The role of Fischer-Tropsch catalysis in Jovian subnebular chemistry

Abstract

We examine the production of methane via Fischer-Tropsch catalysis in an evolving turbulent model of the Jovian subnebula and its implications for the composition of satellitesimals produced in situ. We show that there is a catalytically-active region in the Jovian subnebula from 65 Jupiter radii that moves inwards with time. The pressure range in this region is about 10 −4 to 10 −3 bar and implies that, if transport processes and the cooling of the subnebula are not considered, CO and CO 2 are entirely converted into CH 4 via Fischer-Tropsch catalysis in about 10 1 -10 2 and 10 3 -10 4 years, respectively. On the other hand, the comparison of the chemical conversion times with the viscous timescale of the subdisk in the catalytically-active region implies that only CO can be fully converted into CH 4 , the conversion of CO 2 thus being restricted to a limited production of CH 4 . Moreover, the time required by the Jovian subnebula to cool down from the optimal temperature for Fischer-Tropsch catalysis to the condensation temperature of ices is at least two orders of magnitude higher than the viscous timescale. This implies that any CH 4 produced in the catalytically-active zone will be accreted onto Jupiter long before being incorporated into the forming ices. We then conclude that in an evolving turbulent subnebula, even if Fischer-Tropsch catalysis is active, it has no influence on the composition of the forming satellitesimals that will ultimately take part in the formation of regular icy satellites, in opposition to what has been expected from stationary models.

Formation of Titan in Saturn's subnebula: constraints from Huygens probe measurements

“…As a consequence, the presence of a catalytically active region in the subnebula implies that methane and water formed via Fischer-Tropsch catalysis are accreted with the gas onto the forming Saturn. This conclusion is similar to the one found by Mousis et al (2006b) in the case of the Jovian subnebula. Finally, we conclude that the (homogeneous or heterogeneous) chemical processes that may occur in the subnebula are likely to have no effect on the composition of ices formed in the subnebula: the CO 2 :CO:CH 4 and N 2 :NH 3 ratios in the zone of planetesimal formation (at temperatures lower than 150 K) are the same as those acquired from the solar nebula.…”

“…We first examine the homogeneous gas-phase reactions of C and N volatiles in Saturn's subnebula, following the approach proposed by MGB02. We also investigate the catalytic effects on the gas-phase reactions induced by the presence of iron particles mixed with hydrogen gas in Saturn's subnebula, using the experimental data of Sekine et al (2005) and following the approach proposed by Mousis et al (2006b) for the Jovian subnebula.…”

Serpentinization and the Formation of H2and CH4on Celestial Bodies (Planets, Moons, Comets)

“…They concluded that CH 4 -rich satellitesimals could have formed in the catalytically active region of the subnebula and thus may have played an important role in the origin of Titan's atmosphere. In contrast, investigations led by Mousis et al ( 2006 ) confirmed the likely existence of a catalytically active region in the subnebula but estimated that it has no influence on the composition of the forming satellitesimals, because the produced CH 4 is shown to be accreted by Saturn prior to its trapping in the satellites' building blocks.…”