Cassini-Huygens data are used to re-examine the potential sources of the D/H enhancement over solar, measured in methane, in Titans atmosphere. Assuming that the system is closed with respect to carbon, the use of constraints from the Huygens probe f
or the determination of the current mass of atmospheric methane and the most up-to-date determination of D/H from Cassini/CIRS infrared spectra allow us to show that photochemical enrichment of deuterium is not sufficient to be the sole mechanism yielding the measured D/H value. A possible fractionation between CH3D and CH4 during the escape process may slightly enhance the deuterium enrichment, but is not sufficient to explain the observed D/H value over the range of escape values proposed in the literature. Hence, alternative mechanisms such as a primordial deuterium enrichment must be combined with the photochemical enrichment in Titans atmosphere in order to explain its current D/H value.
