Why does Mercury have no atmosphere, while Titan (the moon of Saturn), which has almost the same dimensions, has one?

Due to its proximity to the Sun, the temperature on the surface of Mercury is high and the thermal velocities of gas molecules are much higher than on the surface of Titan.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.