Formation conditions of Titan

Satellites are generally believed to form in circumplanetary disks (CPDs): a gas disk containing icy and rocky particles that accumulate to form massive moons over time. The discoveries by the Cassini-Huygens mission have led to a revision of the birth environment of the Saturnian system.We aim to constrain the formation circumstances of Titan's building blocks by considering the moon's observed characteristics. We use radiation thermo-chemical CPD models and evaluate them on their capacity to reproduce a Titan-like satellite.To form a moon with Titan's ice-to-rock ratio, we find that the dust-to-gas ratio in the CPD must be in the order of solar nebula values, O(10-2). The ice availability upon accretion is otherwise incompatible with Titan's moment of inertia. Our models predict a large NH3 inventory was available upon Titan's formation, ∼10-20wt.% of the total ice. This is consistent with the hypothesis that the observed N2 in Titan is captured as NH3 and converted by photolysis and shock heating, and is compatible with the possible presence of a conductive layer at 45±15 km as revealed by the Huygens probe.