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The quest for the nearest inhabitable worlds
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| HarbingerDawn | Date: Friday, 21.09.2012, 12:35 | Message # 16 |
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| Quote (LordHaHa) About 0.4 ATM is about where you will die eventually from lack of pressure (not immediately though, over many hours)
How many hours is many? People sustain pressures of less than 0.4 atm for extended periods every day. This includes astronauts on spacewalks (which can last over 8 hours), and people who climb Mount Everest. With enough oxygen, there is nothing that will hurt you about pressures lower than 0.4 atm. The pressure below which humans cannot survive - the Armstrong limit - is 0.0618 atm, though that will kill a human about as quickly as vacuum. If we assume that an altitude of 10,000 ft (~3,300 m) is about as high as you can comfortably go without having to adjust to the altitude, then you can conclude that a human can live indefinitely in a pure oxygen environment with only 0.14 atm of pressure. With acclimatization, even lower pressures can be achieved. The partial pressure of oxygen at the highest altitude at which anyone has ever lived for a long period of time (2 years) is just 0.1 atm.
Therefore, a human can survive - if in good condition and well-acclimated to the low pressure, low oxygen environment - in pressures as low as 0.1 atmospheres.
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| SpaceEngineer | Date: Friday, 21.09.2012, 18:37 | Message # 17 |
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| On the other hand, 10 atm is a pressure at the depth of just 100 meters. I guess modern divers can dive much deeper using special breathing gas. So humans can survive on a temperate planet with 10+ atm with breath mask.
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| smjjames | Date: Friday, 21.09.2012, 19:44 | Message # 18 |
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| Quote (SpaceEngineer) On the other hand, 10 atm is a pressure on the depth of just 100 meters. I guess modern divers can dive much more deep using special breathing gas. So humans can survive on a temperate planet with 10+ atm with breath mask.
Yeah, theres quite a range that we can survive in with technological help.
For gravity though, since wiki says that the typical person could probably withstand up to 5g, so a heavy gravity world with 3 or 4g would be survivable, but hey, we're an extremely adaptable species, so given time, colonists and their descendants could definetly become comfortable on a heavy g planet.
So the question here in this thread is whether we're looking for a planet that is very earthlike or one that isn't real earthlike, but we can survive on the surface in some way .
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| HarbingerDawn | Date: Friday, 21.09.2012, 20:01 | Message # 19 |
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| Quote (smjjames) For gravity though, since wiki says that the typical person could probably withstand up to 5g
Can withstand that, yes. Can stand up in that, no. On a world with much more than 2g, standing, walking, and other physical tasks would become very difficult. I suggest you take a ride in a centrifuge one day if you ever get the opportunity. It can give you perspective on what it's like to move under high g loads.
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| LordHaHa | Date: Friday, 21.09.2012, 20:49 | Message # 20 |
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| Quote (HarbingerDawn) How many hours is many?
0.4 ATM was a value I received for mountain ascents although reevaluating my information here I have to revise that figure for the "death zone floor" downward to 0.37 ATM, which is the pressure at around 26000 feet. The article is also interesting as it documents changes based on season and latitude, which might be interesting to model in the (distant) future in SE (if possible, I know seasons in and of themselves are whole 'nother can of worms).
In any event I recall that its about two days (with some leeway depending on particularly good or bad fitness) for a normal person above 26000 ft (7.925km). With supplemental air mixes (more oxygen) and other technical aids, survivability increases. But even at lower altitudes, physical issues can become pronounced, as well as psychological issues. And then there's the issue of the low temperatures at that altitude, although since we are talking about exoplanets a particular locality may be warm but still have a half density atmosphere.
There was a very good Nova program on low pressure operations around Everest some time ago, the transcript of which can be found here.
As far as habitability, there are many people who do live in somewhat rarefied conditions across the world, with no mechanical assistance at all. They can survive just fine, since they have acclimatized over a long period of time to have better oxygenation capabilities and expanded lung volume. But even so the tendency is to not live towards the death zone amongst populations who can access those areas. If I recall sherpa settlements are towards the rivers that cut through the Himalayan range. At the very least, and the Nova documentary demonstrates, sherpas are still vulnerable to the poor density of the death zone and the immediate area around it.
LordHaHa
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| HarbingerDawn | Date: Friday, 21.09.2012, 21:02 | Message # 21 |
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| Quote (LordHaHa) 0.4 ATM was a value I received for mountain ascents
That data is only relevant for Earth. It assumes a sea-level pressure of 1 atm, a 20.9% fraction of O2, an atmosphere density equal to ours, a planet radius of 6378 km, and a planet mass of 5.97x1024 kg. If our atmosphere was completely oxygen, pressure limits would extend by a factor of nearly 5.
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| LordHaHa | Date: Friday, 21.09.2012, 21:15 | Message # 22 |
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| Quote (SpaceEngineer) On the other hand, 10 atm is a pressure on the depth of just 100 meters. I guess modern divers can dive much more deep using special breathing gas. So humans can survive on a temperate planet with 10+ atm with breath mask.
At 10 ATM you'd have to use something like a Trimix (Helium-Oxygen-Nitrogen) blend. For even denser environments you'd need Hydrox (Hydrogen with a little bit of Oxygen). The latter allows operations down to 650-700m underwater, which would roughly be the equivalent of 65-70 ATM of pressure. Pretty extreme.
Of course with diving you have various complications that can arise from using these chemicals, like HPNS/helium tremors (which would manifest in 50 ATM+ environments on helium-oxygen mixes), nitrogen narcosis, hydrogen narcosis...
LordHaHa
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| LordHaHa | Date: Friday, 21.09.2012, 21:42 | Message # 23 |
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| Quote (HarbingerDawn) That data is only relevant for Earth. It assumes a sea-level pressure of 1 atm, a 20.9% fraction of O2, an atmosphere density equal to ours, a planet radius of 6378 km, and a planet mass of 5.97x1024 kg. If our atmosphere was completely oxygen, pressure limits would extend by a factor of nearly 5.
In terms of operating in high and low pressure environments, as I stated previously, local conditions would allow for different operational parameters. These observations based off of a "very Earth-like" situation, as that is the most detailed reference I have on hand regarding a real, Terran-friendly environment.
I expect that we will come across richer oxygen mixes in some exoplanets of course and that will allow us to reevaluate safe densities on a case by case basis. Also other factors, like ignition ease and intensity (among other things) would have to be evaluated as well, but I digress.
LordHaHa
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| SpaceEngineer | Date: Friday, 21.09.2012, 23:39 | Message # 24 |
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| Quote (LordHaHa) At 10 ATM you'd have to use something like a Trimix (Helium-Oxygen-Nitrogen) blend. For even denser environments you'd need Hydrox (Hydrogen with a little bit of Oxygen).
Such air composition is highly believable for super-earth planets. They may be massive enough to hold helium and even hydrogen, and total mass of atmosphere may be big enough to make surface pressure of 7-10 atm. Local lifeforms may produce oxygen, but it is unlikely that hydrogen-rich atmosphere will have a big amount of oxygen, it will quickly oxidize the hydrogen and hold atmosphere from accumulate explosible amount of oxygen. However, explosion of atmosphere may be a sort of natural disaster, maybe even seasonal.
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| smjjames | Date: Saturday, 22.09.2012, 00:24 | Message # 25 |
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| Quote (SpaceEngineer) Such air composition is highly believable for super-earth planets. They may be massive enough to hold helium and even hydrogen, and total mass of atmosphere may be big enough to make surface pressure of 7-10 atm. Local lifeforms may produce oxygen, but it is unlikely that hydrogen-rich atmosphere will have a big amount of oxygen, it will quickly oxidize the hydrogen and hold atmosphere from accumulate explosible amount of oxygen. However, explosion of atmosphere may be a sort of natural disaster, maybe even seasonal.
How the heck much oxygen would be needed to explode the atmosphere??? 100%??? If that kind of thing is even possible.... Unless you mean an airburst kind of explosion and not the entire atmosphere exploding at once.
I think it would be more likely that life would find some kind of balance between a high oxygen atmosphere vs one that could explode.
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| HarbingerDawn | Date: Saturday, 22.09.2012, 00:28 | Message # 26 |
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| Oxygen burns. It's what it does best. If you have a high enough level of oxygen anything will burn, and even the slightest catalyst will start an immense conflagration. See: Apollo 1 fire.
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Edited by HarbingerDawn - Saturday, 22.09.2012, 00:29 |
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| smjjames | Date: Saturday, 22.09.2012, 00:42 | Message # 27 |
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| I KNOW that, but the whole atmosphere at once? I was asking how high of an oxygen concentration would be needed to do that.
Plus having the entire atmosphere explode is likely to kill off just about anything in it's way (organisms underwater or deep enough underground might survive though). Let alone be a seasonal entiire atmosphere exploding kind of thing.
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| HarbingerDawn | Date: Saturday, 22.09.2012, 00:46 | Message # 28 |
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| Not the entire planet at once, it would start at one point and spread until it encompassed the globe. Could take a whole day even 
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| smjjames | Date: Saturday, 22.09.2012, 01:38 | Message # 29 |
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| Anyways, as a side note to the helium/hydrogen atmospheres for super earths (or perhaps colder titan-like worlds) a helium-methane mix could happen, but we're talking about atmospheres with oxygen in it.
Different atmospheric mixes are to be expected and during the Carboniferous period (giant insects), it got up to 30% oxygen I think. Regarding the high oxygen atmosphere, natural proccesses (lightning, fire, oxidation) would probably keep the atmosphere from getting to the point where the atmosphere has the potential to become a planet wide fireball.
Also, a high oxygen mix creates a different problem, the fact that there is less CO2 as a percentage in the atmosphere means the planet doesn't warm up as much. This is one theory as to why the Earth went into a Snowball Earth period, so much oxygen was pumped out that the planet froze over.
Edited by smjjames - Saturday, 22.09.2012, 01:40 |
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| HarbingerDawn | Date: Saturday, 22.09.2012, 02:09 | Message # 30 |
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| Quote (smjjames) Also, a high oxygen mix creates a different problem, the fact that there is less CO2 as a percentage in the atmosphere means the planet doesn't warm up as much. This is one theory as to why the Earth went into a Snowball Earth period, so much oxygen was pumped out that the planet froze over.
It's also possible that an increase in CO2 could actually lead to a decrease in CO2. Increased temperature could lead to increased evaporation and therefore precipitation, which would start to remove CO2 from the atmosphere, as would the increased wave action likely on the surface of oceans. This would not cool the planet to Snowball levels though, only stabilize the CO2 levels. This is what is thought could have happened after the end of a Snowball Earth period.
hmm, offtopic post is offtopic 
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Edited by HarbingerDawn - Saturday, 22.09.2012, 02:10 |
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