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We use a customized radiative transfer model to show that sharp (∼ 10 m resolution) images of the Venus surface can be achieved at night in spectral windows free of CO 2 absorption found between 1.0 and 1. 2 μ m using a camera at 47 km altitude, just below the planet's optically thick clouds. This is in spite of the Rayleigh scattering by the dense but still semi-transparent lower atmosphere, and the potential for underlying hazes beneath the clouds. The thermal radiation transmitted directly to the camera forms images of spatially varying surface emissivity and/or temperature at the native sensor resolution, platform stability permitting and under reasonable seeing conditions. Near-isotropic Rayleigh scattering dominates in the 1. 0 μ m window. Combined with near-Lambertian reflections off the base of the cloud layer, the diffuse light field builds up a background radiance from surface emission averaged spatially out to several 10s of km, i.e., beyond the camera's field-of-view. At the longer wavelengths (1.1 and 1. 18 μ m windows), the sub-cloud atmosphere itself partially absorbs (hence less direct light), and therefore weakly emits (hence more background light), but the rapidly decreasing Rayleigh scattering compensates and contrast is maintained. In all cases, we demonstrate that the directly-transmitted surface-leaving radiance from the native sensor resolution element (∼ 10 m) is a significant fraction of the total radiance, and thus can be detected above the background light. Extending down to the 0.85 and 0. 90 μ m spectral windows, there is less direct and more background due to the enhanced Rayleigh scattering, but the resulting reduction in contrast can be mitigated by co-adding the ∼ 10 m pixels. This technological advance will open a new era in Venusian geology by enabling discrimination between different surface materials at fine scales. Moreover, potentially active volcanism on our sister planet may be revealed by surface spots that are much hotter than their surroundings. [Display omitted] • A NIR camera can sharply image Venus' surface at night from just below cloud base. • Atmospheric scattering/emission produces a near-uniform glow that reduces contrast. • Enough surface emission is transmitted directly to be detected above the background. • Possible levels of sub-cloud haze excluded from previous studies are accounted for. • Both Venus surface geology and the quest for active volcanology will be transformed. [ABSTRACT FROM AUTHOR]

Abstract: Clouds formed on Jupiter by the impact of July 19, 2009 were observed from the IRTF with the same instrument and near-infrared filters during four observing runs over 50 days, beginning one day after impact, providing comprehensive diagnostics of cloudtop altitude, particle size and opacity and yielding quantitative information on the temporal evolution of the impact cloud (IC). The IC evolved relatively rapidly during the first 26 days after impact (Period I) and relatively slowly for the next 23 days (Period II). For the column volume density of the IC core, analyzed over a range of models with varying Mie-scattering particle radii over 0.1–1.1μm and imaginary indices of refraction (n i ) from 0.001 to the limiting model-constrained value of 0.03, the Period I e-folding timescale is 4–16 times less than for Period II, with a best-fit timescale of of 23 days (Period I) increasing to 117 days (Period II) for the nominal best-fit case of large (0.7–1.1μm) dark (n i =0.01) particles consistent with previous determinations of the size and near-infrared brightness of impact cloud particles (de Pater et al. Icarus 210, 722–741, 2010). Over the entire period, the nominal model mean particle radius ranges from ∼0.85μm one day after impact to 0.89–1.06μm 49 days later, considerably larger than the 0.21–0.28μm particles determined for the Shoemaker Levy 9 impact (SL9; West et al., Science 267, 1296–1301, 1995). The 1.69 and 2.12μm nominal model IC core opacities show timescales averaged over the entire seven-week period of ∼33 and ∼35 days, respectively, ∼60% longer than the 18–23 day timescales of small-particle models (∼0.36μm radius) which are more consistent with the ∼15-day visible timescale reported by Sánchez-Lavega et al. (2011, Icarus 214,462–476). Including the area of the entire IC, we find that the total particle volume over the 49 days changes from ∼0.036 to ∼0.022km3 for the nominal model, corresponding to a variation of the diameter of an equivalent sphere from ∼0.41 to ∼0.35km, close to the ∼10% diameter change over one month reported for SL9 clouds (West et al, ibid). Nominal Period II timescales for opacities and total cloud volume – 50–300 days – are 2–11 times longer than nominal Period I timescales; indeed, values of infinity are consistent with the uncertainties. The relatively long timescales found for total cloud dissipation are consistent with (1) material dispersal by wind shears, and (2) relatively weak sedimentation/coagulation, consistent with SL9 results (West et al., ibid). Finally, the IC thickness of ∼1 scale-height permits a vertical windshear of ∼1m/s, inconsistent with the derived ∼7m/s cloud spreading, thus implying that the meridional shear is the dominant cloud shear component, as reported for visible measurements (Sánchez-Lavega et al. ibid). [Copyright &y& Elsevier]

Abstract: The latitudinal variation of Saturn’s tropospheric composition (NH3, PH3 and AsH3) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6–5.1μm thermal emission spectroscopy on the planet’s nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn’s cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5–2.8bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5–2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric). Saturn’s NH3 spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23–25°S and 42–47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH3 dominates the morphology of the VIMS spectrum, and high-altitude PH3 at p <1.3bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH3 is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH3 shows similar off-equatorial maxima at ±7° with a global abundance of 2–3ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH3 and upper-tropospheric PH3 and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH3, AsH3 and the lower cloud deck). [Copyright &y& Elsevier]

Abstract: A high-speed cyclonic vortex centered on the north pole of Saturn has been revealed by the visual-infrared mapping spectrometer (VIMS) onboard the Cassini–Huygens Orbiter, thus showing that the tropospheres of both poles of Saturn are occupied by cyclonic vortices with winds exceeding 135m/s. High-spatial-resolution (~200km per pixel) images acquired predominantly under night-time conditions during Saturn''s polar winter—using a thermal wavelength of 5.1μm to obtain time-lapsed imagery of discrete, deep-seated (>2.1-bar) cloud features viewed in silhouette against Saturn''s internally generated thermal glow—show a classic cyclonic structure, with prograde winds exceeding 135m/s at its maximum near 88.3° (planetocentric) latitude, and decreasing to <30m/s at 89.7° near the vortex center and<20m/s at 80.5°. High-speed winds, exceeding 125m/s, were also measured for cloud features at depth near 76° (planetocentric) latitude within the polar hexagon consistent with the idea that the hexagon itself, which remains nearly stationary, is a westward (retrograde) propagating Rossby wave – as proposed by Allison (1990, Science 247, 1061–1063) – with a maximum wave speed near 2-bars pressure of ~125m/s. Winds are ~25m/s stronger than observed by Voyager, suggesting temporal variability. Images acquired of one side of the hexagon in dawn conditions as the polar winter wanes shows the hexagon is still visible in reflected sunlight nearly 28 years since its discovery, that a similar 3-lane structure is observed in reflected and thermal light, and that the cloudtops may be typically lower in the hexagon than in nearby discrete cloud features outside of it. Clouds are well-correlated in visible and 5.1μm images, indicating little windshear above the ~2-bar level. The polar cyclone is similar in size and shape to its counterpart at the south pole; a primary difference is the presence of a small (<600km in diameter) nearly pole-centered cloud, perhaps indicative of localized upwelling. Many dozens of discrete, circular cloud features dot the polar region, with typical diameters of 300–700km. Equatorward of 87.8°N, their compact nature in the high-wind polar environment suggests that vertical shear in horizontal winds may be modest on 1000km scales. These circular clouds may be anticyclonic vortices produced by baroclinic instabilities, barotropic instabilities, moist convection or other processes. The existence of cyclones at both poles of Saturn indicates that cyclonic circulation may be an important dynamical style in planets with significant atmospheres. [Copyright &y& Elsevier]

Abstract: Thunderstorm activity on Saturn is associated with optically detectable clouds that are atypically dark throughout the near-infrared. As observed by Cassini/VIMS, these clouds are ~20% less reflective than typical neighboring clouds throughout the spectral range from 0.8μm to at least 4.1μm. We propose that active thunderstorms originating in the 10–20bar water-condensation region vertically transport dark materials at depth to the ~1bar level where they can be observed. These materials in part may be produced by chemical processes associated with lightning, likely within the water clouds near the ~10bar freezing level of water, as detected by the electrostatic discharge of lightning flashes observed by Cassini/RPWS (e.g., Fischer et al. 2008, Space Sci. Rev., 137, 271–285). We review lightning-induced pyrolytic chemistry involving a variety of Saturnian constituents, including hydrogen, methane, ammonia, hydrogen sulfide, phosphine, and water. We find that the lack of absorption in the 1–2μm spectral region by lightning-generated sulfuric and phosphorous condensates renders these constituents as minor players in determining the color of the dark storm clouds. Relatively small particulates of elemental carbon, formed by lightning-induced dissociation of methane and subsequently upwelled from depth – perhaps embedded within and on the surface of spectrally bright condensates such as ammonium hydrosulfide or ammonia – may be a dominant optical material within the dark thunderstorm-related clouds of Saturn. [Copyright &y& Elsevier]

Clouds on Titan result from the condensation of methane and ethane and, as on other planets, are primarily structured by circulation of the atmosphere. At present, cloud activity mainly occurs in the southern (summer) hemisphere, arising near the pole and at mid-latitudes from cumulus updrafts triggered by surface heating and/or local methane sources, and at the north (winter) pole, resulting from the subsidence and condensation of ethane-rich air into the colder troposphere. General circulation models predict that this distribution should change with the seasons on a 15-year timescale, and that clouds should develop under certain circumstances at temperate latitudes (∼40°) in the winter hemisphere. The models, however, have hitherto been poorly constrained and their long-term predictions have not yet been observationally verified. Here we report that the global spatial cloud coverage on Titan is in general agreement with the models, confirming that cloud activity is mainly controlled by the global circulation. The non-detection of clouds at latitude ∼40° N and the persistence of the southern clouds while the southern summer is ending are, however, both contrary to predictions. This suggests that Titan’s equator-to-pole thermal contrast is overestimated in the models and that its atmosphere responds to the seasonal forcing with a greater inertia than expected. [ABSTRACT FROM AUTHOR]

Abstract: We present an analysis of VIRTIS-M-IR observations of 1.74μm emission from the nightside of Venus. The 1.74μm window in the near infrared spectrum of Venus is an ideal proxy for investigating the evolution of middle and lower cloud deck opacity of Venus because it exhibits good signal to noise due to its brightness, good contrast between bright and dark regions, and few additional sources of extinction beside the clouds themselves. We have analyzed the data from the first 407 orbits (equivalent to 407 Earth days) of the Venus Express mission to determine the magnitude of variability in the 1.74μm radiance. We have also performed an analysis of the evolution of individual features over a span of roughly 5–6h on two successive orbits of Venus Express. We find that the overall 1.74μm brightness of Venus has been increasing through the first 407 days of the mission, indicating a gradual diminishing of the cloud coverage and/or thickness, and that the lower latitudes exhibited more variability and more brightening than higher latitudes. We find that individual features evolve with a time scale of about 30h, consistent with our previous analysis. Analysis of the evolution and motion of the clouds can be used to estimate the mesoscale dynamics within the clouds of Venus. We find that advection alone cannot explain the observed evolution of the features. The measured vorticity and divergence in the vicinity of the features are consistent with evolution under the influence of significant vertical motions likely driven by a radiative dynamical feedback. We measure a zonal wind speed of around 65m/s, and a meridional wind speed around 2.5m/s by tracking the motion of the central region of the features. But we also find that the measured wind speeds depend strongly on the points chosen for the wind speed analysis. [Copyright &y& Elsevier]

Abstract: Saturn''s Moon Titan has a thick atmosphere with a meteorological cycle. We report on the evolution of the giant cloud system covering its north pole using observations acquired by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft. A radiative transfer model in spherical geometry shows that the clouds are found at an altitude between 30 and 65km. We also show that the polar cloud system vanished progressively as Titan approached equinox in August 2009, revealing at optical wavelengths the underlying sea known as Kraken Mare. This decrease of activity suggests that the north-polar downwelling has begun to shut off. Such a scenario is compared with the Titan global circulation model of , which predicts a decrease of cloud coverage in northern latitudes at the same period of time. [Copyright &y& Elsevier]