Cooked Volatiles and the Origin of Titan’s Atmosphere: Evidence of Deep Hydrothermal Activity?

Abstract:

As on the terrestrial planets, key clues to the origin of Titan’s enigmatic atmosphere are contained in the abundances of noble gases and stable isotopes in the atmosphere. The Huygens GCMS measured the abundances of ⁴⁰Ar, ³⁶Ar, and ²²Ne (tentatively); as well as the nitrogen and carbon isotopic compositions of atmospheric N₂ and CH₄, respectively. No isotopes of Kr or Xe were detected (<10 ppbv). Cassini CIRS has provided us with the D/H ratio in CH₄. Here, I attempt to explain these data by developing the hypothesis that the noble gases, nitrogen, and methane originated in the rocky core of Titan [1]. The presence of ⁴⁰Ar demonstrates that volatile species can be delivered from the deep interior to the atmosphere. Consistent with [2], I find that Titan’s primordial core should have contained sufficient ³⁶Ar and ²²Ne to explain their reported abundances. By extrapolating this model, I provide a new explanation for why the GCMS failed to detect Kr or Xe, as the predicted mixing ratios of ⁸⁴Kr and ¹³²Xe are ~0.2 ppbv and ~0.01 ppbv, respectively. I find that nitrogen should be outgassed similarly to argon, while krypton can serve as a geochemical proxy for methane, given the similar volatilities of these pairs of substances. This allows me to deduce that geochemical reactions in Titan’s core could have generated enough N₂ and CH₄ from accreted NH₃ and CO₂, respectively. A hydrothermal origin of atmospheric nitrogen is also supported by the similarity in N isotopes between Titan’s N₂ and cometary NH₃ [3]. I find that the isotopic ratios in methane can be explained by low-temperature (~300 K) equilibria with liquid water and the alteration mineral calcite. Looking toward the future, this model predicts ¹²C/¹³C ≈ 84 in dry ice, and D/H ≈ 170 ppm in water ice on Titan’s surface. References: [1] Glein C.R. (2014) Icarus, submitted; [2] Tobie G., et al. (2012) ApJ 752, 125; [3] Mandt K.E., et al. (2014) ApJ Lett. 788, L24.