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

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.

Figures (8)

• Figure 1: The quiescent spectrum of Barnard's Star (GJ 699, shown in red), is shown in comparison with the spectrum of the quiet Sun (from woods2009, shown in black). These spectra are shown at 2 Å resolution and scaled to a common bolometric instellation distance (Figure adapted from france2020). The EUV flux is enhanced relative to the quiet Sun by a factor of 2-10, even for this inactive M dwarf, due to the enhanced EUV/bolometric fraction in low mass stars and the smaller orbital radius of the habitable zone around lower luminosity M dwarf stars
• Figure 2: The three-dimensional MHD stellar wind solution for Barnard's Star. Color contours represent number density values, while selected magnetic field lines are also shown. The solid white circle marks the orbit of the planet.
• Figure 3: Top: the value of the IMF components and the IMF magnitude along the circular orbit at 0.087 AU. Bottom: the stellar wind dynamic pressure along the circular orbit at 0.087 AU. The phase is determined with respect to the temporal longitude as defined by the observed magnetogram and the rotation of the star as observed from the Earth. This also defines the coordinate system of the domain.
• Figure 4: (a)Temperature, (b)Neutral density, and (c)Ion density profiles of the modeled thermosphere for Exo-Mars. In the temperature profile, red and blue line indicate profiles of Exo-Mars and present-day Mars, respectively.
• Figure 5: Left: Shown are two velocity ratios as a function of altitude (y-axis). The solid blue line shows the speed of the upward traveling oxygen divided by the speed of sound. The dashed blue line shows the speed of the upward traveling oxygen divided by the escape velocity at each altitude. The atmosphere reaches the sonic point about 4,000 km before it reaches escape velocity. Right: Shown is the mass flux of oxygen atoms at each altitude. The profile is such that, when integrated over a sphere, it is constant at each altitude ensuring continuity.