Every once in a while in my news feed I run across an article about colonizing other planets, Mars in particular. The most recent one was about an idea that might make it possible to raise the surface temperature by 10C (18F) in a matter of months. That would be enough to melt water in some places, which would be important to those of us who need liquid water to drink and to irrigate crops.
All you have to do is mine the right raw materials and synthesize about two Empire State Buildings worth of a particular form of aerosol particle, and blast it into the atmosphere. You'd have to keep doing this, at some rate, indefinitely since the particles will eventually settle out.
The authors of this idea don't claim that this would make Mars inhabitable, only that it would be a first step. This is fortunate, since there are a few other practical obstacles, even if the particle-blasting part could be made to work:
- The mean surface temperature of Mars is -47C (-53F) as opposed to 14C (52F) for Earth. The resulting -37C (-35F) would not exactly be balmy.
- Atmospheric pressure at the lowest point on Mars is around 14 mbar, compared to about 310 mbar at the top of Mount Everest. Even if the atmosphere of Mars were 100% oxygen, the partial pressure would still be around 20% of what it is atop Everest, and there's a reason they call that the Death Zone. In practice, you'd at least want some water vapor in the mix.
- But of course, the atmosphere on Mars is not 100% oxygen (and even if it were, it wouldn't be for long, since oxygen is highly reactive -- exactly why we need it to breathe). It's actually 0.1% oxygen. There is oxygen in the atmosphere, but it's locked up in carbon dioxide, which makes up about 95% of the atmosphere.
It's at least technically feasible to build small, sealed outposts on the surface of Mars with adequate oxygen and liquid water, at a temperature where people could walk around comfortably, using local materials. Terraforming the whole planet is Not ... Going ... To ... Happen.
But let's assume it does. Somehow, we figure out how to crack oxygen out of surface rocks (there's plenty of iron oxide around; again, there's carbon dioxide in the atmosphere, but nowhere near enough of it) and pump it into the atmosphere at a truly massive scale, far beyond any industrial process that's ever happened on Earth. Mars's atmosphere has a mass of about 2.5x1016 kg, and that would need to increase by a factor of at least five, essentially all of it oxygen, for even the deepest point in Mars to have the same breathability as the peak of Everest.
By comparison, total emissions of carbon dioxide since 1850 are around 2.4×1015 kg and current emissions are around 4×1013 kg per year. In other words, if we could pump oxygen into Mars's atmosphere at the same rate we're pumping carbon dioxide into Earth's atmosphere, it would take about three centuries before the lowest point on Mars had breathable air -- assuming all that oxygen stayed put instead of, say, recombining with the iron (or whatever) it had been split off from or escaping into space.
This is just scratching the surface of the practical difficulties involved in trying to terraform a planet. Planets are big, yo[citation needed].
But then, not always big enough. Broadly speaking, there's a reason that there's lots of hydrogen in Jupiter's atmosphere (about 85%, another 14% helium), while Mars's is mostly carbon dioxide and the Moon has essentially no atmosphere. Jupiter's gravity is strong enough to keep light molecules like molecular hydrogen from escaping on their own or being carried away by the solar wind. Mars's isn't. It can hold onto heavier molecules like carbon dioxide OK, though still with some loss over time, but lighter molecules aren't going to stick around.
Earth is somewhere in the middle. We don't have any loose hydrogen to speak of because it reacts with oxygen (because life), but we also don't have much helium because it escapes.
Blasting oxygen into Mars's atmosphere would work for a while. Probably for a long while, in human terms (to be fair, atmospheric escape on Mars is measured in kg per second, or thousands of tons per year, much smaller than the in-blasting rate would be). In the end, though, trying to terraform Mars means taking oxygen out of surface minerals and sending it into space, with a stopover in the atmosphere.
But there's another wildcard when it comes to establishing a long-term presence on a planet like Mars. Let's put aside the idea of terraforming the atmosphere and stick to enclosed, radiation-shielded, heated spaces with artificially dense air.
The surface gravity of Mars is about 40% of that on Earth. What does that mean? We have no idea. We have some idea of how microgravity (also known as zero-g) affects people. Though fewer than a thousand people have ever been to space, some have spent long enough to study the effects. They're not great. They include loss of muscle and bone, a weakened immune system, decreased production of red blood cells and lots of other, less serious issues.
Obviously, none of this is fatal, there are ways to mitigate most of the effects, and some of them, like decreased muscle mass, may not matter if you're going to spend your whole life in space rather than coming back to earth after a few months (no one has ever spent more than about 14 months in space). But then, that's a problem, too. No one has spent years in microgravity. No one has ever been born in microgravity or grown up in it. We can guess what might happen, but it's a guess.
No one, ever, has spent any significant time in 40% of Earth gravity. The closest is that two dozen people have been to the Moon (16% of Earth gravity), staying at most just over three days. We know even less about the effects of Mars gravity on humans than we do about microgravity, which is only a little bit.
Maybe people would be just fine. Maybe 40% is enough to trigger the same responses as happen normally under full Earth gravity. Maybe it leads to a slow, miserable death as organ systems gradually shut down. Maybe babies can be born and grow to adulthood just as well with 40% gravity as 100%. Leaving aside the ethics of finding that out, maybe it just won't work. Maybe a child raised under 40% gravity is subject to a host of barely-manageable ailments. Maybe they do just great and enjoy a childhood of truly epic dunks at the 4-meter basketball hoop on the dome's playground.
Whatever the answer is, there's absolutely nothing a hypothetical Mars colony could do about it. You can corral a bit of atmosphere into a sealed space and adjust it to be breathable. You can heat a small corner of the new world to human-friendly temperatures. You can separate usable soil out of the salty, toxic surfaces and grow food in the reduced light (the Sun is about 43% as bright on Mars). You can project scenes of a lush, green landscape on the walls.
No matter what you do, the gravity is going to be what it is, and whoever's living there will have to live with it however they can.
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