Impact-induced N2 production from ammonium sulfate: Implications for the origin and evolution of N2 in Titan's atmosphere

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 ((NH4)(2)SO4), for the replenishment of N-2 to the atmosphere in Titan's history. To investigate the conversion of (NH4)(2)SO4 into N-2 by hypervelocity impacts, we measured gases released from (NH4)(2)SO4 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 (NH4)(2)SO4 as a function of peak shock pressure ranging from similar to 8 to similar to 45 GPa. Our results indicate that the initial and complete shock pressures for N-2 degassing from (NH4)(2)SO4 are similar to 10 and similar to 25 GPa, respectively. These results suggest that cometary impacts on Titan (i.e., impact velocity nu(i) > similar to 8 km/s) produce N-2 efficiently; whereas satellitesimal impacts during the accretion (i.e., nu(i) < 4 km/s) produce N-2 only inefficiently. Even when using the proposed small amount of (NH4)(2)SO4 content in the crust (similar to 4 wt.%) (Fortes, AD. et al., 2007. Icarus 188, 139-153), the total amount of N-2 provided through cometary impacts over 4.5 Ga reaches similar to 2-6 times the present atmospheric N-2 (i.e., similar to 7 x 10(20)-2 x 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. (C) 2010 Elsevier Inc. All rights reserved.