What makes the Earth lose weight?

The Earth constitutes a uniquely habitable planetary body, whose present atmospheric composition is quite different from those of other planets in the solar system, as well as from that existing on Earth billions of years ago. Understanding the evolution of our atmosphere over geological times would allow us to not only open a window into Earth's history, it also can help determine if, and under what conditions, Earth-like conditions for habitability can exist.

Several mechanisms have been invoked to explain planetary atmospheric loss. Due to the size of the Earth's magnetosphere, there is no direct interaction between the solar wind and the neutral atmosphere, an interaction that can play a key role in escape in non-magnetized planets, and thermal escape of neutrals is limited to only the lightest elements. Here we explore the possible role played by loss of charged particles from Earth's atmosphere, facilitated via the Earth's ionosphere. This ionospheric outflow provides a pathway for atmospheric migration and escape without requiring direct interaction with solar wind particles or high thermal speeds. The solar EUV photon flux striking the atmosphere, as well as to the electromagnetic driving from the solar wind, both play a key role in regulating this escape. As these are expected to change over geological times, a comprehensive review of escape should consider their variability.

To assess the role of the global magnetic field and solar EUV flux in the overall atmospheric escape, we investigate the evolution of ionospheric outflow under various conditions, designed to mimic both solar and terrestrial historical conditions, ranging from the present to the ancient Earth. We employ the Seven Ion Polar Wind Model (7iPWOM) model, which has the capability of tracking the full evolution of all relevant ion species (H⁺, He⁺, N⁺, O⁺, NO⁺, N2⁺, O2⁺) as they overcome the gravitational bound and escape to space. This first principles model solves for the transport equations along open field lines, and is the first to confirm the key role of outflowing nitrogen ions (and molecular species), previously neglected ionospheric constituents, to the overall polar wind solution. This integrated approach allowed us to estimate the outflow fluence as a function of geomagnetic field strength, solar driving, and atmospheric composition.