Aerosol Opacity Research Articles

An HST/STIS Optical Transmission Spectrum of Warm Neptune GJ 436b

Abstract GJ 436b is a prime target for understanding warm Neptune exoplanet atmospheres and a target for multiple James Webb Space Telescope (JWST) Guaranteed Time Observation programs. Here, we report the first space-based optical transmission spectrum of the planet using two Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) transit observations from 0.53 to 1.03 μm. We find no evidence for alkali absorption features, nor evidence of a scattering slope longward of 0.53 μm. The spectrum is indicative of moderate to high metallicity (∼100–1000× solar), while moderate-metallicity scenarios (∼100× solar) require aerosol opacity. The optical spectrum also rules out some highly scattering haze models. We find an increase in transit depth around 0.8 μm in the transmission spectra of three different sub-Jovian exoplanets (GJ 436b, HAT-P-26b, and GJ 1214b). While most of the data come from STIS, data from three other instruments may indicate this is not an instrumental effect. Only the transit spectrum of GJ 1214b is well fit by a model with stellar plages on the photosphere of the host star. Our photometric monitoring of the host star reveals a stellar rotation rate of 44.1 days and an activity cycle of 7.4 years. Intriguingly, GJ 436 does not become redder as it gets dimmer, which is expected if star spots were dominating the variability. These insights into the nature of the GJ 436 system help refine our expectations for future observations in the era of JWST, whose higher precision and broader wavelength coverage will shed light on the composition and structure of GJ 436b’s atmosphere.

Jupiter's North Equatorial Belt expansion and thermal wave activity ahead of Juno's arrival

AbstractThe dark colors of Jupiter's North Equatorial Belt (NEB, 7–17°N) appeared to expand northward into the neighboring zone in 2015, consistent with a 3–5 year cycle. Inversions of thermal‐IR imaging from the Very Large Telescope revealed a moderate warming and reduction of aerosol opacity at the cloud tops at 17–20°N, suggesting subsidence and drying in the expanded sector. Two new thermal waves were identified during this period: (i) an upper tropospheric thermal wave (wave number 16–17, amplitude 2.5 K at 170 mbar) in the mid‐NEB that was anticorrelated with haze reflectivity; and (ii) a stratospheric wave (wave number 13–14, amplitude 7.3 K at 5 mbar) at 20–30°N. Both were quasi‐stationary, confined to regions of eastward zonal flow, and are morphologically similar to waves observed during previous expansion events.

Moist convection and the 2010–2011 revival of Jupiter’s South Equatorial Belt

The transformation of Jupiter’s South Equatorial Belt (SEB) from its faded, whitened state in 2009-2010 (Fletcher et al., 2011b) to its normal brown appearance is documented via comparisons of thermal-infrared (5–20 µm) and visible-light imaging between November 2010 and November 2011. The SEB revival consisted of convective eruptions triggered over ∼100 days, potentially powered by the latent heat released by the condensation of water. The plumes rise from the water cloud base and ultimately diverge and cool in the stably-stratified upper troposphere. Thermal-IR images from the Very Large Telescope (VLT) were acquired 2 days after the SEB disturbance was first detected as a small white spot by amateur observers on November 9th 2010. Subsequent images over several months revealed the cold, putatively anticyclonic and cloudy plume tops (area 2.5 × 106 km2) surrounded by warm, cloud-free conditions at their peripheries due to subsidence. The latent heating was not directly detectable in the 5-20 µm range. The majority of the plumes erupted from a single source near 140−160∘W, coincident with the remnant cyclonic circulation of a brown barge that had formed during the fade. The warm remnant of the cyclone could still be observed in IRTF imaging 5 days before the November 9th eruption. Additional plumes erupted from the leading edge of the central disturbance immediately east of the source, which propagated slowly eastwards to encounter the Great Red Spot. The tropospheric plumes were sufficiently vigorous to excite stratospheric thermal waves over the SEB with a 20−30∘ longitudinal wavelength and 5-6 K temperature contrasts at 5 mbar, showing a direct connection between moist convection and stratospheric wave activity. The subsidence and compressional heating of dry, unsaturated air warmed the troposphere (particularly to the northwest of the central branch of the revival) and removed the aerosols that had been responsible for the fade. Dark, cloud-free lanes west of the plumes were the first to show the colour change, and elongated due to the zonal windshear to form the characteristic ‘S-shape’ of the revival complex. The aerosol-free air was redistributed and mixed throughout the SEB by the zonal flow, following a westward-moving southern branch and an eastward-moving northern branch that revived the brown colouration over ∼200 days. The transition from the cool conditions of the SEBZ during the fade to the revived SEB caused a 2–4 K rise in 500-mbar temperatures (leaving a particularly warm southern SEB) and a reduction of aerosol opacity by factors of 2–3. Newly-cleared gaps in the upper tropospheric aerosol layer appeared different in filters sensing the ∼700-mbar cloud deck and the 2–3 bar cloud deck, suggesting complex vertical structure in the downdrafts. The last stage of the revival was the re-establishment of normal convective activity northwest of the GRS in September 2011, ∼840 days after the last occurrence in June 2009. Moist convection may therefore play an important role in controlling the timescale and atmospheric variability during the SEB life cycle.

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.

Two-dimensional radiative transfer for the retrieval of limb emission measurements in the martian atmosphere

The remote sounding of infrared emission from planetary atmospheres using limb-viewing geometry is a powerful technique for deriving vertical profiles of structure and composition on a global scale. Compared with nadir viewing, limb geometry provides enhanced vertical resolution and greater sensitivity to atmospheric constituents. However, standard limb profile retrieval techniques assume spherical symmetry and are vulnerable to biases produced by horizontal gradients in atmospheric parameters. We present a scheme for the correction of horizontal gradients in profile retrievals from limb observations of the martian atmosphere. It characterizes horizontal gradients in temperature, pressure, and aerosol extinction along the line-of-sight of a limb view through neighboring measurements, and represents these gradients by means of two-dimensional radiative transfer in the forward model of the retrieval. The scheme is applied to limb emission measurements from the Mars Climate Sounder instrument on Mars Reconnaissance Orbiter. Retrieval simulations using data from numerical models indicate that biases of up to 10K in the winter polar region, obtained with standard retrievals using spherical symmetry, are reduced to about 2K in most locations by the retrieval with two-dimensional radiative transfer. Retrievals from Mars atmospheric measurements suggest that the two-dimensional radiative transfer greatly reduces biases in temperature and aerosol opacity caused by observational geometry, predominantly in the polar winter regions.

Mid-infrared mapping of Jupiter’s temperatures, aerosol opacity and chemical distributions with IRTF/TEXES

Global maps of Jupiter’s atmospheric temperatures, gaseous composition and aerosol opacity are derived from a programme of 5–20 µm mid-infrared spectroscopic observations using the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA’s Infrared Telescope Facility (IRTF). Image cubes from December 2014 in eight spectral channels, with spectral resolutions of R ∼2000−12,000 and spatial resolutions of 2–4° latitude, are inverted to generate 3D maps of tropospheric and stratospheric temperatures, 2D maps of upper tropospheric aerosols, phosphine and ammonia, and 2D maps of stratospheric ethane and acetylene. The results are compared to a re-analysis of Cassini Composite Infrared Spectrometer (CIRS) observations acquired during Cassini’s closest approach to Jupiter in December 2000, demonstrating that this new archive of ground-based mapping spectroscopy can match and surpass the quality of previous investigations, and will permit future studies of Jupiter’s evolving atmosphere. The visibility of cool zones and warm belts varies from channel to channel, suggesting complex vertical variations from the radiatively-controlled upper troposphere to the convective mid-troposphere. We identify mid-infrared signatures of Jupiter’s 5-µm hotspots via simultaneous M, N and Q-band observations, which are interpreted as temperature and ammonia variations in the northern Equatorial Zone and on the edge of the North Equatorial Belt (NEB). Equatorial plumes enriched in NH3 gas are located south-east of NH3-desiccated ‘hotspots’ on the edge of the NEB. Comparison of the hotspot locations in several channels across the 5–20 µm range indicate that these anomalous regions tilt westward with altitude. Aerosols and PH3 are both enriched at the equator but are not co-located with the NH3 plumes. The equatorial temperature minimum and PH3/aerosol maxima have varied in amplitude over time, possibly as a result of periodic equatorial brightenings and the fresh updrafts of disequilibrium material. Temperate mid-latitudes display a correlation between mid-IR aerosol opacity and the white albedo features in visible light (i.e., zones). We find hemispheric asymmetries in the distribution of tropospheric PH3, stratospheric hydrocarbons and the 2D wind field (estimated via the thermal-wind equation) that suggest a differing efficiency of mechanical forcing (e.g., vertical mixing and wave propagation) between the two hemispheres that we argue is driven by dynamics rather than Jupiter’s small seasonal cycle. Jupiter’s stratosphere is notably warmer at northern mid-latitudes than in the south in both 2000 and 2014, although the latter can be largely attributed to strong thermal wave activity near 30°N that dominates the 2014 stratospheric maps and may be responsible for elevated C2H2 in the northern hemisphere. A vertically-variable pattern of temperature and windshear minima and maxima associated with Jupiter’s Quasi Quadrennial Oscillation (QQO) is observed at the equator in both datasets, although the contrasts were more subdued in 2014. Large-scale equator-to-pole gradients in ethane and acetylene are superimposed on top of the mid-latitude mechanically-driven maxima, with C2H2 decreasing from equator to pole and C2H6 showing a polar enhancement, consistent with a radiatively-controlled circulation from low to high latitudes. Cold polar vortices beyond ∼60° latitude can be identified in the upper tropospheric and lower stratospheric temperature maps, suggesting enhanced radiative cooling from polar aerosols. Finally, compositional mapping of the Great Red Spot confirms the local enhancements in PH3 and aerosols, the north–south asymmetry in NH3 gas and the presence of a warm southern periphery that have been noted by previous authors.

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.

Initial results from radio occultation measurements with the Mars Reconnaissance Orbiter: A nocturnal mixed layer in the tropics and comparisons with polar profiles from the Mars Climate Sounder

The Mars Reconnaissance Orbiter (MRO) performs radio occultation (RO) measurements on selected orbits, generally once per day. We have retrieved atmospheric profiles from two subsets of data, yielding a variety of new results that illustrate the scientific value of the observations. One set of measurements sounded the tropics in northern summer at a local time ∼1h before sunrise. Some of these profiles contain an unexpected layer of neutral stability with a depth of ∼4km and a pressure at its upper boundary of ∼160Pa. The mixed layer is bounded above by a temperature inversion and below by another strong inversion adjacent to the surface. This type of structure is observed near Gale Crater, in the Tharsis region, and at a few other locations, whereas profiles in Amazonis Planitia and Elysium Planitia show no sign of a detached mixed layer with an overlying inversion. We supplemented the measurements with numerical simulations by the NASA Ames Mars General Circulation Model, which demonstrate that water ice clouds can generate this distinctive type of temperature structure through their influence on radiative transfer at infrared wavelengths. In particular, the simulations predict the presence of a nocturnal cloud layer in the Tharsis region at a pressure of ∼150Pa (∼10km above the surface), and the nighttime radiative cooling at cloud level is sufficient to produce a temperature inversion above the cloud as well as convective instability below the cloud, consistent with the observations. The second set of measurements sounded mid-to-high northern latitudes in spring, when carefully coordinated observations by the MRO Mars Climate Sounder (MCS) are also available. The differences between the RO and MCS temperature profiles are generally consistent with the expected performance of the two instruments. Within this set of 21 comparisons the average temperature difference is less than 1K where the aerosol opacities are smaller than 10-3km-1, at pressures of 10–50Pa, whereas it increases to ∼2K where the aerosol opacities exceed this threshold, at pressures of 50–300Pa. The standard deviation of the temperature difference is ∼2K, independent of pressure. The second set of RO measurements also provides unique information about the stability of the annual CO2 cycle and the dynamics near the edge of the seasonal CO2 ice cap.

Temperatures and aerosol opacities of the Mars atmosphere at aphelion: Validation and inter-comparison of limb sounding profiles from MRO/MCS and MGS/TES

We exploit the relative stability and repeatability of the Mars atmosphere at aphelion for an inter-comparison of Mars Global Surveyor/Thermal Emission Spectrometer (MGS/TES) and Mars Reconnaissance Orbiter/Mars Climate Sounder (MRO/MCS) nighttime temperature profiles and aerosol opacity profiles in Mars years 25, 26, 29, 30, and 31. Cross-calibration of these datasets is important, as they together provide an extended climatology for this planetary atmosphere. As a standard of comparison we employ temperature profiles obtained by radio occultation methods during the MGS mission in Mars years 24, 25, and 26. We first compare both zonal mean TES limb sounding profiles and zonal mean MCS limb sounding profiles with zonal means of radio occultation temperature profiles for the same season (Ls=70–80°) and latitudes (55–70°N). We employ a statistical z test for quantifying the degree of agreement of temperature profiles by pressure level. For pressures less than 610Pa (altitudes>3km), the ensemble mean temperature difference between the radio occultation and TES limb sounding profiles found in these comparisons was 1.7±0.7K. The ensemble mean temperature difference between radio occultation and MCS profiles was 1.4±1.0K. These differences fall within the formal error estimates for both TES and MCS, validating the accuracy of the instruments and their respective retrieval algorithms. In the second phase of our investigation, we compare aphelion season zonal mean TES limb sounding temperature, water ice opacity, and dust opacity profiles with those obtained at the same latitudes in different years by MCS. The ensemble mean temperature difference found for three comparisons between TES and MCS zonal mean temperature profiles was 2.8±2.1K. MCS and TES temperatures between 610Pa and 5Pa from 55 to 70°N are largely in agreement (with differences<2K) when water ice aerosol opacities are comparable. Temperature differences increase when the opacities are dissimilar; TES profiles exhibit colder temperatures when TES water ice opacities are greater than those observed by MCS. Our comparisons reveal a possible systematic offset of TES and MCS temperatures at the highest altitudes resolved in the TES retrievals; TES temperatures are consistently colder than the corresponding MCS temperatures at pressures⩽1Pa (altitudes⩾58km). We otherwise find no evidence of systematic bias between TES limb sounding and MCS retrieved atmospheric quantities between 610Pa and 1Pa. Inter-annual variability is noted in comparisons of latitudinal temperature gradients from 55 to 70°N, in the amplitude of inversions linked with thermal tides in the middle atmosphere, and in the abundance and vertical distribution of water ice aerosols from 55 to 70°N during the aphelion season.

High‐altitude dust layers on Mars: Observations with the Thermal Emission Spectrometer

AbstractLimb‐scanning observations of Martian atmospheric dust with the Thermal Emission Spectrometer (TES) over 3 Mars years indicate two distinct altitude layers with persistent maxima in the dust mixing ratio vertical profile. The first, lower maximum in the dust distribution profile is the “high‐altitude tropical dust maximum” (HATDM) centered at 20–30 km, previously detected by the Mars Climate Sounder (MCS). Through the observation period, the HATDM followed a repeatable seasonal cycle with a brief absence in early northern spring and reached its highest altitudes and largest amplitude during the dust storm season in southern spring and summer. The HATDM is likely maintained during the day by a combination of convective and topographic updrafts and then degraded at night by scavenging from water ice clouds. The second, upper maximum in the dust distribution profile, which we refer, for convenience, to as the upper dust maximum (UDM), is centered at 45–65 km and is only detected in daytime observations. We see additional evidence of its presence in the limited number of MCS aerosol opacity retrievals available at these altitudes. Comparable dust mixing ratios are nearly absent from this altitude range at night. This upper maximum is generally a northern hemisphere phenomenon, peaking in amplitude in northern summer and nearly absent from the TES observational domain during the dust storm season. We suggest topographic updrafts over Martian volcanoes, small particle size, diurnal transport associated with thermal tides, and scavenging by water ice as probable key factors in the creation of the UDM.

Global maps of anhydrous minerals at the surface of Mars from OMEGA/MEx

We here reassess the global distribution of several key mineral species using the entire OMEGA/Mars Express VIS‐NIR imaging spectrometer data set, acquired from orbit insertion in January 2004 to August 2010. Thirty‐two pixels per degree global maps of ferric oxides, pyroxenes and olivines have been derived. A significant filtering process was applied in order to exclude data acquired with unfavorable observation geometries or partial surface coverage with water and CO2 frosts. Because of strong atmospheric variations over the 3.6 Martian years of observations primarily due to the interannual variability of the aerosol opacity, a new filter based on the atmospheric dust opacity calibrated by the Mars Exploration Rovers measurements has also been implemented. The Fe3+absorption features are present everywhere on the surface, with a variety of intensities indicating distinct formation processes. The pyroxene‐bearing regions are localized in low albedo regions, while the bright regions are spectrally comparable to anhydrous nanophase ferric oxides. The expanded data set increases by a factor of about 2, the number of olivine detections reported in previous OMEGA‐based studies. Olivine is mainly detected in three types of areas over the Martian surface: discontinuous patches on the terraces of the three main basins; smooth inter‐crater plains and smooth crater floors throughout the southern highlands; and crater sand dunes, crater ejectas and extended bedrock exposures in the northern plains. Olivine is also detected in the low albedo pyroxene‐bearing dunes surrounding the northern polar cap.

A comparison of different optimization algorithms for retrieving aerosol optical depths from satellite data: an example of using a dual-angle algorithm

Optimization techniques are often used in remote sensing retrieval of surface or atmospheric parameters. Nevertheless, different algorithms may exhibit different performances for the same optimization problem. Comparison of some classic optimization approaches in this article aims to select the best method for retrieving aerosol opacity, or even for other parameters, from remotely sensed data. Eight frequently used optimization algorithms were evaluated using both simulated data and actual AATSR (advanced along track scanning radiometer) data. Several typical land cover types and aerosol opacity levels were also considered in the simulations to make the tests more representative. It was observed that the absolute error in retrieval would rise after a certain number of iterations due to the round-off error, and the algorithms showed different performances in the inversions without any a priori knowledge. When combined with reasonable a priori knowledge, the selection of various algorithms only slightly affected the retrieval accuracy. Given a summary of all the comparison tests, a special class named ‘trust-region methods’ (TR) was demonstrated to be the optimal choice in general cases. In contrast, some widely used optimization methods in aerosol research, for example, the Levenberg–Marquardt (LM) algorithm, seemed not to display a persuasive performance.

Jovian temperature and cloud variability during the 2009–2010 fade of the South Equatorial Belt

Mid-infrared 7–20 μm imaging of Jupiter from ESO’s Very Large Telescope (VLT/VISIR) demonstrate that the increased albedo of Jupiter’s South Equatorial Belt (SEB) during the ‘fade’ (whitening) event of 2009–2010 was correlated with changes to atmospheric temperature and aerosol opacity. The opacity of the tropospheric condensation cloud deck at pressures less than 800 mbar increased by 80% between May 2008 and July 2010, making the SEB (7–17°S) as opaque in the thermal infrared as the adjacent equatorial zone. After the cessation of discrete convective activity within the SEB in May 2009, a cool band of high aerosol opacity (the SEB zone at 11–15°S) was observed separating the cloud-free northern and southern SEB components. The cooling of the SEBZ (with peak-to-peak contrasts of 1.0 ± 0.5 K), as well as the increased aerosol opacity at 4.8 and 8.6 μm, preceded the visible whitening of the belt by several months. A chain of five warm, cloud-free ‘brown barges’ (subsiding airmasses) were observed regularly in the SEB between June 2009 and June 2010, by which time they too had been obscured by the enhanced aerosol opacity of the SEB, although the underlying warm circulation was still present in July 2010. Upper tropospheric temperatures (150–300 mbar) remained largely unchanged during the fade, but the cool SEBZ formation was detected at deeper levels ( p > 300 mbar) within the convectively-unstable region of the troposphere. The SEBZ formation caused the meridional temperature gradient of the SEB to decrease between 2008 and 2010, reducing the vertical thermal windshear on the zonal jets bounding the SEB. The southern SEB had fully faded by July 2010 and was characterised by short-wave undulations at 19–20°S. The northern SEB persisted as a narrow grey lane of cloud-free conditions throughout the fade process. The cool temperatures and enhanced aerosol opacity of the SEBZ after July 2009 are consistent with an upward flux of volatiles (e.g., ammonia-laden air) and enhanced condensation, obscuring the blue-absorbing chromophore and whitening the SEB by April 2010. These changes occurred within cloud decks in the convective troposphere, and not in the radiatively-controlled upper troposphere. NH 3 ice coatings on aerosols at p < 800 mbar are plausible sources of the suppressed 4.8 and 8.6-μm emission, although differences in the spatial distribution of opacity at these two wavelengths suggest that enhanced attenuation by a deeper cloud ( p > 800 mbar) also occurred during the fade. Revival of the dark SEB coloration in the coming months will ultimately require sublimation of these ices by subsidence and warming of volatile-depleted air.

Modification of Jupiter’s stratosphere three weeks after the 2009 impact

Infrared spectroscopy sensitive to thermal emission from Jupiter’s stratosphere reveals effects persisting 23 days after the impact of a body in late July 2009. Measurements obtained on 2009 August 11 UT at the impact latitude of 56°S (planetocentric), using the Goddard Heterodyne Instrument for Planetary Wind and Composition mounted on the NASA Infrared Telescope Facility, reveal increased ethane abundance and the effects of aerosol opacity. An interval of reduced thermal continuum emission at 11.744 μm is measured ∼60–80° towards planetary east of the impact site, estimated to be at 305° longitude (System III). Retrieved stratospheric ethane mole fraction in the near vicinity of the impact site is enhanced by up to ∼60% relative to quiescent regions at this latitude. Thermal continuum emission at the impact site, and somewhat west of it, is significantly enhanced in the same spectra that retrieve enhanced ethane mole fraction. Assuming that the enhanced continuum brightness near the impact site results from thermalized aerosol debris blocking contribution from the continuum formed in the upper troposphere and indicating the local temperature, then continuum emission by a haze layer can be approximated by an opaque surface inserted at the 45–60 mbar pressure level in the stratosphere in an unperturbed thermal profile, setting an upper limit on the pressure and therefore a lower limit on the altitude of the top of the impact debris at this time. The reduced continuum brightness east of the impact site can be modeled by an opaque surface near the cold tropopause, which is consistent with a lower altitude of ejecta/impactor-formed opacity. The physical extent of the observed region of reduced continuum implies a minimum average velocity of 21 m/s transporting material prograde (planetary east) from the impact.

Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: Vertical and spectral characteristics from Cassini CIRS

Vertical distributions and spectral characteristics of Titan’s photochemical aerosol and stratospheric ices are determined between 20 and 560 cm −1 (500–18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined. In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ∼3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC 3N), as inferred from a composite emission feature at ∼160 cm −1, appear to be located over a narrow altitude range in the stratosphere centered at ∼90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S. There is some evidence of a second ice cloud layer at ∼60 km altitude at 58°S associated with an emission feature at ∼80 cm −1. We speculate that the identify of this cloud may be due to C 2H 6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH 4) in the stratosphere of Titan. Unlike the highly restricted range of altitudes (50–100 km) associated with organic condensate clouds, Titan’s photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3–2.4 at about 160 km to 1.1–1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ∼140 cm −1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics. Finally, volume extinction coefficients N χ E derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios N χ E (1.583 μm)/N χ E (62.5 μm) of roughly 70 and 20, respectively, for Titan’s aerosol and stratospheric ices. The inferred particle cross-section ratios χ E (1.583 μm)/ χ E (62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.

Thermal structure and composition of Jupiter’s Great Red Spot from high-resolution thermal imaging

Thermal-IR imaging from space-borne and ground-based observatories was used to investigate the temperature, composition and aerosol structure of Jupiter’s Great Red Spot (GRS) and its temporal variability between 1995 and 2008. An elliptical warm core, extending over 8° of longitude and 3° of latitude, was observed within the cold anticyclonic vortex at 21°S. The warm airmass is co-located with the deepest red coloration of the GRS interior. The maximum contrast between the core and the coldest regions of the GRS was 3.0–3.5 K in the north–south direction at 400 mbar atmospheric pressure, although the warmer temperatures are present throughout the 150–500 mbar range. The resulting thermal gradients cause counter-rotating flow in the GRS center to decay with altitude into the lower stratosphere. The elliptical warm airmass was too small to be observed in IRTF imaging prior to 2006, but was present throughout the 2006–2008 period in VLT, Subaru and Gemini imaging. Spatially-resolved maps of mid-IR tropospheric aerosol opacity revealed a well-defined lane of depleted aerosols around the GRS periphery, and a correlation with visibly-dark jovian clouds and bright 4.8-μm emission. Ammonia showed a similar but broader ring of depletion encircling the GRS. This narrow lane of subsidence keeps red aerosols physically separate from white aerosols external to the GRS. The visibility of the 4.8-μm bright periphery varies with the mid-IR aerosol opacity of the upper troposphere. Compositional maps of ammonia, phosphine and para-H 2 within the GRS interior all exhibit north–south asymmetries, with evidence for higher concentrations north of the warm central core and the strongest depletions in a symmetric arc near the southern periphery. Small-scale enhancements in temperature, NH 3 and aerosol opacity associated with localized convection are observed within the generally-warm and aerosol-free South Equatorial Belt (SEB) northwest of the GRS. The extent of 4.8-μm emission from the SEB varied as a part of the 2007 ‘global upheaval,’ though changes during this period were restricted to pressures greater than 500 mbar. Finally, a region of enhanced temperatures extended southwest of the GRS during the survey, restricted to the 100–400 mbar range and with no counterpart in visible imaging or compositional mapping. The warm airmass was perturbed by frequent encounters with the cold airmass of Oval BA, but no internal thermal or compositional effects were noted in either vortex during the close encounters.

The impact of Martian aerosols on the retrieval of temperature profiles from PFS measurements

Our purpose was a qualitative assessment of the impact of dust and water ice aerosols on the retrieved temperature profiles and the retrieval process itself in the Martian atmosphere. It aims to quantify the related uncertainties in the atmospheric temperature profiles derived from radiance measurements of the Planetary Fourier Spectrometer (PFS), currently operating on the Mars Express orbiter. In this study the effects of aerosol opacities on simulated data and retrieved temperature profiles were also investigated. From the analysis of the model atmosphere including dust and water ice with different size distributions it results that the dust component affects weighting functions and brightness temperatures less than water ice. A similar situation is also observed when different vertical distributions are considered. Unlike dust, water ice with different sizes of crystals evidently influences weighting functions and brightness temperatures. The impact of the considered water ice vertical distributions on brightness temperatures is noticeable only near 840 cm −1. Considering different dust opacities, the largest differences—5 K maximum—between retrieved temperature profiles were observed close to the surface, regardless assumptions on a size distribution or the refractive indices. Contrary to dust, the different sizes of water ice particles assumed during retrieval stronger affected the retrieved temperature profiles than water ice opacities. Moreover, the differences in the retrieved temperature profiles were amplified while wrong optical properties for dust as well as for water ice aerosol were assumed instead of the nominal case. This means that the wrong assumption can induce an additional source of the retrieval error and lead to unreasonable temperature profiles. In the cases of expected heavily loads water ice crystals, their size distribution in the Martian atmosphere should be known from other observations before the retrieval of the temperature profile is attempted. For the analyzed examples of real PFS measurements the impact of different dust vertical distributions on the retrieval of temperature profile is prominent only in layers close to the surface. However, these differences remain comparable with retrieval errors. All influences of dust on weighting functions, brightness temperatures and during retrieval can be neglected if the noise equivalent radiance (NER) of PFS is taken into account.

Phosphine on Jupiter and Saturn from Cassini/CIRS

The global distribution of phosphine (PH 3) on Jupiter and Saturn is derived using 2.5 cm −1 spectral resolution Cassini/CIRS observations. We extend the preliminary PH 3 analyses on the gas giants [Irwin, P.G.J., and 6 colleagues, 2004. Icarus 172, 37–49; Fletcher, L.N., and 9 colleagues, 2007a. Icarus 188, 72–88] by (a) incorporating a wider range of Cassini/CIRS datasets and by considering a broader spectral range; (b) direct incorporation of thermal infrared opacities due to tropospheric aerosols and (c) using a common retrieval algorithm and spectroscopic line database to allow direct comparison between these two gas giants. The results suggest striking similarities between the tropospheric dynamics in the 100–1000 mbar regions of the giant planets: both demonstrate enhanced PH 3 at the equator, depletion over neighbouring equatorial belts and mid-latitude belt/zone structures. Saturn's polar PH 3 shows depletion within the hot cyclonic polar vortices. Jovian aerosol distributions are consistent with previous independent studies, and on Saturn we demonstrate that CIRS spectra are most consistent with a haze in the 100–400 mbar range with a mean optical depth of 0.1 at 10 μm. Unlike Jupiter, Saturn's tropospheric haze shows a hemispherical asymmetry, being more opaque in the southern summer hemisphere than in the north. Thermal-IR haze opacity is not enhanced at Saturn's equator as it is on Jupiter. Small-scale perturbations to the mean PH 3 abundance are discussed both in terms of a model of meridional overturning and parameterisation as eddy mixing. The large-scale structure of the PH 3 distributions is likely to be related to changes in the photochemical lifetimes and the shielding due to aerosol opacities. On Saturn, the enhanced summer opacity results in shielding and extended photochemical lifetimes for PH 3, permitting elevated PH 3 levels over Saturn's summer hemisphere.

The haze and methane distributions on Uranus from HST-STIS spectroscopy

We analyzed a unique, three-dimensional data set of Uranus acquired with the STIS Hubble spectrograph on August 19, 2002. The data covered a full afternoon hemisphere at 0.1 arc-sec spatial resolution between 300 and 1000 nm wavelength at 1 nm resolution. Navigation was accurate to 0.002 arc-sec and 0.02 nm. We tested our calibration with WFPC2 images of Uranus and found good agreement. We constrained the vertical aerosol structure with radiative transfer calculations. The standard types of models for Uranus with condensation cloud layers did not fit our data as well as models with an extended haze layer. The dark albedo of Uranus at near-infrared methane windows could be explained by methane absorption alone using conservatively scattering aerosols. Ultraviolet absorption from small aerosols in the stratosphere was strongest at high southern latitudes. The uppermost troposphere was almost clear, but showed a remarkable narrow spike of opacity centered on the equator to 0.2° accuracy. This feature may have been related to influx from ring material. At lower altitudes, the feature was centered at 1–2° latitude, suggesting an equatorial circulation toward the north. Below the 1.2 bar level, the aerosol opacity increased some 100 fold. A comparison of methane and hydrogen absorptions contradicted the standard interpretation of methane band images, which assumes that the methane mixing ratio is independent of latitude and attributes reflectivity variations to variations in the aerosol opacity. The opposite was true for the main contrast between brighter high latitudes and darker low latitudes, probing the 1–3 bar region. The methane mixing ratio varied between 0.014 and 0.032 from high to low southern latitudes, while the aerosol opacity varied only moderately with latitude, except for an enhancement at −45° latitude and a decrease north of the equator. The latitudinal variation of methane had a similar shape as that of ammonia probed by microwave observations at deeper levels. The variability of methane challenges our understanding of Uranus and requires reconsideration of previous investigations based on a faulty assumption. Below the 2 bar level, the haze was thinning somewhat. Our global radiative transfer models with 1° latitude sampling fit the observed reflectivities to 2% rms. The observed spectra of two discrete clouds could be modeled by using the background model of the appropriate latitude and adding small amounts of additional opacity at levels near 1.2 bar (southern cloud) and levels as high as 0.1 bar (northern cloud). These clouds may have been methane condensation clouds of low optical depth (∼0.2).

Solar infrared occultation observations by SPICAM experiment on Mars-Express: Simultaneous measurements of the vertical distributions of H 2O, CO 2 and aerosol

The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1–1.7 μm with a spectral resolution of ∼3.5 cm −1. We report results from 24 orbits obtained during MY28 at L s 130°–160°, and the latitude range of 40°–55° N. For these orbits the atmospheric density from 1.43 μm CO 2 band, water vapor mixing ratio based on 1.38 μm absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H 2O ice allows to retrieve particle size ( r eff ∼ 0.5 – 1 μm ) and number density (∼1 cm −3 at 15–30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320°–50° E, where high-altitude clouds at 50–60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius r eff = 0.1 – 0.3 μm , number density N ∼ 10 cm − 3 ) distributed with variance ν eff = 0.1 – 0.2 μm . The vertical optical depth of the clouds is below 0.001 at 1 μm. The atmospheric density profiles are retrieved from CO 2 band in the altitude range of 10–90 km, and H 2O mixing ratio is determined at 15–50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H 2O mixing ratio indicates ∼5 K warmer atmosphere at 25–45 km than predicted by models.