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CousinoMacul
2007-Jan-29, 12:33 AM
Why are Earth's and Titan's atmospheres predominantly nitrogen, while Mars' and Venus' are mostly carbon dioxide?

TriangleMan
2007-Jan-29, 05:07 AM
The main principle at work here is atmospheric escape. The average "escape" velocity of a gas is dependent on the mass of the molecule and the temperature. I think the equation is something like:

v = 157*(T/mass)^1/2 [I can't remember the units you need to use though]

The gas escapes the planet if its speed is enough to overcome the escape velocity of the planet. The speed that a gas needs to escape Mars and Titan is roughly the same, but Titan is farther away, and thus colder. I'm guessing that the difference in temperature is why Titan retains nitrogen but Mars does not. Since the nitrogen escapes Mars carbon dioxide becomes the dominant gas in its atmosphere.

Earth is much more massive than either Mars or Titan so is able to retain nitrogen even though it is warmer.

As for Venus and Earth, according the Wiki article on the atmosphere of Venus the amount of nitrogen (in absolute terms) in both planets' atmospheres is about the same, the percentage is different because of the massive amount of carbon dioxide on Venus. So why is there more carbon doixide on Venus than Earth?

I don't know. I'm now wondering why myself. :think: If I find the answer I'll let you know.

BigDon
2007-Jan-29, 05:25 AM
As for Venus and Earth, according the Wiki article on the atmosphere of Venus the amount of nitrogen (in absolute terms) in both planets' atmospheres is about the same, the percentage is different because of the massive amount of carbon dioxide on Venus. So why is there more carbon doixide on Venus than Earth?

I don't know. I'm now wondering why myself. :think: If I find the answer I'll let you know.

The oceans and life. CO2 is water soluble. Marine creatures sequester CO2 in their skeletons as calcium carbonate. Die and sink it to the bottom.The white cliffs of Dover and all that. Limestone, marble et al. We've had oceans for a really long time. Even before we had free oxigen in the atmosphere we had oceans. Plus there is a geological process screaming in the back of my mind that won't come to the fore. Something about the oxidation of silica into silicon dioxide I think.

Ken G
2007-Jan-29, 06:02 AM
I don't think retention is a key issue, nor life really (though it does play a role). The key gases are well retained (CO2 and N2), and H2O is never well retained (the H comes off too easily). So because of the latter point, you never have much H2O in your atmosphere, so that leaves CO2 and N2 (in that order) as the key players. Earth has a water cycle (life plays a relatively smaller role), which strongly depletes CO2 (lots of chemical reactions involving water will sequester CO2 out of the atmosphere), so that leaves N2 for Earth. Oddly, Titan has almost the same pressure N2, but for a much simpler reason: H2O and CO2 both freeze at that distance to the Sun (witness ice and dry ice on Earth), but N2 does not (even liquid N2 is quite cold), so that leaves N2 for Titan also. (If it wasn't clear, H2O, CO2 and N2 are "outgassed" from the mantle of terrestrial planets via vulcanism, that's the key process for any of those atmospheres.)

Jeff Root
2007-Jan-29, 09:27 AM
Is there an estimated pressure history for Earth's atmosphere?
I would expect that it has been falling more-or-less steadily
since the time of the Moon's formation. Unless the oceans
stabilize it, such that surface water is lost instead, replacing
what is lost from the atmosphere.

-- Jeff, in Minneapolis

CousinoMacul
2007-Feb-01, 11:20 AM
Thanks guys!