What does one theory suggest formed the main atmosphere in the first billion years of the earth's existence?

The theory that suggests intense volcanic activity played a significant role in forming the main atmosphere during the first billion years of Earth's existence highlights the importance of volcanic eruptions in releasing vital gases. During this early period, the planet experienced extensive volcanic activity, which expelled large amounts of carbon dioxide and water vapor into the atmosphere. This volcanic outgassing created a dense atmosphere rich in these gases, while the levels of oxygen were minimal.

The presence of water vapor was particularly crucial, as it eventually led to the formation of oceans when the Earth cooled down enough for it to condense. This initial atmosphere set the stage for further developments in the planet's environment, leading eventually to conditions suitable for life.

The other options do not adequately explain the primary formation of the early atmosphere. For instance, ice caps melting would not have been a factor in the first billion years, as Earth was too hot for ice to form and persist. Solar radiation breaking down gases primarily relates to atmospheric chemistry rather than the initial accumulation of atmospheric components. Lastly, while comets and other celestial objects may have contributed water and other materials to the Earth, the primary formation of the atmosphere is more directly attributed to volcanic activity during this period.