Chemistry in "The Martian"

 

Several chemistry-related topics are essential to the plot of Andy Weir’s “The Martian”. I have listed four below. They all have questions that can be answered to better understand how the main character, Mark Watney, was able to survive on Mars and ultimately return to Earth. General chemistry knowledge, plus a pinch of organic, is all that's necessary.
[Page numbers are from the paperback edition: Broadway Books, 2014. Quotations from the book are in italics.]

1. (p. 3) What fuel does the Mars Ascent Vehicle (MAV) make?
Mark Watney writes in his diary, “The MAV is pretty cool. Turns out, through a neat set of chemical reactions with the Martian atmosphere, for every kilogram of hydrogen you bring to Mars, you can make thirteen kilograms of fuel.”

What could this fuel be?

The MAV (Mars Ascent Vehicle) is the rocket/spaceship that returns the astronauts to Mars orbit at the end of their mission. The MAV is sent to Mars by NASA several years before the astronauts arrive, because it must make its own fuel for the ascent, and that process is S-L-O-W. It would be super expensive to send a bunch of fuel and oxygen to Mars for this purpose.

It doesn’t say so, but the process most likely starts with splitting of CO2 to C + O2 (as in the “oxygenator” in the Hab; see below). Carbon can be burned to generate CO2 again, but the reaction is too slow for a good rocket fuel. Hydrocarbons are much better (used all the way from Robert Goddard’s first liquid-fueled rocket in 1926 to many rockets today). But what kind of hydrocarbon would the MAV use? You must bring the necessary hydrogen to Mars, because there is almost no H2O or any other hydrogen source on Mars.

It’s logical that you would want the cheapest possible fuel that would do the job.* Weir points out that all the hydrogen needed must be shipped from Earth (expensive), while the carbon can be made (along with the O2 needed for combustion) from CO2 on Mars. You want to produce the maximum amount of hydrocarbon with the minimum possible amount of expensive hydrogen.

For example, suppose the fuel is methane, CH4. It could be an excellent rocket fuel, but you would need to send 4 kg of H2 to make 16 kg of CH4. So, you could only make 4 kg of fuel for every 1 kg of hydrogen that you send to Mars.

Weir’s ratio, 13 kg of fuel per 1 kg of H, means the part that isn't hydrogen is 12 kg of C. That is, 1 mole H : 1 mole C, and an empirical formula of CH.

Possible molecular formulas: CH (unstable), C2H2 (acetylene: hard to make, and potentially explosive even in the absence of O2), ..., C6H6 (benzene). Aha! Benzene is a liquid at room temperature. 78 g of benzene (1 mole) contains 72 g C and 6 g H, for the desired ratio of 13 g fuel per 1 g H.

Any other issues? Bad news: benzene freezes at 5.5 °C, and the surface temperature of Mars is usually lower than that. But if the process makes a little toluene (C7H8) along with the benzene, that would increase the H2 needed only slightly (92 g toluene contains 8 g hydrogen, for a ratio of 11.5 g fuel per 1 g H), while lowering the freezing point of the mixture. Also, a catalyst that makes benzene from C + H2 might also make closely related compounds like toluene.

*A CHEM 1202 student could learn the following by looking up published heats of combustion: ΔHc (enthalpy of combustion) for a hydrocarbon CmH2n is approximately equal to m × ΔHc(C) + n × ΔHc(H2). In other words, ΔHc is not very sensitive to the particular hydrocarbon chosen. So, for example, you could use acetylene instead of benzene, but the increase in ΔHc per gram wouldn’t be very big, and acetylene is a gas and is much more difficult to handle safely than benzene.

2. (pp. 24-26) How is basic chemistry related to Mark’s plans for survival?

Andy Weir discusses a LOT of chemistry in this section of the story, but he doesn't do a great job. You can do better.

As the story opens, Mark Watney has been left behind by his Ares 3 crewmates on Mars, because they think he is dead. He recovers from his injury, but realizes that no one knows he is still alive. And he can’t tell anyone, because his communication dish was blown away in the storm.

He faces a crucial test. He assumes that no help could possibly come until the next scheduled NASA Mars mission, Ares 4. Can he survive on Mars until Ares 4 arrives? To stay alive, he must grow a crop that will give him the calories he needs. (He has enough proteins, vitamins, etc.; his biggest need is enough calories to stay alive for several years, most likely from carbohydrates, or carbs.)

Potatoes seem like the best way to produce carbs quickly. But to grow enough of them, he has to convert the entire indoor area of the Hab to potato farming. It turns out that the “limiting reagent” in the potato project is water. There's almost no water on Mars, so he must make it himself. How can he make enough water to support the potato plants?

The “Hab”, Watney’s pressurized environment on Mars, has an “oxygenator” that will split CO2 into C and O2, and a recycling system that captures water vapor and turns it back to H2O. But this isn’t enough water, and he needs that water for himself. How can he make more water on the surface of Mars?

If he had a source of hydrogen (H2), he could combine it with O2 to make more H2O than is already in the Hab, and this could be fed to his potato plants. But this was the problem with the MAV making rocket fuel, as described in problem 1 above: all the H needed must be brought from Earth.

The best source of H for Mark Watney’s potato project, it turns out, is the left-over hydrazine (N2H4) from the Mars descent & ascent modules (MDV and MAV). How would this work? Hydrazine decomposes catalytically to its elements in the presence of Ir; then the H2 can be combined with O2 to make water.

N2H4 → N2 + 2H2

2H2 + O2 → 2H2O

A CHEM 1201 student could easily work out how much hydrazine is needed. However, that isn’t how Mark Watney tries to solve the problem. Here are seven mini-questions (a, b, c, ..., g) based on chemistry discussed in this part of the book.

On p. 24, “50 liters of oxygen [liquid O2] makes 100 liters of molecules that have only one O atom each.” Does it? (a) How much water can you make from 50 L of liquid O2? (To answer this, you will need to look up some properties of the elements and compounds involved.)

On the next page, he discusses how much O2 he will need. The MAV (Mars Ascent Vehicle) collects CO2 from the Martian atmosphere, at a rate of 0.5 liter of liquid CO2 per hour. This works out to about 12.5 L of liquid CO2 in 1 Sol (1 Martian day, slightly longer than an Earth day). (Side point: presumably the MAV does this by using solar energy. But it could only do this during the day, so 0.5 L of CO2 per hour wouldn't work out to 12.5 L per day. Maybe more like 5 L per day. But let's assume his arithmetic is right.)

p. 25: “After 10 Sols, it’ll have made 125 liters of CO2 [liquid], which will make 125 liters of O2 after I feed it through the oxygenator.” (b) If 125 L of liquid CO2 is completely converted to liquid O2, how much will it make?

Then, “That’s enough to make 250 L of water.”  (c) How much liquid H2O can you make from the O2 produced in part (b)?

On p. 26, he moves on to the problem of getting hydrogen to combine with the new O2 to make water. (The problem is NOT how to get them to combine. The problem is how to get them to combine slowly, so they don't blow up the Hab and everything else!) “Each molecule of hydrazine [N2H4] has four hydrogen atoms in it. So each liter of hydrazine has enough hydrogen for two liters of water.” (d) Does it? How much liquid water can you make from 1 L of hydrazine?

(e) In solving the above four problems, you have shown that Andy Weir’s logic is wrong. How would you summarize his logic, and is it close to reality (in your opinion) even if it is the wrong way to calculate amounts of reactants & products (stoichiometry)?

Later on p. 26, he writes, “I’ll spare you the chemistry, but the end result is that five molecules of hydrazine become five molecules of harmless N2 and ten molecules of lovely H2.” (f) What’s a simpler way to write an equation for this reaction? And isn’t it a shame that people assume chemistry is incomprehensible, when it is often easy and fun?

Chemists might be getting used to abuse (getting Weiry, you could say) at this point. But here’s more. On pp. 26-27, he writes, “During this process, it goes through an intermediate step of being ammonia. Chemistry, being the sloppy bitch that it is, ensures there’ll be some ammonia that doesn’t react with the hydrazine, so it’ll just stay ammonia.” (g) Suppose the mechanism for the catalytic decomposition of hydrazine involves ammonia (NH3) as an intermediate. If so, could it also involve reaction of ammonia with hydrazine? What could be the product(s) of such a reaction? (For this problem, you will need to speculate. Consider possible reactions, but make sure they are balanced!)

3. (pp. 164-165) An ideal-gas problem
Here, he says the amount of air needed to increase the pressure in a 1 m3 airlock by 0.2 atm is 285 g. Again, he says he will spare us the math, so you get to do it. How much air is needed to do this?

This is a great question that is also suitable for a CHEM 1201 student.

Spoiler: The number of moles needed (1 cubic meter = 1000 L) at room temperature on Earth (~300 K, or 27 °C), is

n = PV/RT = 0.2 atm × 1000 L / ((0.082057 L atm / mol K) × 300 K) = 8.12 mol.

This is equivalent to 227 g (N2) or 260 g (O2). If it’s 79% N2 and 21% O2 (approximately the composition of Earth's air), it would be ~234 g.

These aren't very close to 285 g. Have we made an incorrect assumption? Maybe the temperature isn’t 27 °C (300 K). If it’s 0 °C instead (273 K, closer to the average temperature on Mars),

n = 0.2 atm × 1000 L / ((0.082057 L atm / mol K) × 273 K) = 8.93 mol.

8.93 mol of O2 is 286 g. So that’s pretty good! But is he using pure O2? No; it looks like a mixture of N2 and O2.

4. (p. 233) An electric vehicle problem
On Mark Watney’s trip to the Mars lander "Pathfinder", his rover traveled 80 km on 18 kWh of energy. How does that compare to modern electric vehicles? Pick your favorite EV, look up its performance in terms of energy used per distance traveled, and compare. What is different about a vehicle traveling on Mars vs. Earth? Would these differences affect the energy efficiency of an EV?