Ozone chemistry on tidally locked M dwarf planets

Abstract

We use the Met Office Unified Model to explore the potential of a tidally locked M dwarf planet, nominally Proxima Centauri b irradiated by a quiescent version of its host star, to sustain an atmospheric ozone layer. We assume a slab ocean surface layer, and an Earth-like atmosphere of nitrogen and oxygen with trace amounts of ozone and water vapour. We describe ozone chemistry using the Chapman mechanism and the hydrogen oxide (HOx, describing the sum of OH and HO2) catalytic cycle. We find that Proxima Centauri radiates with sufficient UV energy to initialize the Chapman mechanism. The result is a thin but stable ozone layer that peaks at 0.75 parts per million at 25 km. The quasi-stationary distribution of atmospheric ozone is determined by photolysis driven by incoming stellar radiation and by atmospheric transport. Ozone mole fractions are smallest in the lowest 15 km of the atmosphere at the substellar point and largest in the nightside gyres. Above 15 km the ozone distribution is dominated by an equatorial jet stream that circumnavigates the planet. The nightside ozone distribution is dominated by two cyclonic Rossby gyres that result in localized ozone hotspots. On the dayside the atmospheric lifetime is determined by the HOx catalytic cycle and deposition to the surface, with nightside lifetimes due to chemistry much longer than time-scales associated with atmospheric transport. Surface UV values peak at the substellar point with values of 0.01 W m−2, shielded by the overlying atmospheric ozone layer but more importantly by water vapour clouds.

Implications of different stellar spectra for the climate of tidally locked Earth-like exoplanets

“…Concerted studies in 3D are required to explore the interaction of the stellar activity with the atmosphere, and in particular, the potential impacts on the O 3 distribution, given that tidal locking gives rise to a permanent day-and nightside, the latter never receiving direct stellar radiation. This work has begun for quiescent host stars (Yates et al 2020), and we are working on extending it to include host star activity. Furthermore, the atmospheric composition in our simulations was kept constant in a simplified Earth-like configuration.…”

“…Potentially important climate processes or mechanisms have also been omitted, such as atmospheric chemistry (e.g. ozone in Yates et al 2020), land-surface impact (e.g. Lewis et al 2018), dust (e.g.…”

“…In this work we use the Met Office general circulation model (GCM), the UM, which has been adapted to a range of exoplanet applications and used for a large number of studies covering hot Jupiters (Mayne et al 2014a(Mayne et al , 2017Amundsen et al 2016;Helling et al 2016;Tremblin et al 2017;Drummond et al 2018aDrummond et al ,b, 2020Lines et al 2018aLines et al ,b, 2019Sainsbury-Martinez et al 2019;Debras et al 2019Debras et al , 2020, mini-Neptunes/Super Earths (Drummond et al 2018c;Mayne et al 2019) and terrestrial planets (Mayne et al 2014b;Boutle et al 2017Boutle et al , 2020Lewis et al 2018;Fauchez et al 2020;Yates et al 2020;Joshi et al 2020;Sergeev et al 2020). For this work, we follow a similar configuration to that of Boutle et al (2017) and Lewis et al (2018), based on the Global Atmosphere 7.0 configuration (Walters et al 2019).…”

“…see Howard et al 2018, for Proxima Centauri). This has important implications for both the atmospheric composition, for example, in terms of stratospheric ozone cycling (Yates et al 2020), and the habitability of planets orbiting such stars.…”

Longitudinally asymmetric stratospheric oscillation on a tidally locked exoplanet

“…A variation of this magnitude is unlikely to be detectable with existing instrumentation, but an ozone column oscillation may be larger in amplitude on a tidally locked planet than on Earth, in line with our simulation results for water vapour. Studies of ozone photochemistry on tidally locked planets (Yates et al 2020) have also shown that the planet's nightside accumulates ozone in its cold traps, resulting in a zonally asymmetrical ozone layer without analogy on Earth and high levels of ozone in one hemisphere. Future work could explore how this tidally locked ozone chemistry would interact with oscillations in dayside production of ozone through stratospheric photochemistry.…”