Mantle degassing and regassing is said to control the habitability of a planet. ..... Substantial regassing began sometime between a few hundred million to 2.5 billion years ago. The report from Washington University, St Louis, MO, explains the process of regassing and states it as a necessity to a planet to support life. However, I read the other day that this process is not necessarily a requirement for a habitable planet. I am sorry I did not save that then but I'll see if I can find it. Nevertheless, the purpose of this report is to date the process of degassing and regassing. There is a beautiful picture of Mt Etna erupting.