Answer:
There are two primary factors for planets having thin atmospheres: size and distance from the Sun. Gravity helps planets and moons to hold on to their atmospheres, so small planets/moons such as Mars and the Moon have thin atmospheres.
Like the sun, the gas giants are composed mainly of hydrogen and helium. Because they are so massive, they exert a much stronger gravitational force than the terrestrial planets. This prevents their gases from escaping, so they have thick atmospheres.
Step-by-step explanation: