Ion escape processes in the solar system and beyond 

Abstract

Understanding the evolution of planetary atmospheres, and particularly the evolution of their composition and eventual habitability, is a major challenge. The evolution of an atmosphere is driven by its interactions with the planetary surface and interior, the influx from space (e.g. meteors), and the atmospheric escape to space in the form of neutral or ionised atoms/molecules, upwelling from the atmosphere and escaping to space. For a planet like Earth, atmospheric escape in the form of neutrals concerns essentially hydrogen whereas heavier species, such as oxygen and nitrogen which constitute 99% of the mass of the terrestrial atmosphere, need to be accelerated as ions in order to reach escape velocities. The ions that outflow from the ionosphere are successively accelerated through a series of energisation mechanisms and can eventually reach velocities above the gravitational escape velocity. Missions like Cluster, MAVEN and Cassini and associated modelling efforts have advanced our understanding of the ion acceleration, circulation in the magnetosphere and escape mechanisms operating on different planetary objects of our solar system, magnetised or unmagnetised. However, several questions remain open, as: (i) What is the exact composition of the escaping populations and how does it change in response to the different driving conditions?  How does it affect the long-term evolution of the composition of a planetary atmosphere and its habitability? (ii) What is the exact degree of plasma recirculation for each ion species, after it has left the ionosphere, versus direct or indirect escape, and what is its dependence on the solar and geomagnetic activity conditions? (iii) What is the effect of a planetary magnetic field on the different escape mechanisms, particularly in view of the conjugate effect of different magnetospheric size / solar wind dynamic pressure / exobase altitude / solar irradiance? (iv) The discovery in recent years of a large number of exoplanets, several of them in the "habitable" zone, raises the question of atmospheric escape mechanisms operating in these environments. Could exoplanets orbiting active K-M stars undergo massive atmospheric escape, removing the constituents of water from their atmospheres under XUV irradiation and making them uninhabitable within a few tens to hundreds of Myr, as some models suggest?