Answer: The terrestrial planets lack hydrogen and helium in their atmosphere because of their relatively low gravity, due to being smaller in size and therefore having lower mass, [ failed to keep these light gases from escaping into space. ]
Some of the hydrogen bonds with other elements, but most of it remains free hydrogen. Both hydrogen and helium are light elements, so they will tend to evaporate into space over time. ... But the gravity of Earth isn't strong enough, so Earth's early atmosphere of helium and free hydrogen evaporated into space.
