Upper atmosphere dynamics and drivers of volatiles loss from terrestrial-type (exo)planets

Abstract

Volatile loss from exoplanetary atmospheres and its possible implications for the longevity of habitable surface conditions is a topic of vigorous debate currently. The vast majority of the habitable zone terrestrial-like exoplanets known to date orbit low-mass M- and K-dwarf stars and are subject to the conditions drastically different to those of terrestrial planets in the Solar System. In particular, they orbit far closer to their host stars than similar planets around G-dwarfs similar to the Sun. Therefore they receive higher X-ray and UV fluxes, even though luminosities of M- and K-dwarfs are lower than those of heavier stars. Furthermore, due to their slower evolution, M-dwarfs retain high activity on the gigayear timescales. The combination of these two effects has led to claims that most terrestrial planets orbiting M-dwarfs may have their atmospheres stripped from the higher X-ray and UV fluxes of their host stars. Opposing this are researchers who point out that volatile inventories for terrestrial exoplanets are ill-constrained, and hence, they may be able to "weather the storm" of these higher X-ray and UV fluxes. In this chapter, we focus on exploring volatile loss in the upper atmospheres of terrestrial planets in our solar system and applications to those in exoplanetary systems around stars of different types.

Figures (22)

• Figure 1: Height scales of atmospheres of Solar System's terrestrial planets. The solid lines show the temperature profiles against altitude, while the background color reflects the atmospheric pressure. Horizontal dashed lines denote the average positions of atmospheric boundaries: tropopause ("t"), stratopause ("s"), and mesopause ("m"). Vertical cyan lines depict the extension of ionospheres ("i") under typical conditions. The "$\odot$" symbol denotes the 100 mbar level.
• Figure 2: Schematic illustrations of (a) ion escape routes from a magnetized planet Seki2001Sci...291.1939S and (b) atmospheric escape mechanisms from unmagnetized planets. In panel (a), ⓐ and ⓑ show outflow and return flow from/to the ionosphere, respectively. The escape routes ⓘi, ⓘii, and ⓘv in the panel (a) result from polar wind and auroral outflows, while routes ⓘ and ⓥ correspond to the plasmaspheric drainage plume and ENA production by charge exchange between the ring current ions and geocorona.
• Figure 3: Plasma element being detached from the plasmasphere through the enhanced magnetospheric convection. From Lemaire2001JASTP..63.1285L.
• Figure 4: Upper atmospheric profiles for neutrals (left panel), ions (middle panel), and the neutral temperature (right panel) for Earth (solid), Venus (dashed), and Mars (dotted). The profiles for the Earth are from the reference atmosphere model NRLMSIS-2.0 Picone2002 and the reference ionosphere model IRI Bilitza2001, both for solar minimum (December 02, 2015), except for CO$_2$, which was modelled with the 1D upper atmosphere model Kompot johnstone2018 and taken from Scherf2025. For Venus and Mars, all profiles are based on solar minimum conditions and were taken from Fox2001 and Fox2009Icar..204..527F, respectively. We note that for the solar maximum, temperatures and atmospheric densities can be slightly higher for all three planets. The solar activity-dependent exobase levels for Venus, Mars, and Earth are schematically highlighted in the figure.
• Figure 5: Two major ion escape channels from Mars observed by MAVEN, i.e., polar plume (red empty arrow) and tailward escape (blue empty arrow), in MSE (Mars-centered Solar Electric) coordinates Dong2015GeoRL..42.8942D.