Titan is roughly the same size as Mercury, yet Titan has an atmosphere while Mercury does not. How can this be?

Question

Quizplus · Accepted Answer

Titan's weaker gravitational field is compensated by its low temperature, which means that it can trap molecules in its atmosphere that would otherwise escape. This process is known as cold trapping, and it happens because the velocity of the molecules decreases with decreasing temperature, making it harder for them to escape Titan's gravity. Mercury, on the other hand, is much hotter, which means that any atmosphere that it may have had in the past was lost to space a long time ago.

Titan Is Roughly the Same Size as Mercury, Yet Titan