The poop contained bacteria, which multiplied once it was mixed with the poop and Martian soil. That’s why he couldn’t grow any more after the depressurization – he still had potatoes he could plant, but all his bacteria died. That’s what I understood, anyway.
The plot you describe above is essentially that of the 2000 Mission to Mars, which is universally regarded as being a pretty terrible film. No one expects any form of extant macroscopic life on Mars, and while looking for signs of life is some part of all planetary exploration missions, it largely consists of looking for waste products or other indicators rather than visible organisms themselves owing to the fact that no other planet or moon in the solar system has a surface that would be amenable to even the most hardy organisms we have observed on Earth. If there is life on Mars or other bodies, it is almost certainly buried below the surface where it would be protected against ultraviolet radiation, underneath an icy surface on Europa or Enceladus, or swimming near thermal vents in Titan’s frigid lakes of liquid hydrocarbons.
The crews’ feces provide biolavailable elements (nitrogen, phosphorus, sulfur, potassium, calcium, magnesium) in addition to the hydrogen and oxygen from water and carbon from the atmosphere, as well as providing a medium to sustain the plants during growth. Potatoes are a hardy crop that require little in the way of excess nutrients. Unfortunately, they are also carbohydrate-rich but poor in proteins and many nutrients (though they are an excellent source of Vitamin C and potassium, and a decent source of fiber if you consume the skin); hence why most cultures for whom Solanium tuberosum is a staple crop consume it along with butterfat, cheese, or some other rich source of proteins, lipids, and B- and D-complex vitamins that the human body cannot synthesize. Although never explicitly stated, the loss of strength and splotchiness of Watney later in the film was an indication of the malnutrition he was suffering by subsisting on only a diet of potatoes.
Stranger
And while he could make more fertile dirt from his own poop, he didn’t have the time to let it ferment and then grow the plants from scratch again.
The book makes clear that he had plenty of multivitamins with him, which means he didn’t really have to worry much about nutrition - just calories.
He was so weak at the end because he was starving, not because he was malnourished.
In the book, Watney had an entire crew’s worth of daily vitamins pills, which he ate with the potatoes and helped stave off the effects of malnutrition. I think he eventually ran out of them, though.
As I have not read the book I can’t comment on what was presented, but a multivitamin is not a panacea for nutrition. Many constituents of multivitamins have limited bioavailability, and without essential amino acids and fatty acids not available in potatoes progressive malnutrition will occur.
Stranger
The vitamins in the book were of the same technology that provided the UV-blocking material in the Hab canvas and Watney’s EVA suit. In other words, it was a bit of a stretch that took some liberties with how vitamins actually work.
Of course, the human body can synthesize Vitamin D just fine provided you can, you know, expose skin to sunlight. Doing so contraindicated on Mars.
I understood the reverse from the book: there was no problem creating more bacteria – his gut basically outputs an endless supply of it – but the potato plants FROZE (and thus DIED) when the habitat was ripped open. And you need LIVING plant cells to start growing a plant.
Okay, so we’ve established I’m not the greatest botanist on the planet. It’s possible I’m not the greatest botanist in the room and I’m alone in this room. But I now know that my earlier belief on how plants grow is wrong. The potato plants we saw in the movie would have been composed largely of native Martian material with only a touch of imported Earth poop.
So follow-up question: how long could Watney have kept the growing cycle going? I’m talking about the cycle of growing potato plants, eating the potatoes, and then using the rest of the plants and his own poop to fertilize the next crop of plants. Was it a perpetual system? Or was he losing important elements in each cycle?
You could be right. But I was thinking that the original potatoes he planted had been frozen in the first place to preserve them during the trip to Mars.
Isn’t there a rule about Earth bacteria and Martian water?
He’s not that worried about nutrients and protein as he’s got the meal kits that he plans to ration. The potatoes are primarily a source of calories. But he wouldn’t be able to feed himself indefinitely. He’s run out of supplements and protein eventually but more to the point, his farm just isn’t big enough. The book touches on this…
My best bet for making calories is potatoes. They grow prolifically and have a reasonable caloric content (770 calories per kilogram). I’m pretty sure the ones I have will germinate. Problem is I can’t grow enough of them. In 62 square meters, I could grow maybe 150 kilograms of potatoes in 400 days (the time I have before running out of food). That’s a grand total of 115,500 calories, a sustainable average of 288 calories per day. With my height and weight, if I’m willing to starve a little, I need 1500 calories per day. Not even close.
So I can’t just live off the land forever. But I can extend my life. The potatoes will last me 76 days.
Potatoes grow continually, so in those 76 days, I can grow another 22,000 calories of potatoes, which will tide me over for another 15 days. After that, it’s kind of pointless to continue the trend. All told it buys me about 90 days.
So now I’ll start starving to death on Sol 490 instead of Sol 400. It’s progress, but any hope of survival rests on me surviving until Sol 1412, when Ares 4 will land.
There’s about a thousand days of food I don’t have. And I don’t have a plan for how to get it.
Watney would be losing essential nutrients each time around, but provided he was using his excrement and urine to directly fertilize the plants he could probably continue the cycle indefinitely, at least long enough to be resupplied or rescued. However, he’d still have to content with progressive malnutrition from the vitamins and fatty acids not being produced by his crop. I was also going to say that you wouldn’t get a good variety of the essential amino acids but upon review it looks like potatoes have a complete complement of the necessary amounts of amino acids…provided, again, that you eat the skin. Knorf is also correct that people can synthesize D-complex vitamins with exposure to sunlight, which is the primary source in terrestrial habitation; I had originally just written B-complex and then added D-complex vitamins without expanding on it, but this is a consideration for space travel and habitation where UVB exposure may be unavailable.
Although this was clearly introduced as a means of having Watney “science the shit out of” another problem, it would have made far more sense that he already had at least a sample crop of a wide array of vegetables and pulses along with in-situ manufacture of fertilizers and irrigation (and a means of removing perchlorates from the Martian soil) as part of an ongoing experiment to evaluate the feasibility and issues with growing foot for future longer term missions. It would have obviates the need to jury-rig a water maker out of toxic propellants and would be a plausible explanation for having enough food for one astronaut to survive autonomously for a couple of years.
Stranger
But this is more interesting.
If that was a real NASA mission, would they have included more food just for this type disaster scenario? It seems like they should include enough food for the whole crew to last until the next ship arrives. As we saw in the beginning of the movie, their escape rocket was almost destroyed. If the whole crew was stranded, there’s no way to make the food last long enough.
I’m sure more food would add more weight, but it doesn’t seem like it would be out of line with the weight of all the equipment at their base.
A calorie-dense food like a protein bar has about 4000-4500 calories/kg. If you go by Watney’s numbers of 1412 sols for 6 people living at near-starvation levels of 1500 calories/sol, that works out to roughly 3000 kg of food. That’s a lot of weight for not much benefit. You still have the problem of how to pick up the astronauts, or how to move them to the next landing site. That was a hugely difficult task with only one astronaut, and becomes essentially impossible with six.
If/when we actually send people to Mars, the contingency plan for what to do if their “MAV” fails will be involve funeral arrangements, not rescue.
If the ascent vehicle were destroyed in flight, it is unlikely that there would be a crew to sustain. As for the storm portrayed in the beginning of The Martian was not realistic; while Mars does have occasional wind storms with speeds up to 100 km/h, the atmosphere is so thin that the dynamic pressure is about the equivalent of a gentle breeze. The greater threat during a storm would be loss of communication, dust ingress, and no ability to collect solar energy for power.
Sending a crewed mission to Mars has a number of substantial challenges including limitations to how much mass can be delivered to the surface; as Mars has a very thin but not negligible atmosphere, it ends up being the most difficult solid body to land a significant amount of payload upon; there is just enough atmosphere to produce substantial dynamic heating, but not enough for effective low speed drag using conventional parachute decelerators for payloads over one metric ton. The recent testing of an advanced deceleration system intended for high mass Martian probes and eventually crewed missions, the Low-Density Supersonic Decelerator, failed twice in testing in the last couple of years. Even when successful, it will probably only scale up to support a payload of a few tens of tons before being of unwieldy size and mass. Until there is an in-situ production and manufacturing infrastructure in space and/or revolutionary advances in propulsion technology, the practical and fiscal limits of the amount of payload that could be sent to Mars will be a significant constraint, and sending hundreds of tons of food to sustain a crew for the couple of years or more between mission opportunities is probably not feasible.
Stranger
It is even worse than that; if we sent a crewed mission to Mars reliant upon conventional propulsion technology and logistics, we probably need to make the system operate in a way that can be controlled by ground control and/or is autonomous so that if the crew is disabled or incapacitated that there is still a chance of abort and recovery. Stories like The Martian or the real life experience of Apollo 13 are inspiring, but they also are dependent upon contrivances (in the case of The Martian) and good luck (on Apollo 13) for the survival of the crew. There are many types of failures or debilitating conditions which no amount of courage, fortitude, or skill could overcome, and reducing the probability of such failures from occurring at an acceptable rate for a mission of more than two and a half years is probably not feasible.
Stranger
To be fair, a cargo of three tons of protein bars wouldn’t need much in the way of a landing. Just slam it into the surface, a safe distance from the base.