Contributions from Accreted Organics to Titan’s Atmosphere: New Insights from Cometary and Chondritic Data.

Since its discovery in the first half of the 20th century, scientists have puzzled over the origins of Titan’s atmosphere. Current models suggest that atmospheric N2 on Titan may have originated from NH3-bearing ice with N-isotopic ratios similar to those observed in NH2 in cometary comae (14N/15N ∼ 136). In contrast, N2 ice appears to be too 15N poor to explain Titan’s atmosphere (14N/15N ∼ 168). Additionally, data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko suggest that the Ar/N2 ratio of outer solar system planetesimals may be too high for a comet-like N2 source on Titan. The Rosetta mission also revealed an astonishing abundance of N-bearing complex organic material. While thermal fractionation of cometary sources during Titan accretion may explain the loss of N2- and Ar-rich ices, more refractory materials such as complex organics would be retained. Later heating in the interior may lead to volatilization of accreted organics, consistent with Cassini–Huygens measurements of 40Ar that suggest outgassing from the interior may have played a role in atmosphere formation. Here, we develop a three endmember mixing model for N isotopes and the 36Ar/14N ratio of Titan’s atmosphere, and consider the implications for the source of atmospheric methane. Our model suggests that Titan’s interior is likely warm, and that N from accreted organics may contribute on the order of 50% of Titan’s present-day nitrogen atmosphere. [ABSTRACT FROM AUTHOR]