See that green glow? It's not the aurora borealis. Although it's technically present in almost all views of the Earth from space, it can only be clearly seen at certain angles and certain times. It's a phenomenon known as airglow, and it's our own atmosphere lighting up like glow-in-the-dark paint.
As we've seen, space agencies are worth it for the art alone. In some of these pictures, in which the Earth is outlined against a backdrop of space, you can see a green band. This is not light traveling through the atmosphere - it's light being generated by the atmosphere.
Ultraviolet radiation is released by the sun, and the atoms in the atmosphere absorb ultraviolet light. Often objects that absorb photons of a certain frequency re-emit atoms of the same frequency, meaning we wouldn't be able to see those re-emitted atoms. Sometimes, though, they absorb these UV photons, use some of the power they're given, re-emit photons at a lower energy. Occasionally, that lower energy photon is in the visible spectrum.
The oxygen in our atmosphere is one of those special types of substances. It only gives off a faint glow, and we can't see it unless it's against a dark background. When conditions are right, like when someone in space photographs the night side of the planet, we can see the particular shade of green it emits. The atmosphere can also glow due to different processes, like chemical interactions between the atoms and bombardment by cosmic rays, but the green band is most prominent.
This kind of glow happens on other planets, too. Since their atmospheres are made up of different atoms, the color is different. Using spectroscopy, scientists can use the color of this glow to understand the chemical make-up of alien atmospheres.
Top Image: NASA
Second Image: NASA
Third Image: NASA