我不确定我可以给一个好技术的答案,但我不确定这是一个平衡,但更多的太阳系的形成的结果。的形成,如果你看看内行星,大部分的气体和冰偏离是由于内部的轨道太打击和早期宇宙中非常活跃的日冕物质抛射,所以你可能得到3类型的行星。1)小岩石行星(地球,水星、金星、火星),2)热木星,这足以氢陷阱虽然他们接近太阳,热,3)超级地球,它有足够的引力陷阱氢。行星大气中大量的氢可能会使氧气不可能形成。氧和氢结合。地球没有引力陷阱一堆氢和一些早期的气氛可能偏离大的流星撞击和日冕物质抛射,更为常见的太阳。知道地球大气层的形成可能是海洋的形成,导致海洋没有大气的情况下是不可能的。海洋38亿年前形成,所以我们必须有一个氛围,至少38亿年。http://people.chem.duke.edu/ jds / cruise_chem /海洋/ ocean1.html年轻的地球大气层主要是(CH4),氨(NH3),水蒸气(H2O)和二氧化碳(CO2)和上面的链接,一旦地球好够酷,水(气)变成水(雨水)我们开始有海洋。随着地球冷却,大概是大气主要是甲烷、氨和二氧化碳。 So, fast forward to the cyanobacteria, which pull CO2 from the air and release O2. Early on, the O2 binds with Iron in the ocean and some of it likely reacts with CH4 and NH3 in the atmosphere when there is lightning. - not crystal clear on that point, but over time, CO2 was pulled from the air, replaced by O2 and O2 reacted with CH4 and NH3, producing more CO2, H20, and N2 - the common elements we have today. Oxygen levels in planets with cyanobacteria, likely depends on how much Iron is dissolved in the oceans and how much hydrogen is in the atmosphere. If there's not enough CO2 to produce enough O2 to saturate the Iron and Hydrogen, the planet probably never gets much O2 in it's atmosphere - so it's all about the ratio of early elements. Super-earths might never get oxygen atmospheres - too much hydrogen. You also get factors like the development of lignin (got that from Neil deGrass' Cosmos) - http://evolution.about.com/od/Cosmos/fl/Cosmos-A-Spacetime-Odyssey-Recap-Episode-109.htm Lignan made trees possible, but nothing could eat trees, so trees captured more and more Carbon, so CO2 levels fell and O2 levels rose. About 100 million years later, termites evolved and the digestion of trees released much of this captured CO2 became possible and Oxygen and CO2 leveled out. Shellfish, to make their shells, take more O2 out of the air than CO2 (Calcium Carbonate has lots of Oxygen in it). That's a slow process, but over time it takes much CO2 out of the air, so over tens of millions of years, parts of the atmosphere are effectively sequestered and trapped in the earths crust, but over time, much of this is also returned to the atmosphere with volcanic activity - so the type of life is a factor and the extent of tectonic activity is a factor as is, the presence of a jupiter and how many comets are likely to hit the planet and the size of the planet. So I think it's more chance, solar system and planetary factors that lead to a planet's atmosphere. To answer your question: > So if we increase the amount of oxygen, it will decrease over time and > if we decrease it, it will be replenished. Tough call regarding Oxygen. CO2 is easier. Because there's a relatively small percentage of CO2 in the air, more CO2 will lead to some of it dissolving in the ocean as carbolic acid, though the warming of the ocean might also slow the oceanic absorption of CO2. Because there's so much more Oxygen in the earth, 21% vs 0.04% of CO2, a measurable increase, say 21% to 22%, you'd probably see a slight increase in dissolved O2 in the ocean, but beyond that, I don't think you'd see that extra oxygen go away anytime soon. My guess is, if such an increase was done, it would last a long time, hundreds of thousands of years if not millions, because oxygen sequestering is pretty slow. Bugs would get slightly bigger pretty quick. That might be the most noticeable effect. If you significantly increase O2 levels, say from 21% to 30%, things would become noticeably more flammable and over time, our lungs would probably get smaller and there would be other effects, probably. But I don't believe there's any sort of equilibrium that would fairly quickly bring it back to 21%. Interesting question though.