Texas A&M University (TAMU) GEOL 104 Physical Geology Exam 1 Practice Exam

Larger planets tend to have thicker atmospheres because their stronger gravity provides a higher escape velocity for gas molecules. With stronger gravity, the atmosphere is held more tightly and gases—especially lighter ones—are less likely to reach speeds that would allow them to escape into space. Smaller planets have weaker gravity, so their gases escape more readily and the atmospheres become thinner or are lost over time. That's why Earth and other larger terrestrial planets can maintain substantial atmospheres, while smaller bodies like Mars have much thinner ones.