# What is the origin of the dominant atmospheric nitrogen content in Earth's atmosphere?

Comparing the atmospheric compositions of Earth with our nearest neighbours:

As the table shows, the Earth's atmospheric nitrogen concentration is 78%, compared to 3.8% for Venus and 2.7% for Mars Image source.

I have read the excellent question and answer Why do Earth and Venus have different atmospheres?, but the answer focuses on the $\ce{CO2}$ content of the planets, this question is about the $\ce{N2}$ content. Also, have read Why do some planets have lots of $\ce{N2}$ and others none?, but in this question, I am looking at how nitrogen concentrations dominated over the other chemicals in Earth's atmosphere, not looking at similarities between planetary processes.

In modern Earth, the nitrogen cycle maintains nitrogen levels, however the early pre-biotic atmosphere, was believed by

most of the scientific community now believes that the early Earth's atmosphere was not reducing. Instead, scientists beleive the atmosphere was full of oxidants, such as $\ce{CO2}$ and $\ce{N2}$.

What is the origin of the dominant atmospheric nitrogen content in Earth's atmosphere?

• It would be fantastic if you could add an image caption/transcript to make your question more accessible to folk with low/no vision. Screen-readers are a thing! :-) – kaberett Nov 3 '14 at 21:50
• @kaberett a written captions is included – user889 Nov 14 '14 at 9:31
• @Michael true, have linked to that good question in my question. – user889 Dec 9 '14 at 6:16

According to the recent paper in Nature Geoscience: Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere, $N_2$ originates from regions of the Earth where plates are converging. Venus and Mars lack plate tectonics and therefore lack $N_2$ in their atmospheres. In other regions of Earth upper mantle, and in Venus and Mars, nitrogen is expected to exist as aqueous ammonium and not enter the atmosphere.

• +1. That certainly is a timely paper with regard to this question! – David Hammen Nov 3 '14 at 20:56
• Excellent paper! precisely answers the question in a way I never expected - definitely accepting this answer. – user889 Nov 4 '14 at 0:05
• +1 Though I'm a bit dubitative regarding the statement that "Venus [...] lack N2 in their atmospheres". 3% of the atmosphere, considering the density of Venus atmosphere is still a lot. I think it corresponds to a partial pressure of more or less 3 bars. – plannapus Nov 4 '14 at 7:23
• @plannapus good point, the paper is discussing the ratio of $N_2$ to noble gases, rather than absolute amount of or percent $N_2$, alternate link: drsamimikhail.files.wordpress.com/2014/02/… ; supplemental material: drsamimikhail.files.wordpress.com/2014/02/… – DavePhD Nov 4 '14 at 12:59

Nitrogen is volatile in most of its forms here on Earth. It is fairly non-reactive with most materials that make up the solid part of Earth, and it is very stable in the presence of solar radiation in the atmosphere. Even though, nitrogen is 4 times more abundant than oxygen in the atmosphere, we must also consider the relative abundances of nitrogen and oxygen over the entire Earth, where oxygen is about 10,000 times more abundant (water, sand). In life forms, nitrogen is less abundant than oxygen, mainly due to the abundance of water.

Venus and Mars, though, have lots of CO2 in the atmosphere. Plant life on Earth (Ocean + Land) and precipitation have taken CO2 out of the atmosphere... plants created the oxygen reserve. Earth is a distinctly unique planet because it undergoes hydrologic, geologic, and biological processes that take CO2 out of the atmosphere.

• This does not explain the origin of atmospheric nitrogen, as per the question. – user889 Nov 3 '14 at 20:19
• @Omen All atomic matter on Earth came from the solar nebula. The point is that the nitrogen molecule is not being consumed because it is very stable in sunlight. It has no place to go and noone wants it. Oxygen is too attractive of a molecule. – farrenthorpe Nov 3 '14 at 23:58
• @farrenthorpe but it's true for the other planets as well. So why the difference? – Gimelist Dec 9 '14 at 6:05
• @Michael sorry could you clarify what you mean? – farrenthorpe Dec 9 '14 at 15:21
• @Michael Earth is distinctly unique because it undergoes hydrologic and biological processes that take CO2 out of the atmosphere. Large gaseous giants still have large reserves of atmospheric molecular hydrogen so they are not a good comparison. Venus and Mars have lots of CO2 in the atmosphere but plant life on Earth (Ocean + Land) and precipitation have taken CO2 out of the atmosphere... plants of course created an Oxygen reserve. – farrenthorpe Dec 9 '14 at 22:44

The two major sources of atmospheric nitrogen are volcanoes and bottom-dwelling denitrifying bacteria, who produce it from nitrate ion dissolved in seawater. If anyone knows the relative importance of these two sources, I'd very much appreciate knowing. Tx.

The reason Earth has so much nitrogen is because it is non reactive and builds up. The origin of the nitrogen is covered in previous answers. Also, Venus has as much nitrogen in its atmosphere as Earth, but it is a much smaller percentage due to the large amount of CO2. Here is a nice graph showing what gases can exist in a planetary atmosphere based on temperature and escape velocity (mass/radius) of a planet http://abyss.uoregon.edu/~js/ast121/lectures/lec14.html .