Molecular Monday: Making Rocket Fuel on Mars

Today’s post is part of a bi-weekly series here on Planet Pailly called Molecular Mondays, where we take a closer look at the atoms and molecules that make up our physical universe.

As most of you now know, I am on a totally-for-real, not-making-this-up mission to visit the planet Mars. Now if you’re planning a mission to Mars, one of the first things you need to figure out is how to get back to Earth. Unless you’re not planning to come back, which is apparently an option.

But if you do want to come home, you’ll probably need to refill the fuel tanks of your spaceship using only the natural resources Mars provides. Believe it or not, this is surprisingly easy to do using a process called the Sabatier reaction (discovered in the early 20^th Century by French chemist Paul Sabatier).

In the Sabatier reaction, hydrogen and carbon dioxide mix together to produce methane, with water being produced as a byproduct. The chemical equation looks like this:

CO₂ + 4H₂ –> CH₄ + 2H₂O

Liquid methane makes a decent rocket fuel, but you still need an oxidizer. To get that, all you have to do is zap that byproduct water with electricity, creating oxygen and hydrogen.

2H₂O –> 2H₂ +O₂

Liquid oxygen is pretty much the best oxidizer you can get, and the “waste” hydrogen can be put back to work keeping the Sabatier reaction going.

I first learned about the Sabatier reaction in Robert Zubrin’s book The Case for Mars, coming soon to my recommended reading series. The only problem, according to Zubrin, who was writing in 1996, is hydrogen. Mars’s atmosphere is almost completely CO₂, but Mars is severely depleted of hydrogen. Zubrin’s solution in his book is to import hydrogen from Earth (still cheaper than trying to ship rocket fuel to Mars).

But since 1996, we’ve learned that Mars has more water than previously thought, most of it frozen just beneath the planet’s surface. So when I read about the Sabatier reaction again, this time in Elon Musk’s paper “Making Humans a Multi-Planet Species,” published in 2017, the hydrogen problem was no longer a problem. We can get it through the electrolysis of Martian water.

Of course for my own Mars mission, I don’t have to worry much about rocket fuel. My spaceship is fueled by pure imagination! But still, if something were to go wrong with my ship, it’s good to have a backup plan.