Stranded on Mars: The science behind The Martian

Maybe you’ve heard about The Martian. Blockbuster movie, came out in 2015, and features Matt Damon’s unclothed behind? Kinda hard to miss!

I have to admit, though, I never saw this movie in theaters. I love space movies and survival stories, so I was excited to watch this one. What was the problem, one might ask? A small inability to keep my mouth shut: I’m notorious for loudly remarking, even in the middle of crowded movie theaters, whenever the laws of physics are being flouted.

So when The Martian – a movie about a biologist leveraging his knowledge of science to survive on a hostile planet – was in theatres, I didn’t go see it. Hollywood’s track record in scientific accuracy is only less impressive than my track record in keeping my mouth shut. But! Snowed in, with no one to complain but my boyfriend, and a DVD of The Martian in hand, I was ready. Good science, bad science, or just plain ugly science? To the best of my knowledge, here’s how Hollywood did.

The Catastrophe

Let’s start with the most immediate problem. Mark Watney is stranded on Mars, completely alone, when his spacesuit is punctured by a broken antenna after a freak accident during an emergency takeoff. Unlike good ol’ Earth, Mars’ atmosphere is thin and mostly composed of carbon dioxide. Less than one percent is oxygen – compared to 21% on Earth [i]. Because the atmospheric pressure on Mars is so low, any oxygen in your spacesuit will quickly escape if that suit is punctured. “Immediate danger of suffocation” should be high on Watney’s priority list! In fact, he’s only saved because not all of his oxygen reserves have escaped by the time he wakes.

However, at this point alarm bells should be ringing in your head. The broken antenna was blown off the space base, known as the ‘Hab’, in a huge sandstorm. “Sure, that sort of thing happens in big storms on Earth all the time,” you might think. But wind is just the motion of air particles, and anyone recalling basic physics can guess that in an atmosphere only a fraction as dense as Earth’s, the motion of some very far-apart air particles is not going to exert very much force [ii]. In fact, the shear stress exerted on the ground by wind is given by:

shear stress = r * u²

where r is the atmospheric density and u is a height-normalized wind velocity [iii]. This means that on Mars (where the atmospheric pressure is about 1% that of Earth’s) you’d need a wind ten times faster than one on Earth to achieve the same wind force and, therefore, the same damage.

On Earth, winds of 55+ mph are necessary to result in “considerable structural damage” like that seen in The Martian [iv]. So we’d need winds of 550+ mph on Mars to achieve the same effect – and the strongest winds on Mars top out at 60 mph, according to NASA[v]. So unless someone attached that antenna to the Hab with scotch tape, there’s no way the winds of a Martian sandstorm were blowing it anywhere – let alone straight through Mark Watney’s spacesuit.

Don’t forget, the crew’s panicked evacuation was sparked by that heavy spaceship beginning to topple over in the storm. In a real, puny Martian wind, nothing doing: the mission would’ve been just fine, with no evacuations and no stabbings.

Oxygen

Luckily, I kept watching after this heresy, because the rest of the movie is pretty interesting. In most survival stories, the difference between life and death comes down to water, shelter, and food. Mark Watney has even more to worry about: the danger of a quick death by catastrophic breach of the Hab, or a slow death by starvation, with no possibility of rescue from Earth before his food runs out.

As long as his shelter holds up, Watney should have enough oxygen: the Hab is supplied with an “oxygenator”. The movie doesn’t exactly go into the chemistry, but I’ll make some educated guesses. The oxygen we breathe is eventually exhaled as carbon dioxide. Theoretically, the bonds in carbon dioxide can be broken and reformed, recovering that precious oxygen and leaving carbon as a byproduct. The only downside is that this takes energy – probably a lot of it. Catalysts (which provide a lower-energy pathway for a reaction) are being developed for the conversion of CO₂ to O₂, so maybe Watney’s oxygenator employs one [vi]. One thing’s for sure, that oxygenator better not break!

Food

Worse than any desert island, Mars is a desert planet: there’s no (available) water and certainly no convenient coconut trees. Watney has the fancy astronaut equivalent of PowerBars, but the amount supplied for their mission – even feeding one person instead of six – isn’t nearly enough to last him until help can potentially arrive. Luckily, NASA provided the mission with a special Thanksgiving dinner, including a dozen real, fresh potatoes. Watney takes Martian soil and mixes it with the freeze-dried poop they’ve been stashing outside as fertilizer, to add essential nutrients (since Martian “soil” is basically very fine sand) [vii]. What isn’t fully explained in the movie, however, is that Watney also needed to add some “fresh” poop to make viable soil, too.

Bacteria play an essential role in making soil suitable for plant life, particularly in nitrogen fixation, in which nitrogen in the form of ammonia (commonly found in fertilizer) is converted into nitrites and nitrates (which plants can use) [viii].  These bacteria are found in soil on Earth; however, the human gut can contain some strains of nitrogen-fixing bacteria, too [ix]. Most bacteria would be killed by freeze-drying and extreme cold, so Watney had better hope that his bowel movements can keep up with his farming endeavors.

Even with all this fresh poop, it isn’t clear that Watney’s soil trick would have worked. Take a look at the ingredients list on Miracle-Gro: it contains not only nitrogen in various forms, but also lots of phosphorus and trace amounts of boron, copper, iron, manganese, molybdenum, and zinc. Phosphorus is another tricky nutrient to deliver to plants, requiring microbes to solubilize it in the right chemical form [x]. Plus, plants on Earth have evolved symbiotic relationships with bacteria living in soil, allowing beneficial bacteria to form colonies in their roots [xi]. Who’s to say that one man’s poop could successfully recreate an intricate ecosystem? Hollywood, apparently.

Water

Watney’s desert island also has a major deficit of water. While the astronauts bring some with them, plants – especially on the scale Watney is proposing – need a lot more than he can spare. His solution? Some chemistry that made the hair on the back of my neck stand up.

Watney has one raw material laying around that he’s not likely to need anytime soon: the rocket fuel hydrazine. Hydrazine (N₂H₄, a liquid at room temperature) decomposes into molecular, gaseous nitrogen and hydrogen in the presence of a catalyst [xii]. Watney burns this hydrogen in the presence of oxygen to get water. Sounds simple, right? On his first try at making water, Watney manages to cause a minor explosion. Since he’s walking around in the Hab without a breathing mask on, there’s probably quite a high percentage of oxygen in the air, allowing the reaction of hydrogen and oxygen to proceed too quickly. After this, Watney makes some “adjustments” (presumably controlling the oxygen level more carefully) and starts it up again.

Setting aside the fact that Watney handles hydrazine pretty cavalierly given its toxicity, and the obvious issues with controlling an exothermic, explosive reaction, we’re still left with the heat from those two exothermic reactions. The decomposition of hydrazine in particular produces so much energy that it must be done in a special chamber that can withstand a rise from room temperature to 800 degrees Celsius, not to mention a doubling in pressure, in a few milliseconds [xiii]. In the well-insulated Hab, the heat produced from creating a few hundred liters of water in a few short weeks isn’t going to dissipate quickly, and it isn’t clear why Watney and his potatoes aren’t roasting where they stand.

Shelter Breach

Ok, this one is more of a continuity error than anything else, I’ll admit. About halfway through the movie, the airlock of the Hab explodes, destroying all of Watney’s careful bacteria cultivation and killing his potatoes. Bad news for Watney. However, he then fixes the hole in the Hab with what appears to be a tarp and duct tape. This huge hole in the side of his only shelter on an uninhabitable planet with storms devastating enough to knock over spacecraft and impale people with flying antennae (according to Hollywood), and he fixes it with duct tape and a glorified plastic bag? I really don’t think that’s the move, dude.

Plus, the whole thing then sits there flapping around as it’s buffeted by the winds. We know the air pressure in the Hab has got to be close to Earth levels, and that the pressure outside is much, much less than that – so that thing should be constantly sucked out, like a plastic bag caught in the universe’s largest vacuum cleaner. But I guess it was more dramatic to have it flapping in the weak Martian wind.

Grade

I didn’t yell at the screen as much as I usually do during a science fiction movie, so The Martian definitely gets at least a passing grade! Sure, a lot of Watney’s plans were far-fetched, but they are based on real science and not mumbo-jumbo [xiv]. Anyone who was actually stranded on Mars in these circumstances would have to depend on a good deal of luck, too. Not to mention how refreshing it is to see a character rely on science and ingenuity to survive, instead of kicking and punching his way out of trouble.

So watch and enjoy – but just remember, if anyone tells you to shut up and watch, don’t listen! You’re just fact-checking for science.

By Kristina Vrouwenvelder (@kvrouwen)

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[i] The Atmosphere of Mars – How Mars Works | HowStuffWorks

[ii] Karl Hille, ‘The Fact and Fiction of Martian Dust Storms’, NASA, 2015

[iii] H. Schlichting and K. Gersten, Boundary-Layer Theory, 8th edn (Springer, 1999).

[iv] ‘Beaufort Scale’, Wikipedia, 2018

[v] Karl Hille, ‘The Fact and Fiction of Martian Dust Storms’, NASA, 2015 

[vi] ‘Splitting Carbon Dioxide Using Low-Cost Catalyst Materials’, ScienceDaily 

[vii] ‘Martian Soil’, Wikipedia, 2018

[viii] ‘Role of Soil Bacteria’

[ix] Katsura Igai and others, ‘Nitrogen Fixation and NifH Diversity in Human Gut Microbiota’, Scientific Reports, 6 (2016), 31942

[x] ‘Phosphate Solubilizing Bacteria’, Wikipedia, 2017 

[xi] S. E. Gould and S. E. Gould, ‘Underground Communities: The Plant Roots That Collect Bacteria’, Scientific American Blog Network

[xii] ‘Hydrazine’, Wikipedia, 2018

[xiii] R. Vieira and others, ‘New Carbon Nanofiber/Graphite Felt Composite for Use as a Catalyst Support for Hydrazine Catalytic Decomposition’, Chemical Communications, 0.9 (2002), 954–55

[xiv] The author of the eponymous novel, Andy Weir, is a computer scientist who wrote the story as an homage to science, endeavoring to make it as accurate as possible. It was initially published on his website and he accepted corrections from his readership – a kind of peer-review process.