The role of Fischer-Tropsch catalysis in Jovian subnebular chemistry

¹ Observatoire de Besançon, CNRS-UMR 6091, 41bis avenue de l'Observatoire, BP 1615, 25010 Besançon Cedex, France e-mail: Olivier.Mousis@obs-besancon.fr
² Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
³ Department of Earth & Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Bunkyo, Tokyo 113-0033, Japan
⁴ Department of Complexity Science & Engineering, Graduate School of Frontier Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 227-8562, Japan

Received: 1 February 2006
Accepted: 4 August 2006

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 CO₂ are entirely converted into CH₄ via Fischer-Tropsch catalysis in about 10¹–10² and 10³–10⁴ 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 CH₄, the conversion of CO₂ thus being restricted to a limited production of CH₄. 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 CH₄ 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.