Stage 2 - Research and Return

When we get to Mars, we're not likely going to turn around and come right back!   The astronaut team will stay on the surface for a considerable length of time for two reasons:

• we'll want them to do a lot of significant research, for there would be little point in just getting there.
• they'll have to wait until Earth and Mars are aligned properly for the return journey (on the return flight, the spacecraft will be launched when Earth is behind Mars in its orbit).

This means that we are going to have to launch from Earth with enough life support (food, water, oxygen) to sustain the crew, and enough propellant (fuel and oxidizer) to keep them healthy for this time, and bring them home.  Or do we?  Certainly they have to have enough supplies, but do they all have to come from Earth?  Could we manufacture some of them on Mars?

Why would we even want to consider manufacturing them there?  Remember the enormous mass ratio between a rocket on the surface of the Earth that launches our astronauts, and the amount of mass that will return?  In the case of the Apollo moon program it was a ratio of almost 600 to 1.  The total mass of the Saturn V launch vehicle was almost 3,000,000 kg while the lunar module was only 5000 kg.  In other words, for every 1 kg we launched from the moon's surface (including the masses of the astronauts, their spacecraft, and the rock samples they returned), it took 600 kg of Saturn V rocket to get it there.

To go to Mars and return, the ratio will be even higher.   If we could produce some of the fuel on Mars then we can use more fuel to get to Mars faster, and more fuel to return.  The faster we can travel, the less serious will be problems caused by long term weightlessness, the easier to provide life support, and the less chance of exposure to radiation.

If we are going to generate fuel on Mars, then we must have a way to do so that is totally reliable.  We also have to be able to use materials that are readily available in the Martian environment.  Remember Scott's Antarctic expedition?  We don't want to leave our crew unable to return from Mars because we have not planned our mission correctly.

The most abundant resource that can be used for fuel on Mars is carbon dioxide, CO₂, which makes up about 95% of the Martian atmosphere.  There may be small amounts of water vapor, or sub-surface water on Mars, but they are probably present in far too small quantities to be counted on, at least for our initial exploration.  Could we do some kind of chemical reaction with the carbon dioxide from the atmosphere that could generate useful fuel and oxidizer?  We might be able to if we use the Sabatier reaction, which is:

CO₂ (g) + 4 H₂ (g) CH₄ (g) + 2 H₂O (g)

Hydrogen could be brought from Earth.  The CO₂ would come from the Martian atmosphere.  The CH₄ (methane) would be liquefied to use as fuel.   The water could be electrolyzed to produce O₂ (g) which could then be used as the oxidizer to burn the methane as propellant.  The methane and oxygen produced could also be used as fuel for a vehicle to explore Mars.  Of course, our astronauts can also breathe the oxygen, and drink some of the water.  So at least initially this looks like a very sensible thing to try.

But there are many questions that need to be answered.  If we have to import the hydrogen from Earth, why not just use it as the fuel directly?  What are the conditions under which the Sabatier reaction occurs?  How much product can we expect to get, and how will the conditions of reaction effect our yield?  What source of energy will be required to carry out this reaction?  What catalysts may be required, and how will this effect the amount of fuel we can produce?

Is this really this hard to figure out? Can't we just mix together 44 g of CO₂ (1 mole) and 8 g of H₂ (4 mol)e and get 16 g of CH₄ (1 mole)   and 36 g of H₂O (2 mol)e – or corresponding proportions by mass?   That would be the ratio of one mole of CO₂ to four moles of H₂ producing one mole of CH₄ and two moles of H₂O as indicated in the balanced reaction.  The answer is a definite NO!  While that is the correctly balanced equation, it ignores a fundamental principle of chemistry known as equilibrium.

To a beginning chemistry student it may seem strange that the balanced equation doesn't really tell what happens in a chemical reaction.  When learning chemistry you spent a lot of time learning to make sure that you started with the same number of atoms of reactant as you ended up with atoms of product.  However, almost all chemical reactions stop before they have created the amount of products indicated by the balanced equation.  In the case of the Sabatier reaction, at 600 ^oC, if you mixed together 1 mole of CO₂ and 4 moles of H₂ in a one litre container, and let them react you would actually end up with the amounts shown in the final row of this table.