BackBreathe in the air, the higher you fly
Twenty per cent of the Earth’s air is oxygen, but only 0.16% of the air on Mars is
PremiumIt’s a good bet you haven’t paid a lot of attention to the density of air. At sea level, it’s about 1.2 kg/cubic metre, meaning a cubic metre of air weighs about 1.2 kg. You’ve probably seen references to how air gets “thinner" or less dense at altitudes. This is why travellers to Ladakh, for example, are advised to take a day or two to “acclimatize" there. The thinner the air, the harder it is to take in enough oxygen to function normally.
Of course, this is even more of a factor for climbers trying to summit Mt. Everest. For at the peak of that mountain, a cubic metre of air weighs only about 400 g—about a third of the sea-level figure. Not only must climbers take the time to acclimatize as they get ever higher up the mountain, but they also often carry oxygen along to help them breathe at those heights.
To understand all this, start with how much air you take in as you breathe: about eight litres a minute. At sea level, that much air weighs about 10 g. Twenty percent of that is oxygen, or about 2 g. So just normal functioning needs 2 g of oxygen per minute of normal breathing. At the top of Everest, you’ll need three minutes, or much deeper breathing, to get that much oxygen into you. Hard work. No wonder most climbers carry tanks of oxygen as they near the top.
Imagine, then, reaching a place where the air around you is so thin that a cubic metre weighs just 18 g. Not over a kg, not 400 g, but a measly 18 g. If you’re still on Earth, the 8 litres you take in each minute will weigh just 0.15 g, of which 0.03 g is oxygen. How hard will you have to work to get your regular 2 g/minute dose of oxygen? No amount of acclimatization will help you survive in the air that thin, even if the air was pure oxygen. You’d have to carry and breathe from oxygen tanks all the time.
There is actually a place where the air density is this low, though it’s nowhere on Earth. This is Mars, and if the thinness of its air wasn’t obstacle enough, there’s an added complication. Twenty per cent of the Earth’s air is oxygen, but only 0.16% of the air on Mars is. So for each minute of breathing on Mars, you will inhale only 0.00024 g—0.24 microgram—of oxygen. The truth is that you’ll be inhaling almost pure carbon dioxide because that’s what makes up 95% of Martian air.
The truth is that you’ll quickly be dead unless you strap on those oxygen tanks you’ve lugged from Earth.
Then again, there’s an instrument called MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment) that travelled to Mars with Nasa’s Perseverance Rover that landed there earlier this year. The team that designed this toaster-sized device describe it as an “electrical tree", which is pretty accurate. Trees on Earth absorb carbon dioxide and release oxygen, and that is exactly what MOXIE is designed to do. Remember that carbon dioxide is made up of carbon and oxygen (CO2). MOXIE “breathes in" carbon dioxide and breaks the molecules into those components, thus producing oxygen. But if that’s just the theory, last April MOXIE went to work and actually produced about 10 g of oxygen in an hour. Just five minutes worth for your average human, sure, but real breathable oxygen all the same. And in doing so, MOXIE showed that future humans on Mars won’t need to carry huge amounts of oxygen from Earth. That would be a nearly impossible job anyway. MOXIE, or really a MOXIE on a larger scale, can produce oxygen for them.
Still, the main reason to produce oxygen there, at least with the first several arrivals on Mars and to allow any long-term human presence on the planet, is not so much because human visitors need to breathe. Even with 100 women and men on Mars, you’d need less than 300 kg of oxygen per day for all their breathing needs. In contrast, to lift a rocket off Mars and carry some of those humans back to Earth—a voyage that will certainly happen over and over again—will need 25,000 kg of oxygen.
There’s no way to carry that much oxygen to Mars. Instead, the scaled-up MOXIE will have to be transported there many months before the first humans arrive and put to work. Even if this hyper-MOXIE can generate a kg of oxygen an hour instead of 10 g, it will still take nearly three years to produce enough oxygen for one rocket lift-off. Then, of course, there’s the need for facilities to store all that oxygen, something else that will need to be transported to Mars. All this, just to give you a sense of what sending humans to Mars will entail. And yet, this column was really prompted not by humans’ Martian needs but by the problems thin air poses to something already on Mars.
I’m referring—you may have guessed—to Ingenuity, a tiny helicopter that also travelled to Mars with Perseverance. But, like MOXIE, this was really—as I wrote in a column on Ingenuity (bit.ly/2XKmdKA)—“just an experiment, designed to answer the question: is flight possible at all on Mars?"
Why was this a question? Precisely because the air on Mars is so thin. Is there enough so that Ingenuity’s blades can lift the chopper off the surface? That was answered in April when Ingenuity flew for half a minute. Since then, it has flown a dozen more times, spending nearly three minutes aloft on some of those flights. It has got as high as 12 m off the ground, and on its ninth flight, in July, it flew over 600 m horizontally.
Yet its 14th flight, which will happen any day now, will be more like a test. This is because the air density on Mars, with the changing of “seasons" there, is falling. Nasa estimates that “in the coming months, we may see densities as low as 0.012 kg per cubic metre during the afternoon hours that are preferable for flight" (go.nasa.gov/2W8VuXc)—meaning, a cubic metre of air will weigh just 12 g. Not 18. Can Ingenuity handle that drop?
The thinning air will need Ingenuity to spin its blades even faster than it has done till now: 2,800 rpm compared to about 2,500 rpm used so far. Would that be possible? Can the little helicopter sustain a rotor speed that high? There are various possible side effects to consider, some potentially damaging to Ingenuity’s delicate mechanisms. Still, on 17 September, Nasa confirmed that Ingenuity did a rotor spin test at 2,800 rpm (bit.ly/3CGTcOK), without taking flight. Thus we know the blades can spin that fast. So Ingenuity is ready to attempt flight again. Though when it happens, the 14th flight will operate at 2,700 rpm, to leave a buffer in case it’s needed at some point. It will be a brief flight—climb to 5 m, fly a short distance sideways and land.
That much will prove that Ingenuity remains able to fly, even as the air on Mars gets still thinner. But it’s probably a good thing Ingenuity doesn’t need any oxygen.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun
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