Where did the earth's first methane-rich atmosphere come from?

From studies of interstellar clouds we know that molecules such as water, methane and ammonia are plentiful. The proto-earth, like the giant planets in our solar system today, probably had an enormous atmosphere very rich in these molecules and may have looked like a mini-Jupiter, until the sun became a T- Tauri star and its monstrous solar wind scoured the inner planets bare of this early atmosphere.