Question: Sorry if this isn't really about neutrinos, but was the Earth formed with it's atmosphere, or did the atmosphere form later?

Susan Cartwright answered on 16 Jun 2015:

The atmosphere formed later. Planets are believed to form when dust orbiting around the star collides and sticks together, making larger objects. In the later stages these collisions can be very violent – it is believed that a late collision with an object the size of Mars kicked enough debris into orbit to form the Moon – and the young Earth is molten. Any original atmosphere would have been destroyed by these events.

Later, when the Earth solidifies, an atmosphere of nitrogen and carbon dioxide develops, partly by emission from volcanoes, partly as a result of comet impacts. Much of the carbon dioxide subsequently dissolves in water and becomes carbonate rock, stripping it out of the atmosphere (this did not happen on Venus, because it’s too hot for water to survive, and Venus’ atmosphere is still 95% CO2) and leaving us with a mostly nitrogen atmosphere. Later still, photosynthesis evolved, producing oxygen as a waste product, and this resulted in our present atmosphere.

This is why astronomers plan to look for spectroscopic signatures of oxygen in the atmospheres of planets around other stars. The only thing keeping Earth’s atmosphere oxygen rich is life (plants) – if we find oxygen in another planet’s atmosphere, chances are life has evolved there.