Did The Martian biologist eat his own shit?

Dec 25, 2015   #Richard Feynman  #Mars  #Biology  #Photosynthesis  #Carl Sagan 

In the movie “The Martian”, Matt Damon plays astronaut Mark Watney who gets stranded on Mars and makes history by practicing open defecation and growing potatoes using his own shit as manure on Martian soil.

From the looks of it, the potato plants turned his excreta into food. That makes for a good cinematic twist, but in reality the potatoes' bulk were made using nuclear energy.

Let me explain.

The purpose of mixing manure with soil to grow plants is to help retain moisture and to lower the pH of the soil to the comfort level of seeds and saplings. The Martian soil is considerably basic, with a pH of around 8.3 and the manure can help lower the pH into the potato’s comfort zone.1 How much can it possibly lower, that I’m not sure, but it is certainly in the right direction. In addition to these, the manure provides some nutrients - nitrogen, phosphorus and potassium (NPK).

But notice that the most important component of potatoes is not made of N, P or K. Where did the carbon in its carbohydrates come from? Watch Richard Feynman explain it.

So the mass of a plant comes from the air, as the plant breathes in carbon di-oxide and breathes out oxygen, using the sunlight to rip out the carbon from the gas. The atmospheric pressure on Mars is about 0.6% of earth’s sea level pressure, so it is pretty hard for all this carbon to come directly from the Martian atmosphere in the raw. If the plants get directly exposed to Martian atmosphere, their water content would boil off at regular temperatures.2

However, the Martian atmosphere is rich in CO2 by proportion (about 96%). So it would be conceivable to have the base station suck in Martian air, compress it to earthly pressures and enrich the oxygen in it in order to make it breathable for us humans. The job of the plants on Mars, however, is to eat this CO2, strip the C out and make food for us. While the ideal situation would be to make use of sunlight in the regular way to photosynthesize the plant material, in the movie, potatoes are grown indoors and so need to be artificially lit. This energy, of course, comes from arrays of solar cells, so eventually the energy being fed to the plants is indeed coming from the sun.

And there you have it. Energy from the Sun is channeled through solar cells and storage batteries into lighting3, and the plants use these photons to strip atmospheric carbon from compressed Martian atmosphere. The station system uses this energy, presumably to compress Martian air for the plants, as well as to enrich the O2 within the station to let Watney breathe.

The Sun is unlikely to be the sole energy source at the base station though and a small nuclear power plant might serve the same purpose. If this is indeed the case, I find the thought that Watney was literally feeding off the trapped energy of atoms in close proximity to his self supremely poetic. The Sun’s energy also has its source in nuclear fusion, but it is so far away and sunlight is so taken for granted on earth that we don’t pay much heed to it. Nevertheless, it is the same poetry there as well.

We’re not only made of starstuff4 as Carl Sagan pointed out, we’re also fed by them, near or far. Is there any greater poetry that can be written about us?


  1. Neutral pH is 7.0. Values lower than 7.0 indicate “acidic” nature and values greater than 7.0 indicate “basic” nature. Pure distilled water is neutral with a pH of 7.0. ↩︎

  2. This is called the Armstrong limit, which is about 6.3kPa, which is about 6% of Earth sea level pressure. The mean pressure on Mars is one tenth of that - at 0.6% of Earth sea level pressure. ↩︎

  3. Inefficient, perhaps, but probably worked for Watney. ↩︎

  4. “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.” - Carl Sagan, Cosmos. ↩︎