14Temperature, Clouds, and Aerosols in Giantand Icy Planets

Robert A. West

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266Temperature Structure and Condensate Clouds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

Temperature Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267Condensate Clouds: Thermochemical Equilibrium Models . . . . . . . . . . . . . . . . . . . . . . . . . 269Photochemistry and Haze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274Cloud and Haze: Relation to Remote Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

Abstract

As observations (transit spectra and secondary eclipse spectra) of extrasolarplanets accumulate, it has become clear that clouds and haze are prevalent inthe Neptune–Jupiter-sized planets. Clouds and haze have a profound influenceon the spectra and must be understood to make sensible interpretation of thedata. High-temperature atmospheres are likely to contain condensates of densematerials (minerals and metals), and this presents a challenge to understand howsuch particles can be maintained high in the atmosphere. Perhaps photochemicalhaze is the more important aerosol at high altitude, but more work needs tobe done to understand its formation and stability. This chapter summarizes thesalient observations and ideas about temperature, clouds, and haze in gas andice giant planets with emphasis on lessons learned from studies in our own solarsystem.

R. A. West (�)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USAe-mail: Robert.a.west@jpl.nasa.gov

© This is a U.S. Government work and not under copyright protection in the US;foreign copyright protection may apply 2018H. J. Deeg, J. A. Belmonte (eds.), Handbook of Exoplanets,https://doi.org/10.1007/978-3-319-55333-7_49

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266 R. A. West

Introduction

The intent of this chapter is to highlight linkages between studies of the giantand icy planets in our solar system (Jupiter, Saturn, Uranus, and Neptune) withstudies of extrasolar planets of similar class. This is mostly a matter of applying theknowledge gained over many decades from extensive and detailed observations ofsolar system planets and related theoretical work and modeling to understand theprocesses important for less-well observed extrasolar planets and to help us sort outwhat the observations of those objects can tell us.

In this chapter we focus on temperature, clouds, and aerosols. Temperature isa fundamental property of a planetary atmosphere and must be understood as partof any investigation of composition or atmospheric dynamics. Clouds and aerosolsare important because they profoundly influence spectral signatures from which wededuce upper atmospheric composition and temperature, and they play a key rolein the atmospheric radiative balance. They also serve as tracers of winds in theatmosphere.

Just as the study of giant planet atmospheres has moved us out of our “comfortzone” with our understanding of the terrestrial atmosphere as a reference point,the study of exoplanet atmospheres moves us further out of our comfort zone.Temperature structure, composition, tidal locking, and photochemistry are amongthe factors that can be far out of the range of solar system familiarity, and sowe must rely on fundamental principles, more comprehensive thermodynamicparameters and line listings for molecular opacities, and innovative thinking. Tobuild a foundation for this process, we list in Table 1 several references of a generalnature for a big-picture understanding of the state of the art of knowledge relevantto this chapter.

In addition to the citations in Table 1, the reader desiring related backgroundinformation is directed to other chapters in this volume, especially the sectionon “Exoplanet Atmospheres” and �Chaps. 16, “Atmospheric Dynamics of Giantsand Icy Planets”, and � 17, “Upper Atmospheres and Ionospheres of Planets andSatellites”

Table 1 Selected general references for temperature, clouds, and aerosols in giant and icy planets

Reference Category Topics

West (2014) Solar system Atmospheres of the giant planets

West et al. (2004) Solar system Clouds and haze (Jupiter)

West et al. (2009) Solar system Clouds and haze (Saturn)

West et al. (1991) Solar system Clouds and haze (Uranus)

West (2000) Exoplanets/solar system Condensates in Jovian atmospheres

Seager and Dotson (2010) Exoplanets Comprehensive survey

Seager (2010) Exoplanet atmospheres Exoplanet atmospheres

Perryman (2014) Exoplanets Comprehensive survey

Heng (2017) Exoplanet atmospheres Theoretical concepts and foundations

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 267

Temperature Structure and Condensate Clouds

Temperature structure and condensate clouds are intimately coupled via the abun-dances of condensable species. For planets in our solar system, the abundances ofcondensable species are enhanced over solar values because they were preferentiallyincorporated into planets in the primitive solar nebula. Presumably the same processapplies to extrasolar planets, but with the understanding that the nebula fromwhich they form can have abundances of heavy elements (generally designated as“metals”) significantly different than those in the protosolar nebula. A convenientway to parameterize in a gross sense is to model abundances of heavy elements asX times solar (relative to hydrogen). Sulfur, nitrogen, and some ratios such as theC/O and Mg/Si are of special significance to composition and cloud formation andare sometimes modeled with an additional parameter (Bond et al. 2010; Petiguraand Marcy 2011; Hu and Seager 2014).

Temperature Profiles

Although temperature is a three-dimensional time-varying field, to first order it isthe global-average vertical temperature profile that controls some chemical speciesmixing ratios and cloud particle densities. The partitioning between CO and CH4is a function of temperature (and therefore altitude), for example. For planets inour solar system, we benefit from high signal/noise observations of well-mixedconstituents that have well-observed bands and lines for methane and collision-induced H2. These provide information on temperature in the upper troposphere andstratosphere for the gas and ice giants, although there is some ambiguity for Uranusand Neptune where the methane mixing ratio is nonuniform due to condensation.We also have the benefit of radio occultation from the spacecraft that have flownby these planets. Radio occultations, coupled with thermal infrared measurements,provide temperature profiles that are spatially well resolved in the vertical dimensionbut only at selected latitudes and times of the occultation measurements. Stellarand solar occultations, observed from the ground and from spacecraft, providetemperature profile information at high altitudes in the thermospheres of the planets(Hubbard et al. 1995; Shemansky and Liu 2012; Koskinen et al. 2013).

Temperature profiles from the types of measurements listed above were usedto produce summary temperature profiles as shown in Fig. 1 for the giant andicy planets of our solar system. Also shown in Fig. 1 are condensate cloud baselevels calculated from knowledge of gas abundances and thermodynamic constants.This will be discussed in more detail in the following section. The region of theatmosphere probed by measurement ranges from about 5 bars to microbars. Atdeeper levels an adiabatic temperature profile is assumed. At high altitudes, abovethe tropopause near 100 mb heating from sunlight absorbed by methane gas andhaze produces a temperature inversion, and at the highest altitudes, additionalheating from a variety of processes (most likely joule heating and auroral energydeposition) produces temperatures that are higher than can be consistent with

268 R. A. West

Fig. 1 Global meantemperature/pressure profilesand condensate cloud basesfor the gas and ice giantplanets in our solar system(Gierasch and Conrath 1993).Not all condensate cloudcandidates appear in thisfigure. Missing are curves forNH4SH and H2S

0.1Pre

ssur

e (b

ars) 0.01

0.001

0.0001

UranusSaturn

Jupiter

CH4NH3

H2O

280240200160

Temperature (K)

120804010

1

Neptune

absorbed sunlight. These processes are likely to be operative on at least somefraction of extrasolar planets depending on the existence and configuration of aplanetary magnetic field and stellar wind parameters. Auroral heating and to someextent joule heating can have large amplitude variations on short time scales,whereas heating from stellar radiation (including the effects of eccentricity) canalso have variability on seasonal time scales.

Extrasolar giant planets close to the host star are tidally locked. The giantplanets in our solar system are rapid rotators relative to their orbital period. Solarheating can therefore be considered to be averaged over longitude. For a tidallylocked planet close to its host star, there is an enormous radiatively driven thermalgradient in longitude, and this drives a strong dayside/nightside flow in the highatmosphere. For the case of WASP-43b, the flow produces an observed signature(an orbital phase lag) in the thermal-IR which serves as an observational basis fordynamical models (Kataria et al. 2015). Similarly, Charnay et al. (2015a, b) studiedsub-Neptune planet GJ1214b, finding that strong meridional circulation can loftcloud particles high in the atmosphere, accounting for the absence of gas-phasespectral signatures. The morphology of the resulting wind field and the distributionof temperature across the planet are unlike anything we are familiar with in oursolar system, although Venus is also a slow rotator, but its atmosphere is not tidallylocked. As a consequence of the tidal locking and resulting flow field, a global-

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 269

average temperature profile is not a good working concept for these planets. Thereare some observational consequences for tidal locking for close-in planets. Thetemperature inferred from atmospheric scale height derived from transit data whichsample the terminator region can be quite different from temperature inferred fromsecondary eclipses which sample the dayside.

Data relevant to exoplanet temperatures are sparse, and temperatures measuredat one location relative to the substellar point do not capture strong temperaturegradients as function of longitude or distance from the substellar location. Transitspectra provide some constraints (Etangs et al. 2008; Bétrémieux and Swain 2017) atthe terminator but only through indirect inference from scale height estimated fromspectral slope. Thermal-IR spectra measured during secondary eclipses provide amore direct measure of temperature but with large uncertainty in pressure due to lowspectral resolution typical of current exoplanet spectra. Thermal spectra can also beobserved at any orbital phase if the target is distant from its host star and if it isbright enough in the thermal infrared. For young planets distant from their host star,thermal emission is produced by residual heat of formation, and inferences abouttemperature structure are based in part on evolutionary models. For these and forplanets whose thermal spectra are not observed, modelers construct thermochemicalequilibrium radiative convective models from known or assumed composition,distance from the host star, and radiation from the host star. These calculationsusually employ a two-stream approximation for radiative flux calculations (Toonet al. 1989). An example of this is EPACRIS, a publically available web interfacefor radiative/convective equilibrium model atmosphere calculations. (http://epacris.ipac.caltech.edu/). For both retrieved (from data) and modeled thermal profiles, thepresence of haze or clouds and the heterogeneous nature of cloudy atmosphereswill complicate the retrievals and produce error in the model result if not fully takeninto account. Some examples of temperature profiles for exoplanet atmospheres areshown in Fig. 2.

The previous paragraphs outline how temperatures are retrieved or estimated forgiant planet atmospheres. Estimation of the associated uncertainties is much moredifficult, and given the small signal/noise ratio of observations typical of exoplanetobservations, the question is of critical importance and still not fully understood.In this regard it is worth reviewing the work of Deming and Seager (2017) whoexamined a now-discarded claim of an exoplanet (hot giant planet HD209458b)with a temperature inversion. Even in our own solar system, the high atmospheresof the giant planets are hotter than predicted from the available solar radiativeinput, and the responsible mechanism(s) is(are) still debated. Auroral heating andjoule heating are certainly important and involve the planetary magnetic field, theplanetary wind field, and the influence from the solar wind. These are largelyunknown for exoplanets.

Condensate Clouds: Thermochemical Equilibrium Models

We use the term “clouds” to indicate condensate clouds and “haze” to indictphotochemical haze (although more generally ionization by charged particle bom-

270 R. A. West

Fig. 2 (Morley et al. 2013)Pressure–temperature (P–T)profiles of GJ 1214b withcondensation curves. Top:solar composition models andcondensation curves. Bottom:50� solar models andcondensation curves.Cloud-free P–T profiles areshown as solid black lines;cloudy (KCl and ZnS clouds)models are shown as dashedlines. The cooler (left) modelsin each panel assume that theabsorbed radiation from thestar is redistributed aroundthe entire planet; the warmer(right) ones assume that theradiation is redistributed overthe dayside only.Condensation curves of allrelatively abundant materialsthat will condense in browndwarf and planetaryatmospheres are shown asdashed colored lines

bardment and thermochemical polymerization are included as contributing pro-cesses). Both can be important depending on the wavelength of interest, pres-sure/temperature profiles, chemical abundances, proximity to the host star, magneticfields, and other factors. In the following sections, we cover the main issues that needto be considered.

A good starting point for understanding condensate clouds requires knowledgeof the temperature-pressure profile, the abundance of condensable constituents,and the appropriate thermodynamic constants. For the gas and ice giants of oursolar system, temperature profiles (a global-average) and condensation lines for therelevant condensables are shown in Fig. 1 and for a hot exoplanet case in Fig. 2.

Thermochemical equilibrium models are helpful in predicting base levels ofcondensate clouds, either by the use of the Clausius–Clapeyron equation (Sánchez-Lavega et al. 2004):

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 271

dPV

dTD

L

T .V 2 � V 1/(1)

or, as most studies do, by minimization of the Gibbs free energy (Lodders andFegley 2002; Visscher et al. 2006, 2010). In Eq. 1 L is the latent heat of thephase transition (in J g�1) and Vi D 1/¡i is the specific volume (phase 1 D vaporphase, 2 D liquid or ice phase) (Sánchez-Lavega et al. 2004). Conceptually ourexpectations about condensate clouds begin with this framework, but the story ismore complex. Clouds of ammonium hydrosulfide, for example, in the atmospheresof Jupiter and Saturn, are formed directly from gas-phase chemistry of NH3 andH2S rather than as a condensate of NH4SH vapor. Whether or not all of the H2S isconsumed in the formation of NH4SH clouds or forms clouds of H2S ice dependson the N/S ratio, and this is still somewhat uncertain for the ice giants of our solarsystem.

Most of the extrasolar Jupiter- and Neptune-class planets discovered to date haveatmospheres much hotter than those in our solar system, and Fig. 2 shows thecomposition of expected condensates in those atmospheres. From thermochemicalequilibrium, we would expect clouds of KCl, ZnS, Na2S, MnS, Cr, MgSiO3, Fe,and Mg2SiO4, depending on temperature and metallicity. Due to the day/nightasymmetry in heating and the associated asymmetry in temperature, the cloudcomposition can be different on the day and night sides, and this difference, whencoupled with the circulation and advection of cloud material, can manifest as anasymmetry in the location of maximum effective temperature or maximum of thelight scattered from the planet (Hu et al. 2015; Parmentier et al. 2016). Figure 3shows how this plays out for a model hot Jupiter with a large azimuthal temperatureasymmetry (Parmentier et al. 2016). Furthermore, Line and Parmentier (2016) showthat patchy clouds “can exactly mimic high mean molecular weight atmospheresand vice. versa over certain wavelength regions, in particular, over the Hubble SpaceTelescope (HST) Wide Field Camera 3 (WFC3) bandpass (1.1–1.7 mu m).” This isone example where caution is needed in the interpretation of data. There are manyothers.

Prediction of cloud base levels for any constituent cannot tell us how verti-cally or optically thick clouds are, vertical or horizontal heterogeneity or cloudmicrophysics. Those topics will be touched on in what follows. Consider theatmosphere of the Earth. The cloud base level for a global-average model of theEarth is at sea level, yet common experience reveals a vast array of cloud behaviorsand complex three-dimensional arrangements constantly changing with time. Thenonhomogeneous nature of clouds has a profound effect on the radiative transferand influences the results of retrievals for gaseous constituents.

The distribution of clouds and their influence on our understanding of theatmospheres is a fundamental question for both solar system planets and extrasolarplanets. The reflecting layer model (RLM) is the simplest model that has often beenused to infer abundances. This model envisions the cloud as having a diffuselyscattering upper boundary that photons do not penetrate. For this model the opticaldepth of gas sampled by photons is readily calculated from the amount of gas

272 R. A. West

Fig. 3 (Parmentier et al. 2016) Pressure–temperature profiles from a model for a planet withTeq D 1,900 K (gray: profiles at all latitudes and longitudes; light green: profiles at the limb westof the substellar point; dark green: profiles at the limb east of the substellar point; blue: profilesat less than 20ı of the antistellar point; red: profiles at less than 20ı of the substellar point). Thecondensation curves of several important species are plotted as dashed lines. At a given pressure,if the temperature is cooler than the condensation temperature, we consider the atmosphere cloudy.For pressures lower than Ptop, we assume that clouds are not present, with this cloud top pressureused as a free parameter

overlying the cloud top. Strong and weak spectral lines have definite and easilycalculated equivalent widths according to this model, and ratios of equivalent widthsfor strong and weak lines would carry the signature of a reflecting layer with aspecific column of overlying gas. Spectra obtained over many years have shownrepeatedly that use of the RLM is not appropriate for the gas and ice giants in oursolar system (see references in Table 1).

A first step to improve our interpretation of spectra envisions a two- or multilayercloud and haze model where photons can penetrate into the cloud and haze, butclouds and haze are considered to be uniform in the horizontal dimensions, andcurvature of the atmosphere is neglected on the length scale of the photon mean-free path (the plane-parallel approximation). Most models of radiative transfer forthe gas and ice giants of the solar system rely on this picture. Typically, the layers ofthe model have a cloud base set by thermochemical equilibrium and with a cloud topat the same or higher altitude. Among the free parameters in the model are cloudoptical thickness, particle single-scattering albedo, and scattering phase function,and these can depend on wavelength. Of course a vertically inhomogeneous cloudmodel can have more than one cloud layer as well as a layer of photochemicalhaze in the stratosphere. The number of free parameters can multiply rapidly, andtheir significance diminishes accordingly. Finding the optimal balance is part of theproblem. The solution to this problem will depend to a large degree on the quality

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 273

of the observations, with high-quality (high signal/noise) observations that involveline or band profiles that sample a large vertical region of the atmosphere providingmore constraints.

Ackerman and Marley (2001) suggested a parametric vertical profile for aerosolparticles that is simple yet captures some of the considerations needed for retrievals.In their approach the cloud base is set by thermochemical equilibrium and cloudparticles extend to higher altitudes density profile calculated from the balancebetween turbulent diffusion and sedimentation in horizontally uniform cloud decks.A key parameter of the Ackerman and Marley concept is labeled frain, defined asthe ratio of the mass-weighted droplet sedimentation velocity to w*, the convectivevelocity scale. With this parameterization (see Fig. 4), one can examine with forwardmodeling a wide range of photon path distributions in the atmosphere to see howradiation samples the atmosphere and also to calculate radiative balance. Althoughdetailed microphysical models for clouds have been constructed, taking into accountprocesses such as homogeneous and heterogeneous nucleation, growth by condensa-tion, coagulation, coalescence, sedimentation, and other processes (Rossow 1978),we do not have enough information to assess the relative importance of these andother processes to make use of the more sophisticated framework. Consequently,such models cannot inform about particle size distributions. Furthermore, theheterogeneous nature of clouds in three dimensions due to atmospheric motions,transport, and sedimentation leads to effects not anticipated by the horizontallyhomogeneous models. In spite of these complications, Wong et al. (2015) proposed

Fig. 4 (Ackerman and Marley 2001). Vertical profiles of mole fraction (mixing ratio by volume,qc) of condensed ammonia from a model of a Jovian ammonia cloud with different values frain andfrom adaptations of other models as labeled. The vertical coordinate is atmospheric pressure. Thedotted line is the temperature prole. The kinks in the condensate proles are caused by ripples in thetemperature profile

274 R. A. West

a cloud parameterization that relates vertical motions with cloud density and particlesize. This technique can be applied to spatially resolved planetary images, but it isunclear how to make it work for disk-integrated measurements.

Photochemistry and Haze

Aerosol particles are formed by photochemical processes (including high-energyphotons from the sun/star and bombardment by energetic particles in the solar/stellarwind, the planetary magnetosphere, and cosmic rays) operating in the high atmo-sphere, generally above the condensate layers, and can have effects on radiativeprocesses and retrievals ranging from negligible to profound, especially in the limb-probing geometry of exoplanet transits. In our solar system, haze aerosol is typicallyrevealed by absorption at UV wavelengths and by stellar and solar occultation, bypolarization, and by strong scattering of the incident light at high phase angles (lowscattering angles).

In our solar system, paths to several photochemical products have been inves-tigated. In Jupiter’s atmosphere, ammonia photolysis leads to the formation ofhydrazine (NH4). The spectral signature of hydrazine ice has, however, not beenobserved. For Saturn, the same process is possible, but due to the colder tem-peratures, the photolysis of ammonia is probably not important. Instead, PH3,a disequilibrium species transported from deeper regions, has been observed insignificant abundance above the cloud tops and can photolyze to produce PH4

(diphosphine), or elemental phosphorus. Neither of these has been observed spec-troscopically.

It is likely that organics are responsible for much of the stratospheric haze in theatmospheres of the gas and ice giants in our solar system and also in exoplanets. Allupper atmospheres of the giant and icy planets in our solar system contain methane(CH4). We do see spectral signatures of methane photolysis products (C2H2, C2H6,and other hydrocarbons), and photochemical models are largely able to account forthese (Gladstone et al. 1996; Moses 2000). With regard to exoplanet atmospheres,photochemistry may be an even more prolific source of haze because many observedgiant planets are close to the parent star and receive a prodigious UV flux. However,CO is favored over CH4 for hot planetary atmospheres, and so it is unclear how wella CO–H2 atmosphere is able to produce haze. Laboratory experiments to addressthis are needed. These and other aspects of exoplanet photochemistry have beendiscussed by several authors (Moses 2014; Moses et al. 2013; Zahnle et al. 2016;Charnay et al. 2015a).

The detailed mechanism and the fractions of C, H, and O in organic haze arestill far from understood, even for the planets in our solar system. Polycyclicaromatic hydrocarbons are suspected key intermediaries (Friedson et al. 2002), andthe simplest, benzene (C6H6), has been observed in the atmospheres of Jupiter andSaturn (Koskinen et al. 2016). Production of C6H6 seems to require ion chemistry,a challenge for chemical models beyond benzene. UV flux from the sun or starcertainly contributes as an energy source to break up the primary molecules, but

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 275

may be a weak source compared to auroral energy deposition at high latitudes forJupiter and Saturn. In order to understand this process in extrasolar planets, wewould need to first understand the magnetic field configuration, stellar wind/planetmagnetosphere interactions, and magnetospheric electron and proton acceleration.

Products of photolysis may be responsible for red/brown colors of clouds wesee at visible wavelengths in the atmospheres of Jupiter and (more muted) Saturn,and, by extension, extrasolar planets. All condensate clouds in our solar system arewhite at visible wavelengths. Several candidates have been suggested to account forthe colors of clouds, including inorganic compounds involving N and S and organiccompounds (West et al. 1986). There are no definitive spectral features in giantplanet spectra to make a convincing assignment. Recently an appealing idea hasemerged from laboratory experiments: chemistry produced by ammonia interactingwith acetylene sedimenting from the stratosphere (Carlson et al. 2016).

Cloud and Haze: Relation to Remote Sensing

Because they influence photon paths through the atmosphere, and because ourunderstanding of the atmospheres of the planets depends largely on photons thatsample the atmosphere, clouds and haze have profound effects on what we knowabout planets and exoplanets. The history of retrievals for giant planets in oursolar system is replete with examples of conflicting or simply wrong estimationsof gaseous abundances and the locations of clouds due in large part to a failure toappreciate how clouds and haze contribute to the reflected spectrum. It is importantto understand the pitfalls as well as the successes.

Clouds and haze affect observations of planets and exoplanets in ways thatdepend strongly on the viewing geometry. Spectra from exoplanets observed intransit are produced by long-path transmission along the terminator limb and arelimited to the high atmosphere due to gas and particle opacity and by refraction(Bétrémieux 2016). Figure 5 shows some model calculations for sample cases ofcloud and haze. Clouds and haze suppress spectral features from gas absorptionby limiting the depth probed. It is difficult to determine whether clouds or haze areresponsible for the suppression of absorption feature, although with spectra coveringa broad spectral range the signature of small haze particles becomes apparent (largeparticles have a spectrally flat opacity except for a few condensed-phase featuressuch as C–H bond absorptions). Aggregate particles are class of haze particlesobserved in the atmosphere of Titan and the polar regions of Jupiter and Saturn(West and Smith 1991). Those particles have a strong wavelength dependenceand high polarization for typical cases where the monomer size is much smallerthan the wavelength. Instruments on the Cassini spacecraft measured spectra asthe spacecraft entered or exited during several solar occultations by Titan, andfrom those measurements (Robinson et al. 2014) simulated spectra of an exoplanettransit for a planet with a Titan-like haze. The spectral slope induced by the haze isprominent in the simulation, indicating that a similar haze in exoplanet atmosphereshould be evident in transit spectra.

276 R. A. West

Fig. 5 (Morley et al. 2013) Comparison of steam and cloudy H-rich atmosphere models in themid-infrared. The 100% water atmosphere model shows water vapor features of a similar amplitudefrom 1 to 20 �m. However, for both of the cloudy models, the clouds become significantly lessoptically thick at longer wavelengths than they are in the near infrared where current data exists.This means that in the mid-infrared, the features are much larger

Spectra from secondary eclipses or from distant planets observed in emittedthermal or scattered starlight carry information from deeper levels. Optically thickcondensate clouds also suppress gaseous spectral features that would otherwise bepresent from upwelling thermal or scattered radiation from levels deeper than theclouds. Spectra of Jupiter in the 5-�m region serve to illustrate. Prior to the GalileoProbe mission to Jupiter, there were competing hypotheses of the distribution ofclouds in hotspot regions. Both could account for brightness temperature whenaveraged over the 5-�m spectral window. One model had a clear atmosphere downto an opaque water cloud; the other had no water cloud but did have an optically thinaerosol layer above the water cloud. Spectral features produced by the latter modelprovided a much better fit to the data (see Fig. 6 of West et al. 1986), and this modelwas later borne out by measurements made by instruments on the probe (Ragent etal. 1998).

Several other types of observations may be employed to infer the existence andproperties of clouds in exoplanet atmospheres. If the thermal emission or scatteredstarlight can be observed as a function of rotational phase or orbital phase, therotational or orbital phase curves carry information on the heterogeneous nature ofthe clouds and/or thermal gradients and on the scattering phase functions of cloudparticles, respectively.

Rotational modulation of an object’s brightness or Doppler shift can be used toconstruct a crude map of brightness features on the surface. The effective resolutionof such a reconstruction is of order 0.1 of a planetary diameter. Crossfield (2014)and Crossfield et al. (2014) used a Doppler technique to reconstruct brightnessvariations on a brown dwarf. Karalidi et al. (2015) developed an intensity-basedmethod (Aeolus) for extrasolar planets. They tested the method using HubbleSpace Telescope images of Jupiter. Similarly, Simon et al. (2016) used precision

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 277

photometry from the Kepler K2 mission along with images of Neptune to explorehow clouds of different sizes and with different rotation rates as a function oflatitude affect the information content of the result. Zhou et al. (2016) were ableto determine the rotation period for a directly imaged planet (2M1207b) fromrotational modulation of its brightness.

With an eye toward future observations of the orbital phase curves of exoplanets,several groups have considered what these measurements can tell us about clouds,haze, and planetary rings. Instruments on spacecraft have provided this informationfor giant planets in our solar system (Tomasko et al. 1978; Smith and Tomasko1984; Pollack et al. 1986; Tomasko and Doose 1984; Rages et al. 1991). Both cloudand haze particles strongly influence the phase curves of the giant planets, althoughRayleigh scattering by gas is also a contributor. Both forward and backscatteringcomponents are evident. The haze aerosols in Titan’s atmosphere are stronglyforward scattering (Rages et al. 1983). The efforts cited above fit observationsover a range of phase angle with model atmospheres having layers of particles andgas. They use scalar or vector (includes linear polarization) radiative transfer codessuch as doubling/adding (Hansen and Hovenier 1974; Evans and Stephens 1991) tohandle multiple scattering, vertical layering, and arbitrary particle phase functions.This approach is needed to understand the observations in terms of cloud opticalproperties and cloud stratigraphy. In addition to doubling/adding, other multiplescattering codes are available for this work such as versions of the discrete ordinatesmethod: DISORT (Lin et al. 2015), LIDORT, and VLIDORT (Spurr 2006).

All too often in exoplanet studies, Lambert’s law (intensity proportional to cosineof the zenith angle of the incident flux) and pure Rayleigh scattering (Madhusudhanand Burrows 2012) have been employed to estimate the phase law. Experience(cited above) with solar system giant planets shows these to be poor approximationsbecause cloud particles dominate the phase curves. To model the phase curveMadhusudhan and Burrows (2012), also examine a forward scattering particlemodel of the form p(cos�) D ¨(1 C g cos �), where ‚ is the angle between theincident and scattered rays and g is the asymmetry parameter, g 2 [0, 1]. Eventhat model does not provide a good fit to the giant planets in our solar system.Cloud particle phase functions for the solid (ice or mineral dust) clouds in thegiant planet atmospheres cannot be computed from first principles (such as Mietheory). Cloud particle models using a double Henyey–Greenstein function of theform p(cos�) D f� HG(g1, cos�) C (1 � f )� HG(g2, cos�) can fit data well, whereHG are Henyey–Greenstein functions with asymmetry parameters g1 and g2 and f 2

[0, 1] (Tomasko et al. 1978). Other studies include works by Burrows et al. (2004),Sudarsky et al. (2005), Dyudina et al. (2005). Mayorga et al. (2016), produced phasecurves for Jupiter in several passbands directly from Cassini images. An effort to putthese in the context of scattering models from the references already cited for giantplanets in our solar system is needed.

Planetary polarization (linear polarization) is a possible diagnostic of cloud andhaze in exoplanet atmospheres if the observations can be made sufficiently precise.Polarization has been observed in some brown dwarfs (Zapatero Osorio et al. 2011;Miles-Páez et al. 2013). Although the dust disk surrounding the star can produce the

278 R. A. West

time-average polarization, a component of the polarization (1.5%) that varies withthe rotation period can be attributed to the star. Several physical mechanisms maycontribute to the polarization signature at thermal-IR wavelengths. These includeZeeman spitting and broadening at frequencies corresponding to the appropriatetransitions, alignment of nonspherical grains in a magnetic field, and scattering bya patchy distribution of particles. De Kok et al. (2011) presented models relevant tothe possibility that a patchy distribution of particles is primarily responsible.

In our solar system, four distinct aerosol types produce polarization. If the par-ticles are liquid as they are for Venus, the spherical shape produces a characteristicsignature in the space defined by scattering angle and wavelength. In this caseexclusively the polarization can be used to put strong constraints on the particlesize distribution and composition (Hansen and Hovenier 1974). If the particles arenot spherical (this covers almost all planetary cases except Venus), the polarizationcan be quite small with little or no diagnostic value unless the particles are smallcompared to the wavelength in which case the polarization approaches Rayleigh-type linear polarization which is very high at scattering angle 90ı. Mid- andlow-latitude stratospheric haze in the giant planet atmospheres is of this type, andthe optical depth is typically much less than 1. A fourth type, abundant in the polarregions of Jupiter and Saturn and at all latitudes on Titan, is composed of aggregatesof many (thousands) of very small (� tens of nm radius) monomers (West andSmith 1991). These particles have Rayleigh-type polarization but can also be highlyforward scattering. Karalidi et al. (2013) presented an analysis of expectations forextrasolar planets similar to Jupiter in terms of polarization properties. West et al.(2015) summarized our current understanding of polarization of the giant planets,Saturn’s rings and Titan.

Future Prospects

The study of exoplanets is a rapidly growing field and will continue to be with datafrom current missions including ground-based, HST, and Spitzer, and new missionssuch as TESS, JWST, WFIRST, and other focused missions under consideration.Spectral data, phase curves, and polarimetry are of particular interest to the studyof clouds and haze. In this regard we can look forward to data with higher spectralresolution, better signal/noise ratio, and spectral coverage. We can also anticipateimportant advances from modeling and laboratory work. Laboratory experimentsare already underway for conditions relevant to the atmospheres of hot Jupitersand warm Neptunes. In particular they can potentially address issues such assoot formation, a complex process involving ion and ion–neutral chemistry that iscurrently beyond the scope of photochemical models. Ground-based observations athigh spectral resolution such as those by Brogi et al. (2016) will continue to advanceand are expected to provide new information relevant to clouds and haze. More canbe done already in modeling phase curves and polarization of exoplanets by makingbetter use of existing measurements and models of giant planet phase curves andpolarization in our own solar system (West et al. 2015). Models of polarization and

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 279

phase curves can also be improved by incorporating existing and future laboratorywork on optical properties of nonspherical mineral dust and ice particles (Pope etal. 1992; Levasseur-Regourd et al. 2015; Muñoz and Hovenier 2015).

Cross-References

�Upper Atmospheres and Ionospheres of Planets and Satellites

References

Ackerman AS, Marley MS (2001) Precipitating condensation clouds in substellar atmospheres.Astrophys J 556(2):872–884. https://doi.org/10.1086/321540

Bétrémieux Y (2016) Effects of refraction on transmission spectra of gas giants: decrease ofthe Rayleigh scattering slope and breaking of retrieval degeneracies. Mon Not R Astron Soc456(4):4051–4060. https://doi.org/10.1093/mnras/stv2955

Bétrémieux Y, Swain MR (2017) An analytical formalism accounting for clouds and other“surfaces” for exoplanet transmission spectroscopy. Mon Not R Astron Soc 467:2834

Bond JC, O’Brien DP, Lauretta DS (2010) The compositional diversity ofextrasolar terrestrial planets. I. In situ simulations. Astrophys J 715(2):1050–1070.https://doi.org/10.1088/0004-637x/715/2/1050

Brogi M, de Kok RJ, Albrecht S, Snellen IAG, Birkby JL, Schwarz H (2016) Rotation and windsof exoplanet hd 189733 b measured with high-dispersion transmission spectroscopy. AstrophysJ 817(2). https://doi.org/10.3847/0004-637x/817/2/106

Burrows A, Sudarsky D, Hubeny I (2004) Spectra and diagnostics for the directdetection of wide-separation extrasolar giant planets. Astrophys J 609(1):407–416.https://doi.org/10.1086/420974

Carlson RW, Baines KH, Anderson MS, Filacchione G, Simon AA (2016) Chromophores fromphotolyzed ammonia reacting with acetylene: application to Jupiter’s great red spot. Icarus274:106–115. https://doi.org/10.1016/j.icarus.2016.03.008

Charnay B, Meadows V, Leconte J (2015a) 3d modeling of Gj1214b’s atmosphere:vertical mixing driven by an anti-Hadley circulation. Astrophys J 813(1):15.https://doi.org/10.1088/0004-637x/813/1/15

Charnay B, Meadows V, Misra A, Leconte J, Arney G (2015b) 3d modeling of GJ1214b’s atmo-sphere: formation of inhomogeneous high clouds and observational implications. Astrophys JLett 813(1):L1. https://doi.org/10.1088/2041-8205/813/1/L1

Crossfield IJM (2014) Doppler imaging of exoplanets and brown dwarfs. Astron Astrophys566:A130. https://doi.org/10.1051/0004-6361/201423750

Crossfield IJM, Biller B, Schlieder JE, Deacon NR, Bonnefoy M, Homeier D,..., Kopytova T(2014) A global cloud map of the nearest known brown dwarf. Nature, 505(7485), 654-C.https://doi.org/10.1038/nature12955

De Kok RJ, Stam DM, Karalidi T (2011) Characterizing exoplanetary atmospheres throughinfrared polarimetry. Astrophys J 741(1). https://doi.org/10.1088/0004-637x/741/1/59

Deming LD, Seager S (2017) Illusion and reality in the atmospheres of exoplanets.https://arxiv.org/abs/1701.00493

Dyudina UA, Sackett PD, Bayliss DDR, Seager S, Throop HB, Dones L (2005)Phase light curves for extrasolar Jupiters and Saturns. Astrophys J 618(2):973–986.https://doi.org/10.1086/426050

280 R. A. West

Etangs ALD, Pont F, Vidal-Madjar A, Sing D (2008) Rayleigh scattering in thetransit spectrum of HD189733b. Astron Astrophys 481(2):L83–L86. https://doi.org/10.1051/0004-6361:200809388

Evans KF, Stephens GL (1991) A new polarized atmospheric radiative-transfer model. J QuantSpectrosc Radiat Transf 46(5):413–423. https://doi.org/10.1016/0022-4073(91)90043-p

Friedson A, Wong AS, Yung Y (2002) Models for polar haze formation in Jupiter’s stratosphere.Icarus 158(2):389–400. https://doi.org/10.1006/icar.2002.6885

Gierasch PJ, Conrath BJ (1993) Dynamics of the atmospheres of the outer planets – post-voyagermeasurement objectives. J Geophys Res Planets 98(E3):5459–5469

Gladstone GR, Allen M, Yung YL (1996) Hydrocarbon photochemistry in the upper atmosphereof Jupiter. Icarus 119(1):1–52. https://doi.org/10.1006/icar.1996.0001

Hansen JE, Hovenier JW (1974) Interpretation of polarization of Venus. J Atmos Sci 31(4):1137–1160. https://doi.org/10.1175/1520-0469(1974)031<1137:Iotpov>2.0.Co;2

Heng K (2017) Exoplanetary atmospheres: theoretical concepts and foundations. PrincetonUniversity Press, Princeton

Hu RY, Seager S (2014) Photochemistry in terrestrial exoplanet atmospheres. III. Photochemistryand thermochemistry in thick atmospheres on super earths and mini Neptunes. Astrophys J784(1):63

Hu RY, Demory BO, Seager S, Lewis N, Showman AP (2015) A semi-analytical model ofvisible-wavelength phase curves of exoplanets and applications to Kepler-7 B and Kepler-10B. Astrophys J 802(1):51. https://doi.org/10.1088/0004-637x/802/1/51

Hubbard WB, Haemmerle V, Porco CC, Rieke GH, Rieke MJ (1995) The occultation of Sao-78505by Jupiter. Icarus 113(1):103–109. https://doi.org/10.1006/icar.1995.1008

Karalidi T, Stam DM, Guirado D (2013) Flux and polarization signals of spatially inhomogeneousgaseous exoplanets. Astron Astrophys 555. https://doi.org/10.1051/0004-6361/201321492

Karalidi T, Apai D, Schneider G, Hanson JR, Pasachoff JM (2015) Aeolus: a Markov Chain MonteCarlo code for mapping ultracool atmospheres. An application on Jupiter and brown dwarf HSTlight curves. Astrophys J 814(1):65. https://doi.org/10.1088/0004-637x/814/1/65

Kataria T, Showman AP, Fortney JJ, Stevenson KB, Line MR, Kreidberg L,... Désert J-M (2015).The atmospheric circulation of the hot jupiter wasp-43b: comparing three-dimensional modelsto spectrophotometric data. Astrophys J 801(2):86. https://doi.org/10.1088/0004-637x/801/2/86

Koskinen TT, Sandel BR, Yelle RV, Capalbo FJ, Holsclaw GM, McClintock WE, Edgington S(2013) The density and temperature structure near the exobase of Saturn from Cassini UVISsolar occultations. Icarus 226(2):1318–1330. https://doi.org/10.1016/j.icarus.2013.07.037

Koskinen TT, Moses JI, West RA, Guerlet S, Jouchoux A (2016) The detectionof benzene in Saturn’s upper atmosphere. Geophys Res Lett 43(15):7895–7901.https://doi.org/10.1002/2016gl070000

Levasseur-Regourd A-C, Renard J-B, Shkuratov YG, Hadamcik E (2015) Laboratory studies.In: Kolokolova L, Hough J, Levasseur-Regourd A-C (eds) Polarimetry of stars and planetarysystems. Cambridge University Press, Cambridge, UK, pp 62–80

Lin Z, Stamnes S, Jin Z, Laszlo I, Tsay SC, Wiscombe WJ, Stamnes K (2015) Improveddiscrete ordinate solutions in the presence of an anisotropically reflecting lower boundary:upgrades of the DISORT computational tool. J Quant Spectrosc Radiat Transf 157:119–134.https://doi.org/10.1016/j.jqsrt.2015.02.014

Line MR, Parmentier V (2016) The influence of nonuniform cloud cover on transit transmissionspectra. Astrophys J 820(1):78. https://doi.org/10.3847/0004-637x/820/1/78

Lodders K, Fegley B (2002) Atmospheric chemistry in giant planets, brown dwarfs,and low-mass dwarf stars – I. Carbon, nitrogen and oxygen. Icarus 155(2):393–424.https://doi.org/10.1006/icar.2001.6740

Madhusudhan N, Burrows A (2012) Analytic models for albedos, phase curves, and polarizationof reflected light from exoplanets. Astrophys J 747(1). https://doi.org/10.1088/0004-637x/747/1/25

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 281

Mayorga LC, Jackiewicz J, Rages K, West RA, Knowles B, Lewis N, Marley MS (2016)JUPITER’S phase variations from cassini: a testbed for future direct-imaging missions. AstronJ 152(6). https://doi.org/10.3847/0004-6256/152/6/209

Miles-Páez PA, Zapatero Osorio MR, Pallé E, Ramírez KP (2013) Linear polarization of rapidlyrotating ultracool dwarfs. Astron Astrophys 556. https://doi.org/10.1051/0004-6361/201321851

Morley CV, Fortney JJ, Kempton EMR, Marley MS, Vissher C, Zahnle K (2013) Quantitativelyassessing the role of clouds in the transmission spectrum of Gj 1214b. Astrophys J 775(1).https://doi.org/10.1088/0004-637x/775/1/33

Moses J (2000) Photochemistry of Saturn’s atmosphere I. Hydrocarbon chemistry and comparisonswith ISO observations. Icarus 143(2):244–298. https://doi.org/10.1006/icar.1999.6270

Moses JI (2014) Chemical kinetics on extrasolar planets. Philos Trans R Soc Math Phys Eng Sci372(2014). https://doi.org/10.1098/rsta.2013.0073

Moses JI, Madhusudhan N, Visscher C, Freedman RS (2013) Chemical consequences of the c/oratio on hot jupiters: examples from wasp-12b, corot-2b, xo-1b, and HD 189733b. Astrophys J763(1). https://doi.org/10.1088/0004-637x/763/1/25

Muñoz O, Hovenier JW (2015) Experimental scattering matrices of clouds of randomly orientedparticles. In: Kolokolova L, Hough J, Levasseur-Regourd A-C (eds) Polarimetry of stars andplanetary systems. Cambridge University Press, Cambridge, pp 130–144

Parmentier V, Fortney JJ, Showman AP, Morley C, Marley MS (2016) Transitions in the cloud com-position of hot Jupiters. Astrophys J 828(1):22. https://doi.org/10.3847/0004-637x/828/1/22

Perryman MAC (2014) The exoplanet handbook. (First paperback edition). Cambridge UniversityPress, Cambridge/New York

Petigura EA, Marcy GW (2011) Carbon and oxygen in nearby stars: keys to protoplanetary diskchemistry. Astrophys J 735(1):41. https://doi.org/10.1088/0004-637x/735/1/41

Pollack JB, Rages K, Baines KH, Bergstralh JT, Wenkert D, Danielson GE (1986) Estimates ofthe bolometric Albedos and radiation balance of Uranus and Neptune. Icarus 65(2–3):442–466.https://doi.org/10.1016/0019-1035(86)90147-8

Pope SK, Tomasko MG, Williams MS, Perry ML, Doose LR, Smith PH (1992) Clouds ofammonia ice – laboratory measurements of the single-scattering properties. Icarus 100(1):203–220. https://doi.org/10.1016/0019-1035(92)90030-b

Ragent B., Colburn DS, Rages KA, Knight TCD, Avrin P, Orton GS.,..., Grams GW (1998).The clouds of Jupiter: results of the Galileo Jupiter mission probe nephelometer experiment.J Geophys Res Planets 103(E10):22891–22909. https://doi.org/10.1029/98je00353

Rages K, Pollack JB, Smith PH (1983) Size estimates of titans aerosols based onVoyager high-phase-angle images. J Geophys Res Space Phys 88(Na11):8721–8728.https://doi.org/10.1029/JA088iA11p08721

Rages K, Pollack JB, Tomasko MG, Doose LR (1991) Properties of scatterers in the troposphereand lower stratosphere of Uranus based on voyager imaging data. Icarus 89(2):359–376.https://doi.org/10.1016/0019-1035(91)90183-T

Robinson TD, Maltagliati L, Marley MS, Fortney JJ (2014) Titan solar occultation observationsreveal transit spectra of a hazy world. Proc Natl Acad Sci U S A 111(25):9042–9047.https://doi.org/10.1073/pnas.1403473111

Rossow WB (1978) Cloud microphysics – analysis of clouds of Earth, Venus, Mars, and Jupiter.Icarus 36(1):1–50. https://doi.org/10.1016/0019-1035(78)90072-6

Sánchez-Lavega AN, Pérez-Hoyos S, Hueso R (2004) Clouds in planetary atmospheres:a useful application of the Clausius–Clapeyron equation. Am J Phys 72(6):767.https://doi.org/10.1119/1.1645279

Seager S (2010) Exoplanet atmospheres: physical processes. Princeton University Press, PrincetonSeager S, Dotson R (2010) Exoplanets. University of Arizona Press, TucsonShemansky DE, Liu X (2012) Saturn upper atmospheric structure from Cassini EUV and FUV

occultations11This article is part of a special issue that honours the work of Dr. Donald M.Hunten FRSC who passed away in December 2010 after a very illustrious career. Can J Phys90(8):817–831. https://doi.org/10.1139/p2012-036

282 R. A. West

Simon AA, Rowe JF, Gaulme P, Hammel HB, Casewell SL, Fortney JJ,..., Marley MS (2016)Neptune’s dynamic atmosphere from Kepler k2 observations: implications for brown dwarflight curve analyses. Astrophys J 817(2). https://doi.org/10.3847/0004-637x/817/2/162

Smith PH, Tomasko MG (1984) Photometry and polarimetry of Jupiter at large phaseangles .2. Polarimetry of the south tropical zone, south equatorial belt, and thepolar-regions from the Pioneer-10 and Pioneer-11 missions. Icarus 58(1):35–73.https://doi.org/10.1016/0019-1035(84)90097-6

Spurr RJD (2006) VLIDORT: a linearized pseudo-spherical vector discrete ordinate radiativetransfer code for forward model and retrieval studies in multilayer multiple scattering media.J Quant Spectrosc Radiat Transf 102(2):316–342. https://doi.org/10.1016/j.jqsrt.2006.05.005

Sudarsky D, Burrows A, Hubeny I, Li AG (2005) Phase functions and lightcurves of wide-separation extrasolar giant planets. Astrophys J 627(1):520–533.https://doi.org/10.1086/430206

Tomasko MG, Doose LR (1984) Polarimetry and photometry of Saturn from Pioneer-11 –observations and constraints on the distribution and properties of cloud and aerosol-particles.Icarus 58(1):1–34. https://doi.org/10.1016/0019-1035(84)90096-4

Tomasko MG, West RA, Castillo ND (1978) Photometry and polarimetry of Jupiter at large phaseangles .1. Analysis of imaging data of a prominent belt and a zone from Pioneer 10. Icarus33(3):558–592. https://doi.org/10.1016/0019-1035(78)90191-4

Toon OB, Mckay CP, Ackerman TP, Santhanam K (1989) Rapid calculation of radiative heatingrates and photodissociation rates in inhomogeneous multiple-scattering atmospheres. J GeophysRes-Atmos 94(D13):16287–16301

Visscher C, Lodders K, Fegley B (2006) Atmospheric chemistry in giant planets, browndwarfs, and low-mass dwarf stars. II. Sulfur and phosphorus. Astrophys J 648(2):1181–1195.https://doi.org/10.1086/506245

Visscher C, Lodders K, Fegley B (2010) Atmospheric chemistry in giant planets, brown dwarfs,and low-mass dwarf stars. III. Iron, magnesium, and silicon. Astrophys J 716(2):1060–1075.https://doi.org/10.1088/0004-637x/716/2/1060

West RA (2000) Condensates in Jovian atmospheres. In: Griffith CA, Marley MS (eds) From giantplanets to cool stars : proceedings of a workshop held at Northern Arizona University, Flagstaff,Arizona, USA, 8–11 June 1999. Astronomical Society of the Pacific, San Francisco

West RA (2014) Atmospheres of the giant planets. In: Spohn T, Breuer D, Johnson TV (eds)Encyclopedia of the solar system, 3rd edn. Elsevier, Amsterdam, pp 743–758

West RA, Smith PH (1991) Evidence for aggregate particles in the atmospheres of titan andJUPITER. Icarus 90(2):330–333. https://doi.org/10.1016/0019-1035(91)90113-8

West RA, Strobel DF, Tomasko MG (1986) Clouds, aerosols, and photochemistry in the Jovianatmosphere. Icarus 65(2–3):161–217. https://doi.org/10.1016/0019-1035(86)90135-1

West RA, Baines KH, Pollack JB (1991) Clouds and aerosols in the Uranian atmosphere. In:Bergstralh JT, Miner ED, Matthews MS (eds) Uranus. University of Arizona Press, Tucson,pp 296–326

West RA, Baines KH, Friedson AJ, Banfield D, Ragent B, Taylor FW (2004) Clouds and haze. In:Bagenal F, Dowling TE, McKinnon WB (eds) Jupiter : the planet, satellites, and magnetosphere.Cambridge University Press, Cambridge, UK, pp 79–104

West RA, Baines KH, Karkoschka E, Sánchez-Lavega A (2009) Clouds and haze in Saturn’satmosphere. In: Dougherty M, Esposito L, Krimigis SM (eds) Saturn from Cassini-Huygens.Springer, Dordrecht, pp 161–180

West RA, Yanamandra-Fisher PA, Korokhin V (2015) Gas giant planets, Saturn’s rings and Titan.In: Kolokolova L, Hough J, Levasseur-Regourd A-C (eds) Polarimetry of stars and planetarySystems. Cambridge Univ. Press, Cambridge, pp 320–339

Wong MH, Atreya SK, Kuhn WR, Romani PN, Mihalka KM (2015) Fresh clouds: a parameterizedupdraft method for calculating cloud densities in one-dimensional models. Icarus 245:273–281.https://doi.org/10.1016/j.icarus.2014.09.042

Zahnle K, Marley MS, Morley CV, Moses JI (2016) Photolytic hazes in the atmosphere of 51 ERIB. Astrophys J 824(2). https://doi.org/10.3847/0004-637x/824/2/137

14 Temperature, Clouds, and Aerosols in Giant and Icy Planets 283

Zapatero Osorio MR, Béjar VJS, Goldman B, Caballero JA, Rebolo R, Acosta-Pulido JA,..., PeñaRamirez K (2011) Near-infrared linear polarization of ultracool dwarfs. Astrophys J 740(1).https://doi.org/10.1088/0004-637x/740/1/4

Zhou Y, Apai D, Schneider GH, Marley MS, Showman AP (2016) Discovery of rota-tional modulations in the planetary-mass companion 2m1207b: intermediate rotationperiod and heterogeneous clouds in a low gravity atmosphere. Astrophys J 818(2):176.https://doi.org/10.3847/0004-637x/818/2/176