Atmospheric Escape - Dominant Atmospheric Escape and Loss Processes On Earth

Dominant Atmospheric Escape and Loss Processes On Earth

Earth is too large to lose a significant proportion of its atmosphere through Jeans escape. The current rate of loss is about three kilograms of hydrogen and 50 grams of helium per second. The exosphere is the high-altitude region where atmospheric density is sparse and Jeans escape occurs. Jeans escape calculations assuming an exosphere temperature of 1,800degrees show that to deplete O+ ions by a factor of e (2.718...) would take nearly a billion years. 1,800degrees is higher than the actual observed exosphere temperature; at the actual average exosphere temperature, depletion of O+ ions would not occur even over a trillion years. Furthermore, most oxygen on Earth is bound as O₂, which is too massive to escape Earth by Jeans escape.

Earth's magnetic field protects it from solar winds and prevents escape of ions, except along open field lines at the magnetic poles. The gravitational attraction of Earth's mass prevents other non-thermal loss processes from appreciably depleting the atmosphere. Yet Earth's atmosphere is two orders of magnitude less dense than that of Venus at the surface. Because of the temperature regime of Earth, CO₂ and H₂O are sequestered in the hydrosphere and lithosphere. H₂O vapor is sequestered as liquid H₂O in oceans, greatly decreasing the atmospheric density. With liquid water running over the surface of Earth, CO₂ can be drawn down from the atmosphere and sequestered in sedimentary rocks. Some estimates indicate that nearly all carbon on Earth is contained in sedimentary rocks, with the atmospheric portion being approximately 1/250,000 of Earth's CO₂ reservoir. If both of the reservoirs were released to the atmosphere, Earth's atmosphere would be even denser than Venus's atmosphere. Therefore, the dominant “loss” mechanism of Earth's atmosphere is not escape to space, but sequestration.