# What makes Earth's atmosphere breathable and other planets not? [closed]

I am no scientist. So, I need a fairly simple and concise answer. What would prevent us from breathing and surviving on Mars or the Moon?

I want to make those two bodies livable as on Earth but still retaining their environments for the most part (i.e. Mars is still a sandy red badlands and the Moon is still a pale rock).

## closed as off-topic by SRM - Reinstate Monica, Mołot, JDługosz, Brythan, FrostfyreJan 11 '17 at 13:20

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question does not appear to be about worldbuilding, within the scope defined in the help center." – SRM - Reinstate Monica, JDługosz, Brythan, Frostfyre
If this question can be reworded to fit the rules in the help center, please edit the question.

• I think you need to do some research on your own. Or, describe why you think Earth's atmosphere is breathable while other planets are not, because it's quite difficult to tell what your level of understanding is on this subject. For instance the Moon's surface is nearly a hard vacuum, so it should be quite obvious why we can't breathe that. – Samuel Jan 11 '17 at 0:44
• Oxygen would be one – paparazzo Jan 11 '17 at 1:59
• I know very little. It took me four tries to pass a science course in my undergrad (Biology failed, Astronomy failed, Environmental Science failed... Environmental Science again... passed with a C). So, any information that can be simplified would be fantastic. – tmfrymier Jan 11 '17 at 2:50
• Take a goldfish out of the water and it probably die within minutes, it uses gill to extract dissolved oxygen inside the water despite the Earth's atmosphere have plenty of it. Same as us humans our body simply isn't evolved to survive the aforementioned world and without oxygen for metabolism our cells will starts dying. Earth is massive enough to trap gas, far away from the sun to sweat and able to produce magnetic field to prevent solar wind from ripping away the atmosphere as it did for moon and probably Mars too. – user6760 Jan 11 '17 at 8:11
• Welcome to the site, tmfryier. Please note that the Worldbuilding SE is dedicated to answering questions about fictional worlds, rather than discussing real-world physics (though that does happen as part of answering questions). If you have a question a specific question about the habitability of a certain atmosphere, feel free to ask another question about it. Otherwise, Google is your friend. There's lots of information about existing celestial bodies their atmosphere. – Frostfyre Jan 11 '17 at 13:19

• No toxic gasses like $\rm SO_2$
• Right pressure (too low and water will boil at body temperature (Armstrong limit), too high and even inert gasses like nitrogen and helium become toxic)
• Right amount of oxygen - partial pressure of oxygen (atmospheric pressure multiplied by fraction of oxygen in air (1 bar * 0.21 for Earth)) needs to be roughly between 0.15 and 1.5 bar

Also a few factors not entirely about breathability:

• Temperature
• Radiation protection (Ozone layer + Earth's magnetic field)

Mars is below the Armstrong limit and has pretty much no oxygen. It is also rather cold and has no radiation protection. Most of that could be fixed, but the lack of a magnetic field will let solar wind deplete any atmosphere you make and making a planet-size magnetic field is a problem...

Moon has insufficient surface gravity to maintain an atmosphere, zero magnetic field and 28-day "days" that would cause wild temperature swings.

In our Solar system, the options seem to be:

• Mars (underground bunkers)
• Venus (acid-resistant airships at 50km altitude)
• Titan (surface, but very cold)

## Because you need something to breathe, in order to breathe.

The thing that prevents us from breathing on other space rocks is either their downright lack of any proper atmosphere (in the case of the moon) or the fact that the atmosphere they do have doesn't have the necessary gases that humans need to constantly process in order to live(namely oxygen).

However this doesn't just mean you'd need to pump a whole lot of breathable air onto a planet/moon to make them habitable. Our moon does not possess enough mass to trap in an atmosphere, it would just float away. Mars does have a strong enough gravitational pull, however since it does not generate a magnetic field, it's atmosphere is constantly stripped by solar winds(causing it to be very thin).

So in order to have a world where humans can breathe on, you need 3 things:

• A rock with a strong enough gravitional pull to maintain an atmosphere.
• A moderate concentration of oxygen within that atmosphere (not too high a concentration though, otherwise everything becomes very flammable, around 20% should do).
• Protection from solar winds (an internally generated magnetic field for example).

OR approaching from the other end, we evolved on Earth, so our respiratory systems are optimised for Earth's atmosphere.
If we'd evolved somewhere else, we'd be optimised for that.

• I think simply knowing “because it’s not like here” is not the qualitative answer he’s looking for. – JDługosz Jan 11 '17 at 8:04