The comet Hale-Bopp.

The origin of our Nitrogen

Hi there! You probably know that our atmosphere is mostly composed of nitrogen, its chemical symbol being N. It appears there as N2, i.e. a molecule of dinitrogen, which is composed of two atoms of nitrogen. It is usual to say nitrogen for dinitrogen, i.e. to make a confusion between the chemical element and the molecule. This compound is essential for the Earth to be habitable. The study I present today, Late delivery of nitrogen to the Earth, addressed the question of the origin of our nitrogen. The authors of this study, i.e. Cheng Chen, Jeremy L. Smallwood, Rebecca G. Martin and Mario Livio, are based at the University of Nevada, and the study has been recently published in The Astronomical Journal.

Nitrogen in our daily life

This title is probably too ambitious. I just will tell you about some aspects of nitrogen (I must confess, I am no chemist at all).
As dinitrogen, it is the main constituent of our atmosphere (some 78%). Moreover, this atom is present in the amino acids, in nucleic acids, i.e. DNA and RNA, and in many industrial compounds. You can find nitrogen in your coffee, you have some in propellants, in explosives,… Its liquid form can be used as a refrigerant,etc.
The overwhelming presence of nitrogen in our atmosphere probably contributes to make it ubiquitous in our daily life.
It is also very present in the universe. Actually, it is estimated to be the seventh in abundance in our Galaxy, i.e. the Milky Way.
Interestingly, it exits under several forms. It can be combined with other elements, for instance in ammonia or in nitric acid, but can also exist as an atom. More precisely, there are several ways it can exist as an atom, since there are two stable isotopic form. And the relative proportion of these two forms is not constant in the Solar System, which may tell you something on the origin of the nitrogen you observe.

Isotopes tell us something about its origin

As an atom, nitrogen has no electric charge, in the sense that the positive and negative charges balance. It is composed of a nucleus, around which 7 electrons orbit. Since these 7 electrons are 7 negative charges, the nucleus must contain 7 protons, to get a total null charge. However, the nucleus also contains neutral particles, i.e. neutrons, and the electric charge does not constrain their abundance. This opens the possibility for several versions of the atom of nitrogen to exist, which differ by the number of neutrons.

That does not mean that you can put as many neutrons as you want in the nucleus, since the element you would create, or Mother Nature would create, would not be necessarily stable. In fact, nitrogen has two stable isotopes, which are denoted 14N and 15N, respectively. xN means that the nucleus is composed of x particles, i.e. 7 protons, which is mandatory to keep the electrical balance, and (x-7) neutrons. So, an atom of 14N is made of 7 electrons, 7 protons, and 7 neutrons, while an atom of 15N is made of 7 electrons, 7 protons, and 8 neutrons.

Our atmosphere presents an isotopic ratio of 15N/14N of 3.676e-3, which means that 14N is overwhelming. However, in the Archean eon, i.e. between 4 and 2.5 billion years ago, the ratio was higher, i.e. 3.786e-3. This number comes from the analysis of Archean sedimentary rocks and crustal hydrothermal systems. However, the isotope 15N is more abundant in the comets. This leaves room for a possible enrichment of the Archean atmosphere in 15N by comets. The authors of this study tried to understand and quantify it.

The dynamical excitation of small bodies brings nitrogen to us

If part of the nitrogen comes from the space, then it should originate behind the nitrogen snow line. What is it? It is the line beyond which, nitrogen survives under a solid form (like ice). As you can understand, you get colder when you go further away from the Sun.

The authors show that the nitrogen snow line is located at some 12 AU (astronomical units), which is somewhere between the orbits of Saturn and Uranus. Small bodies beyond that limit are mostly Trans-Neptunian Objects, i.e. they belong to the Kuiper Belt. You must find a way to put these objects into the orbit of the Earth. Beware that you do not deal with the current Kuiper Belt, but with objects, which were beyond the 12 AU limit some billion years ago.

Interestingly, the authors present in their paper two different but complimentary aspects of this process. The first one is an analytical study of the excitation of the orbits of these objects by secular resonances, while the second one comes from numerical simulations.

Excitation by secular resonances

In physics, a resonance happens when the frequencies of two interacting phenomena get equal, or commensurable. In celestial mechanics, this happens for instance when two objects have the same orbital frequency (example: the Trojan asteroids of Jupiter, sharing the same orbit with the planet), or one object orbits exactly twice as fast as another one.

We speak of secular resonances when the ascending node of the orbit and / or the pericentre is involved. Here, the authors focus on the pericentre, since a resonant behavior involving it would result in the excitation of the eccentricity of the object. It gets resonant with a frequency forced by the system of the outer giant planets.
If a Trans-Neptunian Objects gets an eccentric orbit, then this orbit will become more and more elliptical, and it will be more likely to reach the Earth.

They particularly focused on the so-called ν8 frequency, which results in the most prominent secular resonance in the Kuiper Belt. This process being identified, it must be simulated, to estimate whether the comets undergoing this resonant excitation are likely to hit the Earth or not.

Numerical simulations

For that, they used a well-known simulation code called REBOUND, which is a N-body integrator. In other words, it simulates the motion of several massive bodies, and is particularly suitable for long-term simulations. The authors simulated the motion of 50,000 virtual comets over 100 Myr. These comets were initially uniformly distributed between 38 AU and 45 AU. This resulted in 104 collisions with the Earth.

Using such a numerical code is of high interest, because it not only renders the behavior of the secular resonance which is mentioned above, but also of all the gravitational interactions with the planets. These interactions include mean-motion resonances with Neptune.

10% of our nitrogen may have come from comets

The authors estimate that it can be deduced from their simulations that between the comets delivered between 1022 g and 1023 g of material to the Earth, which would translate between 3.9 x 1019 and 3.9 x 1020 grams of nitrogen. This would represent some 10% of the total nitrogen present on Earth.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

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