# I Requirements for a breathable atmosphere

1. Jan 30, 2017

### Physics quest

Hello all, hope you had happy holidays and new year. I had some questions regarding atmospheres, breathable mostly and what conditions are required on a planet to sustain them.

Could a low gravity planet with the temperature of earth maintain a breathable atmosphere? Could it be given one that could be sustained artificially?

Also, could a planet have an extremely low escape velocity, e.g. something small (my math is not great so I use small numbers for my example) like 3 meters per second (maintained through as a constant force, like say a 3 m/s elevator) being able to leave the planet. could it still have temperatures akin to Earth? What would the gravity be like? Could it maintain any breathable atmosphere?

Thanks all for answers, I am interested in both the facts and theories behind these ideas.

2. Jan 30, 2017

### oz93666

Astronauts in space suits are at about 1/3 rd of an atmosphere of pressure (pure oxygen) ...

At room temperature air molecules have a velocity of 500 m/s (close to 1000 miles per hour) from random thermal motion .. ... the moon could never hold an atmosphere even though escape velocity at surface is 2,380m/sec and gas molecules only move around 500m/sec , the molecules still drift away into space .

To be precise the moon does have a faint atmosphere , 10 million million times less dense than Earth's

3. Jan 30, 2017

### rootone

A low gravity planet (Mars for example) can naturally maintain an atmosphere of heavier gases.
Although it could be possible to enrich the atmosphere of Mars with Oxygen, it's a fairly light gas, some would be lost.
On the other hand 'breathable' doesn't necessarily mean Oxygen.
There are forms of life on Earth, bacteria mainly, that don''t need Oxygen.

4. Jan 31, 2017

### lifeonmercury

Mercury has a surface-bound exosphere and its gases are being constantly replenished by the Sun. This technically counts as an atmosphere too. In fact, the existence of this exosphere could be sufficient to sustain microbial life at certain locations on Mercury.

Back to breathing on Mars. Aside from the thin air and the lack of oxygen, the amount of talcum powder-like dust in the air would be a major health concern due to the extremely tiny particles entering the lungs. Many things would need to be altered on Mars before people could safely venture outside without a pressurized spacesuit or breathing mask.

Last edited: Jan 31, 2017
5. Feb 1, 2017

### newjerseyrunner

I'm going to assume breathable has nothing to do with Earth life and that you are just talking about life in general. So to me, that means that you just need any gas that contains stored energy in high enough density to be metabolized. That makes several atmospheres in our solar system "breathable."

Titan for one has a much thicker atmosphere than Earth, and it's full of hydrocarbons.

Breathable isn't just about how thick the atmosphere is, it's about the chemical composition. As lifeonmercury said, there is an atmosphere on Mercury, but it's completely useless to living things because it's hydrogen. Pluto also has an atmosphere, but again, it's not breathable, it's nitrogen. The point of breathing is to gather energy, those chemicals have none. It's safe to say that atmospheres that are mostly inert gases will not fuel life, even exotic life.

6. Feb 1, 2017

### Physics quest

Thanks for the answers guys, in relation to my question on low gravity planets. If you had constant speed of 3 m/s could you escape Earths orbit? I am not talking about escape velocity, something like a space elevator moving 3 m/s from the surface of Earth to outside its atmosphere?

7. Feb 1, 2017

### newjerseyrunner

Yes, as long as you are going at a constant speed, you can go as slow as you want. A high altitude weather balloon doesn't ascend very quickly.

There is a very big difference between escaping orbit and simply climbing something. Orbiting objects move sideways relative to the surface, an elevator would go straight up.

Be aware though, that the gravity outside of the atmosphere is nearly identical to the gravity at the surface, so you'd have to be held up somehow.

8. Feb 1, 2017

### snorkack

Unless the elevator rotates along with the ground beneath.
A record was set climbing 13 km in 12 hours - up stairs, then presumably back down by elevator and up stairs again.
http://www.guinnessworldrecords.com/world-records/stair-climbing-vertical-height-(12-hours)
At that speed of 13 km per day, climbing to geostationary orbit, 36 000 km from ground, would take about 2800 days, or almost 8 years.
But already after climbing first 6400 km, the local gravity would be less than 0,25 g - so could you adjust gait for more efficient upwards movement?

9. Feb 4, 2017

### jbriggs444

Escape velocity at Earth's surface is around 11 kilometers per second. $E=\frac{1}{2}mv^2$. That comes to something like 60,000,000 Joules per kilogram

At 3 meters per second, the same formula means that you have about 4.5 joules per kilogram. That's a factor of 13 million less.

Gravitational potential energy scales as the inverse of the radius from the center of the gravitating body. You start at 6,000 km away from the center of the earth. If you multiply this by that factor of 13 million that comes to 78 billion kilometers -- some 15 times the radius of Neptune's orbit about the sun. If you climbed that high, you would reach a height where escape velocity from the Earth would be a mere 3 meters per second. [Though you'd still have the Sun to content with].

If you have 800,000 years to spare, you could get that high at 3 meters per second.

It's a rather silly calculation though, since you're already moving at a few hundred meters per second (depending on latitude) before you even get out of bed in the morning.

10. Feb 4, 2017

### snorkack

Which speed increases as you climb.
By the time you reach geostationary orbit, you are on orbit.
Not at escape speed yet. But continuing outside, you are going down. And you will reach escape velocity.

11. Feb 4, 2017

### jbriggs444

If you are climbing a beanstalk or staircase, sure.

12. Feb 6, 2017

### stefan r

You can put a bag around an asteroid or small moon. Then add air.

Deimos (mars moon) has a surface gravity of 0.003 m/ss. you could hollow it out and breath in a chamber inside. Or you could cover the whole thing with an air tight sheet. An elevator to the Lagrange point is only a few kilometers. escape velocity is 5.556 m/s. You could throw a baseball out of orbit.

13. Feb 12, 2017

### Staff: Mentor

Only with closed pressurized structures.

Mars is heavy enough to keep an oxygen atmosphere (although the solar wind is still an issue), hydrogen escapes over geological timescales. Calculation here.

14. Feb 12, 2017

### snorkack

No, it is not safe.
We are breathing air that is mostly inert gases (nitrogen).

15. Feb 13, 2017

### newjerseyrunner

Yes, it can contain inert gas. It simply can't be entirely inert gas. Mercurys atmosphere is entirely hydrogen and Pluto is entirely nitrogen. Those are the useless. Mostly nitrogen like earth is fine because CO2 and O2 are the rest and they have a lot of chemical energy.

16. Feb 13, 2017

### Staff: Mentor

CO2 is tightly bound. Not as much as N2 (which is still used by some microbes), but without catalysts or dissolved in water it doesn't react with anything that occurs naturally.

17. Feb 13, 2017

### jbriggs444

Of course, a hydrogen atmosphere is not useless per se. For instance, a hydrogen atmosphere atop a liquid oxygen lake would have chemical energy aplenty. We happen to live in an environment with an oxidizing atmosphere and a bunch of reducing fuels. One can imagine a reducing atmosphere and a bunch of oxidizing fuels.

I think Asimov did a murder mystery short story in that vein. Google says "The Dust of Death".

18. Feb 13, 2017

### snorkack

On Earth, oxygen in air is in excess and reductants/food are hard to come by. Yet organisms find both food and oxygen.
In an environment where reductants like methane or hydrogen or hydrogen sulphide were present in excess in atmosphere, but oxygen were present in small concentrations, organisms could live by reacting the scarce oxygen with the excess reducers around.