MNRAS 473, 4672–4685 (2018) doi:10.1093/mnras/stx2732 Advance Access publication 2017 October 25 Stratosphere circulation on tidally locked ExoEarths L. Carone,1‹ R. Keppens,2 L. Decin3 and Th. Henning1 1Max-Planck-Institute for Astronomy, Koenigsstuhl 17, D-69117 Heidelberg, Germany 2Centre for Mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium 3Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium Accepted 2017 October 12. Received 2017 October 12; in original form 2017 July 7 Downloaded from https://academic.oup.com/mnras/article/473/4/4672/4564445 by guest on 24 September 2021 ABSTRACT Stratosphere circulation is important to interpret abundances of photochemically produced compounds like ozone which we aim to observe to assess habitability of exoplanets. We thus investigate a tidally locked ExoEarth scenario for TRAPPIST-1b, TRAPPIST-1d, Proxima Centauri b and GJ 667 C f with a simplified 3D atmosphere model and for different stratospheric wind breaking assumptions. These planets are representatives for different circulation regimes for orbital periods: Porb = 1–100 d. The circulation of exoplanets with Porb ≤ 25 d can be dominated by the standing tropical Rossby wave in the troposphere and also in the stratosphere: It leads to a strong equatorial eastward wind jet and to an ‘Anti-Brewer-Dobson’-circulation that confines airmasses to the stratospheric equatorial region. Thus, the distribution of photochemically produced species and aerosols may be limited to an ‘equatorial transport belt’. In contrast, planets with Porb > 25 d, like GJ 667 C f, exhibit efficient thermally driven circulation in the stratosphere which allows for a day side-wide distribution of airmasses. The influence of the standing tropical Rossby waves on tidally locked ExoEarths with Porb ≤ 25 d can, however, be circumvented with deep stratospheric wind breaking alone – allowing for equator-to-pole transport like on Earth. For planets with 3 ≤ Porb ≤ 6 d, the extratropical Rossby wave acts as an additional safeguard against the tropical Rossby wave in case of shallow wind breaking. Therefore, TRAPPIST-1d is less prone to have an equatorial transport belt in the stratosphere than Proxima Centauri b. Even our Earth model shows an equatorial wind jet, if stratosphere wind breaking is ineffi- cient. Key words: methods: numerical – planets and satellites: atmospheres. greater than 20 km; see e.g. Barstow & Irwin 2016; Kreidberg & 1 INTRODUCTION Loeb 2016; Morley et al. 2017). To discuss habitability, we have Tidally locked planets in the habitable zone of cool red dwarf stars to reliably infer the properties of the underlying troposphere and like the TRAPPIST-1 planets, Proxima Centauri b, LHS 1140 b potentially habitable surface from upper atmosphere composition and GJ 667 C f (Anglada-Escudeetal.´ 2013, 2016; Gillon derived from infrared spectra. et al. 2016, 2017; Dittmann et al. 2017; Luger et al. 2017)are The abundances of molecular species and clouds that we can de- our next best hope to study conditions for habitability outside of tect on rocky planets by ‘far remote-sensing’ are modified by the the Solar system. Proxima Centauri b is the nearest exoplanet, general large-scale circulation. On Venus, surface reactions produce located only 1.295 pc away from the Sun. The TRAPPIST-1 planets COS and H2S that are transported upwards as part of the global are only 12 pc away and, in addition, all seven planets transit in front Hadley circulation cells. In the higher atmosphere, the sulphur - of their stars with relatively short orbital periods (Porb = 1.5–18.8 containing compounds are further processed via photochemistry d). Thus, the TRAPPIST-1 planets are particularly suited to search to SO2 and sulphuric acid to form clouds. The global superrotat- for bio-signatures in diverse environments by employing, e.g. in- ing flow results then in a continuous cloud blanket that enshrouds frared spectroscopy with JWST. However, infrared spectroscopy Venus completely (see e.g. Prinn & Fegley 1987,forareviewon mainly probes high atmospheric layers, which would correspond to Venus chemistry and climate). On Earth, the Brewer-Dobson- the stratosphere and mesosphere on Earth (p ≤ 0.1 bar or altitudes circulation in the stratosphere tends to transport the biomarker ozone from its main production regions in the tropical upper strato- sphere towards the polar lower stratosphere (e.g. Dobson 1931; E-mail: [email protected] Brewer 1949; Shaw & Shepherd 2008). Also, the abundances of C 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society ExoEarth’s stratosphere circulation 4673 Downloaded from https://academic.oup.com/mnras/article/473/4/4672/4564445 by guest on 24 September 2021 Figure 1. Different troposphere circulation states identified by Carone, Keppens & Decin (2015) for tidally locked terrestrial planets with respect to orbital period and planetary size, assuming Earth-like atmosphere and thermal forcing. Example planets show a cross-section of the planet’s temperature (colour) and wind flow (grey lines and black arrows) at the top of the troposphere p = 225 mbar and facing the substellar point. Red circles denote the radius and orbital period of the TRAPPIST-1 b,c,d,e,f,g,h planets in this ‘circulation state map’ from left to right. Blue circles denote the position of Proxima Centauri b, LHS 1440 b, GJ 667 C-c and -f, assuming Earth-like density for non-transiting planets. The black to white small circles denote the 3D climate simulations carried out by Carone et al. (2015). CH4 – another biomarker – are affected by the stratospheric Brewer- For each circulation regime, we also investigate two different Dobson-circulation (Cordero & Kawa 2001). On Mars, 3D cir- stratosphere wind breaking assumptions. We chose the following culation is instrumental for the formation of a polar ozone layer representative examples for each circulation regime: a habitable (Montmessin & Lefevre` 2013). In summary, the 3D circulation is TRAPPIST-1b, TRAPPIST-1d, Proxima Centauri b and GJ 667 C crucial for the understanding of the abundances and distribution of f scenario. We will show that, on many tidally locked ExoEarths, aerosols, photochemically produced compounds and clouds in the planetary waves propagate vertically from the troposphere into the upper atmosphere of terrestrial, habitable exoplanets that we will stratosphere with different outcomes in stratosphere circulation: ei- characterize in the near future. ther circulation is confined to an ‘equatorial transport belt’ or circu- In this work, we will investigate stratosphere circulation on tidally lation favours a day side-wide distribution of airmass via thermally locked habitable planets with planetary obliquity zero in the hab- driven equator-to-pole-wards transport. itable zone of ultra-cool dwarf stars for a large range of relevant orbital periods (Porb = 1.5–39 d). We further assume to the first or- der Earth-like atmosphere composition and thermal forcing. Such 2MODEL planets develop highly interesting un-Earth-like circulation patterns We use the model introduced in Carone et al. (2014) for terres- that warrant in-depth investigations, as we already showed for the trial tidally locked planets with greenhouse gas atmospheres. We troposphere (Carone, Keppens & Decin 2014, 2015, 2016). extend the radiative-convective equilibrium temperatures derived We will investigate stratospheric circulation in different tropo- there with a stratosphere extension as described below. The ex- sphere circulation regimes that develop for specific orbital period tended radiative-convective equilibrium temperatures are used in ranges. One of the characteristics of these circulation regimes is that the Newtonian cooling framework as thermal forcing (see section troposphere winds are more or less forced into banded or zonally 2.3 in Carone et al. 2014) in the 3D global circulation model MIT- confined wind structures via two different Rossby waves. The trop- gcm (Adcroft et al. 2004) that solves the primitive hydro-static ical Rossby wave leads to a single equatorial superrotating jet, and equations (see section 2.1 in Carone et al. 2014). We furthermore the extratropical Rossby wave to two high-latitude wind jets. The use a Rayleigh surface friction prescription as described in section specific circulation regimes are (Fig. 1): 2.2 of Carone et al. (2014), where we vary in the following – when appropriate – the maximum surface friction time-scale τ s, fric be- (i) ultra-short orbital periods with a strong mixture of equatorial tween 0.1 and 10 d to establish different circulation states for one superrotation and high-latitude wind jets (Porb ≤ 3d) and the same planet (Carone et al. 2016). (ii) short orbital periods with dominance of either equatorial or high-latitude wind jets (P = 3–6 d) orb 2.1 Stratosphere temperature extension and vertical (iii) intermediate orbital periods with weak superrotation (P = orb resolution 6–25 d) and (iv) long orbital periods (Porb ≥ 25 d) with no standing Rossby We assume for the day-side troposphere the equations outlined in waves and therefore mainly radial wind flow instead of banded wind section 2.4.2 of Carone et al. (2014). These are combined with an structures. adaptation of the ‘modification of the Held and Suarez forcing to MNRAS 473, 4672–4685 (2018) 4674 L. Carone et al. 250 where the substellar point is located at φ = 0, ν = 0. We use for 165 240 165 the remainder of this work the parameters for the troposphere tem- 170 170 350 perature defined in Carone et al. (2014) for the Earth-case (see 230 10-2 175 175 Section 2.4 for more details). The equilibrium temperatures on the = −2 180 220 180 day side are shown in Fig. 2 for the surface gravity g 9.81 m s 300 185 185 and Earth-like atmosphere and surface temperatures. 210 190 190 We retain for the horizontal resolution the original C32 cubed 200 195 195 sphereresolutionasusedinCaroneetal.(2014).