While gravity is the primary force responsible for holding Earth's atmosphere close to its surface, there are other factors that contribute to the phenomenon of the atmosphere extending beyond what gravity alone could retain. These factors are primarily related to the kinetic properties of the gases in the atmosphere and their interactions with other forces.
Thermal Motion: Gases in the atmosphere are in constant motion due to their thermal energy. This motion leads to a phenomenon called "thermal escape," where some of the lighter gas molecules at the upper layers of the atmosphere gain enough kinetic energy to overcome gravity and escape into space. However, the overall effect of thermal escape is relatively small and doesn't significantly deplete the atmosphere over time.
Atmospheric Escape: Certain molecules in the atmosphere, particularly lighter ones like hydrogen and helium, can gain sufficient energy to overcome gravity and escape into space. This process is known as atmospheric escape. While these molecules are continually lost to space, the overall impact on the Earth's atmosphere is minimal. Earth's gravity is strong enough to retain most of the atmospheric gases, including those prone to escape.
Atmospheric Composition: The composition of Earth's atmosphere also plays a role in its extension beyond the gravitational limit. The atmosphere consists of various gases, with the majority being nitrogen (78%) and oxygen (21%). These gases have relatively low molecular weights and are not as prone to escaping compared to lighter gases like hydrogen. The presence of heavier gases, such as carbon dioxide and water vapor, also contributes to the retention of the atmosphere by gravity.
Atmospheric Pressure: The atmosphere exerts pressure on the Earth's surface, which decreases with increasing altitude. However, even at higher altitudes, the pressure is not zero. While the pressure at those altitudes is lower, it still contributes to the overall containment of the atmosphere by gravity.
It's important to note that the concept of Earth's atmosphere extending well beyond what gravity alone could retain is relative. The atmosphere gradually thins out with increasing altitude until it merges with the near-vacuum of outer space. The boundary where the atmosphere becomes negligible is not sharply defined, and its extent depends on the specific definition used for the boundary.
Overall, while gravity is the dominant force in keeping the atmosphere close to Earth's surface, the kinetic properties of gases, atmospheric composition, and other factors collectively contribute to the extension of the atmosphere beyond what gravity alone could retain.