How to Science Your Way Off Mars, Ch. 3 excerpt (Patreon)

Because I'm not posting everything here- that's for those who contributed to the Maretian Kickstarter- but I want you all to see that I am actually writing things.

The book, so far, is at 34,200 words, give or take. I'm pushing ahead as fast as I can.

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Mars has an atmosphere- just about.

That fact is the only reason (aside from authorial fiat) that Cherry Berry was able to turn a head-on collision with a planet into a barely survivable crash landing.

On the descent, after the total disintegration of the mana batteries in the engine room, Cherry only had two partially charged spare mana batteries to feed power to the main engines- enough power, at most, for three seconds at full throttle. She used most of this at once to angle Amicitas away from Mars, or at least to one side. She also used the magical maneuvering thrusters for this purpose, though unlike the larger mana batteries connected to the main engines, the batteries for those were close enough to the crew that they got a trickle of recharge from the magic produced by life in Mark Watney’s universe.

All of this was enough- just- to give Amicitas an angle of attack on re-entry that was more horizontal than vertical. She continued using the maneuvering thrusters, and even the last dregs of the power in the emergency batteries, to maintain the ship’s attitude and to further stretch out the descent, trying to catch any bit of both lift and drag she could manage. She deployed the emergency parachutes, and then the emergency solar array, while still going far too fast, and air resistance ripped them all apart in seconds- but they did slow the ship down a bit, keep the nose up a bit, extend the flight just a bit, which was what Cherry wanted.

By the time Amicitas approached the ground, the ship was still going at speeds which, in Mars’s atmosphere, were supersonic- over three hundred meters per second. Any slower at this point, and the ship’s limited lifting-body shape would not have provided enough lift to slow the vertical rate of descent. By a miracle- the last of the thruster power gave out just before touchdown, and the nose was coming down of its own accord- when the ship made contact it was almost a perfect belly flop. Had the nose been up at that point, the initial impact of the rear section, which broke the ship’s keel and tore a huge hole in the engineering deck, would have caused the ship to cartwheel forwards and tear itself, and its passengers, to shreds.

By a second miracle, it put down on possibly the largest stretch of flat land on the entire planet- Acidalia Planitia, the southernmost extension of the great boreal lowlands which make up most of the northern third of Mars. Thus, while much of the lower part of the outer hull plating and both wingtips got ripped off during the several kilometers it took for the ship to skid to a stop, the nose hit nothing, the ship didn’t get launched off a hillock or canyon wall, and when the ship finally ceased to move, the flight deck and habitat deck were both still airtight.

And then a third thing- I don’t call it a miracle, because, after all, it’s the thing all of the stuff I just described was meant to facilitate- the ship came to a stop a mere ten kilometers away from the one spot on the planet where the crew wouldn’t freeze to death once the sun went down.

Which was a concern, because they landed in the middle of the winter in Mars’s northern hemisphere, and even at the relatively low latitude they landed (30 degrees N), the night temperatures would approach or possibly even surpass -100 Celsius- not survivable even in magic-powered space suits.

And in the daylight, which had not quite half the light of Earth or Equus, the ponies could expect unshielded ultraviolet light and cosmic radiation to be a major concern. And if anything happened to their spacesuits, it would be a race to see which killed them first, decompression or asphyxiation, with carbon dioxide poisoning a distant third place.

Welcome to Mars, and good luck.

Mars is the second most inhabitable world in our solar system after Earth, but that’s saying very little.

To begin with, Mars is only one-tenth Earth’s mass and only about two-thirds as dense. As a result, it has a surface gravity of 3.8 meters per second, or about 40% of Earth’s. Like Earth, it has a crust, a mantle, a liquid outer core and a solid iron inner core (at least, according to seismic readings taken by the Insight Mars lander). Unlike Earth, that outer and inner core don’t create a significant magnetic field, which pretty much leaves the whole planet exposed to solar and cosmic radiation. There are several theories, but the most fundamental one is that Mars, being much smaller than Earth, lost most of its internal heat faster, and having formed farther away from the sun, contains fewer radioactive heavy elements in its inner core than Earth does to create new heat, and thus not enough heat remains for the liquid outer core to undergo convection and thus produce the dynamo effect required for a strong magnetic field.

Because Mars has no magnetic field to hold off the solar wind, and because Mars has at best four-tenths Earth’s gravity, it has trouble holding on to an atmosphere. There are several mechanisms by which a planet can gradually lose air (Earth, for example, loses about nine tons of air each day, almost entirely hydrogen and helium), and Mars is a victim of every last one. The two biggest air thieves, however, are Jeans escape and solar wind ablation.

First, Jeans escape, named after Sir James Jeans (and not after trousers). To put it as simply as possible, heat and motion are pretty much the same thing. The hotter a molecule is, the faster it moves- and smaller molecules, or even individual atoms, move faster when heated than larger ones. If a molecule is small or hot enough, it can achieve escape velocity, or go fast enough and high enough that collisions with other molecules like itself will give it the last push needed to go flying off into the cosmos.

The conditions of gravity and temperature required to keep or release certain gases are pretty much linear with the surface gravity of the planet (or moon) in question. A planet with Earth’s atmospheric temperatures, but with twice the mass, would retain all its hydrogen and helium- and would begin accumulating more from the solar wind. Mars is a lot colder, but it also has a lot less mass, and thus hydrogen and helium also escape.

But Earth has a magnetic field, which protects the atmosphere (except for the parts right around the magnetic poles) from the effects of the solar wind. Mars has no such protection, so it gets a one-two punch from the solar wind. First, the ionized particles of the solar wind, when they hit Mars’s atmosphere, can break apart molecules into individual atoms. In particular this is what happens to water vapor in the upper Martian atmosphere; the solar wind hits it, breaking it up into hydrogen and oxygen. And while O2 would be heavy enough to be retained, an individual oxygen atom isn’t- and it flies off along with the hydrogen.

And then there’s spalling, where the solar wind particles hit an atom or molecule and don’t disrupt it; instead, it acts like a cue ball hitting a bank shot, sending both the solar wind particle and the air particle off into space on escape trajectories.This can happen even to molecules which would otherwise be heavy enough for Mars to hold on to them.

There’s one other factor to consider: Earth’s high gravity hugs its atmosphere relatively close to its surface. Mars, with lower gravity, has a much more diffuse atmosphere that extends higher, making it easier for the solar wind to interact with it.

Add all of this together, and Mars loses air at about twice the rate that Earth does- despite Earth having about two hundred times as much air to lose.

For what it’s worth (not much), about 95% of Mars’s atmosphere is carbon dioxide (the remainder being nitrogen, argon, and trace gases like water vapor). That means there is some greenhouse effect on Mars, but not much (because although Mars has a lot more free carbon dioxide than Earth does, it’s not dense enough to really hold the heat in). It also has a few other effects. For example, although Mars’s air pressure (being strictly a function of mass) is about 0.6% that of Earth’s, its air density is 2% that of Earth’s (because carbon dioxide is a bigger molecule, and thus more reluctant to move or be moved). The speed of sound on Mars is roughly 260 meters per second, as opposed to the roughly 360 meters per second it is on Earth.

The average air pressure on Mars is 610 Pascals. As a point of comparison- the triple point of water- the conditions in which water can exist in solid, liquid and gas at the same time- is 0 degrees Celisus at 611 Pascals. At less than 611 Pa, water can’t be liquid- only solid or gas- and if the air pressure is just above 611 Pa, then liquid water can only exist in an extremely narrow band of temperatures above 0 degrees C. The highest recorded air pressure on Mars, in Hellas crater (the lowest point on the planet, the fifth largest impact crater currently known in the solar system), registered about 1200 Pa, which would allow any liquid water there to boil at only about 20 degrees C. 

Of course, it isn’t often that any point on Mars manages to get a temperature above zero in the first place. 

At its closest approach to the Sun (about 206 million kilometers, as opposed to Earth’s average 150 million kilometer orbit), Mars gets only about half the solar energy the Earth does- about 700 watts per square meter instead of 1400. But unlike Earth, Mars has a significantly elliptical orbit which waxes and wanes over millennia. Right now it’s approaching its maximum eccentricity, where it brushes against the inner edge of the asteroid belt- 250 million kilometers- at the outermost point in its orbit.(That’s a difference of 44 million kilometers, compared to Earth’s five.) Out there the solar energy it gets is closer to 560 watts per square meter. 

All of this- thin atmosphere, distance from the sun- means it gets cold on Mars. How cold? Mars has seasons- its axis is tilted in respect to the sun slightly more than Earth’s is. It has white caps of water ice on each pole. But in each winter that hemisphere’s ice cap swells so massively it can be seen from Earth with hand-held telescopes, and then when spring comes again it shrinks. That growth and shrinkage is caused by Mars’s carbon-dioxide atmosphere getting so cold that it precipitates- that is, it snows dry ice on Mars in the wintertime. That requires sustained temperatures of under -78 degrees C- and in the dark winters at Mars’s poles, it gets below -150 at times, allowing as much as one third of Mars’s entire atmosphere to form a layer of carbon dioxide up to two meters thick.
On Mars’s equator temperatures are generally warmer, but with massive swings between day and night. At Gale Crater, home of the Curiosity rover, the difference between day and night averages roughly seventy degrees Celsius. To put it in terms you could understand, this is like waking up at dawn to frost on the ground, and then by early afternoon being unable to go outside because the temperature is literally hot enough to cook with. In Martian summertime the high temperatures at Gale hit or exceed 0 on a fairly regular basis in temperate zones, but in the wintertime they drop well below the freezing point of carbon dioxide. 

And this is the second most habitable world in our solar system after Earth- because it has a solid surface, some air, a surface temperature which isn’t lethal all the time, and even some water in the form of subsurface ice. That said, without a space suit you’d die of asphyxiation and decompression sickness in a couple of minutes tops. The high concentrations of carbon dioxide wouldn’t even have the chance to poison you.The lethal cold and the long-term problem of unmitigated space radiation are just icing on the cake.

Better get yourself a surface habitat, pronto.