I was thinking today: "How did earth obtain an atmosphere if it didn't have one originally?"

There are likely only a few ways this could happen. The first way is that water was frozen in the dirt and structure of earth always until it formed as water and air and created oceans and an atmosphere here. Another idea is that the water and air were brought here to some degree from some other place or planet for some reason by more evolved beings with access to space travel. another option might be that a water planet or ice ball collided with Earth at some point and some of that water in the form of ice stayed here since then. Any or all of these ideas or other ones I have not thought of must or could have occurred for us to have an atmosphere and oceans and rain and lakes and rivers now. 

The other interesting thing is that water and air are likely to stay either in a gas or liquid or ice form ongoing. However, it is also true that land masses might be more temporary and go beneath the waves so to speak more regularly than we presently think. Whereas Water and ice and Air will tend to remain here on earth unless another mass as big as earth collides with earth at some point and the gravity of the other collider takes some of the water and air with it after it collides with earth. This is what I'm thinking right now.