1 The Structure of Planetary Atmospheres Interactions between the atmosphere, ocean, solid planet, has been averaged over some period of time such as a biosphere, and space are responsible for the evolution of the month, season, year, or longer. The mean surface tem- Earth’s atmosphere. Other planets have simpler global perature is a key parameter of importance for habitability. systems that merely consist of an atmosphere surrounding If the surface temperature is not suitable for liquid water, a rocky surface without a liquid ocean, or, in the case of gas life as we know it will probably not exist there, except giants, atmospheres that thicken inwards towards higher- possibly hidden in the subsurface. density layers and cores. In the first five chapters, we The same principle applies to extreme places on the describe general physical and chemical principles that Earth. For example, the coldest place on Earth’s surface is apply to all or most atmospheres. There are many excellent Vostok, Antarctica, with temperatures ranging from introductory texts on atmospheric physics and chemistry, –14 C (259 K) to a record extreme of –89.2 C (184 K) e.g., Walker (1977), Hartmann (1994), Houghton (2002), (Turner et al., 2009). Consequently, Vostok is frozen and Hobbs (2000), Wallace and Hobbs (2006), and Andrews lifeless. In part, the coldness of Vostok arises from its (2010). These books focus on Earth’s atmosphere. Our height of 3450 m above sea level. How the air tempera- approach here is to discuss the fundamentals of atmospheres ture varies with altitude is one of the most basic features using examples from a variety of planetary atmospheres. of planetary atmospheres, the atmospheric structure, Thus, Chapters 1–5 are an introduction to the concepts of which is our starting point for this book. planetary atmospheres, often with a view to how the basics The Earth’s atmosphere provides the nomenclature apply to issues of atmospheric evolution. We refer back to for describing vertical regions of planetary atmospheres. these fundamentals in subsequent chapters. Terminology for Earth’s atmospheric layers was Some other books also discuss planetary atmospheres. developed in the early 1900s (Lindemann and Dobson, Ingersoll (2013) is an excellent readable primer. Cham- 1923; Martyn and Pulley, 1936). Because of this histor- berlain and Hunten (1987) has a particular emphasis on ical origin, the same terminology only approximately upper atmospheres, Yung and Demore (1999) focus on applies to other planetary atmospheres. photochemistry, Pierrehumbert (2010) has broad insight- Earth’s atmosphere consists of five layers of increas- ful coverage with an emphasis on radiation, while ing altitude (Fig. 1.1). Going upwards, new layers begin Sanchez-Lavega (2010) is a valuable general reference where there is a temperature inflection. text for planetary atmospheres. (1) The troposphere goes from the surface to the tropo- The key uniqueness of this book is that we emphasize pause, which lies at a height of 11 km (0.2 bar) in the US atmospheric evolution, which, needless to say, we con- Standard Atmosphere but varies in altitude from ~ 8kmat sider the most interesting aspect of planetary atmospheres! the poles to ~ 17 km in the tropics. The global average tropopause pressure is 0.16 bar (Sausen and Santer, 2003), varying 0.1 to 0.3 bar from tropics to pole (Hoinka, 1.1 Vertical Structure of Atmospheres 1998). Tropos is ancient Greek for the ‘turning’ of con- 1.1.1 Atmospheric Temperature Structure: vection and pause is Greek for “stop.” An Overview (2) The stratosphere (from Greek stratus for “layered”) A planet’s atmosphere, through its composition and goes from the tropopause to the stratopause at ~ 50 km greenhouse effect, controls climate, which is weather that altitude, where the air pressure is about 100 Pa (1 mbar). Downloaded from https://www.cambridge.org/core. University of Chicago, on 11 Apr 2019 at 04:29:19, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/9781139020558.002 3 The Structure of Planetary Atmospheres 4 500 Quiet Sun Active Sun Exosphere 400 Thermopause 300 Heterosphere Thermosphere Altitude (km) 200 Figure 1.2 Thermal structure of the atmospheres of various planets of the Solar System. The dashed line at 0.1 allows you to 100 see the feature of a common tropopause near ~0.1 bar for the Mesopause Mesosphere thick atmospheres, despite the differences in atmospheric com- Stratopause position. See Robinson and Catling (2014) for sources of data. Stratosphere Homosphere Tropopause Troposphere 0 500 1000 2000 the ground and lifted upwards by buoyancy. Conse- Temperature (K) quently, air parcels convect to places of lower pressure ’ Figure 1.1 The nomenclature for vertical regions of the Earth s where they expand and cool. The net effect of lofted atmosphere, shown schematically. parcels that cool, and sinking ones that warm, is to main- tain an annual average temperature decrease from Earth’s (3) The mesosphere extends from the stratopause to the warm surface to the cold upper troposphere of about mesopause at ~ 85 km altitude, where the air pressure is 6Kkm–1 when globally averaged (see Sec. 1.1.3). about 1–0.1 Pa (0.01–0.001 mbar) Earth’s troposphere contains ~ 80% of the mass of the (4) The thermosphere goes from the mesopause to the atmosphere and this mass, along with the composition of thermopause at about ~ 250–500 km depending on vary- the air, renders the troposphere fairly opaque to thermal- ing ultraviolet from the Sun. Above the thermopause, the infrared (IR) radiation emanating from the planet’s surface. atmosphere becomes isothermal. In general, somewhat below the tropopause, atmospheres (5) The exosphere lies above the thermopause and joins become semi-transparent to thermal-IR radiation. Conse- interplanetary space. Unlike the other layers that are quently, transfer of energy by radiation in the upper tropo- defined by the temperature profile, the exosphere is where sphere replaces convection as the means of upward energy collisions between molecules are so infrequent that they transfer at a radiative–convective boundary. Efficient can usually be neglected. The exobase is the bottom of the emission of thermal-IR radiation to space accounts for exosphere and nearly coincides with the thermopause. the temperature minimum at the tropopauses of many The terminology developed for Earth’s lower atmos- planetary atmospheres, which occurs above the radiative– phere depends upon the presence of the ozone layer in the convective boundary for atmospheres with stratospheres. stratosphere. Ozone absorbs ultraviolet (UV) sunlight, For planets with thick atmospheres, the tropopause which causes temperature to increase with height above temperature minimum occurs where the air has thinned to the troposphere and defines the stratosphere. Other planets, roughly ~ 0.1 bar pressure (Fig. 1.2). Remarkably, this such as Mars, do not have UV absorbers to induce a strato- rule applies to Earth, Titan, Jupiter, Saturn, Uranus, and sphere, so the geocentric nomenclature for atmospheric Neptune, and the mid-to-high latitudes of Venus layers breaks down (Fig. 1.2). However, we generally find (Tellmann et al., 2009), despite vast differences in analogs in other planetary atmospheres for a troposphere, atmospheric composition. This commonality occurs mesosphere, thermosphere, and exosphere. On Titan and because the broadband opaqueness to thermal-IR in upper the giant planets, absorbers of shortwave sunlight (UV, tropospheres is pressure-dependent with similar scaling – visible, and near-infrared) produce stratospheres. varying approximately with the square of the pressure – Convection is the key process in tropospheres. On despite the differences in atmospheric composition. Since Earth, radiation heats the surface, so air is warmed near all these atmospheres have strong and roughly similar Downloaded from https://www.cambridge.org/core. University of Chicago, on 11 Apr 2019 at 04:29:19, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/9781139020558.002 1.1 Vertical Structure of Atmospheres 5 opaqueness (within the same order of magnitude) to Consequently, a molecule excited by absorption of a thermal-IR at moderately deep pressures, such as 1 bar, photon or a rare collision loses internal energy by the weak square-root dependence of pressure on the radiation rather than kinetic energy through collisions. upwardly decreasing opacity ensures that the tropopause The altitude where this happens is the level of radiative minima are all near 0.1 bar, within a factor of 2 or so relaxation (Curtis and Goody, 1956; López-Puertas and (Robinson and Catling, 2014). Thus, a “0.1 bar tropo- Taylor, 2001). pause minimum rule” could apply to exoplanet atmos- Above the mesopause, Earth’s thermosphere is pheres, provided that they have temperature minima at the extremely thin (e.g., <1012 molecules cm–3 above base of stratospheres. Of course, such a rule can’t apply to 120 km, compared to ~ 1019 cm–3 at the surface) and atmospheres without stratospheric absorbers and tropo- responds rapidly to variations in solar radiation shorter pause temperature minima. than ~ 100 nm in wavelength – the extreme ultraviolet In Earth’s stratosphere, ozone (O3) warms the air by (EUV). Daily temperature changes are very large here. absorbing solar UV over a wavelength range of about Similar EUV absorption occurs high up in all planetary 200–300 nm. Stratospheric warming also occurs on the atmospheres, where EUV, x-rays, and chemical reactions giant planets and Titan because aerosols (fine, suspended break up molecules into atoms. The mean free path particles) and methane absorb in the UV, visible, and (which is inversely proportional to density) becomes so near-IR. Warming creates an inversion – a vertical region big that the combination of a continuous flux of ionizing of the atmosphere where temperature increases with radiation and infrequent collision between ions and height (Fig.