Stratospheric Haze Research Articles

Stratospheric aerosols and C6H6 in Jupiter’s south polar region from JWST/MIRI observations

Context. The polar atmosphere of Jupiter is significantly affected by auroral activity, which can induce both thermal and chemical differences compared to the rest of the atmosphere. In particular, auroral activity enhances the production of various hydrocarbons, including benzene. Benzene could be a potential precursor to the formation of the stratospheric hazes. Aims. We investigated the spatial distribution of the benzene abundance across latitudes ranging from 50°S to 81°S and 17°S to 25°S. Additionally, we examined the chemical origin of polar aerosols and their latitudinal distribution. Methods. We employed James Webb Space Telescope (JWST) Mid InfraRed Instrument (MIRI) observations to measure the benzene abundance based on its emission at 674 cm−1. Additionally, we examined the spectral dependence of the aerosol opacity within the 680–760 and 1380–1500 cm−1 spectral ranges, and mapped their distribution from 80°S–50°S. Results. At latitudes lower than 60°S, benzene is found to be up to ten times more abundant compared to lower latitudes. This enhancement of C6H6 is well mixed longitudinally and not particularly concentrated inside the auroral oval. Photochemical models predict a decrease in the abundance as we approach the mid latitudes, but fail at polar latitudes as they do not include ion-neutral chemistry. Moreover, we find that the southern polar atmosphere is enriched with aerosols at ~10 mbar. The optical depth of the aerosols increases at latitudes poleward of ~60°S, similar to the enhancement of C6H6. These aerosols have spectral features similar to the aerosols of Titan and Saturn, and the mass loading is of ~1.2 ± 0.2 × 10−4 g cm−2. Finally, we quantified the impact of these aerosols on the retrieved temperature structure, causing a decrease in the temperature at pressure levels deeper than 10 mbar. Conclusions. We find that the auroral precipitation produces abundant stratospheric aerosols, which must play an important role in the chemistry and dynamics of the planet.

Temporal Variations in Vertical Cloud Structure of Jupiter's Great Red Spot, Its Surroundings and Oval BA From HST/WFC3 Imaging

AbstractIn this study, we present the evolution of the properties and vertical distribution of the hazes in Jupiter's Great Red Spot (GRS), its surroundings and Oval BA from 2015 to 2021. To retrieve the main atmospheric parameters, we model the spectral reflectivity of a number of dynamically and/or spectrally interesting regions with a radiative transfer tool that uses an optimal estimator scheme. The spectra of the selected regions are obtained from high‐resolution Hubble Space Telescope Wide Field Camera 3 images that cover the spectral range from 200 to 900 nm. The a priori model atmosphere used to describe the various Jovian regions is taken from Anguiano‐Arteaga et al. (2021, https://doi.org/10.1029/2021JE006996) for each corresponding area. We find that the biggest variations in the GRS occur in the optical thickness of the stratospheric and tropospheric haze layers starting in 2019 and in the mean size of the tropospheric haze particles in 2018. The absorption spectra of both hazes show little variations among the analyzed regions and years, with the stratospheric haze properties seeming compatible with the chromophore proposed by Carlson et al. (2016, https://doi.org/10.1016/j.icarus.2016.03.008). We report a color change of Oval BA from red to white during these years that, according to our models, can be mostly explained in terms of a decrease in the stratospheric haze optical depth.

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.

Titan Stratospheric Haze Bands Observed in Cassini VIMS as Tracers of Meridional Circulation

We analyzed Cassini data to derive the nature and evolution of circumglobal annuli observed in the stratosphere of Titan, Saturn's largest moon. The annuli were observed between 2004 and 2017 in data acquired by the Visual and Infrared Mapping Spectrometer on board the Cassini spacecraft. We observed a north polar annulus, an equatorial annulus, and several secondary annuli. Pre-Cassini telescopic observations by the Hubble Space Telescope and Keck reported an atmospheric feature consistent with the presence of a south polar annulus between 1999 and 2001, although this feature was not observed by Cassini. Relative to the atmosphere near the annuli, they appear dark at 300–500 nm and bright in methane absorption channels such as the ones at 900 and 1150 nm. The stratosphere seems to rotate around the north pole. Alternatively, it seems to rotate about a point offset from solid-body rotation axis by a few degrees; this point in turn rotates around the solid-body rotation axis.

Vertical Distribution of Aerosols and Hazes Over Jupiter's Great Red Spot and Its Surroundings in 2016 From HST/WFC3 Imaging

AbstractIn this work, we have analyzed images provided by the Hubble Space Telescope's Wide Field Camera 3 (HST/WFC3) in December 2016, with a spectral coverage from the ultraviolet to the near infrared. We have obtained the spectral reflectivity of the GRS and its surroundings, with particular emphasis on selected, dynamically interesting regions. A spectral characterization of the GRS area is performed following two different procedures: (a) in terms of Altitude/Opacity and Color Indices (AOI and CI); (b) by means of automatic spectral classification. We used the NEMESIS radiative transfer suite to retrieve the main atmospheric parameters (e.g., particle vertical and size distributions, refractive indices) that are able to explain the observed spectral reflectivity. The optimal a priori model atmosphere used for the retrievals is obtained from a grid of about 12,000 different atmospheric models, and choosing the one that best fits South Tropical Zone (STrZ) spectra and its observed limb‐darkening. We conclude that the spectral reflectivity of the GRS area is well reproduced with the following layout: (a) a stratospheric haze with its base near the 100 mbar level, with optical depths at 900 nm of the order of unity and particles with a size of 0.3 μm; (b) a more vertically extended tropospheric haze, with τ (900 nm) ∼10 down to 500 mbar and micron sized particles. Both haze layers show a stronger short wavelength absorption, and thus both act as chromophores. The altitude difference between clouds tops in the GRS and surrounding areas is ∼10 km.

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.

Meridional Variations of C2H2 in Jupiter's Stratosphere From Juno UVS Observations

AbstractThe Ultraviolet Spectrograph (UVS) instrument on the Juno mission records far‐ultraviolet reflected sunlight from Jupiter. These spectra are sensitive to the abundances of chemical species in the upper atmosphere and to the distribution of the stratospheric haze layer. We combine observations from the first 30 perijoves of the mission in order to study the meridional distribution of acetylene (C2H2) in Jupiter's stratosphere. We find that the abundance of C2H2 decreases toward the poles by a factor of 2–4, in agreement with previous analyses of mid‐infrared spectra. This result is expected from insolation rates: near the equator, the UV solar flux is higher, allowing more C2H2 to be generated from the UV photolysis of CH4. The decrease in abundance toward the poles suggests that horizontal mixing rates are not rapid enough to homogenize the latitudinal distribution.

Evolution of Saturn’s north polar color and cloud structure between 2012 and 2017 inferred from Cassini VIMS and ISS observations

Cassini/ISS imagery and Cassini/VIMS spectral imaging observations from 0.35 to 5.12 μm show that Saturn’s north polar region (70°–90° N) evolved significantly between 2012 and 2017, with the region poleward of the hexagon changing from dark blue/green to a moderately brighter gold color, except for the inner eye region (88.2°–90° N), which remained relatively unchanged. These and even more dramatic near-IR changes can be reproduced by an aerosol model of four compact layers consisting of a stratospheric haze at an effective pressure near 50 mbar, a deeper haze of putative diphosphine particles typically near 300 mbar, an ammonia cloud layer with a base pressure between 0.4 bar and 1.3 bar, and a deeper cloud of a possible mix of NH4SH and water ice particles within the 2.7 to 4.5 bar region. Between the eye and the hexagon boundary near 75° N were many small discrete bright cloud features that VIMS spectra indicate have increased opacity in the ammonia cloud layer. Our analysis of the background clouds between the discrete features shows that between 2013 and 2016 the effective pressures of most layers changed very little, except for the ammonia ice layer, which decreased from about 1 bar to 0.4 bar near the edge of the eye, but increased to 1 bar inside the eye. Inside the hexagon there were large increases in optical depth, by up to a factor of 10 near the eye for the putative diphosphine layer and by a factor of four over most of the hexagon interior. Inside the eye, aerosol optical depths were very low, suggesting downwelling motions. The high contrast between eye and surroundings in 2016 was due to substantial increases in optical depths outside the eye. The color change from blue/green to gold inside most of the hexagon region can be explained in our model almost entirely by changes in the stratospheric haze, which increased between 2013 and 2016 by a factor of four in optical depth and by almost a factor of three in the short-wavelength peak of its wavelength-dependent imaginary index. A plausible mechanism for increasing aerosol opacity with time is the action of photochemistry as the north polar region became increasingly exposed to solar UV radiation. For 2013 we found an ammonia mixing ratio of about 50×10−6 in the depleted region between 4 bars and the NH3 condensation level (∼ 1 bar), but the NH3 results for 2016 are unclear due to very high retrieval uncertainties associated with increased aerosol opacity. We retrieved a deep abundance of about 5×10−6 for PH3 and a pressure breakpoint (where the PH3 abundance begins to decline with altitude) that coincided with the main cloud top near 300 mbar, except when that cloud opacity was very low, at which point the PH3 breakpoint pressure generally increased substantially, consistent with prior suggestions that the cloud layers shield PH3 from destruction by UV radiation above the clouds. We found an average AsH3 mixing ratio of 2×10−9 with some evidence for a decline with altitude above the main cloud layer.

Experimental Simulation of Titan's Stratospheric Photochemistry: Benzene (C6H6) Ices

AbstractWe performed laboratory experiments to study the photochemical evolution induced by long‐UV irradiation of benzene ices in Titan's atmosphere. The aim of this study was to investigate whether photo‐processed benzene ices could lead to the formation of aerosols analogs to those observed in Titan's stratosphere. Prior to that, spectroscopic properties of amorphous and crystalline benzene ices were studied as a function of temperature, using infrared spectroscopy. UV photolysis experiments (λ > 230 nm) of benzene ices led to the formation of volatile photo‐products, among which fulvene is identified, and of a residue dominated by νCH IR features, demonstrating that pure aromatic‐based polymeric structures are not sufficient to explain the composition of Titan's stratospheric haze layer. However, we provide a characterization of long‐UV‐induced benzene‐containing aerosol analogs, which will contribute to Titan's surface organics layer. These data are of prime interest in the context of the future Dragonfly space mission.

Latitudinal variation of methane mole fraction above clouds in Neptune's atmosphere from VLT/MUSE-NFM: Limb-darkening reanalysis

We present a reanalysis of visible/near-infrared (480–930 nm) observations of Neptune, made in 2018 with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) in Narrow Field Adaptive Optics mode, reported by Irwin et al., Icarus, 311, 2019. We find that the inferred variation of methane abundance with latitude in our previous analysis, which was based on central meridian observations only, underestimated the retrieval errors when compared with a more complete assessment of Neptune's limb darkening. In addition, our previous analysis introduced spurious latitudinal variability of both the abundance and its uncertainty, which we reassess here. Our reanalysis of these data incorporates the effects of limb-darkening based upon the Minnaert approximation model, which provides a much stronger constraint on the cloud structure and methane mole fraction, makes better use of the available data and is also more computationally efficient. We find that away from discrete cloud features, the observed reflectivity spectrum from 800 to 900 nm is very well approximated by a background cloud model that is latitudinally varying, but zonally symmetric, consisting of a H2S cloud layer, based at 3.6–4.7 bar with variable opacity and scale height, and a stratospheric haze. The background cloud model matches the observed limb darkening seen at all wavelengths and latitudes and we find that the mole fraction of methane at 2–4 bar, above the H2S cloud, but below the methane condensation level, varies from 4–---6% at the equator to 2–4% at south polar latitudes, consistent with previous analyses, with a equator/pole ratio of 1.9 ± 0.2 for our assumed cloud/methane vertical distribution model. The spectra of discrete cloudy regions are fitted, to a very good approximation, by the addition of a single vertically thin methane ice cloud with opacity ranging from 0 to 0.75 and pressure less than ~0.4 bar.

Radiative-equilibrium model of Jupiter’s atmosphere and application to estimating stratospheric circulations

Jupiter’s upper troposphere and stratosphere are host to a rich dynamical and chemical activity. This modulates the thermal structure and distribution of trace species and aerosols, which, in turn, impact the atmospheric radiative budget and dynamics. In this paper, we present a computationally efficient 1-D seasonal radiative model, with convective adjustment, of Jupiter’s atmosphere. Our model takes into account radiative forcings from the main hydrocarbons (methane, ethane, acetylene), ammonia, collision-induced absorption, several cloud and haze layers and an internal heat flux. We parametrize four tropospheric cloud and haze layers. Three of them (one tropospheric cloud near 800 mbar, one upper tropospheric haze, one stratospheric haze) are set to be uniform with latitude. On the contrary, we prescribe the spatial distribution of another UV-absorbing “polar” stratospheric haze comprising fractal aggregates based on published observational constraints, as their concentration varies significantly with latitude. We detail sensitivity studies of the equilibrium temperature profile to several parameters (hydrocarbon abundances, cloud particle sizes and optical depths, optical properties of the stratospheric polar haze, etc.). We then discuss the expected seasonal, vertical and meridional temperature variations in Jupiter’s atmosphere and compare the modeled thermal structure to that derived from Cassini and ground-based thermal infrared observations.We find that the equilibrium temperature in the 5–30 mbar pressure range is very sensitive to the chosen stratospheric haze optical properties, sizes and number of monomers. One of the three sets of optical properties tested yields equilibrium temperatures that match well, to first order, the observed ones. In this scenario, the polar haze significantly warms the lower stratosphere (10–30 mbar) by up to 20 K at latitudes 45–60°, and reproduces an observed north–south asymmetry in stratospheric temperature. The polar haze also acts to shorten significantly the radiative timescales, estimated by our model to 100 (Earth) days at the 10-mbar level. At pressures lower than 3 mbar, our modeled temperatures systematically underestimate the observed ones by ∼5 K. This might suggest that other processes, such as dynamical heating by wave breaking or by eddies, or a coupling with thermospheric circulation, play an important role. An alternate possibility is that the uncertainty on the abundance of hydrocarbons is responsible for this mismatch. In the troposphere, we can only match the observed lack of meridional gradient of temperature by varying the internal heat flux with latitude.We then exploit knowledge of heating and cooling rates (using our radiative seasonal model combined to observational constraints on the temperature) to diagnose the residual-mean circulation in Jupiter’s stratosphere. This is done under the assumption that the eddy heat flux convergence term is negligible. In the Earth’s stratosphere, the residual-mean circulation obtained with this method represents well, on a seasonal scale, the transport of tracers in regions where wave breaking and dissipation are weak. However, on Jupiter, in the lower stratosphere (5–30 mbar), the residual-mean circulation strongly depends on the assumed properties of the stratospheric haze. Our main conclusion is that it is crucial to improve our knowledge on the different radiative forcing terms (in particular regarding the stratospheric haze properties) to increase our confidence in the estimated circulation. By extension, this will also be crucial for future 3D GCM studies.

Saturn's south polar cloud composition and structure inferred from 2006 Cassini/VIMS spectra and ISS images

We used 0.85–5.1 μm 2006 observations by Cassini's Visual and Infrared Mapping Spectrometer (VIMS) to constrain the unusual vertical structure and compositions of cloud layers in Saturn's south polar region, the site of a powerful vortex circulation, shadow-casting cloud bands, and evidence for ammonia ice clouds without lightning. Finding ammonia ice spectral signatures in polar regions is surprising because over most of Saturn an overlying haze layer of unknown composition but significant optical depth completely obscures it, unless penetrated by significant convection, confirmed by lightning, as in the Great Storm of 2010–2011 (Sromovsky et al. 2013, Icarus 226, 402–418), and in the storms of Storm Alley region (Baines et al. 2009, Planet. & Space Sci. 57, 1650–1658). This is clarified by our radiation transfer modeling of VIMS spectra of the south polar background and discrete features, using a 4-layer model that includes (1) a stratospheric haze, (2) a top tropospheric layer of non-absorbing (possibly diphosphine) particles near 300 mbar, with a fraction of an optical depth (much less than found elsewhere on Saturn), (3) a moderately thicker layer (1–2 optical depths) of NH3 ice particles near 900 mbar, and (4) extending from 5 bars up to 2–4 bars, an assumed optically thick layer where NH4SH and H2O are likely condensables. The ammonia layer is the main modulator of pseudo-continuum I/F in reflected sunlight. That layer has about one optical depth in background clouds, but about double that in the brightest clouds, and about half that in discrete dark clouds. What makes the 3-μm absorption unexpectedly apparent in these polar clouds is the relatively low optical depth of the top tropospheric cloud layer, which can be an order of magnitude less than in non-polar regions on Saturn, perhaps because of polar downwelling and/or lower photochemical production rates. We found changes in the PH3 vertical profile and AsH3 mixing ratio that support the existence of downwelling within 2° of the pole. We also found evidence for step-wise decreases in optical depth of the stratospheric haze near 87.9° S and in the putative diphosphine layer near 88.9° S, and evidence against the idea that deep convective eyewalls are responsible for the shadows observed near the same latitudes. In 752-nm Cassini images we identified moderately bright features extending from shadow-producing boundaries when those boundaries rotated to the opposite side of Saturn's pole. Under those observing conditions an illuminated eyewall should produce a bright feature extending towards the pole. Instead, the features extend away from the pole, as expected for what we call antishadows, which are bright features produced by light illuminating a translucent layer from below. This provides strong qualitative evidence that both shadows and antishadows are produced by small step changes in the optical depth of the overlying translucent aerosol layers.

Rotational Light Curves of Jupiter from Ultraviolet to Mid-infrared and Implications for Brown Dwarfs and Exoplanets

Abstract Rotational modulations are observed on brown dwarfs and directly imaged exoplanets, but the underlying mechanism is not well understood. Here we analyze Jupiter’s rotational light curves at 12 wavelengths from the ultraviolet (UV) to the mid-infrared (mid-IR). The peak-to-peak amplitudes of Jupiter’s light curves range from subpercent to 4% at most wavelengths, but the amplitude exceeds 20% at 5 μm, a wavelength sensing Jupiter’s deep troposphere. Jupiter’s rotational modulations are primarily caused by discrete patterns in the cloudless belts instead of the cloudy zones. The light-curve amplitude is controlled by the sizes and brightness contrasts of the Great Red Spot (GRS), expansions of the North Equatorial Belt (NEB), patchy clouds in the North Temperate Belt (NTB), and a train of hot spots in the NEB. In reflection, the contrast is controlled by upper tropospheric and stratospheric hazes, clouds, and chromophores in the clouds. In thermal emission, the small rotational variability is caused by the spatial distribution of temperature and opacities of gas and aerosols; the large variation is caused by the NH3 cloud holes and thin-thick clouds. The methane-band light curves exhibit opposite-shape behavior compared with the UV and visible wavelengths, caused by a wavelength-dependent brightness change of the GRS. Light-curve evolution is induced by periodic events in the belts and longitudinal drifting of the GRS and patchy clouds in the NTB. This study suggests several interesting mechanisms related to distributions of temperature, gas, hazes, and clouds for understanding the observed rotational modulations on brown dwarfs and exoplanets.

Jupiter’s para-H2 distribution from SOFIA/FORCAST and Voyager/IRIS 17–37 µm spectroscopy

Spatially resolved maps of Jupiter’s far-infrared 17–37 µm hydrogen-helium collision-induced spectrum were acquired by the FORCAST instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA) in May 2014. Spectral scans in two grisms covered the broad S(0) and S(1) absorption lines, in addition to contextual imaging in eight broad-band filters (5–37 µm) with spatial resolutions of 2–4″. The spectra were inverted to map the zonal-mean temperature and para-H2 distribution (fp, the fraction of the para spin isomer with respect to the ortho spin isomer) in Jupiter’s upper troposphere (the 100–700 mbar range). We compared these to a reanalysis of Voyager-1 and -2 IRIS spectra covering the same spectral range. Tropospheric temperature contrasts match those identified by Voyager in 1979, within the limits of temporal variability consistent with previous investigations. Para-H2 increases from equator to pole, with low-fp air at the equator representing sub-equilibrium conditions (i.e., less para-H2 than expected from thermal equilibration), and high-fp air and possible super-equilibrium at higher latitudes. In particular, we confirm the continued presence of a region of high-fp air at high northern latitudes discovered by Voyager/IRIS, and an asymmetry with generally higher fp in the north than in the south. Far-IR aerosol opacity is not required to fit the data, but cannot be completely ruled out. We note that existing collision-induced absorption databases lack opacity from (H2)2 dimers, leading to under-prediction of the absorption near the S(0) and S(1) peaks. There appears to be no spatial correlation between para-H2 and tropospheric ammonia, phosphine and cloud opacity derived from Voyager/IRIS at mid-infrared wavelengths (7–15 µm). We note, however, that para-H2 tracks the similar latitudinal distribution of aerosols within Jupiter’s upper tropospheric and stratospheric hazes observed in reflected sunlight, suggesting that catalysis of hydrogen equilibration within the hazes (and not the main clouds) may govern the equator-to-pole gradient, with conditions closer to equilibrium at higher latitudes. This gradient is superimposed onto smaller-scale variations associated with regional advection of para-H2 at the equator and poles.

Titan Science with the James Webb Space Telescope

The James Webb Space Telescope (JWST), scheduled for launch in 2018, is the successor to the Hubble Space Telescope (HST) but with a significantly larger aperture (6.5 m) and advanced instrumentation focusing on infrared science (0.6–28.0 μm). In this paper, we examine the potential for scientific investigation of Titan using JWST, primarily with three of the four instruments: NIRSpec, NIRCam, and MIRI, noting that science with NIRISS will be complementary. Five core scientific themes are identified: (1) surface (2) tropospheric clouds (3) tropospheric gases (4) stratospheric composition, and (5) stratospheric hazes. We discuss each theme in depth, including the scientific purpose, capabilities, and limitations of the instrument suite and suggested observing schemes. We pay particular attention to saturation, which is a problem for all three instruments, but may be alleviated for NIRCam through use of selecting small sub-arrays of the detectors—sufficient to encompass Titan, but with significantly faster readout times. We find that JWST has very significant potential for advancing Titan science, with a spectral resolution exceeding the Cassini instrument suite at near-infrared wavelengths and a spatial resolution exceeding HST at the same wavelengths. In particular, JWST will be valuable for time-domain monitoring of Titan, given a five- to ten-year expected lifetime for the observatory, for example, monitoring the seasonal appearance of clouds. JWST observations in the post-Cassini period will complement those of other large facilities such as HST, ALMA, SOFIA, and next-generation ground-based telescopes (TMT, GMT, EELT).

Mid-infrared spectroscopy of Uranus from the Spitzer Infrared Spectrometer: 1. Determination of the mean temperature structure of the upper troposphere and stratosphere

On 2007 December 16–17, spectra were acquired of the disk of Uranus by the Spitzer Infrared Spectrometer (IRS), ten days after the planet’s equinox, when its equator was close to the sub-Earth point. This spectrum provides the highest-resolution broad-band spectrum ever obtained for Uranus from space, allowing a determination of the disk-averaged temperature and molecule composition to a greater degree of accuracy than ever before. The temperature profiles derived from the Voyager radio occultation experiment by Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92, 14987–15001) and revisions suggested by Sromovsky et al. (Sromovsky, L.A., Fry, P.A., Kim, J.H. [2011]. Icarus 215, 292–312) that match these data best are those that assume a high abundance of methane in the deep atmosphere. However, none of these model profiles provides a satisfactory fit over the full spectral range sampled. This result could be the result of spatial differences between global and low-latitudinal regions, changes in time, missing continuum opacity sources such as stratospheric hazes or unknown tropospheric constituents, or undiagnosed systematic problems with either the Voyager radio-occultation or the Spitzer IRS data sets. The spectrum is compatible with the stratospheric temperatures derived from the Voyager ultraviolet occultations measurements by Herbert et al. (Herbert, F. et al. [1987]. J. Geophys. Res. 92, 15093–15109), but it is incompatible with the hot stratospheric temperatures derived from the same data by Stevens et al. (Stevens, M.H., Strobel, D.F., Herbert, F.H. [1993]. Icarus 101, 45–63). Thermospheric temperatures determined from the analysis of the observed H2 quadrupole emission features are colder than those derived by Herbert et al. at pressures less than ∼1μbar. Extrapolation of the nominal model spectrum to far-infrared through millimeter wavelengths shows that the spectrum arising solely from H2 collision-induced absorption is too warm to reproduce observations between wavelengths of 0.8 and 3.3mm. Adding an additional absorber such as H2S provides a reasonable match to the spectrum, although a unique identification of the responsible absorber is not yet possible with available data. An immediate practical use for the spectrum resulting from this model is to establish a high-precision continuum flux model for use as an absolute radiometric standard for future astronomical observations.

Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere

Mid-infrared spectral observations Uranus acquired with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope are used to determine the abundances of C2H2, C2H6, CH3C2H, C4H2, CO2, and tentatively CH3 on Uranus at the time of the 2007 equinox. For vertically uniform eddy diffusion coefficients in the range 2200–2600cm2s−1, photochemical models that reproduce the observed methane emission also predict C2H6 profiles that compare well with emission in the 11.6–12.5μm wavelength region, where the υ9 band of C2H6 is prominent. Our nominal model with a uniform eddy diffusion coefficient Kzz=2430cm2s−1 and a CH4 tropopause mole fraction of 1.6×10−5 provides a good fit to other hydrocarbon emission features, such as those of C2H2 and C4H2, but the model profile for CH3C2H must be scaled by a factor of 0.43, suggesting that improvements are needed in the chemical reaction mechanism for C3Hx species. The nominal model is consistent with a CH3D/CH4 ratio of 3.0±0.2×10−4. From the best-fit scaling of these photochemical-model profiles, we derive column abundances above the 10-mbar level of 4.5+01.1/−0.8×1019molecule-cm−2 for CH4, 6.2±1.0×1016molecule-cm−2 for C2H2 (with a value 24% higher from a different longitudinal sampling), 3.1±0.3×1016molecule-cm−2 for C2H6, 8.6±2.6×1013molecule-cm−2 for CH3C2H, 1.8±0.3×1013molecule-cm−2 for C4H2, and 1.7±0.4×1013molecule-cm−2 for CO2 on Uranus. A model with Kzz increasing with altitude fits the observed spectrum and requires CH4 and C2H6 column abundances that are 54% and 45% higher than their respective values in the nominal model, but the other hydrocarbons and CO2 are within 14% of their values in the nominal model. Systematic uncertainties arising from errors in the temperature profile are estimated very conservatively by assuming an unrealistic “alternative” temperature profile that is nonetheless consistent with the observations; for this profile the column abundance of CH4 is over four times higher than in the nominal model, but the column abundances of the hydrocarbons and CO2 differ from their value in the nominal model by less than 22%. The CH3D/CH4 ratio is the same in both the nominal model with its uniform Kzz as in the vertically variable Kzz model, and it is 10% lower with the “alternative” temperature profile than the nominal model. There is no compelling evidence for temporal variations in global-average hydrocarbon abundances over the decade between Infrared Space Observatory and Spitzer observations, but we cannot preclude a possible large increase in the C2H2 abundance since the Voyager era. Our results have implications with respect to the influx rate of exogenic oxygen species and the production rate of stratospheric hazes on Uranus, as well as the C4H2 vapor pressure over C4H2 ice at low temperatures.

Stratospheric aerosols on Jupiter from Cassini observations

We retrieved global distributions and optical properties of stratospheric aerosols on Jupiter from ground-based NIR spectra and multiple-phase-angle images from Cassini Imaging Science Subsystem (ISS). A high-latitude haze layer is located at ∼10–20mbar, higher than in the middle and low latitudes (∼50mbar). Compact sub-micron particles are mainly located in the low latitudes between 40°S and 25°N with the particle radius between 0.2 and 0.5μm. The rest of the stratosphere is covered by the particles known as fractal aggregates. In the nominal case with the imaginary part of the UV refractive index 0.02, the fractal aggregates are composed of about a thousand 10-nm-size monomers. The column density of the aerosols at pressure less than 100mbar ranges from ∼107cm−2 at low latitudes to ∼109cm−2 at high latitudes. The mass loading of aerosols in the stratosphere is ∼10−6gcm−2 at low latitudes to ∼10−4gcm−2 in the high latitudes. Multiple solutions due to the uncertainty of the imaginary part of the refractive index are discussed. The stratospheric haze optical depths increase from ∼0.03 at low latitudes to about a few at high latitudes in the UV wavelength (∼0.26μm), and from ∼0.03 at low latitudes to ∼0.1 at high latitudes in the NIR wavelength (∼0.9μm).

Retrieval of jovian cloud structure from the Cassini ISS limb-darkening data: I. Continuum scattering phase functions for cloud and haze in the South Tropical Zone

We have deduced the scattering properties of aerosols in the jovian upper troposphere by analyzing imaging data obtained at a wide variety of solar phase angles (4–140°) by the Cassini Imaging Science Subsystem (ISS) Narrow Angle Camera (NAC) during its flyby of Jupiter. The limb-darkening curves along the South Tropical Zone (STrZ) are extracted from CB2 images (effective wavelength: 750nm) to constrain the single scattering phase functions of aerosols. The best-fit Mie scattering phase function for cloud is obtained with the real part of the refractive index nr,cloud=1.85 and the effective radius reff,cloud=0.3μm. The best-fit combination of nr,cloud and reff,cloud would strongly suggest the idea that the abundant small particle population in the upper troposphere is not composed of pure NH3 ice. Although the optical properties of the stratospheric haze are not well constrained compared with those of cloud, the haze is found to be optically thin (<0.06) and to be strongly forward scattering (effective radius reff,haze=0.5μm).We compare our results with the scattering phase function at red wavelength (640nm) for the STrZ derived by Tomasko et al. (Tomasko, M.G., West, R.A., Castillo, N.D. [1978]. Icarus 33, 558–592), which was deduced from analysis of the Pioneer 10 Imaging Photopolarimeter (IPP) data (12–150° for solar phase angles). Our new Mie scattering phase function can reproduce the Pioneer 10 observations well. In contrast, their scattering phase function described by the double Henyey–Greenstein function does not reproduce the Cassini ISS observations. This is attributed to the fact that their scattering phase function is underconstrained, primarily due to a considerable gap in observations for an intermediate solar phase angle range (34–109°).Our new Mie scattering phase function has advantages over that of Tomasko et al. (Tomasko, M.G., West, R.A., Castillo, N.D. [1978]. Icarus 33, 558–592).(1)Since the Cassini ISS data do not have a large gap in solar phase angle, the new Mie scattering phase function is better constrained.(2)The Mie scattering phase function can be applied easily to different wavelengths.With such characteristics, we now have a reliable baseline scattering phase function that can be used to interpret the ever-changing appearance of jovian clouds as changes of the vertical cloud structure and/or distribution of chromophores in the atmosphere.

The three-micron spectral feature of the Saturnian haze: Implications for the haze composition and formation process

We retrieve a 3.4-μm spectral feature pertaining to the Saturnian haze from Cassini/VIMS Procyon occultation data obtained at 12 different altitudes from approximately 0.7 to 0.0018mbar (Nicholson et al., 2006; Bellucci, 2008; Bellucci et al., 2009). The occultation occurred at 55°N latitude, at the edge of the “polar hood” region, where the extinction from stratospheric aerosols becomes strongest. The improved retrieval procedure eliminates the influence of strong adjacent CH4 absorption bands. We derive optical-depth spectra (hereafter called τ-spectra) of the haze on a relative scale and find that the τ-spectra exhibit a broad opacity peak at 3.36–3.41μm. At the 0.7–0.025mbar levels, the opacity is maximum at 3.41μm, which approximately coincides with the 3.4-μm aliphatic C–H stretching bands typical of the solid state of hydrocarbons such as C5H12, C6H12, C6H14 and C7H14, but not limited to these molecules. At the 0.015–0.0018mbar levels, however, maximum opacity occurs at 3.36μm, similarly to the τ-spectra of Titan's haze (Kim et al., 2011), suggesting similar chemical composition and possibly a similar formation mechanism occurring in the upper stratospheres of Saturn and Titan. We also find that with mole fractions on the order of 1ppb, C6H12, C6H14 and C7H14 can be saturated near the lower levels of the occultation, suggesting that condensation of these or other heavier hydrocarbons could contribute to the formation of the stratospheric haze particles at these altitudes on Saturn.