On the Origin of Titan’s Nitrogen

Today we travel to Titan, one of Saturn’s many moons. A thick orangy haze obscures the surface. This haze–Titan’s atmosphere–is mostly nitrogen. Today’s question: where did that nitrogen come from?
Before we delve into this, let’s have a quick lesson on origins in general, in terms of the solar system. Billions of years ago, our sun was surrounded by gas and tiny flecks of rock and metal. These tiny bits of material would stick together, and when they grew large enough, attracted even more material with their gravitational pull. In a process called accretion, they would accumulate into what were essentially baby planets (or in the case of Titan, baby moons) called planetesimals. These were the ‘building blocks’ of the modern solar system.
These areas of gas and dust also would have surrounded young, forming gas giant planets. Early Saturn would have had its own; the Saturnian subnebula, a fancy term for today. Your other fancy technical term of the day is for the gas and dust surrounding the young sun, the protosolar nebula (PSN).
So, there are two basic ways for Titan to have formed: planetesimal(s) formed in the PSN that ended up orbiting Saturn as one moon after all the collisions and gravitational interactions the early solar system entailed, or it formed within the Saturnian subnebula much as the planets did around the sun.
Titan as seen by the Cassini spacecraft. Image Credit: NASA/JPL-Caltech/Space Science Institute.
Titan as seen by the Cassini spacecraft. Image Credit: NASA/JPL-Caltech/Space Science Institute.
So how could we figure this out?
Enter today’s article: Protosolar Ammonia as the Unique Source of Titan’s Nitrogen by Kathleen E. Mandt, Olivier Mousis, Jonathan Lunine, and Daniel Gautier. Published in 2014 in Astrophysical Journal Letters, this article shows the reader a method of answering the proposed question using isotopes.
As you might remember from high school chemistry, isotopes are elements with a different number of neutrons than you might expect. This does not change what the element is, but it does change some other properties, such as the mass. You can also tell the difference between isotopes of the same element: in our case, this is going to be very helpful.
We are looking at nitrogen, more specifically, nitrogen-14 and nitrogen-15. One thing we can measure is the ratio of these two isotopes, nitrogen-14/nitrogen-15.
Scientists suspect that the PSN ratio from molecular nitrogen was about 435. Without going into the chemistry of it, the Saturnian subnebula is expected to have similar results. Ammonia ices on comets (which formed in the PSN, but with different chemical conditions than molecular nitrogen in the PSN),  are measured to have ratios around 130, and Earth’s ratio is 272.
And Titan? Titan’s is 167.7.
This is clearly not in the  435 range. The authors did a calculation to see if Titan could have started off with a ratio similar to Earth’s and have chemical processes change the ratio to its current value. However, none of the processes that could do this make much impact, even in the 4.6 billion years since the start of the solar system. They also calculated that at most, Titan could have started with a ratio of 190. In other words, this lines up with ammonia ice from the PSN, chemically broken down into molecular nitrogen. So, Titan’s nitrogen is likely from the ammonia ices of the early solar system. Earth’s nitrogen was also suspected to have originated from the ammonia ice of comets, but this seems unlikely, as the nitrogen ratio in our atmosphere has likely remained unchanged. As often in science, realizing the likely solution to one question leads us to question something else.
Bibliography
Bennett, Jeffrey et al. The Cosmic Perspective: The Solar System. 7th ed. San Francisco: Addison-Wesley. Print.
A textbook used for reference about the accretion process, as well as isotopes.
Jet Propulsion Labratory and California Institute of Technology. “Titan’s Building Blocks Might Pre-date Saturn. JPL/CalTech, 23 June 2014. Web.
An article discussing today’s paper, from which I took the image in this post.
Mandt, Kathleen E., et al. “Protosolar ammonia as the unique source of Titan’s nitrogen.” The Astrophysical Journal Letters 788.2 (2014): L24.
Today’s main article.
National Aeronautics and Space Administration. “About Saturn & Its Moons: Titan.” Cassini Solstice Mission. N.p., n.d. Web.
A page discussing Titan, with much information on the moon in its tabs.

 

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