The role of Fischer–Tropsch catalysis in the origin of methane-rich Titan

Abstract

Fischer–Tropsch catalysis, which converts CO and H 2 into CH 4 on the surface of iron catalyst, has been proposed to produce the CH 4 on Titan during its formation process in a circum-planetary subnebula. However, Fischer–Tropsch reaction rate under the conditions of subnebula have not been measured quantitatively yet. In this study, we conduct laboratory experiments to determine CH 4 formation rate and also conduct theoretical calculation of clathrate formation to clarify the significance of Fischer–Tropsch catalysis in a subnebula. Our experimental result indicates that the range of conditions where Fischer–Tropsch catalysis proceeds efficiently is narrow ( T ∼ 500 – 600 K ) in a subnebula because the catalysts are poisoned at temperatures above 600 K under the condition of subnebula (i.e., H 2/CO = 1000). This suggests that an entire subnebula may not become rich in CH 4 but rather that only limited region of a subnebula may enriched in CH 4 (i.e., CH 4-rich band formation). Our experimental result also suggests that both CO and CO 2 are converted into CH 4 within time significantly shorter than the lifetime of the solar nebula at the optimal temperatures around 550 K. The calculation result of clathration shows that CO 2-rich satellitesimals are formed in the catalytically inactive outer region of subnebula. In the catalytically active inner region, CH 4-rich satellitesimals are formed. The resulting CH 4-rich satellitesimals formed in this region play an important role in the origin of CH 4 on Titan. When our experimental data are applied to a high-pressure model for subnebula evolution, it would predict that there should be CO 2 underneath the Iapetus subsurface and no thick CO 2 ice layer on Titan's icy crust. Such surface and subsurface composition, which may be observed by Cassini–Huygens mission, would provide crucial information on the origin of icy satellites.