Comparative Climatology of Terrestrial Planets

1. INTRODUCTION Clouds and hazes are found in every substantial solar system atmosphere and are likely ubiquitous in extrasolar planetary atmospheres as well. They provide sinks for volatile compounds and influence both the deposition of incident flux and the propagation of emitted thermal radiation . Consequently they affect the atmospheric thermal profile, the global climate, the spectra of scattered and emitted radiation, and the detectability by direct imaging of a planet. As other chapters in this book attest, clouds and hazes are a complex and deep subject. In this chapter we will broadly discuss the roles of clouds and hazes as they relate to the study of exoplanet atmospheres. In particular, we will focus on the challenge of exoplanet cloud modeling and discuss the impact of condensates on planetary climates. The terms “clouds” and “hazes” are sometimes used interchangeably. Here we use the term “cloud” to refer to condensates that grow from an atmospheric constituent when the partial pressure of the vapor exceeds its saturation vapor pressure. Such supersaturation is typically produced by atmospheric cooling, and cloud particles will generally evaporate or sublimate in unsaturated conditions. A general framework for such clouds in planetary atmospheres is provided by Sánchez-Lavega et al. (2004). By “haze” we refer to condensates of vapor produced by photochemistry or other nonequilibrium chemical processes. This usage is quite different from that of the terrestrial water cloud microphysics literature where the distinction depends on water droplet size and atmospheric conditions. Because exoplanetary atmospheres can plausibly span such a wide range of compositions as well as temperature and pressure conditions, a large number of species may form clouds. Depending on conditions, clouds in a solar composition atmosphere can include exotic refractory species such as Al2O3, CaTiO3, Mg2SiO4, and Fe at high temperature, and Na2S, MnS, and of course H2O at lower temperatures. Many other species condense as well, including CO2 in cold, 367 Clouds and Hazes in Exoplanet Atmospheres Mark S. Marley NASA Ames Research Center Andrew S. Ackerman NASA Goddard Institute for Space Studies Jeffrey N. Cuzzi NASA Ames Research Center Daniel Kitzmann Zentrum für Astronomie und Astrophysik, Technische Universität Berlin Clouds and hazes are commonplace in the atmospheres of solar system planets and are likely ubiquitous in the atmospheres of extrasolar planets as well. Clouds affect every aspect of a planetary atmosphere, from the transport of radiation, to atmospheric chemistry, to dynamics, and they influence — if not control — aspects such as surface temperature and habitability. In this review we aim to provide an introduction to the role and properties of clouds in exoplanetary atmospheres. We consider the role clouds play in influencing the spectra of planets as well as their habitability and detectability. We briefly summarize how clouds are treated in terrestrial climate models and consider the far simpler approaches that have been taken so far to model exoplanet clouds, the evidence for which we also review. Since clouds play a major role in the atmospheres of certain classes of brown dwarfs, we briefly discuss brown dwarf cloud modeling as well. We also review how the scattering and extinction efficiencies of cloud particles may be approximated in certain limiting cases of small and large particles in order to facilitate physical understanding. Since clouds play such important roles in planetary atmospheres, cloud modeling may well prove to be the limiting factor in our ability to interpret future observations of extrasolar planets. Marley M. S., Ackerman A. S., Cuzzi J. N., and Kitzmann D. (2013) Clouds and hazes in exoplanet atmospheres. In Comparative Climatology of Terrestrial Planets (S. J. Mackwell et al., eds.), pp. 367–391. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816530595-ch15. 368 Comparative Climatology of Terrestrial Planets Mars-like atmospheres and NH3 in the atmospheres of cool giants, such as Jupiter and Saturn. In Earth-like atmospheres water clouds are likely important, although Venus-like conditions and sulfuric-acid or other clouds are possibilities as well. Depending on atmospheric temperature, pressure, and composition the range of possibilities is very large. Furthermore, not all clouds condense directly from the gas to a solid or liquid phase as the same species. For example, in the atmosphere of a gas giant exoplanet solid MnS cloud particles are expected to form around 1400 K from the net reaction H2S + Mn → MnS(s) + H2 (Visscher et al., 2006). Clouds strongly interact with incident and emitted radiative fluxes. The clouds of Earth and Venus increase the planetary Bond albedo (the fraction of all incident flux that is...