  We describe a scenario of Titan's formation matching the constraints imposed
by its current atmospheric composition. Assuming that the abundances of all
elements, including oxygen, are solar in the outer nebula, we show that the icy
planetesimals were agglomerated in the feeding zone of Saturn from a mixture of
clathrates with multiple guest species, so-called stochiometric hydrates such
as ammonia hydrate, and pure condensates. We also use a statistical
thermodynamic approach to constrain the composition of multiple guest
clathrates formed in the solar nebula. We then infer that krypton and xenon,
that are expected to condense in the 20-30 K temperature range in the solar
nebula, are trapped in clathrates at higher temperatures than 50 K. Once
formed, these ices either were accreted by Saturn or remained embedded in its
surrounding subnebula until they found their way into the regular satellites
growing around Saturn. In order to explain the carbon monoxide and primordial
argon deficiencies of Titan's atmosphere, we suggest that the satellite was
formed from icy planetesimals initially produced in the solar nebula and that
were partially devolatilized at a temperature not exceeding 50 K during their
migration within Saturn's subnebula. The observed deficiencies of Titan's
atmosphere in krypton and xenon could result from other processes that may have
occurred both prior or after the completion of Titan. Thus, krypton and xenon
may have been sequestrated in the form of XH3+ complexes in the solar nebula
gas phase, causing the formation of noble gas-poor planetesimals ultimately
accreted by Titan. Alternatively, krypton and xenon may have also been trapped
efficiently in clathrates located on the satellite's surface or in its
atmospheric haze.
