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 temperature of the protosaturnian nebula enabled Titan to acquire the moderately volatile compounds 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
A rate of heat loss
B proximity to the Sun
C availability of methane and ammonia
D distance from its planet
The paragraph discusses Titan’s thick atmosphere and explains the conditions under which a body can have a thick atmosphere.
According to the last two sentences of the paragraph, Titan was able to acquire an atmosphere because of a prevailing low temperature, but Ganymede and Callisto could not because they were at a higher temperature. Because the reason for this difference in temperature was their respective distances from the sun, Choice B is correct. The passage says nothing about differences in rate of heat loss, availability of methane andammonia, or distance from their planets, and it explicitly states that the three moons are the same size.