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Observations of trace gases over the south pole of Titan indicate that the moon’s middle-atmospheric circulation extends to an altitude of at least 600 kilometres, which is higher than previously thought and requires active chemistry and dynamics in the upper atmosphere. Seasonal upheavals in Titan's atmosphere When NASA's Cassini probe reached the Saturnian system in 2004, one of its goals was to observe the seasonal behaviour of Saturn's biggest moon, Titan, which has a nitrogen-rich atmosphere comparable to Earth's. As the seasons pass, those observations are bearing fruit. Here, Nicholas Teanby et al . report a build-up of trace gases over Titan's south pole approximately two years after the observation of a post-equinox circulation reversal in the middle atmosphere, from which they conclude that middle-atmosphere circulation must extend to at least 600 kilometres above the moon's surface, rather than the 450–500 kilometres previously suspected. These findings would require active upper-atmosphere chemistry, and an alternative explanation for the detached layer of haze at 450–500 kilometres that was formerly regarded as an atmospheric boundary. Saturn’s moon Titan has a nitrogen atmosphere comparable to Earth’s, with a surface pressure of 1.4 bar. Numerical models reproduce the tropospheric conditions very well but have trouble explaining the observed middle-atmosphere temperatures, composition and winds 1 , 2 . The top of the middle-atmosphere circulation has been thought to lie at an altitude of 450 to 500 kilometres, where there is a layer of haze that appears to be separated from the main haze deck 3 . This ‘detached’ haze was previously explained as being due to the co-location of peak haze production and the limit of dynamical transport by the circulation’s upper branch 4 . Here we report a build-up of trace gases over the south pole approximately two years after observing the 2009 post-equinox circulation reversal, from which we conclude that middle-atmosphere circulation must extend to an altitude of at least 600 kilometres. The primary drivers of this circulation are summer-hemisphere heating of haze by absorption of solar radiation and winter-hemisphere cooling due to infrared emission by haze and trace gases 5 ; our results therefore imply that these effects are important well into the thermosphere (altitudes higher than 500 kilometres). This requires both active upper-atmosphere chemistry, consistent with the detection of high-complexity molecules and ions at altitudes greater than 950 kilometres 6 , 7 , and an alternative explanation for the detached haze, such as a transition in haze particle growth from monomers to fractal structures 8 .

The loss of water from Mars to space is thought to result from the transport of water to the upper atmosphere, where it is dissociated to hydrogen and escapes the planet. Recent observations have suggested large, rapid seasonal intrusions of water into the upper atmosphere, boosting the hydrogen abundance. We use the Atmospheric Chemistry Suite on the ExoMars Trace Gas Orbiter to characterize the water distribution by altitude. Water profiles during the 2018–2019 southern spring and summer stormy seasons show that high-altitude water is preferentially supplied close to perihelion, and supersaturation occurs even when clouds are present. This implies that the potential for water to escape from Mars is higher than previously thought.

Saturn's slow seasonal evolution was disrupted in 2010–2011 by the eruption of a bright storm in its northern spring hemisphere. Thermal infrared spectroscopy showed that within a month, the resulting planetary-scale disturbance had generated intense perturbations of atmospheric temperatures, winds, and composition between 20° and 50°N over an entire hemisphere (140,000 kilometers). The tropospheric storm cell produced effects that penetrated hundreds of kilometers into Saturn's stratosphere (to the 1-millibar region). Stratospheric subsidence at the edges of the disturbance produced \"beacons\" of infrared emission and longitudinal temperature contrasts of 16 kelvin. The disturbance substantially altered atmospheric circulation, transporting material vertically over great distances, modifying stratospheric zonal jets, exciting wave activity and turbulence, and generating a new cold anticyclonic oval in the center of the disturbance at 41°N.

ESA’s Jupiter Icy Moons Explorer (JUICE) will provide a detailed investigation of the Jovian system in the 2030s, combining a suite of state-of-the-art instruments with an orbital tour tailored to maximise observing opportunities. We review the Jupiter science enabled by the JUICE mission, building on the legacy of discoveries from the Galileo, Cassini, and Juno missions, alongside ground- and space-based observatories. We focus on remote sensing of the climate, meteorology, and chemistry of the atmosphere and auroras from the cloud-forming weather layer, through the upper troposphere, into the stratosphere and ionosphere. The Jupiter orbital tour provides a wealth of opportunities for atmospheric and auroral science: global perspectives with its near-equatorial and inclined phases, sampling all phase angles from dayside to nightside, and investigating phenomena evolving on timescales from minutes to months. The remote sensing payload spans far-UV spectroscopy (50-210 nm), visible imaging (340-1080 nm), visible/near-infrared spectroscopy (0.49-5.56 μm), and sub-millimetre sounding (near 530-625 GHz and 1067-1275 GHz). This is coupled to radio, stellar, and solar occultation opportunities to explore the atmosphere at high vertical resolution; and radio and plasma wave measurements of electric discharges in the Jovian atmosphere and auroras. Cross-disciplinary scientific investigations enable JUICE to explore coupling processes in giant planet atmospheres, to show how the atmosphere is connected to (i) the deep circulation and composition of the hydrogen-dominated interior; and (ii) to the currents and charged particle environments of the external magnetosphere. JUICE will provide a comprehensive characterisation of the atmosphere and auroras of this archetypal giant planet.

The cloud that appeared above the south pole of Saturn’s satellite Titan in early 2012 is found to be composed of micrometre-sized particles of frozen hydrogen cyanide, indicating a dramatic cooling of Titan’s atmosphere to temperatures about 100 degrees less than predicted by atmospheric circulation models. Unseasonal ice clouds on Titan In May 2012 the Imaging Science Subsystem on-board the Cassini spacecraft identified a large cloud-like structure above the dark southern pole of Saturn's largest moon Titan. Since then the cloud has been seen at very high altitudes (300 km) and high southern latitudes in regions of the atmosphere where temperatures were too high to allow the condensation of any of Titan's known trace gases. This mysterious cloud has now been characterized using near-infrared spectra from Cassini's Visual and Infrared Mapping Spectrometer. The data reveal a cloud composed of micrometre-sized hydrogen cyanide (HCN) ice particles and are consistent with dramatic cooling of Titan's atmosphere inside the winter polar vortex in early 2012. Such a cooling — to temperatures a hundred degrees colder than predicted by circulation models — contrasts with the high-altitude warming observed previously in the polar vortex. Titan’s middle atmosphere is currently experiencing a rapid change of season after northern spring arrived in 2009 (refs 1 , 2 ). A large cloud was observed 3 for the first time above Titan’s southern pole in May 2012, at an altitude of 300 kilometres. A temperature maximum was previously observed there, and condensation was not expected for any of Titan’s atmospheric gases. Here we report that this cloud is composed of micrometre-sized particles of frozen hydrogen cyanide (HCN ice). The presence of HCN particles at this altitude, together with temperature determinations from mid-infrared observations, indicate a dramatic cooling of Titan’s atmosphere inside the winter polar vortex in early 2012. Such cooling is in contrast to previously measured high-altitude warming in the polar vortex 1 , and temperatures are a hundred degrees colder than predicted by circulation models 4 . These results show that post-equinox cooling at the winter pole of Titan is much more efficient than previously thought.

Saturn’s largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter polar vortices. Following Titan’s 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-polar subsidence and winter polar vortex formation. Throughout 2010–2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the polar vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2–6 years post-equinox. The long time-frame to reach a stable vortex configuration results from the high infrared opacity of Titan’s trace gases and the relatively long atmospheric radiative time constant. Winter polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan. The polar hot-spot appeared in Titan after equinox in 2010 suddenly cooled in early 2012, which wasn’t predicted by models. Here the authors use observations to show that the increase in trace gases during the hot-spot resulted in radiative cooling feedback.

Atmospheric circulation patterns derived from multi-spectral remote sensing can serve as a guide for choosing a suitable entry location for a future in situ probe mission to the Ice Giants. Since the Voyager-2 flybys in the 1980s, three decades of observations from ground- and space-based observatories have generated a picture of Ice Giant circulation that is complex, perplexing, and altogether unlike that seen on the Gas Giants. This review seeks to reconcile the various competing circulation patterns from an observational perspective, accounting for spatially-resolved measurements of: zonal albedo contrasts and banded appearances; cloud-tracked zonal winds; temperature and para-H 2 measurements above the condensate clouds; and equator-to-pole contrasts in condensable volatiles (methane, ammonia, and hydrogen sulphide) in the deeper troposphere. These observations identify three distinct latitude domains: an equatorial domain of deep upwelling and upper-tropospheric subsidence, potentially bounded by peaks in the retrograde zonal jet and analogous to Jovian cyclonic belts; a mid-latitude transitional domain of upper-tropospheric upwelling, vigorous cloud activity, analogous to Jovian anticyclonic zones; and a polar domain of strong subsidence, volatile depletion, and small-scale (and potentially seasonally-variable) convective activity. Taken together, the multi-wavelength observations suggest a tiered structure of stacked circulation cells (at least two in the troposphere and one in the stratosphere), potentially separated in the vertical by (i) strong molecular weight gradients associated with cloud condensation, and by (ii) transitions from a thermally-direct circulation regime at depth to a wave- and radiative-driven circulation regime at high altitude. The inferred circulation can be tested in the coming decade by 3D numerical simulations of the atmosphere, and by observations from future world-class facilities. The carrier spacecraft for any probe entry mission must ultimately carry a suite of remote-sensing instruments capable of fully constraining the atmospheric motions at the probe descent location.

Past spacecraft observations of Venus have found considerable spatial and temporal variations of water vapour abundance above the clouds. Previous searches for variability below the clouds at 30–45 km altitude found no large scale latitudinal gradients, but lacked the spatial resolution to detect smaller scale variations. Here we interpret results from the VIRTIS imaging spectrometer on Venus Express, remotely sounding at near‐infrared ‘spectral window’ wavelengths, as indicating that the water vapour abundance at 30–40 km altitude varies from 22 to 35 ppmv (±4 ppmv). Furthermore, this variability is correlated with cloud opacity, supporting the hypothesis that its genesis is linked to cloud convection. It is also possible to fit the observations without requiring spatial variation of water abundance, but this places a strong constraint on the spectral dependence of the refractive index data assumed for the lower cloud particles, for which there is as yet no supporting evidence.

The Composite InfraRed Spectrometer (CIRS) instrument onboard the Cassini spacecraft performed 8.4 million spectral observations of Titan at resolutions between 0.5–15.5 cm - 1 . More than 3 million of these were acquired at a low spectral resolution (SR) (13.5–15.5 cm - 1 ), which have excellent spatial and temporal coverage in addition to the highest spatial resolution and lowest noise per spectrum of any of the CIRS observations. Despite this, the CIRS low-SR dataset is currently underused for atmospheric composition analysis, as spectral features are often blended and subtle compared to those in higher SR observations. The vast size of the dataset also poses a challenge as an efficient forward model is required to fully exploit these observations. Here, we show that the CIRS FP3/4 nadir low-SR observations of Titan can be accurately forward modelled using a computationally efficient correlated- k method. We quantify wavenumber-dependent forward modelling errors, with mean 0.723 nW cm - 2 sr - 1 /cm - 1 (FP3: 600–890 cm - 1 ) and 0.248 nW cm - 2 sr - 1 / cm - 1 (FP4: 1240–1360 cm - 1 ), that can be used to improve the rigour of future retrievals. Alternatively, in cases where more accuracy is required, we show observations can be forward modelled using an optimised line-by-line method, significantly reducing computation time.

Aerosols are ubiquitous in planetary atmospheres in the Solar System. However, radiative forcing on Jupiter has traditionally been attributed to solar heating and infrared cooling of gaseous constituents only, while the significance of aerosol radiative effects has been a long-standing controversy. Here we show, based on observations from the NASA spacecraft Voyager and Cassini, that gases alone cannot maintain the global energy balance in the middle atmosphere of Jupiter. Instead, a thick aerosol layer consisting of fluffy, fractal aggregate particles produced by photochemistry and auroral chemistry dominates the stratospheric radiative heating at middle and high latitudes, exceeding the local gas heating rate by a factor of 5–10. On a global average, aerosol heating is comparable to the gas contribution and aerosol cooling is more important than previously thought. We argue that fractal aggregate particles may also have a significant role in controlling the atmospheric radiative energy balance on other planets, as on Jupiter. Aerosols play an important role in Earth’s radiative balance, but their influence on the climate of giant planets is unclear. Here, the authors show that gases alone cannot maintain the energy balance in the middle atmosphere of Jupiter, instead proposing that an aerosol layer dominates radiative heating.