Impact Devolatilization of Ammonium Sulfate: Implications for the Origin of N2 in Titan’s Atmosphere

Introduction: Titan has a thick atmosphere composed primarily of N 2 with a few percent of CH 4. One of the most puzzling aspects of Titan's atmosphere is the origin of N 2 because of the near absence of non-radiogenic noble gases (e.g., 36 Ar) in the atmosphere, highly suggestive of that the nitrogen was captured as NH 3 and other non-N 2-bearing compounds in the satellitesimals [1]. Although several studies have investigated the mechanism responsible for converting NH 3 to N 2 in the primitive atmosphere of Titan generated during the accretion [e.g., 2, 3], it is still unclear how and when the production of N 2 has occurred. In this study, we assess the role of shock-induced devolatilization of Titan's icy crust and mantle by hypervelocity impacts of cometary bodies for the origin of N 2 in the atmosphere. Although the chemical compositions of Titan's crust and mantle are still uncertain, ammonium sulfate ((NH 4) 2 SO 4) is considered as one of the major components when primordial NH 3 reacts with sulfate-rich water in the ocean during the accretion and the differentiation [4, 5]. We conduct laboratory experiments of hypervelocity impacts onto ammonium sulfate to investigate whether the conversion of ammonium sulfate to N 2 occurs or not. Then, we measure the efficiency of N 2 production as a function of peak shock pressure and discuss whether the impact devolatilization of Titan's crust can explain the present amount of N 2 in Titan's atmosphere. Experimental: We conducted laboratory experiments of hypervelocity impact using a laser gun method [6]. The configuration of our experimental system is shown in Fig 1. A quadruple mass spectrometer (QMS; BGM-202, Qulee) was used for the gas analysis of shock-induced gas species. About 10 Pa of helium gas was introduced into the vacuum chamber as an internal standard for quantification of N 2 production. We used a gold (Au) foil (Nilaco, 99.95% purity) as an impactor, and isotopic-labeled ammonium sulfate powder compressed at 20 MPa (15 N>99%; ISOTEC) as a target for identification of the N 2 production. The porosity of the compressed target