Scientific summary

The Earth constitutes a habitable planetary body whose present atmospheric composition is quite different from those of other planets in the Solar System, as well as from that which existed 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, but also help determine if, and under what circumstances, Earth-like conditions for habitability can exist on other planetary bodies. There are several main factors that control atmospheric evolution, such as the initial redox state of the planet, biospheric reactions, sub-surface activities, a net particle escape of atmospheric constituents out to space. The atmosphere is also impacted by the stellar environment through radiative and particle fluxes, where the latter may be controlled by the geomagnetic field.

Due to the size of the Earth’s magnetosphere, at present-day there is no direct interaction between the solar wind and the neutral atmosphere, an interaction that could have played a key role in atmospheric escape in non-magnetized planets like Mars. In addition, at present, thermal escape of neutrals from the Earth is limited to the lightest elements (hydrogen and helium). The escape of heavier elements like oxygen and nitrogen may be facilitated via the Earth’s ionosphere, the charged particle component of the Earth’s upper atmosphere. This ionospheric outflow has been observed by spacecraft for present-day Earth, and is regulated by the the Earth’s magnetic field, solar extreme ultra-violet (EUV) photon flux striking the upper atmosphere, and the electromagnetic driving from the solar wind as it interacts with the Earth’s magnetosphere.

A long standing question in studying planetary atmospheres and their connection to their host star is whether planetary magnetic fields play a critical role in allowing the planet to retain a habitable atmosphere. The uncertainty in addressing this question comes from two directions. On the one hand, we have limited knowledge of when planets gain or lose their ability to create a global core dynamo field, even for the much studied bodies like the Earth, Mars, and the Moon. On the other hand, our understanding of how such fields, through their coupling to plasmas lying within and outside of the magnetosphere, facilitate (or prevent) atmospheric escape, is still lacking.

The ISSI team addresses the problem of atmospheric loss of heavy elements on cur- rent and geological scales, involving processes starting from surface-air interactions, atmospheric chemistry, circulation patterns, ionization by solar radiation, and ionosphere-magnetosphere and solar wind interactions. Our multi-disciplinary team includes experts from space sciences and planetary sciences, paleomagnetism, and Earth and atmospheric sciences, in order to tackle this problem from all angles presently afforded by the scientific community.