GRE General Test: RC-537149 GRE Reading Comprehension

Saturn’s giant moon Titan is the only planetary satellite with a significant atmosphere and the only body in the solar system other than Earth that has a thick atmosphere dominated by molecular nitrogen. For a long time, the big question about Titan’s atmosphere was how it could be so thick, given that Jupiter’s moons Ganymede and Callisto, which are the same size as Titan, have none. The conditions for acquiring and retaining a thick nitrogen atmosphere are now readily understood. The low tempera- ture of the protosaturnian nebula enabled Titan to acquire the moderately volatile com- pounds methane and ammonia (later converted to nitrogen) in addition to water. The higher temperatures of Jupiter’s moons, which were closer to the Sun, prevented them from acquiring such an atmosphere.
According to the passage, Titan differs atmospherically from Ganymede and Callisto because of a difference in