Atmospheric Escape Rates from Mars - If it Orbited an Old M-Dwarf Star

TL;DR

This study models atmospheric escape processes for a Mars-like exoplanet orbiting an old M dwarf star, finding that such planets likely lose their atmospheres rapidly due to high escape rates driven by stellar radiation and wind.

Contribution

It provides the first detailed modeling of atmospheric escape for a Mars-like planet around an old M dwarf, incorporating stellar EUV spectra and magnetic field assumptions.

Findings

Escape rates are dominated by thermal processes.

Atmospheres would be lost in less than tens of millions of years.

Recent planets around Barnard's star likely lack significant atmospheres.

Abstract

Atmospheric escape is an important process that influences the evolution of planetary atmospheres. A variety of physical mechanisms can contribute to escape from an atmosphere, including thermal escape, ion escape, photochemical escape, and sputtering. Here we estimate escape rates via each of these processes for a hypothetical Mars-like exoplanet orbiting Barnard's star (an old, inactive M dwarf star). We place the planet at an orbital distance that receives the same total stellar flux as it does in our solar system. We use the measured stellar extreme ultraviolet (EUV) spectrum and assumptions on the star's magnetic field to determine both the high-energy radiation and the stellar wind environment around the planet. This information is used to model the response of the planet's thermosphere, exosphere and magnetosphere using a variety of models that have been validated against solar system observations. We find overall escape rates that are dominated by thermal processes and elevated by 2-5 orders of magnitude relative to present-day Mars, suggesting that a Mars-like planet orbiting Barnard's star would not retain a significant atmosphere for more than 10's of millions of years. Recently reported planets around Barnard's star should also not have retained significant atmospheres. By extension, Mars-like planets orbiting any M dwarf near the 'Habitable Zone' should not retain atmospheres for extended periods of time.