  Cassini-Huygens data are used to re-examine the potential sources of the D/H
enhancement over solar, measured in methane, in Titan's atmosphere. Assuming
that the system is closed with respect to carbon, the use of constraints from
the Huygens probe for 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 Titan's atmosphere in order to
explain its current D/H value.
