Question: Callisto is the Solar System's third largest moon after Titan, which is a moon of which planet named for the Roman god of agriculture?

Neptune is the eighth and farthest known planet from the Sun in the Solar System.  In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet.  Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune.  Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU .  It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune's trident. <SEP> The Nice ( ) model is a scenario for the dynamical evolution of the Solar System.  It is named for the location of the Observatoire de la Côte d'Azur, where it was initially developed, in Nice, France.  It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary gas disk.  In this way, it differs from earlier models of the Solar System's formation.  This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies including the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune.  Its success at reproducing many of the observed features of the Solar System means that it is widely accepted as the current most realistic model of the Solar System's early evolution, although it is not universally favoured among planetary scientists.  Later research revealed a number of differences between the original Nice model's predictions and observations of the current Solar System, for example the orbits of the terrestrial planets and the asteroids, leading to its modification. <SEP>  It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System not to be properly differentiated.  Callisto was discovered in 1610 by Galileo Galilei.  At in diameter, Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass.  It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about .  It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated.  Callisto's rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand nearly still in Callisto's sky. Callisto (Jupiter IV) is the second-largest moon of Jupiter, after Ganymede.  It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt. <SEP> Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter.  It is a gas giant with an average radius about nine times that of Earth.  Although it has only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive.  Saturn is named after the Roman god of agriculture; its astronomical symbol (♄) represents the god's sickle.