Impact-induced N 2 production from ammonium sulfate: Implications for the origin and evolution of N 2 in Titan’s atmosphere

Abstract

Chemical reactions and volatile supply through hypervelocity impacts may have played a key role for the origin and evolution of both planetary and satellite atmospheres. In this study, we evaluate the role of impact-induced N 2 production from reduced nitrogen-bearing solids proposed to be contained in Titan’s crust, ammonium sulfate ((NH 4) 2SO 4), for the replenishment of N 2 to the atmosphere in Titan’s history. To investigate the conversion of (NH 4) 2SO 4 into N 2 by hypervelocity impacts, we measured gases released from (NH 4) 2SO 4 that was exposed to hypervelocity impacts created by a laser gun. The sensitivity and accuracy of the measurements were enhanced by using an isotope labeling technique for the target. We obtained the efficiency of N 2 production from (NH 4) 2SO 4 as a function of peak shock pressure ranging from ∼8 to ∼45 GPa. Our results indicate that the initial and complete shock pressures for N 2 degassing from (NH 4) 2SO 4 are ∼10 and ∼25 GPa, respectively. These results suggest that cometary impacts on Titan (i.e., impact velocity v i > ∼8 km/s) produce N 2 efficiently; whereas satellitesimal impacts during the accretion (i.e., v i < 4 km/s) produce N 2 only inefficiently. Even when using the proposed small amount of (NH 4) 2SO 4 content in the crust (∼4 wt.%) (Fortes, A.D. et al., 2007. Icarus 188, 139–153), the total amount of N 2 provided through cometary impacts over 4.5 Ga reaches ∼2–6 times the present atmospheric N 2 (i.e., ∼7 × 10 20–2 × 10 21 [mol]) based on the measured production efficiency and results of a hydrodynamic simulation of cometary impacts onto Titan. This implies that cometary impacts onto Titan’s crust have the potential to account for a large part of the present N 2 through the atmospheric replenishment after the accretion.