Haze Opacity Research Articles

Photochemical Hazes Can Trace the C/O Ratio in Exoplanet Atmospheres

Photochemical hazes are suspected to obscure molecular features, such as water, from detection in the transmission spectra of exoplanets with atmospheric temperatures <800 K. The opacities of laboratory produced organic compounds (tholins) from Khare et al. have become a standard for modeling haze in exoplanet atmospheres. However, these tholins were grown in an oxygen-free, Titan-like environment that is very different from typical assumptions for exoplanets, where C/O ∼ 0.5. This work presents the 0.13–10 μm complex refractive indices derived from laboratory transmission measurements of tholins grown in environments with different oxygen abundances. With the increasing uptake of oxygen, absorption increases across the entire wavelength range, and a scattering feature around 6 μm shifts toward shorter wavelengths and becomes more peaked around 5.8 μm, due to a C = O stretch resonance. Using GJ 1214 b as a test case, we examine the transmission spectra of a sub-Neptune planet with C/O ratios of solar, 1, and 1000 to evaluate the effective differences between our opacities and those of Khare. For an atmosphere with solar hydrogen and helium abundances, we find a difference of 200–1500 ppm, but for high-metallicity (Z = 1000) environments, the difference may only be 20 ppm. The 1–2 μm transmission data for GJ 1214 b rule out the Titan-like haze model, and are more consistent with C/O = 1 and C/O = solar haze models. This work demonstrates that using haze opacities that are more consistent with underlying assumptions about bulk atmospheric composition are important for building self-consistent models that appropriately constrain the atmospheric C/O ratio, even when molecular features are obscured.

Deep Clouds on Jupiter

Jupiter’s atmospheric water abundance is a highly important cosmochemical parameter that is linked to processes of planetary formation, weather, and circulation. Remote sensing and in situ measurement attempts still leave room for substantial improvements to our knowledge of Jupiter’s atmospheric water abundance. With the motivation to advance our understanding of water in Jupiter’s atmosphere, we investigate observations and models of deep clouds. We discuss deep clouds in isolated convective storms (including a unique storm site in the North Equatorial Belt that episodically erupted in 2021–2022), cyclonic vortices, and northern high-latitude regions, as seen in Hubble Space Telescope visible/near-infrared imaging data. We evaluate the imaging data in continuum and weak methane band (727 nm) filters by comparison with radiative transfer simulations, 5 micron imaging (Gemini), and 5 micron spectroscopy (Keck), and conclude that the weak methane band imaging approach mostly detects variation in the upper cloud and haze opacity, although sensitivity to deeper cloud layers can be exploited if upper cloud/haze opacity can be separately constrained. The cloud-base water abundance is a function of cloud-base temperature, which must be estimated by extrapolating 0.5-bar observed temperatures downward to the condensation region near 5 bar. For a given cloud base pressure, the largest source of uncertainty on the local water abundance comes from the temperature gradient used for the extrapolation. We conclude that spatially resolved spectra to determine cloud heights—collected simultaneously with spatially-resolved mid-infrared spectra to determine 500-mbar temperatures and with improved lapse rate estimates—would be needed to answer the following very challenging question: Can observations of deep water clouds on Jupiter be used to constrain the atmospheric water abundance?

Investigating temporal changes in Jupiter’s aerosol structure with rotationally-averaged 2015–2020 HST WFC3 images

Annual imaging observations of Jupiter from 2015 onward by the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) as part of the Outer Planet Atmospheres Legacy (OPAL) program (Simon et al., 2015) have provided a valuable data set from which Jupiter’s temporal variability can be characterized and modeled. Spectral samples at discrete filter bands from the UV to short near-IR wavelengths also provide an opportunity to connect temporal changes in reflectivity with temporal changes in cloud structure. To improve constraints on that cloud structure we used center-to-limb variations that were noise-reduced by rotational averaging during each annual OPAL observation sequence. Fitting observations at multiple and distinctly different observer zenith angles helps to make up for the lack of multiple methane-band filters in discerning the vertical distribution of aerosols. The short-wavelength limit of the OPAL observations also provides better constraints than ground-based observation on the properties of Jupiter’s chromophore, i.e., the compound that creates Jupiter’s variety of red colorations to clouds and hazes composed mainly of colorless condensates. We chose four latitudes to investigate with radiative transfer models based on a simple 3-layer aerosol structure consisting of a deep putative NH3 cloud layer, with a physically thin chromophore layer that has a fitted location at or near the top of the deep cloud, and an overlying stratospheric haze layer. Our chosen planetocentric latitudes were 5.5°S (Equatorial Zone South,EZS), 0° (Equatorial Zone, EZ), 10°N (North Equatorial Belt, NEB), and 21.75°N (North Temperate Belt, NTB). Two major color transformations occurred during the 2015–2020 period we analyzed. One is a dramatic dimming of the center of the Equatorial Zone at short wavelengths, especially after 2017, and a significant reddening of that region. The second is a significant reddening of part of the North Temperate Belt (near 22°N) between 2016 and 2017. Both reddening events can be modeled by doubling of the optical depth of the chromophore layer during the transitions. Comparison with prior 2017 model results for the NEB, EZ, and NTB have revealed agreement with Braude et al. (2020) on the vertical location of the chromophore layer but disagreements on cloud-top pressures and the vertical distribution of stratospheric haze particles. We identified what seems to be common tendency of models based on HST/WFC3 observations to either infer very little stratospheric haze opacity or to put that opacity at too high a pressure.

Probing the atmosphere of WASP-69 b with low- and high-resolution transmission spectroscopy

Consideration of both low- and high-resolution transmission spectroscopy is key for obtaining a comprehensive picture of exoplanet atmospheres. In studies of transmission spectra, the continuum information is well established with low-resolution spectra, while the shapes of individual lines are best constrained with high-resolution observations. In this work, we aim to merge high- with low-resolution transmission spectroscopy to place tighter constraints on physical parameters of the atmospheres. We present the analysis of three primary transits of WASP-69 b in the visible (VIS) channel of the CARMENES instrument and perform a combined low- and high-resolution analysis using additional data from HARPS-N, OSIRIS/GTC, and WFC3/HST already available in the literature. We investigate the Na I D1 and D2 doublet, Hα, the Ca II infra-red triplet (IRT), and K I λ7699 Å lines, and we monitor the stellar photometric variability by performing long-term photometric observations with the STELLA telescope. During the first CARMENES observing night, we detected the planet Na I D2 and D1 lines at ~7 and ~3σ significance levels, respectively. We measured a D2/D1 intensity ratio of 2.5 ± 0.7, which is in agreement with previous HARPS-N observations. Our modelling of WFC3 and OSIRIS data suggests strong Rayleigh scattering, solar to super-solar water abundance, and a highly muted Na feature in the atmosphere of this planet, in agreement with previous investigations of this target. We use the continuum information retrieved from the low-resolution spectroscopy as a prior to break the degeneracy between the Na abundance, reference pressure, and thermosphere temperature for the high-resolution spectroscopic analysis. We fit the Na I D1 and D2 lines individually and find that the posterior distributions of the model parameters agree with each other within 1σ. Our results suggest that local thermodynamic equilibrium processes can explain the observed D2 /D1 ratio because the presence of haze opacity mutes the absorption features.

Tracking Short-term Variations in the Haze Distribution of Titan’s Atmosphere with SINFONI VLT

Abstract While it has long been known that Titan’s haze and atmosphere are dynamic on seasonal timescales, recent results have revealed that they also exhibit significant subseasonal variations. Here, we report on observations of Titan acquired over an eight-month period between 2014 April and 2015 March with the Spectrograph for Integral Field Observations in the Near Infrared instrument on the Very Large Telescope using adaptive optics. These observations have an average five-day cadence, permitting interrogation of the short-period variability of Titan’s atmosphere. Disk-resolved spectra in the H and K bands (1.4–2.4 μm) were analyzed with the PyDISORT radiative transfer model to determine the spatial distribution and variation of stratospheric haze opacity over subseasonal timescales. We observed a uniform decrease in haze opacity at 20°N and an increase in haze opacity at 250–300°E and ∼40°N over the span of our observations. Globally, we found variations on the order of 5%–10% on timescales of weeks, as well as a steady, global increase in the amount of haze over timescales of months. The observed variations in haze opacity over the short timescales of our observations were of similar magnitude to long-period variations attributed to seasonal variation, suggesting rapid dynamical processes that may take part in the distribution of hazes in Titan’s atmosphere.

Impact of photochemical hazes and gases on exoplanet atmospheric thermal structure

ABSTRACT We investigate the impact of photochemical hazes and disequilibrium gases on the thermal structure of hot Jupiters, using a detailed 1D radiative-convective model. We find that the inclusion of photochemical hazes results in major heating of the upper and cooling of the lower atmosphere. Sulphur containing species, such as SH, S2, and S3, provide significant opacity in the middle atmosphere and lead to local heating near 1 mbar, while OH, CH, NH, and CN radicals produced by the photochemistry affect the thermal structure near 1 $\mu$bar. Furthermore, we show that the modifications on the thermal structure from photochemical gases and hazes can have important ramifications for the interpretation of transit observations. Specifically, our study for the hazy HD 189733 b shows that the hotter upper atmosphere resulting from the inclusion of photochemical haze opacity imposes an expansion of the atmosphere, thus a steeper transit signature in the ultraviolet–visible part of the spectrum. In addition, the temperature changes in the photosphere also affect the secondary eclipse spectrum. For HD 209458 b, we find that a small haze opacity could be present in this atmosphere, at pressures below 1 mbar, which could be a result of both photochemical hazes and condensates. Our results motivate the inclusion of radiative feedback from photochemical hazes in general circulation models for a proper evaluation of atmospheric dynamics.

Safety profiles of terahertz scanning in ophthalmology

Terahertz (THz) technology has emerged recently as a potential novel imaging modality in biomedical fields, including ophthalmology. However, the ocular biological responses after THz electromagnetic exposure have not been investigated. We conducted a rabbit study to evaluate the safety profiles of THz scanning on eyes, at a tissue, cellular, structural and functional level. Eight animals (16 eyes) were analysed after excessive THz exposure (control, 1 h, 4 h, and 1 week after continuous 4-h exposure; THz frequency = 0.3 THz with continuous pulse generated at 40 µW). We found that at all the time points, the corneas and lens remained clear with no corneal haze or lens opacity formation clinically and histopathologically. No thermal effect, assessed by thermographer, was observed. The rod and cone cell-mediated electroretinography responses were not significantly altered, and the corneal keratocytes activity as well as endothelial viability, assessed by in-vivo confocal microscopy, was not affected. Post-exposed corneas, lens and retinas exhibited no significant changes in the mRNA expression of heat shock protein (HSP)90AB1), DNA damage inducible transcript 3 (DDIT3), and early growth response (EGR)1. These tissues were also negative for the inflammatory (CD11b), fibrotic (fibronectin and α-smooth muscle actin), stress (HSP-47) and apoptotic (TUNEL assay) responses on the immunohistochemical analyses. The optical transmittance of corneas did not change significantly, and the inter-fibrillar distances of the corneal stroma evaluated with transmission electron microscopy were not significantly altered after THz exposure. These results provide the basis for future research work on the development of THz imaging system for its application in ophthalmology.

Prospects for Characterizing the Haziest Sub-Neptune Exoplanets with High-resolution Spectroscopy

Abstract Observations to characterize planets larger than Earth but smaller than Neptune have led to largely inconclusive interpretations at low spectral resolution due to hazes or clouds that obscure molecular features in their spectra. However, here we show that high-resolution spectroscopy (R ∼ 25,000–100,000) enables one to probe the regions in these atmospheres above the clouds where the cores of the strongest spectral lines are formed. We present models of transmission spectra for a suite of GJ 1214b–like planets with thick photochemical hazes covering 1–5 μm at a range of resolutions relevant to current and future ground-based spectrographs. Furthermore, we compare the utility of the cross-correlation function that is typically used with a more formal likelihood-based approach, finding that only the likelihood-based method is sensitive to the presence of haze opacity. We calculate the signal-to-noise ratio (S/N) of these spectra, including telluric contamination, Required to robustly detect a host of molecules such as CO, CO2, H2O, and CH4 and photochemical products like HCN as a function of wavelength range and spectral resolution. Spectra in the M band require the lowest S/Nres to detect multiple molecules simultaneously. CH4 is only observable for the coolest models (T eff = 412 K) and only in the L band. We quantitatively assess how these requirements compare to what is achievable with current and future instruments, demonstrating that characterization of small cool worlds with ground-based high-resolution spectroscopy is well within reach.

Constraints on Neptune's haze structure and formation from VLT observations in the H-band

A 1-dimensional microphysics model has been used to constrain the structure and formation of haze in Neptune's atmosphere. These simulations were coupled to a radiative-transfer and retrieval code (NEMESIS) to model spectral observations of Neptune in the H-band performed by the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) in 2013. It was found that observations in the H-band and with emission angles ≤60° are largely unaffected by the imaginary refractive index of haze particles, allowing a notable reduction of the free parameters required to fit the observations. Our analysis shows a total haze production rate of (2.61 ± 0.18) × 10−14 kg m−2 s−1, about 10 times larger than that found in Uranus's atmosphere, and a particle electric charge of q = 8.6 ± 1.1 electrons per μm radius at latitudes between 5 and 15° S. This haze production rate in Neptune results in haze optical depths about 10 times greater than those in Uranus. The effective radius reff was found to be 0.22 ± 0.01 and 0.26 ± 0.02 μm at the 0.1 and 1-bar levels, respectively, with haze number densities of 8.48+1.78−1.31 and 9.31+2.52−1.91 particles per cm3. The fit at weak methane-absorbing wavelengths reveals also the presence of a tropospheric cloud with a total optical depth >10 at 1.46 μm. The tropospheric cloud base altitude was found near the 2.5-bar level, although this estimation may be only representative of the top of a thicker and deeper cloud.Our analysis leads to haze opacities about 3.5 times larger than that derived from Voyager-2 observations (Moses et al., 1995). This larger opacity indicates a haze production rate 2 times larger at least. To study this difference haze opacity or production rate, we performed a timescale analysis with our microphysical model to estimate the time required for haze particles to grow and settle out. Although this analysis shows haze timescales (~15 years) shorter than the time lapsed between Voyager-2 observations and 2013, the solar illumination at the top of the atmosphere has not varied significantly during this period (at the studied latitudes) to explain the increase in haze production. This difference in haze production rate derived for these two periods may arise from: a) the fact that in our analysis we employed spectral observations in the infrared (H-band), while Moses et al. (1995) used photometric images taken at 5 different filters in the visible. While high-phase-angle Voyager observations are more sensitive to small haze particles and at altitudes above the 0.1-bar level, the haze constraints derived from VLT spectra in H-band are limited to pressures greater than 0.1 bar. As a result of the different phase angles of the two set of observations, differences in the estimation of M0 may arise from the use of Mie phase functions as well. b) our 1-dimensional model does not account for latitudinal redistributions of the haze by dynamics. A possible meridional transport of haze with wind velocities greater than ~0.03 m s−1 would result in dynamics timescales shorter than 15 years and thus might explain the observed variations in the haze production rate during this period.Compared with our estimations, photochemical models point to even larger production rates on Neptune (by a factor of 2.4). Assuming that the photochemical simulations are correct, we found that this discrepancy can be explained if haze particles evaporate before reaching the tropospheric-cloud levels. This scenario would decrease the cumulative haze opacity above the 1-bar level, and thus a larger haze production rate would be required to fit our observations. However, to validate this haze vertical structure future microphysical simulations that include the evaporation rates of haze particles are required.

Deflating Super-puffs: Impact of Photochemical Hazes on the Observed Mass–Radius Relationship of Low-mass Planets

Abstract The observed mass–radius relationship of low-mass planets informs our understanding of their composition and evolution. Recent discoveries of low-mass, large-radius objects (“super-puffs”) have challenged theories of planet formation and atmospheric loss, as their high inferred gas masses make them vulnerable to runaway accretion and hydrodynamic escape. Here we propose that high-altitude photochemical hazes could enhance the observed radii of low-mass planets and explain the nature of super-puffs. We construct model atmospheres in radiative-convective equilibrium and compute rates of atmospheric escape and haze distributions, taking into account haze coagulation, sedimentation, diffusion, and advection by an outflow wind. We develop mass–radius diagrams that include atmospheric lifetimes and haze opacity, which is enhanced by the outflow, such that young (∼0.1–1 Gyr), warm (T eq ≥ 500 K), low-mass objects (M c &lt; 4 M ⊕) should experience the most apparent radius enhancement due to hazes, reaching factors of three. This reconciles the densities and ages of the most extreme super-puffs. For Kepler-51b, the inclusion of hazes reduces its inferred gas mass fraction to &lt;10%, similar to that of planets on the large-radius side of the sub-Neptune radius gap. This suggests that Kepler-51b may be evolving toward that population and that some warm sub-Neptunes may have evolved from super-puffs. Hazes also render transmission spectra of super-puffs and sub-Neptunes featureless, consistent with recent measurements. Our hypothesis can be tested by future observations of super-puffs’ transmission spectra at mid-infrared wavelengths, where we predict that the planet radius will be half of that observed in the near-infrared.

Photochemical Hazes in Sub-Neptunian Atmospheres with a Focus on GJ 1214b

Abstract We study the properties of photochemical hazes in super-Earth/mini-Neptune atmospheres with particular focus on GJ 1214b. We evaluate photochemical haze properties at different metallicities between solar and 10,000× solar. Within the four-order-of-magnitude change in metallicity, we find that the haze precursor mass fluxes change only by a factor of ∼3. This small diversity occurs with a nonmonotonic manner among the different metallicity cases, reflecting the interaction of the main atmospheric gases with the radiation field. Comparison with relative haze yields at different metallicities from laboratory experiments reveals a qualitative similarity to our theoretical calculations and highlights the contributions of different gas precursors. Our haze simulations demonstrate that higher metallicity results in smaller average particle sizes. Metallicities at and above 100× solar with haze formation yields of ∼10% provide enough haze opacity to satisfy transit observations at visible wavelengths and obscure sufficiently the H2O molecular absorption features between 1.1 and 1.7 μm. However, only the highest-metallicity case considered (10,000× solar) brings the simulated spectra into closer agreement with transit depths at 3.6 and 4.5 μm, indicating a high contribution of CO/CO2 in GJ 1214b’s atmosphere. We also evaluate the impact of aggregate growth in our simulations, in contrast to spherical growth, and find that the two growth modes provide similar transit signatures (for D f = 2), but with different particle size distributions. Finally, we conclude that the simulated haze particles should have major implications for the atmospheric thermal structure and for the properties of condensation clouds.

Aggregate Hazes in Exoplanet Atmospheres

Abstract Photochemical hazes have frequently been used to interpret exoplanet transmission spectra that show an upward slope toward shorter wavelengths and weak molecular features. While previous studies have only considered spherical haze particles, photochemical hazes composed of hydrocarbon aggregate particles are common throughout the solar system. We use an aerosol microphysics model to investigate the effect of aggregate photochemical haze particles on the transmission spectra of warm exoplanets. We find that the wavelength dependence of the optical depth of aggregate particle hazes is flatter than for spheres because aggregates grow to larger radii. Consequently, while spherical haze opacity displays a scattering slope toward shorter wavelengths, aggregate haze opacity can be gray in the optical and near-infrared, similar to those assumed for condensate cloud decks. We further find that haze opacity increases with increasing production rate, decreasing eddy diffusivity, and increasing monomer size, although the magnitude of the latter effect is dependent on production rate and the atmospheric pressure levels probed. We generate synthetic exoplanet transmission spectra to investigate the effect of these hazes on spectral features. For high haze opacity cases, aggregate hazes lead to flat, nearly featureless spectra, while spherical hazes produce sloped spectra with clear spectral features at long wavelengths. Finally, we generate synthetic transmission spectra of GJ 1214b for aggregate and spherical hazes and compare them to space-based observations. We find that aggregate hazes can reproduce the data significantly better than spherical hazes, assuming a production rate that is limited by delivery of methane to the upper atmosphere.

An absolute sodium abundance for a cloud-free 'hot Saturn' exoplanet.

Broad absorption signatures from alkali metals, such as the sodium (Na I) and potassium (K I) resonance doublets, have long been predicted in the optical atmospheric spectra of cloud-free irradiated gas giant exoplanets1-3. However, observations have revealed only the narrow cores of these features rather than the full pressure-broadened profiles4-6. Cloud and haze opacity at the day-night planetary terminator are considered to be responsible for obscuring the absorption-line wings, which hinders constraints on absolute atmospheric abundances7-9. Here we report an optical transmission spectrum for the 'hot Saturn' exoplanet WASP-96b obtainedwith the Very Large Telescope, which exhibits the complete pressure-broadened profile of the sodium absorption feature. The spectrum is in excellent agreement with cloud-free, solar-abundance models assuming chemical equilibrium. We are able to measure a precise, absolute sodium abundance of logεNa = [Formula: see text], and use it as a proxy for the planet's atmospheric metallicity relative to the solar value (Zp/Zʘ = [Formula: see text]). This result is consistent with the mass-metallicity trend observed for Solar System planets and exoplanets10-12.

Observations of the global haze redistribution on Titan from 2006 to 2015 with OSIRIS at Keck

We observed Titan with the OH Suppressing InfraRed Imaging Spectrograph (OSIRIS) at the W. M. Keck observatory from 2006 through 2015 using adaptive optics. The sunlight scattered by atmospheric haze was spatially resolved in the 2.0 µm (K) band window, and the spectra were analyzed with a radiative transfer model to determine the vertical (altitude) and meridional (latitudinal) variation in the haze distribution over this time period. This study complements recent work by Karkoschka (2016) in the season of observations, in the time span and sampling interval, in wavelength coverage and spectral resolution, as well as in the radiative transfer methodology and analysis. We observe the largest meridional gradient in haze opacity above 20 km toward the northern hemisphere in January 2010. Individual observations can show significant deviations from a relatively smooth linear gradient in haze across the entire disk. The variation in haze below 20 km is rarely well-described by a simple model and there is a systematically smaller amount of haze opacity retrieved from the equator to 10° S when observing the disk with a sub-observer longitude near 150° W. This correlation with longitude suggests one of the following; a localized decrease in haze scattering, a localized increase in gas opacity, or a systematic over-estimate of the surface albedo in this region.

The detection of benzene in Saturn's upper atmosphere

AbstractThe stratosphere of Saturn contains a photochemical haze that appears thicker at the poles and may originate from chemistry driven by the aurora. Models suggest that the formation of hydrocarbon haze is initiated at high altitudes by the production of benzene, which is followed by the formation of heavier ring polycyclic aromatic hydrocarbons. Until now there have been no observations of hydrocarbons or photochemical haze in the production region to constrain these models. We report the first vertical profiles of benzene and constraints on haze opacity in the upper atmosphere of Saturn retrieved from Cassini Ultraviolet Imaging Spectrograph stellar occultations. We detect benzene at several different latitudes and find that the observed abundances of benzene can be produced by solar‐driven ion chemistry that is enhanced at high latitudes in the northern hemisphere during spring. We also detect evidence for condensation and haze at high southern latitudes in the polar night.

Stratospheric benzene and hydrocarbon aerosols detected in Saturn’s auroral regions

Context. Saturn’s polar upper atmosphere exhibits significant auroral activity; however, its impact on stratospheric chemistry (i.e. the production of benzene and heavier hydrocarbons) and thermal structure remains poorly documented. Aims. We aim to bring new constraints on the benzene distribution in Saturn’s stratosphere, to characterize polar aerosols (their vertical distribution, composition, thermal infrared optical properties), and to quantify the aerosols’ radiative impact on the thermal structure. Methods. Infrared spectra acquired by the Composite Infrared Spectrometer (CIRS) on board Cassini in limb viewing geometry are analysed to derive benzene column abundances and aerosol opacity profiles over the 3 to 0.1 mbar pressure range. The spectral dependency of the haze opacity is assessed in the ranges 680–900 and 1360–1440 cm −1 . Then, a radiative climate model is used to compute equilibrium temperature profiles, with and without haze, given the haze properties derived from CIRS measurements. Results. On Saturn’s auroral region (80 ◦ S), benzene is found to be slightly enhanced compared to its equatorial and mid-latitude values. This contrasts with the Moses & Greathouse (2005, J. Geophys. Res., 110, 9007) photochemical model, which predicts a benzene abundance 50 times lower at 80 ◦ S than at the equator. This advocates for the inclusion of ion-related reactions in Saturn’s chemical models. The polar stratosphere is also enriched in aerosols, with spectral signatures consistent with vibration modes assigned to aromatic and aliphatic hydrocarbons, and presenting similarities with the signatures observed in Titan’s stratosphere. The aerosol mass loading at 80 ◦ S is estimated to be 1−4 × 10 −5 gc m −2 , an order of magnitude less than on Jupiter, which is consistent with the order of magnitude weaker auroral power at Saturn. We estimate that this polar haze warms the middle stratosphere by 6 K in summer and cools the upper stratosphere by 5 K in winter. Hence, aerosols linked with auroral activity can partly account for the warm polar hood observed in Saturn’s summer stratosphere.

Seasonal variation of Titan’s haze at low and high altitudes from HST-STIS spectroscopy

The Space Telescope Imaging Spectrograph accumulated image cubes of Titan in five years between 1997 and 2004 that we calibrated and analyzed. The observations probe Titan’s early northern fall to early winter. Methane bands between 543 and 990nm wavelength are well resolved spectrally, and Titan’s latitudinal and center-to-limb reflectivity variations are resolved spatially. A principal component analysis revealed two large components and two small components of less significance. The first principal component describes a variation of Titan’s haze below 80±20km altitude. Haze particles change their size, opacity, and/or shape of the single scattering phase function. The largest and smallest opacities occurred both in 1997 at high southern latitudes and northern latitudes, respectively. The hemispherical asymmetry switched sign in 2002 at low latitudes, in 2003 at mid latitudes, and in early 2004 at high latitudes. The seasonal amplitude increased almost linearly with distance from the Equator. Tropical latitudes had slightly lower opacities than the annual and global average if the observed variation is seasonally symmetric and shaped like a sine curve. The cause for the variation may be condensation of gases onto aerosols seasonally driven by atmospheric dynamics. The second principal component describes a variation of haze opacity at altitudes above 150±50km. Largest and smallest opacities both occurred in 2004 at northern and high southern latitudes, respectively. The asymmetry switched in late 2001. Tropical latitudes had significantly higher haze opacity than the annual and global average, opposite to the case at low altitudes. The cause for the high-altitude variation may be aerosols transported at varying speeds driven by atmospheric dynamics. We present a seasonal model that completely describes the haze parameters at each altitude, latitude, and time. It compares fairly well with Cassini results obtained since 2004. The north–south asymmetry may reverse in 2016 at high altitudes and 2017 through 2018 at low altitudes. The observed variations are significant for modeling photometric data of Titan’s surface. They describe several characteristics of Titan’s haze variations that can be compared with results from Global Circulation Models.

Titan’s surface composition and atmospheric transmission with solar occultation measurements by Cassini VIMS

Solar occultation measurements by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) reveal the near-infrared transmission of Titan’s atmosphere down to an altitude of ∼40km. By combining these observations with VIMS reflectance measurements of Titan’s surface and knowledge of haze and gas opacity profiles from the Huygens probe, we constrain a simple model for the transfer of radiation in Titan’s atmosphere in order to derive surface reflectance in the methane windows used for compositional analysis. The advantages of this model are twofold: (1) it is accurate enough to yield useful results, yet simple enough to be implemented in just a few lines of code, and (2) the model parameters are directly constrained by the VIMS occultation and on-planet measurements. We focus on the 2.0, 2.7, 2.8 and 5.0μm windows, where haze opacity is minimized, and diagnostic vibrational bands exist for water ice and other candidate surface species. A particularly important result is the strong atmospheric attenuation at 2.7μm compared to 2.8μm, resulting in a reversal of apparent spectral slope in a compositionally diagnostic wavelength range. These results show that Titan’s surface reflectance is much “bluer” and more closely matched by water ice than the uncorrected spectra would indicate, although the majority of Titan’s surface has a spectrum consistent with mixtures (either intimate or areal) of water ice and haze particles precipitated from the atmosphere. Compositions of geologic units can be accurately modeled as mixtures ranging from predominantly water ice (Sinlap crater ejecta and margins of dark equatorial terrain) to predominantly organic-rich (Tui Regio and Hotei Regio), with particles in the size range ∼10–20μm. In distinguishing between hypothesized formation mechanisms for Tui and Hotei Regio, their organic-rich composition favors a process that concentrates precipitated haze particles, such as playa lake evaporite deposition (Barnes, J.W., Bow, J., Schwartz, J., Brown, R.H., Soderblom, J.M., Hayes, A.G., Vixie, G., Le Mouélic, S., Rodriguez, S., Sotin, C., Jaumann, R., Stephan, K., Soderblom, L.A., Clark, R.N., Buratti, B.J., Baines, K.H., Nicholson, P.D. [2011]. Icarus, 216, 136–140). In other places, kilometer-scale exposures of nearly pure water ice bedrock on Titan’s surface indicate relatively locally rapid erosion compared to rates of accumulation of solid hydrocarbons precipitated from the atmosphere. Somewhat surprisingly, Titan’s vast equatorial dune fields appear slightly enriched in water ice compared to the surrounding bright regions, but the spectrum of the dune material itself may nonetheless be consistent with a predominantly organic haze-derived composition.

Neptune’s cloud and haze variations 1994–2008 from 500 HST–WFPC2 images

The analysis of all suitable images taken of Neptune with the Wide Field Planetary Camera 2 on the Hubble Space Telescope between 1994 and 2008 revealed the following results. The activity of discrete cloud features located near Neptune’s tropopause remained roughly constant within each year but changed significantly on the time scale of ∼5 years. Discrete clouds covered 1% of the disk on average, but more than 2% in 2002. The other ∼99% of the disk probed Neptune’s hazes at lower altitudes. At red and near-infrared wavelengths, two dark bands around −70° and 10° latitude were perfectly steady and originated in the upper two scale heights of the troposphere, either by decreased haze opacity or by an increased methane relative humidity. At blue wavelengths, a dark band between −60° and −30° latitude was most obvious during the early years, caused by dark aerosols below the 3-bar level with single scattering albedos reduced by ∼0.04, and this contrast was constant between 410 and 630 nm wavelength. The dark band decayed exponentially with a time constant of 5 ± 1 years, which can be explained by settling of the dark aerosols at a rate of 1 bar pressure difference per year. The other latitudes brightened with the same time constant but lower amplitudes. The only exception was a darkening event in the 15–30° latitude region between 1994 and 1996, which coincides with two dark spots observed in the same region during the same time period, the only dark spots seen since Voyager. The dark aerosols had a similar latitudinal distribution as the discrete clouds near the tropopause, although both were separated by four scale heights. Photometric analysis revealed a phase coefficient of 0.0028 ± 0.0010 mag/deg for the 0–2° phase-angle range observable from Earth. Neptune’s sub-Earth latitude varied by less than 3° throughout the observation period providing a data set with almost constant viewing geometry. The trends observed up to 2008 continued into 2010 based on images taken with the Wide Field Camera 3.

Far-infrared opacity sources in Titan’s troposphere reconsidered

We use Cassini far-infrared limb and nadir spectra, together with recent Huygens results, to shed new light on the controversial far-infrared opacity sources in Titan’s troposphere. Although a global cloud of large CH 4 ice particles around an altitude of 30 km, together with an increase in tropospheric haze opacity with respect to the stratosphere, can fit nadir and limb spectra well, this cloud does not seem consistent with shortwave measurements of Titan. Instead, the N 2–CH 4 collision-induced absorption coefficients are probably underestimated by at least 50% for low temperatures.