If the atmosphere was thin, then the sun rays would penetrate it better and it would heat it up much more, it would absorb it and everything would be hotter, a lot hotter. If it was thick everything would be colder, if it was much thicker then the earth could become very frigid, never ice cold because the nuclear reaction in its core keep it pretty warm, but not hot enough for us.If there was a little gravity some of it or all of it would drift off into space and we wouldn't have one. And lastly if there was ton gravity the atmosphere would be denser. All the atoms would be pulled closer together and the atmosphere or the skies would be much hotter, but it would still protect us. Hope it helps.