Final answer:
Titan's atmosphere does not contain carbon dioxide or water vapor due to the moon's extreme temperatures, which cause these compounds to freeze instead of being in vapor form. The atmosphere of Titan includes primarily nitrogen and features a methane and ethane cycle analogous to Earth's water cycle.
Step-by-step explanation:
The atmosphere of Venus is mostly carbon dioxide, and the atmosphere of the Earth has water vapor. These gases are absent in the atmosphere of Titan, Saturn's moon, due to the extreme temperatures on Titan that are too low for either carbon dioxide or water to exist in vapor form.
Instead, Titan's atmosphere consists primarily of nitrogen. Unlike Venus and Earth, Titan has lakes and rivers of liquid hydrocarbons, like methane and ethane, indicating a cycle of evaporation and condensation similar to the water cycle on Earth but with hydrocarbons instead.
Radar and infrared imaging from the Cassini orbiter show that Titan has a remarkably active surface with erosional features, which suggests that atmospheric methane can condense and fall as rain, just as water does on Earth, creating a low-temperature equivalent of the water cycle.