location. That means making future landings at the same spot on Mars (and the obvious risk of creating a “hive of scum and rogues” like Mos Eisley). Which is a bit limiting.
More problematic is the fact that resources at the site will also be limited. Glaciers are big, but they are not infinite. Either you have to repeat the process somewhere else at some point, or you have to design the entire chemical plant so that it can be packed up and moved.
None of this is insurmountable. We may have to accept a certain level of Mars infrastructure becoming a stranded asset as travel continues. But from a chemistry perspective, Musk’s plan should at least work for initial recon, pending a few choices about whether or not to refuel before the first passengers.