I used to think that most terrestrial planet atmospheres were secondary (from out gassing). However, after reading this https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140017487.pdf and noticing that the ratio of CO2 to N2 is far higher on Venus than that just expected from out gassing, I have done a 180.

I'm thinking that Earth has a primarily secondary atmosphere because of the Moon creating event and smaller planets like Mars just aren't massive enough to retain original nitrogen, etc.

Would this cast a shadow on the idea of early extreme stellar activity ripping off atmospheres of close in planets?

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    $\begingroup$ So you take $CO_2$ as proxy for outgassing and $N_2$ as proxy for primordial atmosphere? Or why is it of all the ratio of the two, that makes you your head scratch? $\endgroup$ Oct 22, 2017 at 23:43
  • $\begingroup$ There's a ton of information fit into those pages, so the article takes some time to read. That said, on your last sentence, The article mentions twice, once early on and in more detail, on page 11, right column from the top. It suggests significant atmosphere loss based on Xe-129 concentration on both Mars and Earth likely continuing after the giant impact that's thought to have created the moon. Also, no Xenon has been detected on Venus, suggesting all 3 planets lost significant atmosphere in the first 100(ish) million years after planet formation presumably by the solar wind. $\endgroup$
    – userLTK
    Oct 23, 2017 at 10:48
  • $\begingroup$ How would you define "original atmosphere" ? What about (potentially) elements or molecules which have precipitated out due to cooling? $\endgroup$ Oct 23, 2017 at 13:00
  • $\begingroup$ I'm showing my ignorance here, but I was under the impression that shortly after planet formation a new star will have an event that clears most of the "debris" out of the system (at least the inner zone where rocky planets form). This is when I was thinking that rocky planets would lose any atmosphere they already have. The reason I mention the ratio of CO2 to N2 is because I read here abyss.uoregon.edu/~js/ast121/lectures/lec14.html , for example, that out gassing produces the molecules in about a 4.6:1 ratio. Where does the rest of the CO2 come from? $\endgroup$ Oct 23, 2017 at 21:32

1 Answer 1


Too long for a comment, so . . .

It's important to remember that the field of primordial atmospheres has a good bit of uncertainty to it. I've tried to read up on it as a layman and there's a good deal of contradicting theories and not much certainty. Your first article addresses this uncertainty several times.

The reason I mention the ratio of CO2 to N2 is because I read here, for example, that out gassing produces the molecules in about a 4.6:1 ratio.

Could you pull the quote to that 4.6:1 ratio. I didn't see it in that article and that sounds much too accurate for an estimate in this field. 4.6 to 1 suggests about a 2% range of uncertainty. I find that hard to believe. Certainly a ratio with that kind of accuracy should cite a respected research study.

And noticing that the ratio of CO2 to N2 is far higher on Venus than that just expected from out gassing

What ratio is expected from outgassing? Your 2nd article mentions a neat and tidy percentages:

The composition of outgassing is similar for Venus, Earth and Mars and is composed of 58% H2O, 23% CO2, 13% SO2, 5% N2 and traces of noble gases (Ne, Ar, Kr). The latter evolution of this outgassing is driven primarily by the surface temperature and chemistry of the planet.

But I call BS, cause one, this doesn't site a source and some basic google-research on Earth volcanic activity suggests the numbers are far from that neat and tidy - see charts below.

enter image description here Source


enter image description here Source

The 2nd source, above, doesn't even mention Nitrogen (N2), Wikipedia does, though they don't give percentages beyond 60% for water, 10-40% for CO2.

Earth's volcanic gas today is probably not a good measuring stick because of 3 plus billion years of life (some nitrogen fixing, lots of long chain carbon molecules) and some 4 billion years of plate tectonics.

And to my knowledge, a sample of volcanic gas from Venus has never been studied. In fact, there are no known active volcanoes on Venus at all. Source. Outgassing can perhaps be estimated by atmospheric content, but I'm skeptical on the accuracy of those numbers they gave. I think your first article from NASA far more reliable. They quote a ton of sources and several times in the article, indicate legitimate uncertainty.

What I took away from the NASA article:

Yes, when the solar system was very young, the t-tauri phase of the sun blew most of the atmosphere off the 4 inner planets (or 3 Earth-like if you want to ignore Mercury, which the first article does). This is backed up in part by the lack of Xe-129 which should form as a gas based on the 15 million year half life of I-129. If the atmospheres weren't blown off, we should see a certain percentage Xe-129 in the atmospheres of Earth/Mars/Venus. The percentage is much too low, suggesting the blow-off did happen over the first, perhaps 100 million years, continuing well after the end of the T-tauri phase, when the sun was still very active with solar flares.

The giant impact that is thought to have created the Moon (about 30 million years after planet formation) may have blown off a chunk of Earth's atmosphere as well, but the Solar wind probably did too.

Where the primordial atmosphere came from? Whether out gassing or comets (or a combination of both), I don't think that's a resolved question. What seems true, based on the first article, is that Venus's water evaporated and it lost it's water by a combination of photo-chemistry and it's higher temperature and perhaps some jeans escape. No water on it's surface may have slowed down any hope it had for plate tectonics and that reduced the rate that it outgassed. (a general summary of the NASA article). To get a better idea of Venus history, they'd like some rocky samples, but they don't have any.

Earth maintained it's oceans, perhaps because it had more water to begin with, or a lower temperature or combination of the two. The lunar impact may have sped up Earth's outgassing as it liquified the entire crust for thousands, perhaps as long as a couple million years. It's unclear if Venus ever had it's crust liquified.

But after Earth cooled after the giant impact, it had enough water for oceans and oceans made it possible to form carbonate rock and remove much of the CO2 from it's atmosphere. Life assisted with the removal of CO2 and fixed some nitrogen too. Ammonia also dissolves in water, so much of the nitrogen that came to Earth by comets could have been stored in the oceans, while ammonia that came to Venus by comet would have been vaporized as gas and light enough where some of it would inevitably be lost to jeans escape and subject to photo-chemical dissociation, and, a weaker magnetic field.

Earth and Venus have similarish amounts of CO2, it's just that Earth has most of it's CO2 stored in carbonate rock. Earth and Venus may have similar amounts of Nitrogen too, though Venus atmosphere has about 4 times as much as Earth's atmosphere, currently, that doesn't take into account Nitrogen in the oceans and fixed or bonded within the Earth.

I don't think Venus "extra" CO2 is a mystery because it didn't have oceans long enough to pull the CO2 out of it's atmosphere. We also don't know enough about the materials that formed Venus might have been different than those that formed Earth, and we don't know that much about the process that followed, so I don't think it's fair way to say it has too high a CO2 to N2 ratio.

It's entirely possible that the giant impact did blow away a portion of Earth's atmosphere, leaving it somewhat atmosphere depleted compared to Venus, but your NASA article points out that there's some uncertainty on how much atmosphere the Earth had after the giant impact leading to uncertainty on how quickly the Earth cooled.

It could, I think, work in the other direction, where the enormous heat of impact and liquification of the crust may have enabled more atmosphere to outgas, giving Earth a thicker atmosphere early on but post impact, where, the Venus equivalent of that gas might still be trapped inside Venus.

There's a lot about this subject that remains unresolved. This is just my takeaway from the articles. Take this answer with a grain of salt.


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