Formation conditions of Titan

<p>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.</p> <p>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.</p> <p>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<sup>-2</sup>). The ice availability upon accretion is otherwise incompatible with Titan's moment of inertia. Our models predict a large NH<sub>3</sub> inventory was available upon Titan's formation, ∼10-20wt.% of the total ice. This is consistent with the hypothesis that the observed N<sub>2</sub> in Titan is captured as NH<sub>3</sub> 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.</p>

Cassini-Huygens’ exploration of the Saturn system: 13 years of discovery

The Cassini-Huygens mission to Saturn provided a close-up study of the gas giant planet, as well as its rings, moons, and magnetosphere. The Cassini spacecraft arrived at Saturn in 2004, dropped the Huygens probe to study the atmosphere and surface of Saturn’s planet-sized moon Titan, and orbited Saturn for the next 13 years. In 2017, when it was running low on fuel, Cassini was intentionally vaporized in Saturn’s atmosphere to protect the ocean moons, Enceladus and Titan, where it had discovered habitats potentially suitable for life. Mission findings include Enceladus’ south polar geysers, the source of Saturn’s E ring; Titan’s methane cycle, including rain that creates hydrocarbon lakes; dynamic rings containing ice, silicates, and organics; and Saturn’s differential rotation. This Review discusses highlights of Cassini’s investigations, including the mission’s final year.