Understanding Titan’s Atmospheric Isotope Inventory through Laboratory Photolysis Experiments using Vacuum Ultraviolet Photons from Advanced Light Source Synchrotron

Abstract:

Titan, Saturn’s planet-like moon with a thick atmosphere consists mainly of N₂ (98.4 %) and CH₄ (1.4%). It is debated whether the N₂ is primordial, or the NH₃, which later converted to N₂ by physic-chemical processes and, if NH₃ is primordial, what is the source of that material: Saturnian-subnebula or the comets? N₂ is enriched in ¹⁵N (¹⁴N/¹⁵N = 160 compared to 272 for Earth) and in geochemical terminology, d¹⁵N_air = 700 ‰ (parts per thousand with respect to ambient air). On the same scale the solar wind and Jupiter’s atmosphere are ~ -400 ‰ (depleted in ¹⁵N). The comets (NH₃ and HCN) and insoluble organic matter in meteorites are also enriched in ¹⁵N in the range up to a few thousand ‰. On the contrary, the carbon isotopic ratio in CH₄ in Titan is similar to the other solar system bodies (¹²C/¹³C~ 89).

We have performed extensive low temperature (80 K) photodissociation of N₂ and CO (in presence of H₂) at VUV wavelengths to measure the isotopic fractionation in the products. The integrated instantaneous fractionation in the product NH₃ is about 1000 ‰ over the N₂ dissociation regime (80-100 nm), which arise due to quantum mechanical selection rules. CO₂ and CH₄, the products of CO photodissociation, show contradictory results for two elements. While product O (trapped in CO₂) is enriched by few thousand ‰, there is no significant C isotopic enrichment in CH₄. These laboratory measurements along with the measurements by Cassini–Huygens spacecraft constrain the origin of volatiles in Titan’s atmosphere and indicate that Titan accreted comet-like NH₃ and CH₄, which are the 1st generation photolysis products (of the remaining materials after the formation of gas giants) in the solar nebula. Later, NH₃ converted to N₂ in a bulk fashion (within Titan) and retained mostly identical isotopic composition. ¹⁵N enrichment measured in HCN in the present day atmosphere (d¹⁵N_air > 1500 ‰), is possibly from the 2^nd generation N₂ photolysis in Titan’s modern atmosphere.