Answer:
the most likely source is volcanoes
Explanation:
One theory suggests that the early atmosphere came from intense volcanic activity, which released gases that made the early atmosphere very similar to the atmospheres of Mars and Venus today. These atmospheres have:
a large amount of carbon dioxide
little or no oxygen
small amounts of other gases, such as ammonia and methane
Volcanic activity also released water vapour, which condensed as the Earth cooled to form the oceans. Nitrogen was probably also released by volcanoes which gradually built up in the atmosphere because it is unreactive.
