The planets of the Solar System were constructed from patches of gas and dust that clumped together under the force of gravity. The newly formed planets were surrounded in residual material. This residual material, according to the widely accepted theory, eventually coalesced into the various moons that populate our corner of the universe. Since our solar system's moons formed around their host planets, both would have been constructed under the same gravitational influences. Scientists have long thought that Saturn, with 62 named moons, constructed its natural satellites in the aforementioned manner. However, a new study suggests that Titan, Saturn's enormous planet-like moon, may have been on the production line before Saturn itself.
The finding comes from a joint NASA and ESA-endorsed study looking at Titan’s nitrogen-rich atmosphere. The authors claim that the gas found on Titan originated in the distant Oort cloud, the "home of the comets" which is comprised of all the rocky debris surrounding our Sun that orbits far beyond Neptune. The main implication of this study is that a crucial part of Titan’s atmosphere was formed very early in the Solar System’s development, rather than coming together after the emergence of Saturn.
Kathleen Mandt, of Southwest Research Institute in San Antonio, led the team, which took a detailed look at the isotope ratio of Titan’s nitrogen. Elements come in different forms depending on their number of isotopes. The ratio between one isotope relative to another of the same element is a good indicator of the conditions under which they formed and on what timescale. The researchers found that the ratio between Titan’s nitrogen-14 and nitrogen-15 isotopes is significantly different. Ultimately, the team discovered that our solar system is not old enough for the nitrogen isotope ratio to have changed this significantly i.e., Titan has not been around long enough to account for such a change, so it must have formed elsewhere.
The isotope ratio seems to be much more consistent with that of the Oort cloud, where innumerable comets and asteroids have rested for billions of years.
"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 said on NASA’s JPL website.
Moreover, this new evidence means that we may have to update the theory surrounding for formation of our own planet's sky. It was believed that the nitrogen in Earth’s atmosphere, like Titan, was the result of ammonia ice that was brought into the inner solar system by travelling comets. This new research gives further support to the advancing view that comet fragments and meteorites were not responsible for delivering the element to Earth. Further, it seems to be the case that the Earth and Titan received their nitrogen from two different sources. Now it is "uncertain whether the building blocks of Titan" formed within the Saturnian dust cloud, according to the research team in their paper abstract.
Mandt commented: "Some have suggested that meteorites brought nitrogen to Earth, or that nitrogen was captured directly from the disk of gas that formed the sun. This is an interesting puzzle for future investigations."
Titan has been watched closely by the Cassini-Huygens mission since 2004. The Cassini spacecraft has made over 100 passes of Saturn’s moon since it arrived in the area a decade ago and even landed a probe (Huygens) on its surface in 2005. Mandt’s group used atmospheric data gathered by this mission to come to their conclusions, which they published in the peer-reviewed ‘Astrophysical Journal Letters’.
In August 2014, ESA’s Rosetta probe is due to arrive at a comet approaching near to our Sun. This rendezvous will give Mandt’s team the opportunity to expand on their theories once Rosetta begins to investigate the comet's ingredients.