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I've watched The Martian and I've read the book, yet I still don't understand one thing.

If he ran out of horizontal space for crops, why didn't Mark Watney start digging downward?

In the book he used part of the Hab's floor canvas to make a "mars tent" for sleeping and working (which was dropped completely from original release and present as pretty big 100% Earth-made tent in directors cut), so he could make a "hatch" without losing the Hab completely. Digging downward through soil, rocks, etc would be a problem at first, but Mark is creative, he could make some kind of pickaxe. In a few month he would have an underground area for planting more potatoes (plus in the book he had peas and corn, yet he chose potatoes only... for some reason), which would give him way better chances of survival till Ares 4 (and a stronger hit, when the Hab actually blew up).

So, why didn't he dig?

P.S.: I'm not asking this on scientific communities of Stack Exchange, because, frankly, considering Martian radiation, very weak sunlight and pretty bad (although not completely unusable) soil, the book/movie is not really a hard science... Not hard enough, at least.

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    Why potatoes and not peas and/or corn? The biggest difference I can see between the food production is that peas and corn require pollination, tuber growth does not. Possibly also the amount of food that could be produced from original seed stock might factor in. You want more potatoes, you just mound more dirt to cover more of the stem. I assume they have artificial lighting in the hab, that could be focused on plants, and scientists have generally concurred that growing food, as depicted, is not unrealistic. – PoloHoleSet Mar 29 '17 at 15:43
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    I'm also not sure why you deem growing food, in general, to be unrealistic because of Martian sunlight, but growing food underground to be a possibility. – PoloHoleSet Mar 29 '17 at 15:45
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    "Not ideal" <> "unrealistic" – PoloHoleSet Mar 29 '17 at 15:55
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    @MrScapegrace: Actual botanists have taken a look at the crop thing and have said that it's feasible. We have actually grown plants using sterile soil mixed with manure (alas not human). So that part is hard science. The consensus is that the only part that's not hard science is the weather in the beginning. The rest that you dismiss as not hard science have been verified as hard science by actual scientists involved in the particular fields – slebetman Mar 29 '17 at 18:12
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    Have just been reading this book. He indeed says he chooses potatoes because they have the highest calorie content. He already has all the vitamins and protein he needs, he only needs calories. While variety might have been nice, it would have limited the days he would be able to live. – EvSunWoodard Mar 29 '17 at 20:44
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Because he would need to make an airtight seal between the walls of the Hab and the ground.

What is shown in the movie is that the Hab is a prefabricated 'tent', and Mark shifts a significant amount of Martian rock dust into that space through the airlock and makes soil that can support a potato by adding "biological materials" and water.

In this scenario he doesn't have to make this airtight, it is already. To dig down into the ground he would have to make a reliable seal between the Hab materials and dusty rocky ground, which would be extremely hard.

I'm also not entirely convinced that the extra area that he might be able to generate would be easy or even sustainable. For this to be worthwhile he would have to dig down and make a cave that extends beyond the border of the Hab - with all the physical work removing the material out of the Hab via an airlock, which would be backbreaking. Then he'd have to provide enough lighting and water. He has a limited amount of hydrazine to make water - which would be absorbed not only into his new planting soil, but also (uselessly) into the walls and roof of his cave.

His air appears to be a relatively replenishable resource from the the 'airmaker'/oxygenator that the story mentions, which is presumably solar powered. The water is not infinitely replenishable, and every time he goes outside to get rid of a bag of rock from his mining he dumps 1 airlock full of water vapor.

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    No, he never creates a seal between Hab material and dirt in the book. – sirjonsnow Mar 29 '17 at 14:58
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    @sirjonsnow .... I agree, it seems implausible to me that you could create a seal with a small amount of superglue. You might create one with almost literally gallons of it, but even then if the rock had any cracks you would have a slow (but perhaps manageable) leak. – iandotkelly Mar 29 '17 at 15:01
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    Aside from air considerations etc, digging and swinging a pick-axe consumes a massive amount of calories. Didn't read the book: was it a savings over digging to scrape/gather the regolith? – Yorik Mar 29 '17 at 17:28
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    @slebetman I'm guessing that he's thinking of when Mark makes the "Bedroom", cutting chunks out of the hab, and gluing the top to the "floor". Not realizing that the "floor" was the floor of the hab, rather than the surface of Mars. – Patrick M Mar 29 '17 at 18:41
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    @MrScapegrace go outside and try to glue a piece of canvas to some dirt to form an airtight seal and let me know how that works out for you... – Austin Philp Mar 30 '17 at 21:48
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The question on the potatoes was directly addressed in the book. They gave the most calories per area of farm land.

Couple other points where you appear to have miss interpreted/remembered some details. The hab is not sealed to the ground. When they got there they laid out the floor materials, staked them down, attached the canvas for the sides and top, and hooked up the other components. It is a free standing tent with a semi rigid floor and lots of accessories to keep the people in it alive and comfortable.

When Mark made his bedroom/working tent, he took a portion of the canvas from the hab top & sides, and the floors of the emergency tents from the rovers, and remanufactured things to get a larger single tent in place of the two smaller ones. There were several references in the book to the Hab ceiling being lower on one side after this because of the missing canvas.

Also the hab was providing more than just atmosphere containment for the farm. It also had light and heat. I am pretty sure there was a line in the book about the hab using UV lights for the health of the crew, which made them good grow lights for the plants. Trying to replicate any of those conditions outside of the controlled environment of the hab would have been enormously more difficult, both in terms of technical feasibility, and the level of effort expended.

Keep in mind that his primary issue was calorie deficiency. He had plenty of vitamins and supplements to last for years if he could find enough calories to keep his body running. Anything that involved additional physical exertion, without directly and immediately contributing to his survivability, would have made the problem worse.

  • I seem to recall him also having some kind of glue for the seams. But that wouldn't help much with the martian soil. – Wayne Werner Mar 31 '17 at 15:48
  • Correct. He had a glue for sealing the canvas material to itself, and/or patching his suit. – Rozwel Mar 31 '17 at 16:46

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