Why does Earth and Venus have an atmosphere but the Moon and Mercury have a much smaller one?
It is a bit of everything. The moon does not have an atmosphere partly because of its mass. However, it is also without an atmosphere because it was blasted out of earth due to a collision between the earth and a Mars size object during earth’s formation. When blasted out, it was unable to form an atmosphere as it did not contain the ingredients to develop the atmosphere. Where Mercury is concerned, it is due to its close proximity to the sun. Strong solar wind and solar flares have since blown the atmosphere away eons ago.
