Tag Archives: Habitability

Earth

The origin of our Nitrogen

Leave a comment

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.

Outline

Nitrogen in our daily life
Isotopes tell us something about its origin
The dynamical excitation of small bodies brings nitrogen to us
10% of our nitrogen may have come from comets
The study and its authors

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.

Asteroids: Main Belt

Our water comes from far away

Leave a comment

Hi there! Can you imagine that our water does not originally come from the Earth, but from the outer Solar System? The study I present you today explains us how it came to us. This is Origin of water in the inner Solar System: Planetesimals scattered inward during Jupiter and Saturn’s rapid gas accretion by Sean Raymond and Andre Izidoro, which has recently been published in Icarus.

Outline

From the planetary nebula to the Solar System
No water below this line!
Planet encounter and gas drag populate the Asteroid Belt
Making the exoplanets habitable
To know more…

From the planetary nebula to the Solar System

There are several competing scenarios, which describe a possible path followed by the Solar System from its early state to its current one. But all agree that there was originally a protoplanetary disk, orbiting our Sun. It was constituted of small particles and gas. Some of the small particles accreted to form the giant planets, first as a massive core, then in accreting some gas around. The proto-Jupiter cleared a ring-shaped gap around its orbit in the disk, Saturn formed as well, the planets migrated, in interacting with the gas. How fast did they migrate? Inward? Outward? Both? Scenarios diverge. Anyway, the gas was eventually ejected, and the protoplanetary disk was essentially cleared, except when it is not. There remains the telluric planets, the giant planets, and the asteroids, many of them constituting the Main Belt, which lies between the orbits of Mars and Jupiter.
If you want to elaborate a fully consistent scenario of formation / evolution of the Solar System, you should match the observations as much as possible. This means matching the orbits of the existing objects, but not only. If you can match their chemistry as well, that is better.

No water below this line!

The origin of water is a mystery. You know that we have water on Earth. It seems that this water comes from the so-called C-type asteroids. These are carbonaceous asteroids, which contain a significant proportion of water, usually between 5 and 20%. This is somehow the same water as on Earth. In particular, it is consistent with the ratios D/H and 15N/14N present in our water. D is the deuterium, it is an isotope of hydrogen (H), while 15N and 14N are two isotopes of nitrogen (N).

These asteroids are mostly present close to the outer boundary of the Main Belt, i.e. around 3.5 AU. An important parameter of a planetary system is the snow line: below a given radius, the water cannot condensate into ice. That makes sense: the central star (in our case, the Sun) is pretty hot (usually more than pretty, actually…), and ice cannot survive in a hot environment. So, you have to take some distance. And the snow line of the Solar System is currently lose to 3.5 AU, where we can find these C-type asteroids. Very well, there is no problem…

But there is one: the location of the snow line changes during the formation of the Solar System, since it depends on the dynamical structure of the disk, i.e. eccentricity of the particles constituting it, turbulence in the gas, etc. in addition to the evolution of the central star, of course. To be honest with you, I have gone through some literature and I cannot tell you where the snow line was at a given date, it seems to me that this is still an open question. But the authors of this study, who are world experts of the question, say that the snow line was further than that when these C-types asteroids formed. I trust them.

And this raises an issue: the C-types asteroids, composed of at least 5% of water, have formed further than they are. This study explains us how they migrated inward, from their original location to their present one.

Planet encounter and gas drag populate the Asteroid Belt

The authors ran intensive numerical simulations, in which the asteroids are massless particles, but with a given radius. This seems weird, but this just means that the authors neglected the gravitational action of the asteroids on the giant planets. The reason why they gave them a size in that it influences the way the gas drag (remember: the early Solar System was full of gas) affects their orbits. This size actually proved to be a key parameter. So, these asteroids were affected by the gas and the giant planets, but in the state they were at that time, i.e. initially Jupiter and Saturn were just slowly accreting cores, and when these cores of solid material reached a critical size, then they were coated by a pretty rapid (over a few hundreds of kyr) accretion of gas. The authors considered only Jupiter in their first simulations, then Jupiter and Saturn, and finally the four giant planets. Their different parameters were:

  • the size of the asteroids (planetesimals),
  • the accretion velocity of the gas around Jupiter and Saturn,
  • the evolution scenario of the early Solar System. In particular, the way the giant planets migrated.

Simulating the formation of the planet actually affects the orbital evolution of the planetesimals, since the mass of the planets is increasing. The more massive the planet, the most deviated the asteroid.

And the authors succeed in putting C-type asteroids with this mechanism: when a planetesimal encounters a proto-planet (usually the proto-Jupiter), its eccentricity reaches high numbers, which threatens its orbital stability around the Sun. But the gas drag damps this eccentricity. So, these two effects compete, and when ideally balanced this results in asteroids in the Main-Belt, on low eccentric orbits. And the authors show that this works best for mid-sized asteroids, i.e. of the order of a few hundreds of km. Below, Jupiter ejects them very fast. Beyond, the gas drag is not efficient enough to damp the eccentricity. And this is consistent with the current observations, i.e. there is only one C-type asteroid larger than 1,000 km, this is the well-known Ceres.

However, the scenarios of evolution of the Solar System do not significantly affect this mechanism. So, it does not tell us how the giant planets migrated.

Once the water ice has reached the main asteroid belt, other mechanism (meteorites) carry it to the Earth, where it can survive thanks to our atmosphere.

Making the exoplanets habitable

This study proposes a mechanism of water delivery, which could be adapted to any planetary system. In particular, it tells us a way to make exoplanetary planets habitable. Probably more to come in the future.

To know more…

  • The study, presented by the first author (Sean N. Raymond) on his own blog,
  • The website of Sean N. Raymond,
  • The IAU page of Andre Izidoro.
  • And I would like to mention Pixabay, which provides free images, in particular the one of Cape Canaveral you see today. Is this shuttle going to fetch some water somewhere?

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