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Oxygen escape from Earth’s atmosphere since the Great Oxidation Event
Two factors influence atmospheric escape: solar forcing and the planetary environment. The younger Sun had a faster and denser solar wind, and a more intense EUV radiation, depositing more energy in the ionosphere than today. The younger Earth had a hotter atmosphere and its magnetic field has fluctuated over time.
We developed a semi-empirical model that extrapolates current escape rates to past conditions, considering the seven main escape processes for terrestrial planets: Jeans escape, photochemical escape, ion pickup, sputtering, cross-field ion loss, polar wind, and cusp escape. Using empirical formulas, a physical scaling and a magnetic field model, we determine the past escape rates of each mechanism.
As the Earth remained magnetised since the Great Oxidation Event, 2.45 billion years ago, oxygen escape was dominated by ion escape through open magnetic field lines connecting the polar ionosphere to the interplanetary medium (polar wind and cusp escape).
The oxygen escape rate 2.45 billion years ago was more than one order of magnitude higher than now, with a total escape rate above 1027 s-1 or few hundreds of kilograms per second. The total oxygen loss during this time corresponds to about 50% of the current atmospheric oxygen content, allowing the Earth to maintain an atmosphere rich in oxygen.
Understanding key parameters of atmospheric stability
BIRA-IASB’s semi-empirical model isolates the key parameters that drive oxygen escape from a magnetised planet like Earth. One of the central elements is the balance between the planetary magnetic field pressure and the pressure of the solar wind.
This pressure ratio shapes the magnetospheric geometry, affecting the proportion of open versus closed magnetic field lines. In the distant past, a lower ratio of Earth’s magnetic to solar wind pressure caused atmospheric regions connected to open field lines to expand.
This expansion allowed more atmospheric oxygen to escape through the polar regions, especially as a stronger solar wind provided more energy to accelerate ions. However, this increase in ion escape was not limitless. The ionosphere’s ability to produce ions is finite and reached a saturation point around 1.5 billion years ago. Prior to that, even as the area of open field lines grew, the limited ionospheric ion supply kept escape rates constrained.
By accounting for all known oxygen escape mechanisms, this model offers new insights into the long-term evolution of Earth’s atmosphere. By refining our understanding of atmospheric escape drivers, it also enhances our ability to assess the evolution and stability of other planetary atmospheres.
