6 ms·

AFAIK Nitrogen in the initial atmosphere rules this out.

perl4ever 7y ago
Is there a succinct explanation of why some terrestial bodies in the solar system (Earth, Titan, Triton, Pluto) have mostly nitrogen atmospheres, but others (Mars, Venus) are mostly carbon dioxide instead? I mean, aside from specifics about each, what's the pattern that you would apply to an arbitrary planet?
- croddin 7y ago
  I think it is due the magnetosphere shielding from solar wind. Earth has a strong magnetosphere, while neither mars or venus do. For titan, wikipedia says "Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the solar wind.", not sure about Triton and Pluto but they are much further from the sun so that probably helps solar wind not strip away as much nitrogen.
- Rooster61 7y ago
  A relative lack of effect from the solar wind, either through magnetic shielding (Earth, Titan) or sheer distance from the Sun (Triton, Pluto). Mars and Venus are both in the inner Solar System, and both have very weak magnetic fields.
- elihu 7y ago
  According to wikipedia, Venus' atmosphere is about 3.5% nitrogen and its pressure is about 91 atmospheres. So, technically Venus doesn't have a lack of nitrogen (it appears to have about 3 times as much as Earth), it has an extreme abundance of carbon dioxide. Something interesting I didn't know is that apparently Venus has almost no plain oxygen. Mars, on the other hand, doesn't have much nitrogen. Perhaps that's attributable to the relative lightness of N2, which would be more prone to float off into space than the somewhat heavier CO2.
  - gnulinux 7y ago
    Earth atmosphere is 0.04% CO2. Why does Earth have such little CO2? Is maybe used by life on early Earth?
    - perl4ever 7y ago
      Is that a rhetorical question? I assume it's something to do with: https://en.wikipedia.org/wiki/Limestone https://en.wikipedia.org/wiki/Limestone
    - rozab 7y ago
      Yep. https://en.wikipedia.org/wiki/Great_Oxidation_Event https://en.wikipedia.org/wiki/Great_Oxidation_Event
    - mattkrause 7y ago
      I’m pretty sure we still have plants, though I admittedly haven’t been out of the house much. A lot of carbon was also captured during the aptly-named Carboniferous Period 360-300 million years ago. Woody plants had just evolved, but microbes that could break them down had not. Thus, coal. An earlier drop in sea levels also caused carbonate (as in limestone, dolomite, and iron ores) to precipitate out. The combination of these effects pushed oxygen levels about 50% above present, allowing very large animals to evolve.
      - saagarjha 7y ago
        > Woody plants had just evolved, but microbes that could break them down had not. Thus, coal. Does this mean we’ll never have coal again?
        anticensor 6y ago
        We will, but the balance is much more against coal now. We will have it consumed before it is replaced.
        mattkrause 7y ago
- pdonis 7y ago
  > Is there a succinct explanation of why some terrestial bodies in the solar system (Earth, Titan, Triton, Pluto) have mostly nitrogen atmospheres, but others (Mars, Venus) are mostly carbon dioxide instead? For Earth, the answer is simple: life. In the absence of life, a nitrogen-oxygen atmosphere is thermodynamically unstable. Our best current understanding is that the atmosphere of the early Earth, before life developed, was mostly methane and ammonia, which are commonly observed in the atmospheres of the outer planets and their satellites. Once life developed, the methane and ammonia were gradually chemically converted by life to water and nitrogen. For the others you mention, oxygen is not present, and the nitrogen is combined with methane (plus traces of carbon monoxide or carbon dioxide). I have not seen detailed thermodynamic calculations, but if there are very few oxygen atoms present, nitrogen plus methane should be thermodynamically stable. The unique feature of Earth as compared with these others would be the presence of a sufficient quantity of oxygen atoms to oxidize the nitrogen to ammonia in the early atmosphere before life developed. For Mars, it is close enough to the Sun and small enough to make its temperature too high for it to retain significant amounts of nitrogen or methane (it has traces of both, IIRC), so CO2, which is significantly heavier, is the only significant component left. For Venus, a significant amount of nitrogen gas is present (about four times as much as in Earth's atmosphere--Venus is significantly larger than Mars so its gravity can keep the nitrogen from escaping), but this is masked by the enormous amount of CO2 present (about 95 atmospheres worth), which AFAIK is believed to be due to all of the calcium carbonate in the surface rocks having been driven into the atmosphere due to a runaway greenhouse effect raising the temperature.
  - neuronic 7y ago
    When solar system planets formed from stellar dust, do we assume that said dust was uniform in composition and merely position, solar dynamics and interplanetary dynamics such as collisions (formation of Moon) led to the differences in chemical composition? For example, it's the outer planets that are gas giants. Venus composition doesn't seem too different from Earth but it's much closer to Sol, so it's more exposed with much more energy absorbed etc.
    - pdonis 7y ago
- T-hawk 7y ago
  It's not a pattern of ratio or composition, it's a pattern of the quantity of each individual component. The individual quantities then constitute the composition. For CO2, the pattern is temperature. Titan, Triton, Pluto are well below CO2's freezing point so it stays on the surface as ice instead of atmosphere. At these temperatures, nitrogen is the heaviest abundant molecule that remains gaseous, followed by methane. If you're warm enough for gaseous CO2, you have it. The only exception is Earth and that's because plant life absorbed it and emitted O2 instead. For nitrogen, methane, argon, the pattern is mass. Only Venus and Earth have enough mass to hold on to those, compared to heavier CO2. Venus and Earth have similar quantities of those. Venus's excess of CO2 comes from temperature high enough to evaporate carbon and oxygen from rock compounds.
  - saberdancer 7y ago
    Titan has enough mass to keep a decent nitrogen based atmosphere as well. It is probably closest to Earth with the exception that methane is a water analogue on Titan.

Is there a succinct explanation of why some terrestial bodies in the solar system (Earth, Titan, Triton, Pluto) have mostly nitrogen atmospheres, but others (Mars, Venus) are mostly carbon dioxide instead? I mean, aside from specifics about each, what's the pattern that you would apply to an arbitrary planet?

croddin 7y ago
I think it is due the magnetosphere shielding from solar wind. Earth has a strong magnetosphere, while neither mars or venus do. For titan, wikipedia says "Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the solar wind.", not sure about Triton and Pluto but they are much further from the sun so that probably helps solar wind not strip away as much nitrogen.
Rooster61 7y ago
A relative lack of effect from the solar wind, either through magnetic shielding (Earth, Titan) or sheer distance from the Sun (Triton, Pluto). Mars and Venus are both in the inner Solar System, and both have very weak magnetic fields.
elihu 7y ago
According to wikipedia, Venus' atmosphere is about 3.5% nitrogen and its pressure is about 91 atmospheres. So, technically Venus doesn't have a lack of nitrogen (it appears to have about 3 times as much as Earth), it has an extreme abundance of carbon dioxide. Something interesting I didn't know is that apparently Venus has almost no plain oxygen. Mars, on the other hand, doesn't have much nitrogen. Perhaps that's attributable to the relative lightness of N2, which would be more prone to float off into space than the somewhat heavier CO2.
- gnulinux 7y ago
  Earth atmosphere is 0.04% CO2. Why does Earth have such little CO2? Is maybe used by life on early Earth?
  - perl4ever 7y ago
    Is that a rhetorical question? I assume it's something to do with: https://en.wikipedia.org/wiki/Limestone https://en.wikipedia.org/wiki/Limestone
  - rozab 7y ago
    Yep. https://en.wikipedia.org/wiki/Great_Oxidation_Event https://en.wikipedia.org/wiki/Great_Oxidation_Event
  - mattkrause 7y ago
    I’m pretty sure we still have plants, though I admittedly haven’t been out of the house much. A lot of carbon was also captured during the aptly-named Carboniferous Period 360-300 million years ago. Woody plants had just evolved, but microbes that could break them down had not. Thus, coal. An earlier drop in sea levels also caused carbonate (as in limestone, dolomite, and iron ores) to precipitate out. The combination of these effects pushed oxygen levels about 50% above present, allowing very large animals to evolve.
    - saagarjha 7y ago
      > Woody plants had just evolved, but microbes that could break them down had not. Thus, coal. Does this mean we’ll never have coal again?
      - anticensor 6y ago
        We will, but the balance is much more against coal now. We will have it consumed before it is replaced.
      - mattkrause 7y ago
pdonis 7y ago
> Is there a succinct explanation of why some terrestial bodies in the solar system (Earth, Titan, Triton, Pluto) have mostly nitrogen atmospheres, but others (Mars, Venus) are mostly carbon dioxide instead? For Earth, the answer is simple: life. In the absence of life, a nitrogen-oxygen atmosphere is thermodynamically unstable. Our best current understanding is that the atmosphere of the early Earth, before life developed, was mostly methane and ammonia, which are commonly observed in the atmospheres of the outer planets and their satellites. Once life developed, the methane and ammonia were gradually chemically converted by life to water and nitrogen. For the others you mention, oxygen is not present, and the nitrogen is combined with methane (plus traces of carbon monoxide or carbon dioxide). I have not seen detailed thermodynamic calculations, but if there are very few oxygen atoms present, nitrogen plus methane should be thermodynamically stable. The unique feature of Earth as compared with these others would be the presence of a sufficient quantity of oxygen atoms to oxidize the nitrogen to ammonia in the early atmosphere before life developed. For Mars, it is close enough to the Sun and small enough to make its temperature too high for it to retain significant amounts of nitrogen or methane (it has traces of both, IIRC), so CO2, which is significantly heavier, is the only significant component left. For Venus, a significant amount of nitrogen gas is present (about four times as much as in Earth's atmosphere--Venus is significantly larger than Mars so its gravity can keep the nitrogen from escaping), but this is masked by the enormous amount of CO2 present (about 95 atmospheres worth), which AFAIK is believed to be due to all of the calcium carbonate in the surface rocks having been driven into the atmosphere due to a runaway greenhouse effect raising the temperature.
- neuronic 7y ago
  When solar system planets formed from stellar dust, do we assume that said dust was uniform in composition and merely position, solar dynamics and interplanetary dynamics such as collisions (formation of Moon) led to the differences in chemical composition? For example, it's the outer planets that are gas giants. Venus composition doesn't seem too different from Earth but it's much closer to Sol, so it's more exposed with much more energy absorbed etc.
  - pdonis 7y ago
T-hawk 7y ago
It's not a pattern of ratio or composition, it's a pattern of the quantity of each individual component. The individual quantities then constitute the composition. For CO2, the pattern is temperature. Titan, Triton, Pluto are well below CO2's freezing point so it stays on the surface as ice instead of atmosphere. At these temperatures, nitrogen is the heaviest abundant molecule that remains gaseous, followed by methane. If you're warm enough for gaseous CO2, you have it. The only exception is Earth and that's because plant life absorbed it and emitted O2 instead. For nitrogen, methane, argon, the pattern is mass. Only Venus and Earth have enough mass to hold on to those, compared to heavier CO2. Venus and Earth have similar quantities of those. Venus's excess of CO2 comes from temperature high enough to evaporate carbon and oxygen from rock compounds.
- saberdancer 7y ago
  Titan has enough mass to keep a decent nitrogen based atmosphere as well. It is probably closest to Earth with the exception that methane is a water analogue on Titan.