5 Escape of Atmospheres to Space

5 Escape of Atmospheres to Space So far, our discussion of atmospheric evolution has circumstances: Jeans’ escape, where individual mol- concentrated on atmosphere and climate fundamentals. ecules evaporate into a collisionless exosphere, and Climate constrains possible life and, as we will see later hydrodynamic escape, which is a bulk outflow with a in this book, the way that climate is thought to have velocity driven by atmospheric heating that induces an evolved can explain many environmental differences upward pressure gradient force (e.g., Johnson et al., between Earth, Venus, and Mars. Climate is closely tied 2013d; Walker, 1982). (ii) Suprathermal (or nonthermal) to the composition of a planet’s atmosphere, which deter- escape is where individual atoms or molecules are mines the greenhouse effect. Consequently, to understand boosted to escape velocity because of chemical reactions how climate has changed over time, we must consider or ionic interactions. Finally, (iii) impact erosion is where how atmospheric composition has evolved. In turn, we atmospheric gases are expelled en masse as a result of must examine how atmospheric gases can be lost. large body impacts, such as the cumulative effect of Gases are lost at an atmosphere’s upper and lower asteroids hits. Of these three types, nonthermal escape is boundaries: the planet’s surface and interplanetary space. generally slow because if it were fast the molecules would In this chapter, we consider the latter. Studies of the Solar collide and the escape would be in the thermal category. System have shown that some bodies are vulnerable to Theory suggests that the two mechanisms that can most atmospheric escape (Hunten, 1990). Indeed, many smaller efficiently cause substantial atmospheric loss are hydro- objects, e.g., most moons and essentially all asteroids, are dynamic escape driven by stellar irradiation (Lammer airless because of escape, making the theory of atmos- et al., 2008; Sekiya et al., 1981; Sekiya et al., 1980a; pheric escape crucial for explaining differences in surface Watson et al., 1981; Zahnle et al., 1990; Zahnle and volatiles. Escape processes can help us understand the Kasting, 1986) and impact erosion (Griffith and Zahnle, lack of atmospheres on the Moon and Mercury, the barren 1995; Melosh and Vickery, 1989; Walker, 1986; Zahnle nature of the Galilean satellites versus Titan (Griffith and et al., 1992). In addition, hydrodynamic escape from early Zahnle, 1995; Gross, 1974; Zahnle et al., 1992), why the hydrogen-rich atmospheres on the terrestrial planets is atmosphere of Mars is thin (Brain and Jakosky, 1998; relevant for observations of noble gases and their iso- Melosh and Vickery, 1989; Zahnle, 1993b), the red color topes, as discussed in Ch. 6, because such escape can of the Martian surface (Hartman and McKay, 1995; Hun- drag along heavier gases. ten, 1979c), the lack of oceans on Venus (Kasting and In this chapter, we focus particularly on the escape of Pollack, 1983) (see Ch. 13), and possibly the oxidizing hydrogen, for two reasons. First, hydrogen is the lightest nature of the Earth’s atmosphere and surface (Catling gas and consequently the most prone to escape. Second, et al., 2001) (See Ch. 10). later in the book, we will see that substantial loss of We can group various types of atmospheric escape hydrogen can affect the redox chemistry of a planet’s into three categories following Catling and Zahnle (2009). atmosphere and surface, changing the chemical character (i) Thermal escape is when irradiation from a parent star of a planet. Rocky planets, as a whole, become more (or, less commonly, a very high heat flux from a planet or oxidized when hydrogen escapes to space. This oxidation moon interior) heats an atmosphere, causing atmospheric occurs irrespective of whether the hydrogen is transported molecules to escape to space. Two end-member approxi- through the atmosphere as H2,H2O, CH4, HCN, NH3,or mations of thermal escape are appropriate under different some other H-bearing compound. Oxidation occurs Downloaded from https://www.cambridge.org/core. University of Chicago, on 19 Apr 2018 at 03:28:44, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/9781139020558.006 129 Escape of Atmospheres to Space 130 because the hydrogen atom that escapes ultimately Later, the Irish physicist George Stoney (who gave us derives from some oxidized form of hydrogen such as the term electron) understood that a few gas particles in water (H2O), water of hydration in silicate rocks (–OH), the high-velocity tail of a Maxwell–Boltzmann distribu- or hydrocarbons (–CH). It was in these compounds that tion of velocities would have sufficient energy to escape hydrogen was originally incorporated into planets like the from a planet’s upper atmosphere even if an average Earth. Consequently, when hydrogen escapes, matter particle did not (Stoney, 1898, 1900a, b, c, 1904). This somewhere on a planet’s surface or subsurface is irrevers- process is nowadays called Jeans’ escape after Sir James ibly oxidized. Jeans, who described its physics in The Dynamical Oxidation is most obvious if we consider hydrogen Theory of Gases (1954, first edition 1904). At that time, that escapes after atmospheric water vapor undergoes in the early twentieth century, balloon soundings in photolysis. Consider water vapor photolysis and escape Earth’s lower atmosphere were extrapolated to the entire in the upper atmosphere of the Earth. In this case, the upper atmosphere, which was assumed to be isothermal at oxygen left behind can oxidize the Earth’s surface so that ~220 K. The hot, 1000–2000 K thermosphere was any further oxygen produced (by photolysis and hydrogen unknown. Consequently, Jeans incorrectly calculated an escape) is less likely to be taken up by the crust and more exceedingly low escape rate of hydrogen. liable to remain in the atmosphere. However, today’s Later, the Space Age provided data from rocket abiotic production rate of oxygen is ~102 times smaller soundings. As a result, in the next major treatment of than the rate of O2 production from photosynthesis and, atmospheric escape, Spitzer (1952) corrected Jeans’ hence, plays a negligible role in the atmospheric oxygen earlier mistake by using more realistic thermospheric budget. It is nonetheless important to understand such temperatures. From the 1950s to the present day, data abiotic oxygen, both because of its possible effect on very have become directly available on the number density of early life on this planet and because of its future signifi- hydrogen and the temperature in the upper atmosphere. cance in interpreting spectra that may be obtained from Measurements include satellite drag through the thermo- exoplanets. sphere, in situ mass spectrometer measurements, and The effect of the escape of hydrogen in oxidizing images of the geocorona, which is a glow at the Lyman- surfaces is also widely considered to be responsible for α wavelength (121 nm) caused by resonant scattering of the oxidized states of Venus and Mars, as illustrated by solar ultraviolet (UV) by a cloud of atomic hydrogen that the red color of the Martian surface (Hartman and surrounds the Earth. UV images taken by spacecraft show McKay, 1995; Hunten, 1979c). Ancient hydrogen escape the hydrogen atoms. Atoms are on ballistic trajectories has also been proposed as a means of oxidizing the back to Earth, escaping, or in orbit (Fig. 5.1). Earth’s atmosphere, crust, and mantle (Catling et al., For astrobiology, we note that about half of the hydro- 2001; Kasting et al., 1993a; Zahnle et al., 2013) (see gen atoms seen in Fig. 5.1 derive from decomposition of Ch. 10). methane (CH4), ~90% of which enters the atmosphere from the biosphere. Most of the other half of the H atoms originates from the photodissociation of water 5.1 Historical Background to vapor. In Fig 5.1, we catch a glimpse of some of the Atmospheric Escape 93 000 tonnes of hydrogen that escape each year (or The idea of the escape of gases from the Earth’s atmos- 3 kg/s) from the Earth. phere is as old as kinetic theory and has an unusual In the past 60 years, planetary exploration and astron- history. A Scottish amateur scientist, John Waterston omy have widened our perspective of both atmospheric (1811–1883), first developed a theory of gases in which escape and of aeronomy, the study of processes in the the mean kinetic energy of each species was proportional rarefied atmosphere from the stratosphere to interplanet- to temperature, and he also introduced the notion of ary space. Space science led to the recognition of atmospheric escape (Haldane, 1928, pp. 209–210). How- suprathermal escape, hydrodynamic escape, and impact ever, the Royal Society rejected Waterston’s paper erosion, as discussed in various reviews (e.g., Ahrens, describing kinetic theory in 1845, and it remained 1993; Chamberlain, 1963; Hunten, 1990, 2002; Hunten unknown until Lord Rayleigh rediscovered the manu- and Donahue, 1976; Hunten et al., 1989; Johnson et al., script in 1891. By then, Waterston’s ideas had been 2008c; Lammer, 2013; Shizgal and Arkos, 1996; Strobel, overtaken by the work of Clausius, Maxwell, and Boltz- 2002; Tinsley, 1974; Walker, 1977). Recently, the dis- mann, while Waterston disappeared in 1883, presumed to covery of exoplanets has made atmospheric escape a have drowned near Edinburgh. fundamental consideration in understanding exoplanetary Downloaded from https://www.cambridge.org/core. University of Chicago, on 19 Apr

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