Nitrogen in the atmosphere of Saturn's moon Titan's was formed at temperatures far colder than Saturn itself, throwing a major curveball into ideas of solar system formation, including the origins of our own atmosphere.
Like Earth, Titan's atmosphere is mainly nitrogen, and thought to serve as quite a good model of conditions on the early Earth. Most of it is nitrogen 14 (7 protons, 7 neutrons) but it also contains nitrogen 15 (7 and 8). Both are stable and while nitrogen 14 would be slightly more inclined to escape Titan's gravity to space, calculations by Dr Kathleen Mandt of Southwest Research Institute indicate this would not have occurred to an extent that would alter the composition over the lifetime of the solar system.
"When we looked closely at how this ratio could evolve with time, we found that it was impossible for it to change significantly. Titan's atmosphere contains so much nitrogen that no process can significantly modify this tracer even given more than four billion years of solar system history," Mandt says.
Consequently, Mandt could compare Titan's nitrogen ratio with that of other objects in the solar system. In the Astrophysical Journal Letters she reveals that Titan's atmosphere matches the nitrogen found in ammonia in comets from the outer reaches of the solar system, but not on Earth.
“Titan's atmosphere must have originated as ammonia ice formed in the Protosolar Nebula under conditions similar to that of cometary formation,” the paper concludes. The comets of the outer reaches of the solar system are thought to predate the planets, including Saturn. Mandt has previously found that the methane in Titan's atmosphere is much younger, less than a billion years old.
At one point it was theorized that both Titan and Earth had atmospheres that would match the make-up of comets, but studies of comets undermined this idea, and the NASA/ESA Cassini-Huygens mission showed Titan's nitrogen ratio is different from the Earth's. The question then became whether Titan started that way, or had it evolved over time. Mandt claims to have solved that problem.
To complicate matters however, it is thought that comets born in the Oort Cloud and Kuiper Belt differ in their Hydrogen and Nitrogen isotopic ratios. Later this year the ESA's Rosetta mission will encounter comet 67P/Churyumov-Gerasimenko, thought to be a Kuiper Belt comet, providing an opportunity to test this idea. If, as anticipated, Kuiper Belt comets have a different isotopic ratio it will raise questions as to how Titan could resemble objects so much further out, rather than its (relatively speaking) neighbors.
The other challenge is to uncover the origin of our own atmosphere. At one time our own nitrogen rich atmosphere was thought to have been brought here by comets, but the difference with the isotopic ratio for Oort cloud comets undermined this, and the findings on Titan make it less credible still. "Some have suggested that meteorites brought nitrogen to Earth, or that nitrogen was captured directly from the disk of gas that formed the sun,”says Mandt This is an interesting puzzle for future investigations.”