I'm trying to get a basic understanding of Earth's origins in order to teach an advanced oceanography course to high school students this summer. The course starts with one lecture on the origins of the universe, solar system, the earth, and the ocean. Why did Earth lose its first atmosphere but keep its second? As I understand, the Earth had a "first atmosphere" that was made up of mostly hydrogen and helium accumulated from the solar nebula. The earth was too small and too hot to hold on to these gases when our young star began to generate energetic solar winds and the first atmosphere was "blown off". Later a second atmosphere was created by outgassing (volcanic eruptions) and impact degassing (vaporizing asteroids/comets). This atmosphere stayed. Why? Was it because the Earth was now more massive and commanded a stronger gravitational hold of this atmosphere? Was it because this atmosphere was made up of heavier molecules that could not escape to space as easily? Was it because the Earth had cooled, so these molecules did not have the energy to escape? Was it because the sun's solar winds became weaker? Was it because the Earth's magnetic field formed and acted as a shield that protected this second atmosphere? Thanks in advance for your help!