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1. INTRODUCTION The field of exoplanet atmosphere characterization stands on a divide. On the one side are the dozens of hot Jupiter-mass (MJup) exoplanets with published atmosphere observations. On the other side lie the emerging population of super Earths and anticipated Earth-mass (M ) planets, with atmospheres as yet to be characterized. Super Earths are loosely defined to be planets >1 M and (7) Here, kB is Boltzmann’s constant, Texo is the exospheric temperature, m is molecular or atomic mass, G is the gravitational constant, Mp is the planet mass, and Rp is the planet radius. This simple equation shows that thermal escape is more likely to occur for lower-mass species and hotter exospheres, and will be slowed for larger-mass planets and planets of higher density (with larger M/R ratios). The thermal velocities of particles of a given mass are not, however, identical, and instead follow a Maxwellian distribution in which the high-energy tail of particles is more likely to escape. Slower atoms and molecules then move to fill the high-velocity tail. This process is called “Jeans escape.” The Jeans escape flux can be calculated by integrating the Maxwellian velocity distribution over the upward hemisphere for escape velocities consistent with the exobase temperature. The Jeans escape flux is typically given in units of cm–2 s–1 and can be expressed as ( ) c c B c Jeans c n 2k T B 1 e m 2 −λ Φ = + λ π (8) where nc is the number density of particles at the exobase, and the subscript c refers to exobase properties. The factor B effectively slows down the escape by accounting for the moisture-laden air that condenses, and typically can be found from the surface (fog) up to 18 km altitude, depending on the location of the tropopause. On Mars, both water and CO2 ice clouds may be present, forming near 10 km and the colder 50 km altitude respectively. On Venus, however, the “cloud” particles are produced photochemically at high altitudes (near 80–90 km) via a UV-driven reaction with sulfur dioxide, water, and atomic oxygen from the photolysis of CO2. These particles remain small at higher altitudes but can collide and coagulate to form larger particles at lower altitudes. This results in a photochemically produced planetwide cloud deck from 45 to 70 km altitude with fine haze above it up to 90 km. Clouds can both reflect and absorb radiation, and can affect the planetary energy balance by producing net cooling or warming effects under different conditions. Perhaps the most intuitive effect of both water and CO2 clouds is to enhance reflectivity in the visible, which increases the planetary albedo. On Earth, low and middle liquid water clouds are strong visible-light reflectors but poor absorbers in the visible and infrared, and so provide a net cooling effect. However, high ice clouds, which are often thin, allow visible light to reach the surface and absorb more infrared radiation than liquid water, providing a net heating effect. At night, when clouds are not illuminated and cannot enhance the planet’s albedo, they instead serve as greenhouse warming agents, by absorbing escaping infrared radiation and reradiating it back toward the surface. Thick CO2 ice clouds behave slightly differently from low water clouds in that although they also significantly increase the albedo of a planet, CO2 ice is more efficient at scattering escaping infrared radiation back toward the planetary surface, and so can potentially produce a net warming of the atmosphere, despite its high visible reflectivity. This CO2 blanketing behavior has been invoked as a potential means of both warming early Mars (Forget and Pierrehumbert, 1997) and of extending the outer limits of the habitable zone. Cloud formation can also affect climate, and atmospheric evolution, by modifying the tropospheric lapse rate (the rate at which temperature decreases with altitude). When a cloud forms, a gas condenses or freezes into the liquid or solid state, thereby releasing latent heat into the surrounding atmosphere . Condensation and latent heat release reduces the rate at which moist air parcels cool as they rise, reducing the atmospheric temperature lapse rate. One consequence of this in a water-rich troposphere is that the tropopause cold trap gets pushed higher into the atmosphere. This is why Earth’s tropopause is much higher in the tropics than at the poles. In an extreme case a high tropopause may eventually allow water to rise to an atmospheric region where it is vulnerable to UV photolysis and...

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