Modelling of thunderclouds and lightning generation on Titan

The likelihood of lightning generation in Titan's troposphere is investigated by theoretical consideration and a numerical one-dimensional time-dependent thundercloud model. The main cloud electrification hypotheses proposed for terrestrial thunderstorms are examined taking into account recent knowledge concerning Titan's atmospheric state, especially condensation, and the lower ionosphere. Titan's thunderclouds may be quite rare because of the difficult methane nucleation and low convection energy, but once moist convection is triggered, a massive cloud containing slowly falling solid and liquid methane particles may be expected. In this case the cloud rapidly attaches a large amount of free electrons, which an abundant in Titan's troposphere due to the low abundance of electrophilic species. The entirely negative space charge in the cloud causes within a few minutes a temporary maximum electric field of 2 x 10(6) V m(-1) which is sufficient to initiate 20 km long negative cloud-to-ground lightning strokes in Titan's lower troposphere. The collisional charging, on the other hand, appears to be less efficient since the charge transfer itself may be limited at Titan's cold temperatures and no substantial charge redistribution takes place in the cloud due to the weak updraft and gravitation. (C) 2001 Elsevier Science Ltd. All rights reserved.