Science highlights from Huygens:
#4. The origin of Titan's nitrogen atmosphere
During its descent to the surface, the GCMS measured isotopic ratios and trace species in the atmosphere. One of the objectives for the GCMS was to search for heavy, noble gases such as argon-36 (36Ar), argon-38 (38Ar), krypton (Kr), and xenon (Xe).
These primordial gases have been detected and measured in meteorites, in the atmospheres of Earth, Mars, Venus (to some extent), and Jupiter. Differing patterns of relative abundances and isotopic ratios of the gases provide insights into the origin and evolution of these objects. As a result, their measurements in the atmosphere of Titan were eagerly anticipated.
Scientists had theorised that these noble gases were present throughout the solar nebula, and should therefore have been incorporated into both Saturn and Titan during the early stages of planet formation. In the context of the origin of nitrogen, 36Ar is of particular importance, and the GCMS found that the ratio of 36Ar to nitrogen was about one million times less than is found in the Sun.
Direct condensation of gases in the young Titan would have resulted in the capture of 36Ar, as well as nitrogen, in solar proportions. However, the depleted ratio detected by the GCMS on Huygens implies that the nitrogen was captured as ammonia (NH3) or in other nitrogen-bearing compounds.
The rarity of noble gases on Earth has long been viewed as strong support for the atmosphere having been formed by the impacts of gas-rich planetesimals, and the near absence of noble gases from Titan provides more support for this hypothesis.