The water in Earth's atmosphere primarily originated from a combination of processes involving the planet's formation, volcanic activity, and the release of water vapor from the Earth's interior. Here's a summary of how water came to be present on Earth:
Early Formation: Approximately 4.6 billion years ago, the Earth formed through the accretion of cosmic dust and debris. During this phase, the planet was extremely hot, and any water that may have been present initially likely existed in the form of vapor.
Outgassing: As the Earth's interior underwent intense volcanic activity, a process called outgassing occurred. Volcanoes released gases, including water vapor, carbon dioxide, nitrogen, methane, and others, into the early atmosphere. The water vapor, being lighter than the other gases, rose and accumulated in the atmosphere over time.
Cometary Bombardment: In the early stages of the solar system's formation, comets, which are icy bodies, bombarded the Earth. These comets contained significant amounts of water ice. The impacts of comets delivered substantial quantities of water to Earth, contributing to the overall water content on the planet.
Subsequent Water Cycling: Once water was present on the Earth's surface, a continuous cycle of water movement, known as the water cycle or hydrological cycle, began. The water evaporated from oceans, lakes, and other bodies of water and entered the atmosphere as water vapor. This vapor then condensed to form clouds and eventually fell back to the Earth as precipitation (rain, snow, etc.), replenishing the water sources.
It's important to note that the exact proportions of water contributed by each of these processes are still topics of scientific investigation and debate. However, the consensus among scientists is that a combination of outgassing, cometary impacts, and subsequent water cycling played significant roles in bringing water to Earth and maintaining it in the atmosphere and hydrosphere over billions of years.