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Introduction Titan is the only moon in the solar system with a thick atmosphere, dominated by nitrogen and organic compounds and methane- and ethane-based climatic cycles similar to the hydrological cycle on Earth. Hence, Titan is a prime target for planetary and astrobiological researches. Heaviest organic materials resulting from atmospheric chemistry (including high atomic number aerosols) precipitate onto the surface and are subject to geological processes (e.g., eolian and fluvial erosion) that lead to the formation of a variety of landscapes, including dune fields, river networks, mountains, labyrinth terrains, canyons, lakes and seas analogous to their terrestrial counterparts but in an exotic context. Its optically thick atmosphere, however, prevents the surface from being probed in the entirety of the near-infrared (NIR) range, and its composition is still largely unknown, or largely debated at the least, preventing to fully understand and quantify the geological processes at play. Incident and reflected solar radiations are indeed strongly affected by gaseous absorption and aerosol scattering in the NIR. Only where the methane absorption is the weakest, a few transmission windows allow the detection of radiation coming from the low atmosphere and the surface, making possible to retrieve the surface albedo. In the 0.88-5.11 μm range (VIMS-IR channel), the Visual and Infrared Mapping Spectrometer (VIMS) instrument on board the Cassini spacecraft has shown that the surface can be observed in eight narrow transmission windows centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.69, and 2.78 μm, and in the 5.0-5.11 μm interval. Even in these transmission windows, residual gaseous absorption and increasing scattering from aerosols with decreasing wavelength make the analysis of the surface signal and the retrieving of surface albedo complex and delicate. In order to retrieve the surface albedo in the atmospheric windows in the most possible rigorous way, we have developed a radiative transfer (RT) model with up-to-date gaseous abundances profiles and absorption coefficients and improved photochemical aerosol optical properties. We validated our model using in situ observations of Huygens-DISR (Descent Imager / Spectral Radiometer) acquired during descent and once landed. We then applied our RT model to the Selk crater area (the Dragonfly mission landing area) in order to map the surface albedo and discuss the surface properties of the different geomorphological units of the region. Radiative transfer Our RT model is based on the SHDOM solver to solve the RT equations using the plan-parallel approximation. Vertical abundance profiles and absorption lines of CH4 and isotopes, CO, C2H2 and HCN are implemented using the most recent studies. Correlated-k coefficients are used to calculate gases absorption coefficients at VIMS-IR spectral sampling and resolution. Aerosols extinction profile and single scattering albedo are described using a fractal code developed by [1], allowing the aerosol fractal dimension to be varied. Aerosols phase function is modified using a multi-angular VIMS sequence (Sébastien Rodriguez, personal communication). Our model is validated using the in situ observations of Huygens-DISR acquired during the complete descent sequence and once landed. Application We applied our RT model to the Selk crater region by inverting aerosol opacity and surface albedo over 4 VIMS cubes (1578266417_1, 1575509158_1, 1578263500_1, 1578263152_1) acquired over the area. We built local maps of aerosol opacities and surface albedos of the Selk region by combining the 4 VIMS cubes on a geographically projected mosaic (see the mosaic of the 4 raw VIMS observations in Fig. 1). A few longitudinal profiles of the retrieved atmospheric properties are shown in Fig. 2. Slopes and seams between cubes of the aerosol opacities, originally due to varying observation geometries between flybys, have been entirely corrected, confirming the robustness of our RT model and making the retrieved surface albedo more reliable. Retrieved surface albedo have been then corrected for the photometry using in-situ observations ([3]). The resulting albedo maps of the regions are highly contrasted and homogeneous, most of the seams between cubes (due to residual surface photometry) being corrected (Fig. 3). Conclusion We developed and validated a new RT model for Cassini-VIMS observations of Titan with up-to-date atmospheric optical description. Coupled with an efficient inversion scheme, our model can be apply to the complete VIMS dataset for the retrieval of Titan’s atmospheric opacities and surface albedos at regional and global scales. References [1] Rannou, P., McKay, C., & Lorenz, R. 2003, Planetary and Space Science, 51, 963 [2] Karkoschka, E., Schröder, S. E., Tomasko, M. G., & Keller, H. U. 2012, Planetary and Space Science, 60, 342

Uranus’ atmosphere, once thought to be bland and static, has, in recent years, been shown to be anything but that. Radiative transfer retrieval analysis of high-resolution telescope observations has uncovered a dynamic atmosphere, displaying seasonal change and latitudinal variability. Uranus’ atmosphere is enshrouded in a global cloud/haze, meaning a robust aerosol layer model is required to probe any variability observed in its discrete features. One such example is its north polar hood, a bright ‘cap-like’ feature enshrouding the polar region northwards of ~45°N latitude (Fig. 1). Figure 1: A false colour HST/WFC3 image of Uranus taken in 2018 displaying the north polar hood at the top right of the disc. However, using remotely-sensed observations leads to a highly degenerate problem, resulting in competing aerosol models. Here we employ one such holistic aerosol model, derived by Prof. Patrick Irwin, in combination with the NEMESIS radiative transfer retrieval code. We utilise both space-based and ground-based observations to analyse the development of this hood over time, using the Minnaert approximation (Eqn. 1) to carry out a limb-darkening analysis of our observations to provide further constraint on our retrievals (demonstrated by Irwin et al., 2021). I/F = (I/F)0μ0kµk-1 (1) We demonstrate latitudinal variability in the methane volume mixing ratio via retrievals on HST/STIS and VLT/MUSE data. We then provide definitive evidence that a change in aerosol layers is a direct cause of brightening observed in the hood over time, and we display retrieval results on HST/WFC3 data spanning 2014 - 2021 to reveal what we find this change to be. This change is currently hypothesised as an increase in opacity of the middle (~1 - 2 bar) haze layer in the holistic model. These results strengthen the case for the holistic aerosol model and provide important context for the upcoming orbiter-probe mission to Uranus. Further scrutiny of this holistic aerosol model by employing it to the modelling of other discrete features will be valuable future work.

NASA’s Dragonfly mission will send a rotorcraft lander to the surface of Titan in the mid-2030s. Dragonfly's science themes include investigation of Titan’s prebiotic chemistry, habitability, and potential chemical biosignatures from both water-based “life as we know it” (as might occur in the interior mantle ocean, potential cryovolcanic flows, and/or impact melt deposits) and potential “life, but not as we know it” that might use liquid hydrocarbons as a solvent (within Titan’s lakes, seas, and/or aquifers). Consideration of both of these solvents simultaneously led to our initial landing site in Titan’s equatorial dunes and interdunes to sample organic sediments and water ice, respectively. Ultimately, Dragonfly's traverse target is the 80 km diameter Selk Crater, at 7° N, where we seek previously liquid water that has mixed with surface organics. Our science goals include determining how far prebiotic chemistry has progressed on Titan and what molecules and elements might be available for such chemistry. We will also determine the role of Titan’s tropical deserts in the global methane cycle. We will investigate the processes and processing rates that modify Titan’s surface geology and constrain how and where organics and liquid water can mix on and within Titan. Importantly, we will search for chemical biosignatures indicative of past or extant biological processes. As such, Dragonfly, along with Perseverance, is the first NASA mission to explicitly incorporate the search for signs of life into its mission goals since the Viking landers in 1976.

International audience; A new digital terrain model of the Huygens landing 1 site on Saturn's largest moon • We create a new digital terrain model (DTM) of the Huygens landing site that 14 offers the best available resolution of river valleys on Titan. 15 • The complexity of the data set requires a tailor-made reconstruction procedure 16 that is detailed. 17 • The workflow uses reprocessed Huygens/DISR images and an automated shape-18 from-motion algorithm to improve an earlier DTM. Abstract 20 River valleys have been observed on Titan at all latitudes by the Cassini-Huygens 21 mission. Just like water on Earth, liquid methane carves into the substrate to form a com-22 plex network of rivers, particularly stunning in the images acquired near the equator by 23 the Huygens probe. To better understand the processes at work that form these land-24 scapes, one needs an accurate digital terrain model (DTM) of this region. The first and 25 to date the only existing DTM of the Huygens landing site was produced by the United 26 States Geological Survey (USGS) from high-resolution images acquired by the DISR (De-27 scent Imager/Spectral Radiometer) cameras onboard the Huygens probe and using the 28 SOCET SET photogrammetric software. However, this DTM displays inconsistencies, 29 primarily due to non optimal viewing geometries and to the poor quality of the origi-30 nal data, unsuitable for photogrammetric reconstruction. We investigate a new approach, 31 benefiting from a recent reprocessing of the DISR images correcting both the radiomet-32 ric and geometric distortions. For the DTM reconstruction, we use MicMac, a photogram-33 metry software based on automatic open-source shape-from-motion algorithms. To over-34 come challenges such as data quality and image complexity (unusual geometric config-35 uration), we developed a specific pipeline that we detailed and documented in this ar-36 ticle. In particular, we take advantage of geomorphic considerations to assess ambigu-37 ity on the internal calibration and the global orientation of the stereo model. Besides the 38 novelty in this approach, the resulting DTM obtained offers the best spatial sampling 39 of Titan's surface available and a significant improvement over the previous results. 40

HST/STIS observations of Uranus in 2015 show that the depletion of upper tropospheric methane has been relatively stable and that the polar region has been brightening over time as a result of increased aerosol scattering. This interpretation is confirmed by near-IR imaging from HST and from the Keck telescope using NIRC2 adaptive optics imaging. Our analysis of the 2015 spectra, as well as prior spectra from 2012, shows that there is a factor of three decrease in the effective upper tropospheric methane mixing ratio between 30\deg N and 70\deg N. The absolute value of the deep methane mixing ratio, which probably does not vary with latitude, is lower than our previous estimate, and depends significantly on the style of aerosol model that we assume, ranging from a high of 3.5$\pm$0.5% for conservative non-spherical particles with a simple Henyey-Greenstein phase function to a low of 2.7%$\pm$0.3% for conservative spherical particles. Our previous higher estimate of 4$\pm$0.5% was a result of a forced consistency with occultation results of Lindal et al. (1987, JGR 92, 14987-15001). That requirement was abandoned in our new analysis because new work by Orton et al. (2014, Icarus 243, 494-513) and by Lellouch et al. (2015, Astron. & AstroPhys. 579, A121) called into question the occultation results. For the main cloud layer in our models we found that both large and small particle solutions are possible for spherical particle models. At low latitudes the small-particle solution has a mean particle radius near 0.3 $\mu$m, a real refractive index near 1.65, and a total column mass of 0.03 mg/cm$^2$, while the large-particle solution has a particle radius near 1.5 $\mu$m, a real index near 1.24, and a total column mass 30 times larger. The pressure boundaries of the main cloud layer are between about 1.1 and 3 bars, within which H$_2$S is the most plausible condensable., Comment: There are 55 pages, 36 figures, 13 tables, and supplemental information. Version 2 corrects labeling errors in two figures (25L and 34), corrects an error in the referenced index of refraction of H2S, corrects associated comparisons with fitted values in several sentences, and adds a new conclusion paragraph to better summarize the comparison

The factors contributing to the initial selection of a dune site near the Selk impact structure on Titan as the first landing site for the Dragonfly mission are described. These include arrival geometry and aerodynamic/aerothermodynamic considerations, illumination, and Earth visibility, as well as the likely presence of exposed deposits of water-rich material, potentially including materials where molten ice has interacted with organics. Cassini observations of Selk are summarized and interpreted: near-infrared reflectance and microwave emission data indicate water-rich materials in and around the crater. Radar topography data shows the rim of Selk to have slopes on multi-km scales reaching only ∼2° degrees, an order of magnitude shallower than early photoclinometric estimates.

Prompted by the detection of stratospheric cloud layers by Cassini’s Composite Infrared Spectrometer (CIRS; see Anderson, C.M., Samuelson, R.E. [2011]. Icarus 212, 762–778), we have re-examined the observations made by the Descent Imager/Spectral Radiometer (DISR) in the atmosphere of Titan together with two constraints from measurements made outside the atmosphere. No evidence of thin layers (

The DISR (Descent Image/Spectral Radiometer) imagers took about 300 images during the descent of the Huygens probe in Titan’s atmosphere. We combined them into a photometrically calibrated mosaic of Titan’s surface within 100 km of the Huygens landing site with a resolution ranging from 1 km at 100 km distance from the landing site to 50 cm near the landing site. We analyzed how the reflectivity of each location varies with the changing phase angle and emission angle during the descent. We found strong variations with the emission angle but no significant variation of the surface phase function. The latter is possibly obscured by the diffuse nature of illumination by Titan’s sky at visible wavelengths. We constructed a map of this emission angle dependence, which represents a measure of surface reflectivity and roughness of the terrain. Titan’s surface probed by the images shows various terrain types with a small correlation between reflectivity and roughness. We propose that variations in the emission angle dependence might be correlated with average tilt angles of the surface. We detected elevated terrain in the bright highland north of the landing site as published by Soderblom et al. (Soderblom, L.A. et al. [2007]. Planet. Space Sci. 55, 2015–2024), but their high ridges in the lakebed appear mostly flat in our analysis. We integrated the color information from the DISR spectrometers into our mosaic to create a color map. We also compared the features seen in maps of the reflectivity, the emission angle dependence, and color to features seen in Cassini RADAR images. Finally, we provide a refined version of the images from Huygens after landing down to a resolution of 2 mm.

During the descent of the Huygens probe in Titan’s atmosphere, the Descent Imager/Spectral Radiometer (DISR) acquired spectra of 3660 locations within 250 km of the landing site. Each spectrum consisted of 200 resolution elements between 480 and 960 nm wavelength. With the help of radiative transfer models, contributions from the atmosphere and surface were separated. In eight methane windows, the data were combined into a map of Titan’s surface reflectivity with 250 km diameter near the landing site. Principal component analysis revealed three significant components, a brightness component that is consistent with a mosaic based on DISR imaging of much higher spatial resolution, a spectral slope component, and a spectral curvature component. The brightness component has stronger contrasts at longer wavelengths, or brighter areas have a larger spectral slope, consistent with previous results (Keller et al. [2008]. Planet. Space Sci. 56, 728–752). The second component corresponds to small differences in spectral slopes that are not correlated with features seen before except for an area with unusual high spectral slope found by the same authors and confirmed here. Our map of the second component gives another important parameter in characterizing and understanding Titan’s surface. The third principal component is somewhat noisy and describes variation in the spectral curvature that have never seen before at similar wavelengths. These variations require processes to differentiate surface spectra. To extend this work to longer wavelengths, 62 spectra from 850 to 1600 nm wavelength were investigated too, although the much lower number of spatial resolution points revealed only two significant components in the principal component analysis. They correlate with the first two components found in the shorter wavelength data. We also compare our results with an observation by Cassini’s Visible Imager/Mapping Spectrometer (VIMS) that imaged part of our investigated area with 4096 spatial resolution elements. Both data sets are complementary. DISR data extend to about 1500 nm wavelength while most surface features are seen in the VIMS data beyond 1500 nm.

The altitude and zonal motion of the Huygens probe descending through Titan’s atmosphere was determined early under the assumption of no meridional motion (Bird et al. [2005]. Nature 438, 800–802). By comparing images taken during the descent, Karkoschka et al. (Karkoschka et al. [2007]. Planet. Space Sci. 55, 1895–1935) determined the meridional motion of Huygens, which was generally much smaller than its zonal motion. Here, we present a comprehensive geometrical analysis of all images taken during the descent that is four times more accurate than the previous study. The result is a meridional wind profile across Titan’s troposphere with northward winds by up to 0.4 m/s with an average of 0.1 m/s above 1 km altitude, and southward winds below, peaking at 0.9 m/s near 0.4 km altitude. The imaging data extend down to 0.22 km altitude, although additional information came from the horizontal impact speed near 0.8 m/s southward (Schroder et al. [2012]. Planet. Space Sci. 73, 327–340). There is a region between 5 and 8 km altitude with no significant meridional wind. In the stratosphere, the average meridional wind was 1.2 ± 1.5 m/s northward, and zero meridional motion is possible down to 15 km altitude. We present the difference between the zonal speeds of Huygens and the wind that was ignored in previous publications and amounts to up to 7 m/s. We determined the three rotational angles of Huygens for the times of each exposure that showed surface features. During 26 exposures, the swing speed of Huygens was fast enough to smear images. Inferred swing speeds were up to 20°/s during the calm phase of the descent, consistent with up to 40°/s swings reported before during the rough phase. The improved geometric calibration of images allowed identification of many features also seen in Cassini radar images. This comparison yields the location of the Huygens LandingSite as 192.34 ± 02° West and 10.47 ± 0.02° South.

Revised calibration and processing of 1600 images of Uranus by Voyager 2 revealed dozens of discrete features south of −45° latitude, where only a single feature was known from Voyager images and none has been seen since. Tracking of these features over five weeks defined the southern rotational profile of Uranus with high accuracy and no significant gap. The profile has kinks unlike previous profiles and is strongly asymmetric with respect to the northern profile by Sromovsky et al. (Sromovsky, L.A., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A. [2012]. Icarus 220, 694–712). The asymmetry is larger than that of all previous data on jovian planets. A spot that included the South Pole off-center rotated with a period of 12.24 h, 2 h outside the range of all previous observations of Uranus. The region between −68° and −59° latitude rotated almost like a solid body, with a shear that was about 30 times smaller than typical shears on Uranus. At lower latitudes, features were sheared into tightly wound spirals as Voyager watched. The zone at −84° latitude was exceptionally bland; reflectivity variations were only 18 ppm, consistent with a signal-to-noise ratio estimated at 55,000. The low noise was achieved by smoothing over dozens of pixels per image and averaging 1600 images. The presented data set in eight filters contains rich information about temporal evolution and spectral characteristics of features on Uranus that will be the basis for further analysis.

While landing on Titan, several instruments onboard Huygens acquired measurements that indicate the probe did not immediately come to rest. Detailed knowledge of the probe's motion can provide insight into the nature of Titan's surface. Combining accelerometer data from the Huygens Atmospheric Structure Instrument (HASI) and the Surface Science Package (SSP) with photometry data from the Descent Imager/Spectral Radiometer (DISR) we develop a quantitative model to describe motion of the probe, and its interaction with the surface. The most likely scenario is the following. Upon impact, Huygens created a 12 cm deep hole in the surface of Titan. It bounced back, out of the hole onto the flat surface, after which it commenced a 30-40 cm long slide in the southward direction. The slide ended with the probe out of balance, tilted in the direction of DISR by around 10 degrees. The probe then wobbled back and forth five times in the north-south direction, during which it probably encountered a 1-2 cm sized pebble. The SSP provides evidence for movement up to 10 s after impact. This scenario puts the following constraints on the physical properties of the surface. For the slide over the surface we determine a friction coefficient of 0.4. While this value is not necessarily representative for the surface itself due to the presence of protruding structures on the bottom of the probe, the dynamics appear to be consistent with a surface consistency of damp sand. Additionally, we find that spectral changes observed in the first four seconds after landing are consistent with a transient dust cloud, created by the impact of the turbulent wake behind the probe on the surface. The optical properties of the dust particles are consistent with those of Titan aerosols from Tomasko et al. (P&SS 56, 669). We suggest that the surface at the landing site was covered by a dust layer, possibly the 7 mm layer of..., 31 pages, 14 figures

From STIS observations of Uranus in 2012, we found that the methane volume mixing ratio declined from about 4% at low latitudes to about 2% at 60 deg N and beyond. This is similar to that found in the south polar regions in 2002, in spite of what appears to be strikingly different convective activity in the two regions. Keck and HST imaging observations close to equinox imply that the depletions were simultaneously present in 2007, suggesting they are persistent features. The depletions appear to be mainly restricted to the upper troposphere, with depth increasing poleward from about 30 deg N, reaching ~4 bars at 45 deg N and perhaps much deeper at 70 deg N. The latitudinal variations in degree and depth of the depletions are important constraints on models of meridional circulation. Our observations are qualitatively consistent with previously suggested circulation cells in which rising methane-rich gas at low latitudes is dried out by condensation and sedimentation of methane ice particles as the gas ascends to altitudes above the methane condensation level, then is transported to high latitudes, where it descends and brings down methane depleted gas. Since this cell would seem to inhibit formation of condensation clouds in regions where clouds are actually inferred from spectral modelling, it suggests that sparse localized convective events may be important in cloud formation. The small-scale latitudinal variations we found in the effective methane mixing ratio between 55 deg N and 82 deg N have significant inverse correlations with zonal mean latitudinal variations in cloud reflectivity in near-IR Keck images taken before and after the HST observations. If the CH4/H2 absorption ratio variations are interpreted as local variations in para fraction instead of methane mixing ratio, we find that downwelling correlates with reduced cloud reflectivity., Comment: 20 pages, 19 figures, 4 tables, on-line supplemental material available

We determined Titan's reflectivity spectrum near the Huygens' landing site from observations taken with the Descent Imager/Spectral Radiometer below 500 m altitude, in particular the downward-looking photometer and spectrometers. We distinguish signal coming from illumination by sunlight and the lamp onboard Huygens based on their different spectral signatures. For the sunlight data before landing, we find that spatial variations of Titan's reflectivity were only ∼0.8%, aside from the phase angle dependence, indicating that the probed area within ∼100 m of the landing site was very homogeneous. Only the very last spectrum taken before landing gave a 3% brighter reflectivity, which probably was caused by one bright cobble inside its footprint. The contrast of the cobble was higher at 900 nm wavelength than at 600 nm. For the data from lamp illumination, we confirm that the phase function of Titan's surface displays a strong opposition effect as found by Schroder and Keller (2009. Planetary and Space Science 57, 1963–1974). We extend the phase function to even smaller phase angles (0.02°), which are among the smallest phase angles observed in the solar system. We also confirm the reflectivity spectrum of the dark terrain near the Huygens' landing site between 900 and 1600 nm wavelength by Schroder and Keller (2008. Planetary and Space Science 56, 753–769), but extend the spectrum down to 435 nm wavelength. The reflectivity at zero phase angle peaks at 0.45±0.06 around 750 nm wavelength and drops down to roughly 0.2 at both spectral ends. Our reflectivity of 0.45 is much higher than all previously reported values because our observations probe lower phase angles than others. The spectrum is very smooth except for a known absorption feature longward of 1350 nm. We did not detect any significant variation of the spectral shape along the slit for exposures after landing, probing a 25×4 cm2 area. However, the recorded spectral shape was slightly different for exposures before and after landing. This difference is similar to the spectral differences seen on scales of kilometers (Keller et al., 2008. Planetary and Space Science 56, 728–752), indicating that most observations may probe spatially variable contributions from two basic materials, such as a dark soil partially covered by bright cobbles. We used the methane absorption features to constrain the methane mixing ratio near the surface to 5.0±0.3%, in agreement with the 4.92±0.24% value measured in situ by Niemann et al. (2005. Nature 438, 779–784), but smaller than their revised value of 5.65±0.18% (Niemann et al., 2010. Journal of Geophysical Research 115, E12006). Our results were made possible by an in depth review of the calibration of the spectroscopic and photometric data.

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.

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).

We use 11 data sets of methane transmission measurements within 0.4–5.5 μm wavelength to model the methane transmission for temperature and pressure conditions in the jovian planets. Eight data sets are based on published laboratory measurements. Another two data sets come from two spectrometers onboard the Huygens probe that measured methane absorption inside Titan’s atmosphere ( Tomasko et al., 2008b, PSS 56, 624 ), and we provide a refined analysis. The last data set is a set of new Jupiter images by the Hubble Space Telescope to measure atmospheric transmission with Ganymede as the light source. Below 1000 nm wavelength, our resulting methane absorption coefficients are generally close to those by Karkoschka (1998, Icarus 133, 134) , but we add descriptions of temperature and pressure dependence. One remaining inconsistency occurs between 882 and 902 nm wavelength where laboratory data predict larger absorptions in the jovian atmospheres than observed. We present possible explanations. Above 1000 nm, our analysis of the Huygens data confirms methane absorption coefficients by Irwin et al. (2006, Icarus 181, 309) at their laboratory temperatures. Huygens data also confirm Irwin’s model of extrapolation to Titan’s lower pressures. However, their model of extrapolation to Titan’s lower temperatures predicts absorption coefficients up to 100 times lower than measured by Huygens. For each of ∼3700 wavelengths, we present a temperature dependence that is consistent with all laboratory data and the Huygens data. Since the Huygens data probe similar temperatures as many observations of Saturn, Uranus, Neptune, and Titan, our methane model will allow more reliable radiative transfer models for their atmospheres.

After a preliminary analysis of the low-frequency data collected with the electric antenna of the Permittivity, Wave and Altimetry (PWA) experiment onboard the Huygens Probe that landed on Titan on 14 January, 2005, it was anticipated in a previous article [Béghin et al., 2007. A Schumann-like resonance on Titan driven by Saturn's magnetosphere possibly revealed by the Huygens Probe. Icarus, 191, 251–266] that the Extremely Low-Frequency (ELF) signal at around 36 Hz observed throughout the descent, might have been generated in the upper ionosphere of Titan, driven by a plasma instability mechanism associated with the co-rotating Kronian plasma flow. The involved process was proposed as the most likely source of a Schumann resonance in the moon's atmospheric cavity, the second eigenmode of which is actually found by models to occur at around 36 Hz. In this paper, we present a thorough analysis of this signal based upon the Huygens Probe attitude data deduced from the Descent Imager Spectral Radiometer (DISR), and relevant measurements obtained from the Radio Plasma Wave Science (RPWS) experiment and from the magnetometer (MAG) onboard Cassini orbiter during flybys of Titan. We have derived several coherent characteristics of the signal which confirm the validity of the mechanism initially proposed and provide new and significant insights about such a unique type of Schumann resonance in the solar system. Indeed, the 36 Hz signal contains all the characteristics of a polarized wave, with the measured electric field horizontal component modulated by the antenna rotation, and an altitude profile in agreement with a Longitudinal Section Electric (LSE) eigenmode of the atmospheric cavity. In contrast to Earth's conditions where the conventional Transverse Magnetic mode is considered, the LSE mode appears to be the only one complying with the observations and the unexpected peculiar conditions on Titan. These conditions are essentially the lack of any lightning activity that can be ascertained from Cassini observations, the presence of an ionized layer centered around 62 km altitude that was discovered by the PWA instrumentation, and the existence of a subsurface conducting boundary which is mandatory for trapping ELF waves. A simple theoretical model derived from our analysis places tentatively consequential constraints on the conductivity profile in the lower ionosphere. It is also consistent with the presence of a conductive water ocean below an icy crust some tens of kilometers thick. Fil: Béghin, C.. Universite d’Orleans; Francia Fil: Canu, P.. Universite de Versailles-Saint Quentin en Yvelines; Francia Fil: Karkoschka, E.. University of Arizona; Estados Unidos Fil: Sotin, C.. California Institute Of Technology; Estados Unidos Fil: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Kurth, W. S.. University of Iowa; Estados Unidos Fil: Berthelier, J. J.. Institut Pierre Simon Laplace; Francia Fil: Grard, R.. European Space Agency; Países Bajos Fil: Hamelin, M.. Institut Pierre Simon Laplace; Francia Fil: Schwingenschuh, K.. Austrian Academy of Sciences; Austria Fil: Simões, F.. Institut Pierre Simon Laplace; Francia

The Descent Imager/Spectral Radiometer (DISR) of the Huygens probe was in an excellent position to view aspects of rain as it descended through Titan's atmosphere. Rain may play an important part of the methane cycle on Titan, similar to the water cycle on Earth, but rain has only been indirectly inferred in previous studies. DISR detected two dark atmospheric layers at 11 and 21 km altitude, which can be explained by a local increase in aerosol size by about 5–10%. These size variations are far smaller than those in rain clouds, where droplets grow some 1000-fold. No image revealed a rainbow, which implies that the optical depth of raindrops was less than ∼ 0.0002 / km . This upper limit excludes rain and constrains drizzle to extremely small rates of less than 0.0001 mm/h. However, a constant drizzle of that rate over several years would clear the troposphere of aerosols faster than it can be replenished by stratospheric aerosols. Hence, either the average yearly drizzle rate near the equator was even less ( 0.1 mm / yr ), or the observed aerosols came from somewhere else. The implied dry environment is consistent with ground-based imaging showing a lack of low-latitude clouds during the years before the Huygens descent. Features imaged on Titan's surface after landing, which might be interpreted as raindrop splashes, were not real, except for one case. This feature was a dewdrop falling from the outermost baffle of the DISR instrument. It can be explained by warm, methane-moist air rising along the bottom of the probe and condensing onto the cold baffle.

The descent imager/spectral radiometer (DISR) instrument aboard the Huygens probe into the atmosphere of Titan measured the brightness of sunlight using a complement of spectrometers, photometers, and cameras that covered the spectral range from 350 to 1600 nm, looked both upward and downward, and made measurements at altitudes from 150 km to the surface. Measurements from the upward-looking visible and infrared spectrometers are described in Tomasko et al. [2008a. Measurements of methane absorption by the descent imager/spectral radiometer (DISR) during its descent through Titan's atmosphere. Planet. Space Sci., this volume]. Here, we very briefly review the measurements by the violet photometers, the downward-looking visible and infrared spectrometers, and the upward-looking solar aureole (SA) camera. Taken together, the DISR measurements constrain the vertical distribution and wavelength dependence of opacity, single-scattering albedo, and phase function of the aerosols in Titan's atmosphere. Comparison of the inferred aerosol properties with computations of scattering from fractal aggregate particles indicates the size and shape of the aerosols. We find that the aggregates require monomers of radius 0.05 μm or smaller and that the number of monomers in the loose aggregates is roughly 3000 above 60 km. The single-scattering albedo of the aerosols above 140 km altitude is similar to that predicted for some tholins measured in laboratory experiments, although we find that the single-scattering albedo of the aerosols increases with depth into the atmosphere between 140 and 80 km altitude, possibly due to condensation of other gases on the haze particles. The number density of aerosols is about 5/cm3 at 80 km altitude, and decreases with a scale height of 65 km to higher altitudes. The aerosol opacity above 80 km varies as the wavelength to the −2.34 power between 350 and 1600 nm. Between 80 and 30 km the cumulative aerosol opacity increases linearly with increasing depth in the atmosphere. The total aerosol opacity in this altitude range varies as the wavelength to the −1.41 power. The single-scattering phase function of the aerosols in this region is also consistent with the fractal particles found above 60 km. In the lower 30 km of the atmosphere, the wavelength dependence of the aerosol opacity varies as the wavelength to the −0.97 power, much less than at higher altitudes. This suggests that the aerosols here grow to still larger sizes, possibly by incorporation of methane into the aerosols. Here the cumulative opacity also increases linearly with depth, but at some wavelengths the rate is slightly different than above 30 km altitude. For purely fractal particles in the lowest few km, the intensity looking upward opposite to the azimuth of the sun decreases with increasing zenith angle faster than the observations in red light if the single-scattering albedo is assumed constant with altitude at these low altitudes. This discrepancy can be decreased if the single-scattering albedo decreases with altitude in this region. A possible explanation is that the brightest aerosols near 30 km altitude contain significant amounts of methane, and that the decreasing albedo at lower altitudes may reflect the evaporation of some of the methane as the aerosols fall into dryer layers of the atmosphere. An alternative explanation is that there may be spherical particles in the bottom few kilometers of the atmosphere.

International audience; New low-temperature methane absorption coefficients pertinent to the Titan environment are presented as derived from the Huygens DISR spectral measurements combined with the in-situ measurements of the methane gas abundance profile measured by the Huygens Gas Chromatograph/Mass Spectrometer (GCMS). The visible and near-infrared spectrometers of the descent imager/spectral radiometer (DISR) instrument on the Huygens probe looked upward and downward covering wavelengths from 480 to 1620 nm at altitudes from 150 km to the surface during the descent to Titan's surface. The measurements at continuum wavelengths were used to determine the vertical distribution, single-scattering albedos, and phase functions of the aerosols. The gas chromatograph/mass spectrometer (GCMS) instrument on the probe measured the methane mixing ratio throughout the descent. The DISR measurements are the first direct measurements of the absorbing properties of methane gas made in the atmosphere of Titan at the pathlengths, pressures, and temperatures that occur there. Here we use the DISR spectral measurements to determine the relative methane absorptions at different wavelengths along the path from the probe to the sun throughout the descent. These transmissions as functions of methane path length are fit by exponential sums and used in a haze radiative transfer model to compare the results to the spectra measured by DISR. We also compare the recent laboratory measurements of methane absorption at low temperatures [Irwin et al., 2006. Improved near-infrared methane band models and k-distribution parameters from 2000 to 9500 cm -1 and implications for interpretation of outer planet spectra. Icarus 181, 309-319] with the DISR measurements. We find that the strong bands formed at low pressures on Titan act as if they have roughly half the absorption predicted by the laboratory measurements, while the weak absorption regions absorb considerably more than suggested by some extrapolations of warm measurements to the cold Titan temperatures. We give factors as a function of wavelength that can be used with the published methane coefficients between 830 and 1620 nm to give agreement with the DISR measurements. We also give exponential sum coefficients for methane absorptions that fit the DISR observations. We find the DISR observations of the weaker methane bands shortward of 830 nm agree with the methane coefficients given by Karkoschka [1994. Spectrophotometry of the jovian planets and Titan at 300- to 1000-nm wavelength: the methane spectrum. Icarus 111, 174-192]. Finally, we discuss the implications of our results for computations of methane absorption in the atmospheres of the outer planets.

To coincide with the flyby of the Pluto-bound New Horizons probe, Jupiter was the target of intensive observation, starting in February 2007, from a battery of ground-based telescopes and the Hubble Space Telescope (HST). Weeks into the project, on 25 March, an intense disturbance developed in Jupiter's strongest jet at 23° North latitude, lasting to June 2007. This type of event is rare — the last ones were seen in 1990 and 1975. The onset of the disturbance was captured by the HST, and the development of two plumes was followed in unprecedented detail. The two plumes (bright white spots in the small infrared image on the cover) towered 30 km above the surrounding clouds. The nature of the power source for the jets that dominate the atmospheres of Jupiter and Saturn is a controversial matter, complicated by the interplay of local and planet-wide meteorological factors. The new observations are consistent with a wind extending deep into the atmosphere, well below the level reached by solar radiation. In the larger cover image, turbulence caused by the plumes can be seen in the band that is home to the jet. Observations and modelling of two plumes in Jupiter's atmosphere that erupted at the same latitude as the strongest jet (23° North) are reported. Based on dynamical modelling it is concluded that the data are consistent only with a wind that extends well below the level where solar radiation is deposited. The atmospheres of the gas giant planets (Jupiter and Saturn) contain jets that dominate the circulation at visible levels1,2. The power source for these jets (solar radiation, internal heat, or both) and their vertical structure below the upper cloud are major open questions in the atmospheric circulation and meteorology of giant planets1,2,3. Several observations1 and in situ measurements4 found intense winds at a depth of 24 bar, and have been interpreted as supporting an internal heat source. This issue remains controversial5, in part because of effects from the local meteorology6. Here we report observations and modelling of two plumes in Jupiter’s atmosphere that erupted at the same latitude as the strongest jet (23° N). The plumes reached a height of 30 km above the surrounding clouds, moved faster than any other feature (169 m s-1), and left in their wake a turbulent planetary-scale disturbance containing red aerosols. On the basis of dynamical modelling, we conclude that the data are consistent only with a wind that extends well below the level where solar radiation is deposited.

Titan's vast equatorial fields of RADAR-dark longitudinal dunes seen in Cassini RADAR synthetic aperture images correlate with one of two dark surface units discriminated as “brown” and “blue” in Visible and Infrared Mapping Spectrometer (VIMS) color composites of short-wavelength infrared spectral cubes (RGB as 2.0, 1.6, 1.3 μm). In such composites bluer materials exhibit higher reflectance at 1.3 μm and lower at 1.6 and 2.0 μm. The dark brown unit is highly correlated with the RADAR-dark dunes. The dark brown unit shows less evidence of water ice suggesting that the saltating grains of the dunes are largely composed of hydrocarbons and/or nitriles. In general, the bright units also show less evidence of absorption due to water ice and are inferred to consist of deposits of bright fine precipitating tholin aerosol dust. Some set of chemical/mechanical processes may be converting the bright fine-grained aerosol deposits into the dark saltating hydrocarbon and/or nitrile grains. Alternatively the dark dune materials may be derived from a different type of air aerosol photochemical product than are the bright materials. In our model, both the bright aerosol and dark hydrocarbon dune deposits mantle the VIMS dark blue water ice-rich substrate. We postulate that the bright mantles are effectively invisible (transparent) in RADAR synthetic aperture radar (SAR) images leading to lack of correlation in the RADAR images with optically bright mantling units. RADAR images mostly show only dark dunes and the water ice substrate that varies in roughness, fracturing, and porosity. If the rate of deposition of bright aerosol is 0.001–0.01 μm/yr, the surface would be coated (to optical instruments) in hundreds-to-thousands of years unless cleansing processes are active. The dark dunes must be mobile on this very short timescale to prevent the accumulation of bright coatings. Huygens landed in a region of the VIMS bright and dark blue materials and about 30 km south of the nearest occurrence of dunes visible in the RADAR SAR images. Fluvial/pluvial processes, every few centuries or millennia, must be cleansing the dark floors of the incised channels and scouring the dark plains at the Huygens landing site both imaged by Descent Imager/Spectral Radiometer (DISR).

The Descent Imager/Spectral Radiometer (DISR) provided 376 images during the descent to Titan and 224 images after landing. Images of the surface had scales between 150 m/pixel and 0.4 mm/pixel, all of which we assembled into a mosaic. The analysis of the surface and haze features in these images and of other data gave tight constraints on the geometry of the descent, particularly the trajectory, the tip and tilt, and the rotation of the Huygens probe. Huygens moved on average in the direction of 2ring operator north of east from 145 to 50 km altitude, turning to 5ring operator south of east between 30 and 20 km altitude, before turning back to east. At 6.5 km altitude, it reversed to WNW, before reversing back to SE at 0.7 km altitude. At first, Huygens was tilting slowly by up to 15ring operator as expected for a descent through layers of changing wind speeds. As the winds calmed, tilts decreased. Tilts were approximately retrieved throughout the main-parachute phase, but only for 160 specific times afterwards. Average swing rates were 5ring operator/s at high and low altitudes, but 13ring operator/s between 110 and 30 km altitude. Maximum swing rates were often above 40ring operator/s, far above the design limit of 6ring operator/s, but they caused problems only for a single component of DISR, the Sun Sensor. The excitation of such high swing rates on the stabilizer parachute is not fully understood. Before the parachute exchange, the rotational rate of Huygens smoothly approached the expected equilibrium value of 3 rotations per vertical kilometer, although clockwise instead of counterclockwise. Starting at 40 s after the parachute exchange until landing, Huygens rotated erratically. Long-term averages of the rotational rate varied between 2.0 and 4.5 rotations/km. On time scales shorter than a minute, some 100 strong rotational accelerations or decelerations created azimuthal irregularities of up to 180ring operator, which caused DISR to take most exposures at random azimuths instead of pre-selected azimuths. Nevertheless, we reconstructed the azimuths throughout the 360 rotations during the descent and for each of some 3500 DISR exposures with a typical accuracy near 2ring operator. Within seconds after landing, the parachute moved into the field of view of one of the spectrometers. The observed light curve indicated a motion of the parachute of 0.3 m/s toward the SSE. DISR images indicated that the probe did not penetrate into the surface, assuming a level ground. This impact of Huygens must have occurred on major rocks or some elevated area. The unexpected raised height increases ice-rock sizes by 40% with respect to estimations made in 2005 [Tomasko, M.G., Archinal, B., Becker, T., Bezard, B., Bushroe, M., Combes, M., Cook, D., Coustenis, A., de Bergh, C., Dafoe, L.E., Doose, L., Doute, S., Eibl, A., Engel, S., Gliem, F., Grieger, B., Holso, K., Howington-Kraus, E., Karkoschka, E., Keller, H.U., Kirk, R., Kramm, R., Kuppers, M., Lanagan, P., Lellouch, E., Lemmon, M., Lunine, J., McFarlane, E., Moores, J., Prout, G.M., Rizk, B., Rosiek, M., Rueffer, P., Schroder, S.E., Schmitt, B., See, C., Smith, P., Soderblom, L., Thomas, N., West, R., 2005. Rain, winds and haze during the Huygens probe's descent to Titan's surface. Nature 438, 765–778]. During the 70-min surface phase, the tilt of Huygens was 3ring operator, changing by a small fraction of a degree. The apparent horizon looking south to SSW from the landing site was 1–2ring operator above the theoretical horizon, sloping by 1ring operator up to the left (east). Our best guess puts the horizon as a 1–2 m high hill in 30–50 m distance. We detected the refraction from warm, rising air bubbles above our illuminated spot. Bright, elongated, cm-sized objects appear occasionally on the surface. If real, they could be rain drop splashes or fluffy particles blown across Titan's surface.

Jupiter's remaining White Oval changed color in late 2005 and became noticeably red in early 2006, as reported by amateur observers. We present wind and color analyses from high spatial resolution images taken with the Hubble Space Telescope Advanced Camera for Surveys in April 2006. These images suggest that the recent color change was tied to a strengthening of this storm, as implied by increased vorticity, causing it to become more like the Great Red Spot. From a historical perspective, the current activity may be consistent with the generation of new anticyclones at this latitude in the coming months and years.

We analyzed 134 images of Saturn taken by the Hubble Space Telescope between 1991 and 2004. The images cover wavelengths between 231 and 2370 nm in 30 filters. We combined some 10 million calibrated reflectivity measurements into 18,000 center-to-limb curves. We used the method of principal component analysis to find the main latitudinal and temporal variations in Saturn's atmosphere and their spectral characteristics. The first principal variation is a strong latitudinal variation of the aerosol optical depth in the upper troposphere. This structure shifts with Saturn's seasons, but the structure on small scales of latitude stays constant. The second principal variation is a variable optical depth of stratospheric aerosols. The optical depth is large at the poles and small at mid- and low latitudes with a steep gradient in-between. This structure remains essentially constant in time. The third principal variation is a variation in the tropospheric aerosol size, which has only shallow gradients with latitude, but large seasonal variations. Thus, aerosol sizes and their phase functions inferred at a particular season are not representative of Saturn's atmosphere at other seasons. Aerosols are largest in the summer and smallest in the winter. The fourth principal variation is a feature of the tropospheric aerosols with irregular latitudinal structure and fast variability, on the time scale of months. Spherical aerosols do not display the spectral characteristic of that feature. We suspect that variations in the shape of aerosols may play a role. We found a spectral feature of the imaginary index of aerosols, which darkens them near 400 nm wavelength. While we can describe Saturn's variations quite accurately, our presented model of Saturn's average atmosphere is still uncertain due to possible systematic offsets in methane absorption data and limitations of the knowledge about the shape of aerosols. In order to compare our results with those from comparable investigations, which used less than 30 filters, we fit models to spectral subsets of our data. We found very different best-fitting models, depending on the subset of filters, indicating a high sensitivity of results on the spectral sampling.

Based on 87 resolved Voyager images of the five innermost satellites of Neptune, their shapes were measured and fit by tri-axial ellipsoids with the semi-axes of 48 × 30 × 26 km for Naiad, 54 × 50 × 26 km for Thalassa, 90 × 74 × 64 km for Despina, 102 × 92 × 72 km for Galatea, and 108 × 102 × 84 km for Larissa. Thomas and Veverka published a similar shape for Larissa (104 × 89 km, J. Geophys. Res. 96, 19261–19268, 1991). The other satellites had no published shapes. Using Voyager photometry of the six inner satellites by the same authors and the revised sizes, including the published size of Proteus, the reflectivity within this inner system was found to vary by about 30%. Geometric albedos in the visible are estimated between 0.07 for Naiad and 0.10 for Proteus. The rotational lightcurves of these satellites seem to be due to satellite shapes.

Hubble Space Telescope imaging of Uranus in 1994, 1997, 1998, and 2000 revealed 13 cloud features, allowing the first measurements of wind velocities at northern latitudes not accessible to the Voyager cameras and new measurements of southern-latitude wind velocities determined during the 1986 Voyager encounter. Images acquired with the Keck 10-meter telescope adaptive optics system in June 2000 also showed some of the same features. Wind speeds inferred from feature motions—along with additional measurements by Karkoschka (1998, Science 280 ) and Voyager measurements (Smith et al . 1986, Science 246 )—indicate a zonal wind profile that is asymmetric with respect to the equator. Small but consistent differences are seen between the newer data and a profile determined from Voyager data: nearly all the newer measurements have slightly slower velocities than expected. We cannot yet determine whether the source of these differences is a slowly changing Uranian wind profile or subtle latitudinal structure in a temporally constant profile. If Uranus' winds vary with time, this may indicate unusual atmospheric dynamics created by the 98° tilt of the planet's rotation axis.

The ring model of Irvine (1966, J. Geophys. Res. 71 , 2931–2937) is used to explain the longitudinal brightness variation of the e ring of Uranus observed by Voyager 2 and with the Hubble Space Telescope, while basic ring properties are based on occultation data. The observations span a factor of 600 in the range of phase angles (0.03–20°) which allows the rejection of previous simpler ring models. Particles of the e ring are more densely packed than estimated previously. The average volume filling factor is at least 0.06, corresponding to mean particle separations of about twice their diameters or less. Plots are presented which allow the prediction of photometric properties of all uranian rings for various earth-based geometries.

Photometric properties of 4 rings and 16 satellites of Uranus are presented, based on 41 Hubble Space Telescope images taken in 1997. Up to 25 filters per object covered the wavelength range 0.27–2 μm. The whole range of phase angles observable from Earth (0.03–3°) was probed. Reflectivities were calibrated on an absolute scale to typically 4% accuracy. Below 1 μm, derived geometric albedos of major satellites are typically 30% higher than previously reported due to previously unobserved steep upturns of the phase curve shortward of 0.2° phase angle, possibly caused by coherent backscatter. Portia is slightly oblong and Juliet and Belinda are very oblong based on their rotational lightcurves. Puck displays a weak spectral feature indicative of water ice absorption. Cordelia and Ophelia were recovered very close to the positions predicted from their gravitational influence on the e ring. They have not been observed since 1986. Subtle color variations within the satellite system were confirmed. Throughout the uranian system, the slope of phase curves below 1° phase angle displays a strong correlation with the albedo and a strong anticorrelation with the slope at higher phase angles. Similar correlations were found for Hapke parameters. The brightness of 20 members of the uranian system is presented in functional form for a wide range of wavelengths, phase angles, and orbital longitudes.

The discovery of S/1986 U 10 is presented including photometry and orbital elements. The new satellite orbits only 1200 km outside Belinda's orbit and very close to the 44:43 eccentric resonance with Belinda. Its radius of about 15 km is much smaller than that expected from the trend seen for the other 15 regular uranian satellites. The 10 uranian satellites previously discovered by Voyager 2 are about 20% brighter than determined previously. One of these satellites, Puck, had a measured size which was slightly revised to 81±2 km. The first size measurements for the other nine satellites yielded sizes 40% larger on average and up to 60% larger than previously estimated. Most of these satellites are nonspherical. Juliet and Belinda may be the most oblong satellites in the Solar System among satellites with measured shapes.

Hubble Space Telescope (HST) images of Uranus taken in 1997 reveal that Voyager's view of the southern hemisphere was not representative of the rest of the planet. The banded structure with hemispherical asymmetry, presented in 25 filters between 0.27 and 2.03 μm wavelength, caused Uranus' full disk to darken by as much as 35% between 1985 and 2001, and probably much more over a full uranian season. The amplitude is strongly wavelength dependent. Hemispherical albedo asymmetries are as large as a factor of two over low latitudes and could be larger when they include higher latitudes that will be observable soon. Previous models of Uranus' atmosphere do not apply to northern latitudes. Some observed brightness variations of Uranus, explained as physical variations in previous studies, can now be fully explained by the geometry of changing subsolar latitudes. HST images taken between 1994 and 2000 show the latitudinal albedo structure and the number of discrete clouds in both hemispheres remaining roughly constant, although a slight darkening of high southern latitudes at some filters probing the methane haze indicates a slow decrease in its optical depth. A similar conclusion applies when including Voyager images of 1986. Physical changes in Uranus' atmosphere were many times faster before 1982 than since, and a similar rapid change can be expected in the near future.

We analyze observations made in August and November 1995 during the Earth and Sun crossings of Saturn's ring plane, respectively. The August 1995 observations combine data taken with the Adonis adaptive optics system at the European Southern Observatory (ESO) and images from the Hubble Space Telescope (HST). The November 1995 data are based on HST images only. We report here the detections of four new objects (three in August, one in November) orbiting near, or within, the F ring of Saturn. Two of the objects observed at ESO in August 1995 are most probably S/1995 S5 and S/1995 S6, reported by P. D. Nicholson et al. (1996, Science 272, 509–515) from the HST observations on August 10, 1995. The third object, S/1995 S20 cannot be clearly linked with any other objects reported by other observers. An elongated object, or arc, is tracked in November 1995, and can be connected to one of the arcs also reported by Nicholson et al. Our combined measurements improve the determination of the orbital parameters of S/1995 S5 and the arc, indicating that these objects orbit, within the error bars (≲±140 km), in the F ring. We discuss the nature and origin of these F-ring features. We propose that they are clouds of regolith ejecta resulting from collisions between large particles, or “parent bodies,” within the F ring. From the available constraints (brightness and lifetime of the objects), we show that the observations are consistent with the presence of several hundred 1-km-sized (and/or several thousand 100-m-sized) unseen parent bodies embedded in the F ring, each of which is covered by a regolith layer tens of centimeters to ∼1 m in thickness.

New measurements of Titan's hemispheric brightness asymmetry from HST images from 260 to 1040 nm show that the contrast is strongest near 450 nm (blue) and, with the opposite sign, at 889 nm (methane band). Comparison with a full Titan year of disk-integrated albedo data indicates that the seasonal cycle in asymmetry is smooth, but is insufficient to explain the variation in albedo, and a twice-per-year global albedo enhancement 50% larger than the hemispheric asymmetry amplitude is required, as noted by other workers. We also report measurements of limb-darkening (strongest at red wavelengths) and note that the transition zone between the “hemispheres” lies in the northern, brighter hemisphere at present. Comparison of models to the HST data set indicates that a change in aerosol number density above 70 km, and largely below 120 km, is the likely mechanism and is probably driven by aerosol transport by meridonal and vertical winds.

We measured the location, size, and shape of Titan's shadow in five images taken with the Hubble Space Telescope in 1995. We inferred the altitude of Titan's optical limb at wavelengths of 337–954 nm to an accuracy of 15 km. At each wavelength, altitudes are constant north of −5° and also constant but lower south of −50° latitude, with a linear transition region inbetween. The amplitude of the variation of altitude with latitude increases from close to zero at wavelength 337 nm to 130 km at 954 nm. We conclude that Titan's aerosols are larger (0.3 μm mean radius) at northern latitudes where we probe the detached haze layer than at southern latitudes (0.1 μm south of −50°) where we probe the main haze layer below. The geometric cross section of aerosols at probed altitudes (∼300 km) does not show a significant latitudinal variation. The wavelength dependence of the size of Titan's shadow is not consistent with a spherical shape of aerosols, but gives evidence of their fractal nature.

Photometric properties of nine uranian satellites and four rings, based on six Hubble Space Telescope images taken in 1995, are presented. Derived albedos are consistent with previous data taken at the same phase angle of 1°, but inconsistent with most Voyager-based estimates extrapolated from observations at phase angles above 15°. The shape of phase functions in the range 1–90° is similar to that of asteroids. Darker surfaces have steeper phase functions than brighter ones, except for the four brightest satellites, which have the same phase function. Puck's geometric albedo in the visible is 0.11 ± 0.015, much larger than the Voyager-based value of 0.074 ± 0.008. The satellites smaller than Puck may be 10% larger than Voyager-based estimates. Ring particles have a geometric albedo of 0.061 ± 0.006, much larger than the Voyager-based value of 0.032 ± 0.003. The longitudinal variation of brightness of the ϵ ring indicates that the mean separation of particles in the ring is four to five times their diameter. While the uranian rings and satellites seemed to be all gray heretofore, the wide wavelength range of this study, 340–910 nm, detected their subtle, distinct colors. Rings and the minor satellites are brown, Miranda is blue, Umbriel is red, and Ariel, Titania, and Oberon are yellow. Rings and minor satellites belong spectrally to M-type asteroids.

Recent observations demonstrate that near-infrared spectroscopy can probe Titan's surface through its haze. In a 5-week period during September and October 1993 we observed Titan's methane windows at 1.1, 1.3, 1.6, and 2 μm. At 1.1 and 1.3 μm observations were consistent with observations in 1992 at the same phases reported by Lemmon et al. (1993, Icarus 103, 329-332). Our new observations indicate that Titan was brighter near eastern elongation than near western elongation by 23 ± 2% at 1.6 μn and 32 ± 3% at 2 μn. With almost daily observations at 2 μn during one orbit, we observed Titan to be dark near western elongation, to brighten as it approached eastern elongation, and to darken as it returned to western elongation. We determine that the observed light-curve is due to surface albedo variations and Titan's rotational period is 15.950 ± 0.025 days. By considering the work of other observers we constrain the rotational period to be 15.949 ± 0.006 days; this constraint is consistent with synchronous rotation. Models of Titan's surface reflectivity are inconsistent with the presence of a strong 2-μm water ice absorption feature and do not require the presence of any surface absorption features; however, we cannot rule out models of the surface as dirty water ice or silicates that have been suggested by others.

International audience; This study presents an approximate model for the atypical Schumann resonance in Titan's atmosphere that accounts for the observations of electromagnetic waves and the measurements of atmospheric conductivity performed with the HASI-PWA (Huygens Atmospheric Structure and Permittivity, Wave and Altimetry) instrumentation during the descent of the Huygens Probe through Titan's atmosphere in January 2005. After many years of thorough analyses of the collected data, several arguments enable us to claim that the Extremely Low Frequency (ELF) wave observed at around 36 Hz displays all the characteristics of the second harmonic of a Schumann resonance. On Earth, this phenomenon is well known to be triggered by lightning activity. Given the lack of evidence of any thunderstorm activity on Titan, we proposed in early works a model based on an alternative powering mechanism involving the electric current sheets induced in Titan's ionosphere by the Saturn's magnetospheric plasma flow. The present study is a further step in improving the initial model and corroborating our preliminary assessments. We first develop an analytic theory of the guided modes that appear to be the most suitable for sustaining Schumann resonances in Titan's atmosphere. We then introduce the characteristics of the Huygens electric field measurements in the equations, in order to constrain the physical parameters of the resonating cavity. The latter is assumed to be made of different structures distributed between an upper boundary, presumably made of a succession of thin ionized layers of stratospheric aerosols spread up to 150 km and a lower quasi-perfect conductive surface hidden beneath the non-conductive ground. The inner reflecting boundary is proposed to be a buried water-ammonia ocean lying at a likely depth of 55 - 80 km below a dielectric icy crust. Such estimate is found to comply with models suggesting that the internal heat could be transferred upwards by thermal conduction of the crust, while convective processes cannot be ruled out.

Full-disk albedo spectra of the jovian planets and Titan were derived from observations at the European Southern Observatory in July 1993. The spectra extend from 300- to 1000-nm wavelength at 1-nm resolution. The signal-to-noise ratio is approximately 1000 in the visible. The accuracy is 2% for relative and 4% for absolute ulbedos. Colors and magnitudes were also determined. Some 40-60 Raman scattering features are visible in the spectrum of each jovian planet. A Raman scattering model with five parameters adjusted for each planet can explain these features. A methane absorption spectrum is given which fits methane features in the spectra of the jovian planets and Titan. It differs from room-temperature laboratory spectra but it is consistent with some limited laboratory data at cold temperatures. Three new, weak methane bands were detected in the spectra of Uranus and Neptune. A strong absorption hand in Jupiter's spectrum is possibly due to water, confirming a strong depletion of oxygen in the probed part of Jupiter's atmosphere.

We observed Saturn with the Hubble Space Telescope at wavelengths 0.30-0.89 μm for the purpose of determining the distribution of hazes. In the stratosphere, haze optical depths in the ultraviolet are essentially zero for midnorthern latitudes, are small (∼0.2) at low latitudes and mid-southern latitudes, but large (almost unity) above 70° north. The optical depth falls off sharply in the visible due to the small radii of the stratospheric aerosols (∼0.15 μm). The latitudinal distribution of tropospheric haze was found mostly consistent with previous investigations. It is completely different from the distribution of stratospheric haze since optical depths in the troposphere strongly increase from the north pole to the equator. In the ultraviolet, the stratospheric aerosols are darker than tropospheric aerosols. Latitudinal albedo and color variations in the visible, defining Saturn's belt and zone structure, can be explained by variations in the size of tropospheric aerosols (radii 1-2 μm). The unusual blue-green color of mid-southern latitudes in 1991 may be due to smaller radii (∼0.5 μm) in the troposphere. At wavelength 0.30 μm we found an indication of a gaseous absorption of ∼0.02 optical depth in the upper part of the stratosphere.

We have detected time variation in Titan's geometric albedo in methane windows at 0.94, 1.08, and 1.28 μm, relative to its albedo in adjacent methane bands, of 8 ± 5%, 14 ± 3%, and 22 ± 3%, respectively. We attribute these changes to a surface feature or a feature near the surface. Our observations are consistent with synchronous rotation and can be explained by a higher surface albedo by 0.1 on Titan's leading hemisphere.