| Prior to Cassini-Huygens, it was debated how Titan acquired its earth-like atmosphere of
nitrogen [1]. This talk will review the history of Titan’s atmosphere, models, and the unique
role of Cassini-Huygens in understanding the origin and evolution of an atmosphere of
nitrogen on Titan. After hydrogen and helium, nitrogen is the fourth most abundant
element in the solar system. In the colder outer solar system beyond 5 AU, nitrogen
is bound to hydrogen in the giant planets. Thus ammonia (NH3), not N2, is the
dominant reservoir of nitrogen in these objects. The satellites that form in the relatively
warm and dense subnebula of the gas giant planets, Jupiter and Saturn, may acquire
nitrogen as NH3 during their accretion [2], although some models had proposed N2,
not NH3, as the stable form of nitrogen in the subnebulae. The latter is reflected
in the atmosphere of Triton, which almost certainly accreted nitrogen directly as
N2, since N2 can be the stable form of nitrogen in the very cold environment of
Neptune. Before Cassini-Huygens, it was debated whether Titan, the largest moon
of Saturn, also acquired its nitrogen directly as N2, putting it in the same class
as Neptune’s moon Triton half its size, or the nitrogen on Titan was secondary
atmosphere, produced from a nitrogen bearing molecule, putting Titan in the class
with terrestrial planets. The evidence from Cassini-Huygens to be discussed in this
talk leaves no doubt that Titan’s nitrogen atmosphere is secondary [3]. Probable
scenarios of the sustenance, evolution and reduction or demise of this atmosphere will
also be explored. References: [1]Owen T. (2000), Planet. Space Sci. 48, 747-752.
[2]Prinn R.G., Fegley B. (1981), Astrophys J. 249, 308-317. [3]Atreya S.K., Lorenz
R.D., Waite J.H. (2009), pp 177-199, in Titan (R.H. Brown et al., eds.) Springer. |