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The history of in-situ Venus exploration has been limited to a few opportunities with different probes that were capable to operate, for short periods of time, under the extreme atmospheric conditions of the planet. Among these missions, the VeGa balloons deployed in the Venus atmosphere in the mid-eighties of previous century revealed the advantages of using this concept for investigating the atmosphere of Venus. In this regard, the recent studies for the 2023-2030 Planetary Decadal Survey [1-3] have pointed the potential of using balloon platforms for planetary science exploration, considering that the different technologies required for these missions are currently mature enough to develop long-lived and possibly even altitude-varying probes or more specifically, aerobots.In this work, we present an early concept of a lightweight radiometer for future balloon missions to Venus. Its primary scientific objectives are: i) to measure solar and ii)thermal infrared fluxes and their deposition in the cloud layer, iii) to characterize the variability of the cloud structure and its constituents, and iv) to detect and characterize atmospheric lightning events. Those investigations will allow us to understand the role of each objective in determining the atmospheric structure and the driving circulation of the planet.Due to the limitations on resources for this kind of platforms, the key characteristics of the proposed instrument are its high scientific performance and the scarce resources needs: low accommodation volume, size, and mass; low power and data volume consumption. The radiometer combines different spectral bandpass channels (from UVA to IR) with particular orientations and field of view (FoV) selected to meet the scientific objectives. The instrument also incorporates a visible camera to provide context images for cloud investigations.The Spanish National Institute of Aerospace Technology (INTA) has established a long-term strategy in the last decade with the program InMARS [4] that is devoted to developing high-performance, low-power, miniature sensors designed for in-situ planetary missions [5-10]. Within this program, we have developed an intensive selection, qualification, and screening activity in our particular technological roadmap called CERES (Compact Electronic Resources for the Exploration of Space), which allowed INTA to acquire critical technologies, components (including mixed ASICs [11-12]), materials and procedures for such instrumentation developments.[1] K.H. Baines et al, 2020. White Pape. [2] Martha S. Gilmore et al, 2020. Venus Flagship Mission Decadal Study Final Report [3] Joseph O’Rourke, ADVENTS misión concept study. [4] I.Arruego et al. IPPW 2018. Boulder. Colorado. USA. [5] H. Guerrero et al. EGU 2010. Geophysical Research Abstracts Vol. 12, EGU2010-13330, 2010. [6] I. Arruego et al. DREAMS-SIS. ASR 2017. 60 (1): 103-120. [7] V. Apéstigue et al. Sensors.2022. [8] D. Rodionov et al. Sixth International Workshop on the Mars Atmosphere: Modelling and Observations. 2017. Granada. Spain. [9] D. Scaccabarozzi et al. IEEE MetroAeroSpace proccedings. 2019. Torino.Italy. [10] A. Russu et al. Proc. SPIE 11129. [11] S. Sordo-Ibáñez et al. IEEE Transactions on Nuclear Science, vol. 63, pp. 2379-2389, 2016. [12] S. Sordo-Ibáñez et al. IEEE Transactions on Magnetics, vol. 51, pp. 1-4, 2015

On May 31, 2020 a short-lived convective storm appeared in one of the small cyclones of Jupiter's South Temperate Belt (STB) at planetographic latitude 30.8S. The outbreak was captured by amateur astronomer Clyde Foster in methane-band images, became widely known as Clyde's Spot, and was imaged at very high resolution by the Junocam instrument on board the Juno mission 2.5 days later. Junocam images showed a white two-lobed cyclonic system with high clouds observed in the methane-band at 890 nm. The storm evolved over a few days to become a dark feature that showed turbulence for months, presented oscillations in its drift rate, and slowly expanded, first into a Folded Filamentary Region (FFR), and later into a turbulent segment of the STB over a timescale of one year. On August 7, 2021, a new storm strikingly similar to Clyde's Spot erupted in a cyclone of the STB. The new storm exhibited first a similar transformation into a turbulent dark feature, and later transformed into a dark cyclone fully formed by January 2022. We compare the evolution into a FFR of Clyde's Spot with the formation of a FFR observed by Voyager 2 in 1979 in the South South Temperate Belt (SSTB) after a convective outburst in a cyclone that also developed a two-lobed shape. We also discuss the contemporaneous evolution of an additional cyclone of the STB, which was similar to the one were Clyde's Spot developed. This cyclone did not exhibit visible internal convective activity, and transformed from pale white in 2019, with low contrast with the environment, to dark red in 2020, and thus, was very similar to the outcome of the second storm. This cyclone became bright again in 2021 after interacting with Oval BA. We present observations of these phenomena obtained by amateur astronomers, ground-based telescopes, Hubble Space Telescope and Junocam. This study reveals that short-lived small storms that are active for only a few days can produce complex longterm changes that extend over much larger areas than those initially covered by the storms. In a second paper [In tilde urrigarro et al., 2022] we use the EPIC numerical model to simulate these storms and study moist convection in closed cyclones. We are very thankful to the large community of amateur observers operating small telescopes that submit their Jupiter observations to databases such as PVOL and ALPO-Japan. We are also grateful to two anonymous reviewers for their comments that improved the clarity of this paper. This work has been supported by Grant PID2019-109467GB-I00 funded by MCIN/AEI/10.13039/501100011033/and by Grupos Gobierno Vasco IT1366-19. PI acknowledges a PhD scholarship from Gobierno Vasco. GSO and TM were supported by NASA with funds distributed to the Jet Propulsion Laboratory, California Institute of Technology under contract 80NM0018D0004. C. J. Hansen was sup-ported by funds from NASA, USA to the Juno mission via the Planetary Science Institute. IOE was supported by a contract funded by Europlanet 2024 RI to navigate Junocam images, now available as maps in PVOL at http://pvol2.ehu.eus. Europlanet 2024 RI has received funding from the European Unions Horizon 2020 research and innovation programme under grant agreement No 871149. G.S. Orton, S. R. Brueshaber, T. W. Momary, K. H. Baines and E. K. Dahl were visiting Astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration. In addition, support from NASA Juno Participating Scientist award 80NSSC19K1265 was provided to M.H. Wong. This work has used data acquired from the NASA/ESA Hubble Space Telescope (HST) , which is operated by the Association of 807 Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These HST observations are associated with several HST observing programs: GO/DD 14661 (PI: M.H. Wong) , GO/DD 15665 (PI: I. de Pater) , GO/DD 15159 (PI: M. H. Wong) , GO/DD 15502 (PI: A. Simon) , GO/DD 14661 (PI: M. H. Wong) , GO/DD 16074 (PI: M.H. Wong) , GO/DD 16053 (PI: I. de Pater) , GO/DD 15929 (PI: A. Simon) , GO/DD 16269 (PI: A. Simon) . PlanetCam observations were collected at the Centro Astronomico Hispanico en Andalucia (CAHA) , operated jointly by the Instituto de Astrofisica de Andalucia (CSIC) and the Andalusian Universities (Junta de Andalucia) . This work was enabled by the location of the IRTF and Gemini North telescopes within the Mauakea Science Reserve, adjacent to the summit of Maunakea. We are grateful for the privilege of observing Kaawela (Jupiter) from a place that is unique in both its astronomical quality and its cultural signifi-cance. This research has made use of the USGS Integrated Software for Imagers and Spectrometers (ISIS) . Voyager 2 images were accessed through The PDS Ring-Moon Systems Nodes OPUS search service.

Cassini/ISS imagery and Cassini/VIMS spectral imaging observations from 0.35 to 5.12 microns show that between 2012 and 2017 the region poleward of the Saturn's northern hexagon changed from dark blue/green to a moderately brighter gold color, except for the inner eye region (88.2 deg - 90 deg N), which remained relatively unchanged. These and even more dramatic near-IR changes can be reproduced by an aerosol model of four compact layers consisting of a stratospheric haze at an effective pressure near 50 mbar, a deeper haze of putative diphosphine particles typically near 300 mbar, an ammonia cloud layer with a base pressure between 0.4 bar and 1.3 bar, and a deeper cloud of a possible mix of NH4SH and water ice particles within the 2.7 to 4.5 bar region. Our analysis of the background clouds between the discrete features shows that between 2013 and 2016 the effective pressures of most layers changed very little, except for the ammonia ice layer, which decreased from about 1 bar to 0.4 bar near the edge of the eye, but increased to 1 bar inside the eye. Inside the hexagon there were large increases in optical depth, by up to a factor of 10 near the eye for the putative diphosphine layer and by a factor of four over most of the hexagon interior. Inside the eye, aerosol optical depths were very low, suggesting downwelling motions. The high contrast between eye and surroundings in 2016 was due to substantial increases in optical depths outside the eye. The color change from blue/green to gold inside most of the hexagon region can be explained in our model almost entirely by changes in the stratospheric haze, which increased between 2013 and 2016 by a factor of four in optical depth and by almost a factor of three in the short-wavelength peak imaginary index., 47 pages, 38 figures, 7 tables

The identity of the coloring agent(s) in Jupiter's atmosphere and the exact structure of Jupiter's uppermost cloud deck are yet to be conclusively understood. The Cr\`{e}me Br\^ul\'ee model of Jupiter's tropospheric clouds, originally proposed by Baines et al. (2014) and expanded upon by Sromovsky et al. (2017) and Baines et al. (2019), presumes that the chromophore measured by Carlson et al. (2016) is the singular coloring agent in Jupiter's troposphere. In this work, we test the validity of the Cr\`{e}me Br\^ul\'ee model of Jupiter's uppermost cloud deck using spectra measured during the Juno spacecraft's 5$^{\mathrm{th}}$ perijove pass in March 2017. These data were obtained as part of an international ground-based observing campaign in support of the Juno mission using the NMSU Acousto-optic Imaging Camera (NAIC) at the 3.5-m telescope at Apache Point Observatory in Sunspot, NM. We find that the Cr\`{e}me Br\^ul\'ee model cloud layering scheme can reproduce Jupiter's visible spectrum both with the Carlson et al. (2016) chromophore and with modifications to its imaginary index of refraction spectrum. While the Cr\`{e}me Br\^ul\'ee model provides reasonable results for regions of Jupiter's cloud bands such as the North Equatorial Belt and Equatorial Zone, we find that it is not a safe assumption for unique weather events, such as the 2016-2017 Southern Equatorial Belt outbreak that was captured by our measurements., Comment: 38 pages, 21 figures; Accepted for publication in AAS Planetary Science Journal

We report results incorporating the optical properties of the red-tinted photochemically-generated aerosols of Carlson et al. (2016, Icarus 274, 106–115) in spectral models of Jupiter's Great Red Spot (GRS). This material - created in laboratory GRS simulations from acetylene reacting with photolytic products of ammonia produced by ~0.2-μm radiation, similar to solar radiation on Jupiter within and above the upper troposphere, provides an excellent match to 0.35–1.05-μm spectra of the core of the Great Red Spot obtained by the Visual Infrared Mapping Spectrometer (VIMS) during the 2000–2001 Cassini-Huygens flyby. Radiative transfer models of GRS spectra acquired near the central-meridian (CM) and limb by the visual channel of the Cassini/VIMS near closest approach on December 31, 2000 and January 2, 2001, respectively, show remarkable agreement for model morphologies where the following conditions all exist: (1) most of the optical depth of the Carlson et al. (2016) chromophore resides near the top or above the main cloud layer, rather than being uniformly distributed within it, (2) the chromophore consists of relatively small particles in the 0.1–0.2 μm range, and (3) the 1-μm optical depth of the chromophore layer is small, of the order of 0.1–0.2. For such models, the chromophore layer mass abundance is 32–40 μgm cm−2. Consideration of the availability of the acetylene and ammonia parent gas material near the observed chromophore layers gives powerful support for the chromophore residing at the top of the main cloud in the upper troposphere rather than residing as a detached layer in the stratosphere. Under steady-state formation/loss conditions, consideration of plausible eddy diffusion coefficients pertaining to the relatively quiescent Jovian upper atmosphere yield untenable vertical transport times of more than several centuries required to supply the acetylene needed to form the chromophore layer, with stratospheric chromophore models requiring more than half a millennium. Consideration of the convective nature of the GRS and possible presence of acetylene-generating thunderstorms yields upper-troposphere chromophore layer formation times ranging from 11 years to 1.5 months for (1) plausible eddy diffusion coefficients ranging from 104 to 106 cm2 s−1 and (2) efficient conversion of C2H2 and NH3 into the Carlson et al. (2016) chromophore. Thus, the enhanced red coloring of the GRS may be due to the combined effects of (1) the relatively high, 0.2-bar cloudtop where ammonia ice and gas are delivered to prime photo-dissociation altitudes, by (2) powerful convection, which also delivers acetylene from depth created by (3) lightning, which is itself a by-product and indicator of powerful convection, and aided by (4) the vortex nature of the GRS, which helps to confine and concentrate the chromophore as it forms over time within the GRS anticyclone.

In this study we performed a new analysis of ground-based observations that were taken on 17 April 2011 using the 10-metre Keck telescope on Mauna Kea, Hawaii. Emissions from H 3 + , a major ion in Saturn’s ionosphere, were previously analyzed from these observations, indicating that peaks in emission at specific latitudes were consistent with an influx of charged water products from the rings known as ‘ring rain’. Subsequent modeling showed that these peaks in emission are best explained by an increase in H 3 + density, rather than in column-averaged H 3 + temperatures, as a local reduction in electron density (due to charge exchange with water) lengthens the lifetime of H 3 + . However, what has been missing until now is a direct derivation of the H 3 + parameters temperature, density and radiative cooling rates, which are required to confirm and expand on existing models and theory. Here we present measurements of these H 3 + parameters for the first time in the non-auroral regions of Saturn, using two H 3 + lines, Q(1,0 − ) and R(2,2). We confirm that H 3 + density is enhanced near the expected ‘ring rain’ planetocentric latitudes near 45°N and 39°S. A low H 3 + density near 31°S, an expected prodigious source of water, may indicate that the rings are ‘overflowing’ material into the planet such that H 3 + destruction by charge-exchange with incoming neutrals outweighs its lengthened lifetime due to the aforementioned reduction in electron density. Derived H 3 + temperatures were low while the density was high at 39°S, potentially indicating that the ionosphere is most affected by ring rain in the deep ionosphere. Saturn’s moon Enceladus, a known water source, is connected with a dense region of H 3 + centered on 62°S, perhaps indicating that charged water from Enceladus is draining into Saturn’s southern mid-latitudes. We estimated the water product influx using previous modeling results, finding that 432 - 2870 kg s − 1 of water delivered to Saturn’s mid-latitudes is sufficient to explain the observed H 3 + densities. Assuming that our Saturn northern Spring measurement represents all seasons, and that the rings are able to reorganize over time, the ring rain mechanism alone will drain Saturn’s rings to the planet in 292 − 124 + 818 million years.

The interest in the possibility of life on Venus is driven not just by curiosity about life originating in another Earth-like environment, but because of the possibility that life may be playing a critical role in the planet’s present, and possibly its past, atmospheric state. The brilliance of Venus in the night sky (as viewed from Earth) is due to its highly reflective cloud cover, about 28 km thick at the equator. Its spectral albedo is about 90% at wavelengths > 500 nm, but it drops gradually to about 40% around 370 nm before rising slightly at shorter wavelengths. This albedo drop is due to the presence of several absorbers in the atmosphere and the cloud cover. A very large fraction of the energy absorbed by Venus is at ultraviolet wavelengths with sulfur dioxide above the clouds contributing to the absorption below 330 nm; however, the identities of the other absorbers remain unknown. The inability to identify the absorbers that are responsible for determining the radiative energy balance of Venus over the last century is a major impediment to understanding how the planet “works”, a major component of NASA’s efforts in planetary exploration. Limaye et al. (Astrobiology 18, 1181-1198, 2018) presented a hypothesis suggesting that cloud-based microbial life could be contributors to the spectral signatures of Venus’ clouds, building upon previous suggestions of the possibility of life in the clouds of Venus.Four interconnected themes for the exploration of Venus as an astrobiology target are: – (i) investigations focused on the likelihood that liquid water existed on the surface in the past leading to the potential for the origin and evolution of life, (ii) investigations into the potential for habitable zones within Venus’ clouds and Venus-like atmospheres, (iii) theoretical investigations into how active aerobiology may impact the radiative energy balance of Venus’ clouds and Venus-like atmospheres, and (iv) application of these investigative themes towards better understanding the atmospheric dynamics and habitability of exoplanets. These themes can serve as a basis for proposed Venus Astrobiology Objectives and suggestions for measurements for future missions, as per the goals and objectives developed by the Venus Exploration Analysis Group (VEXAG), which is sponsored by NASA to plan for the future exploration of Venus. A Venus Collection to be published in Astrobiology journal in 2021 will include papers from the “Habitability of the Venus Cloud Layer”, Moscow (October 2019) workshop.

A lightweight, low-power instrument package to measure,in situ,both (1) the local gaseous environment and (2) the composition and microphysical properties of attendant venusian aerosols is presented. This Aerosol-Sampling Instrument Package (ASIP) would be used to explore cloud chemical and possibly biotic processes on future aerial missions such as multiweek balloon missions and on short-duration ( A quadrupole ion-trap mass spectrometer (QITMS; Madzunkov and Nikolić,J Am Soc Mass Spectrom25:1841–1852, 2014) fed alternately by (1) an aerosol separator that injects only aerosols into a vaporizer and mass spectrometer and (2) the pure aerosol-filtered atmosphere, achieves the compositional measurements. Aerosols vaporized 2SO4aerosols, to better than 20% in 2SO4-relative abundances of 2 × 10−9. An integrated lightweight, compact nephelometer/particle-counter determines the number density and particle sizes of the sampled aerosols.

Venus Corona and Tessera Explorer (VeCaTEx) would use an aerobot to descend repeatedly beneath the dense clouds for imaging targeted area of the surface in the near infrared to address six of the prime investigations prioritized by VEXAG. The technologies needed could be matured during the next decade.

International audience; More than 85% of the 23 investigations developed by VEXAG are largely accomplished via a NF mission centered on a variable-altitude balloon (aerobot) supported by a science/comm orbiter. Circling Venus >15 times over ~90 days, the aerobot repeatedly visits 52-62 km alts as it semi-continuously samples a host of environmental & surface parameters.

We present a case for the exploration of Venus as an astrobiology target—(1) investigations focused on the likelihood that liquid water existed on the surface in the past, leading to the potential for the origin and evolution of life, (2) investigations into the potential for habitable zones within Venus’ present-day clouds and Venus-like exo atmospheres, (3) theoretical investigations into how active aerobiology may impact the radiative energy balance of Venus’ clouds and Venus-like atmospheres, and (4) application of these investigative approaches toward better understanding the atmospheric dynamics and habitability of exoplanets. The proximity of Venus to Earth, guidance for exoplanet habitability investigations, and access to the potential cloud habitable layer and surface for prolonged in situ extended measurements together make the planet a very attractive target for near term astrobiological exploration.

The authors and co-signers of the Terrestrial Planets Comparative Climatology (TPCC) mission concept white paper advocate that planetary science in the next decade would greatly benefit from comparatively studying the fundamental behavior of the atmospheres of Venus and Mars, contemporaneously and with the same instrumentation, to capture atmospheric response to the same solar forcing, and with a minimum of instrument-related variability. Thus, this white paper was created for the 2023-2032 Planetary Science Decadal Survey process. It describes the science rationale for such a mission, and a mission concept that could achieve such a mission., 8 pages including cover page with one figure on cover page

We describe the prediction, design, execution and calibration of stellar and solar occultation observations of Saturn's rings by the Visual and Infrared Mapping Spectrometer (VIMS) instrument on the Cassini spacecraft. Particular attention is paid to the technique developed for onboard acquisition of the stellar target and to the geometric and photometric calibration of the data. Examples of both stellar and solar occultation data are presented, highlighting several aspects of the data as well as the different occultation geometries encountered during Cassini's 13 year orbital tour. Complete catalogs of ring stellar and solar occultations observed by Cassini-VIMS are presented, as a guide to the standard data sets which have been delivered to the Planetary Data System's Ring Moon Systems Node., 90 Pages, 22 Figures, Accepted for Publication in Icarus

This white paper, written in support of NASA's 2023-2032 Planetary Decadal Survey, outlines 10 major questions that focus on the origin, evolution, and current processes that shape the atmospheres of Uranus and Neptune. Prioritizing these questions over the next decade will greatly improve our understanding of this unique class of planets, which have remained largely unexplored since the Voyager flybys. Studying the atmospheres of the Ice Giants will greatly inform our understanding of the origin and evolution of the solar system as a whole, in addition to the growing number of exoplanetary systems that contain Neptune-mass planets., Comment: 8 pages, 0 figures, White Paper submitted to the Astrobiology and Planetary Science Decadal Survey

A 5° latitude band on Saturn centered near planetocentric latitude 36°S is known as “Storm Alley” because it has been for several extended periods a site of frequent lightning activity and associated thunderstorms, first identified by Porco et al. (2005). The thunderstorms appeared as bright clouds at short and long continuum wavelengths, and over a period of a week or so transformed into dark ovals (Dyudina et al., 2007). The ovals were found to be dark over a wide spectral range, which led Baines et al. (2009) to suggest the possibility that a broadband absorber such as soot produced by lightning could play a significant role in darkening the clouds relative to their surroundings. Here we show that an alternative explanation, which is that the clouds are less reflective because of reduced optical depth, provides an excellent fit to near infrared spectra of similar features obtained by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) in 2008, and leads to a plausible scenario for cloud evolution. We find that the background clouds and the oval clouds are both dominated by the optical properties of a ubiquitous upper cloud layer, which has the same particle size in both regions, but about half the optical depth and physical thickness in the dark oval regions. The dark oval regions are also marked by enhanced emissions in the 5-µm window region, a result of lower optical depth of the deep cloud layer near 3.1–3.8 bar, presumably composed of ammonium hydrosulfide (NH4SH). The bright storm clouds completely block this deep thermal emission with a thick layer of ammonia (NH3) clouds extending from the middle of the main visible cloud layer probably as deep as the 1.7-bar NH3 condensation level. Other condensates might also be present at higher pressures, but are obscured by the NH3 cloud. The strong 3-µm spectral absorption that was displayed by Saturn’s Great Storm of 2010–2011 (Sromovsky et al., 2013) is weaker in these storms because the contrast is muted by the overlying cloud deck that these less intense storms do not fully penetrate. Our speculated evolutionary scenario that seems consistent with these results is that deep convection produces lightning and bright clouds of large ammonia particles that rise up into the mid level of the overlying visible deck, pushing out the particles in that layer with the outflow at the top of the convective towers. When the convective pulse subsides, these large particles fall out of the column within a week or so, leaving behind less optical depth than background clouds, making them appear darker because they are less reflective. However, this simple picture does not explain all details of the phenomenon, e.g. the irregular morphology of the bright convective regions and the stable regular shapes of the dark ovals that are formed in their wake.

We used 0.85 - 5.1 micron 2006 observations by Cassini's Visual and Infrared Mapping Spectrometer (VIMS) to constrain the unusual vertical structure and compositions of cloud layers in Saturn's south polar region, the site of a powerful vortex circulation, shadow-casting cloud bands, and spectral evidence of ammonia ice clouds without the lightning usually associated with such features. We modeled spectral observations with a 4-layer model that includes (1) a stratospheric haze, (2) a top tropospheric layer of non-absorbing (possibly diphosphine) particles near 300 mbar, with a fraction of an optical depth (much less than found elsewhere on Saturn), (3) a moderately thicker layer (1 - 2 optical depths) of ammonia ice particles near 900 mbar, and (4) extending from 5 bars up to 2-4 bars, an assumed optically thick layer where NH4SH and H20 are likely condensables. What makes the 3-micron absorption of ammonia ice unexpectedly apparent in these polar clouds, is not penetrating convection, but instead the relatively low optical depth of the top tropospheric cloud layer, perhaps because of polar downwelling and/or lower photochemical production rates. We did not find any evidence for optically thick eyewalls that were previously thought to be responsible for the observed shadows. Instead, we found evidence for small step-wise decreases in optical depth of the stratospheric haze near 87.9 deg S and in the putative diphosphine layer near 88.9 deg S, which are the likely causes of shadows and bright features we call antishadows. We found changes as a function of latitude in the phosphine vertical profile and in the arsine mixing ratio that support the existence of downwelling within 2 deg of the pole., 28 pages, 21 figures, 7 tables

We present Keck-NIRSPEC observations of Saturn's H3+ aurora taken over a period of a month, in support of the Cassini mission's ‘Grand Finale’. These observations produce two-dimensional maps of Saturn's H3+ temperature and ion winds for the first time. These maps show surprising complexity, with different morphologies seen in each night. The H3+ ion winds reveal multiple arcs of 0.5–1 km s−1 ion flows inside the main auroral emission. Although these arcs of flow occur in different locations each night, they show intricate structures, including mirrored flows on the dawn and dusk of the planet. These flows do not match with the predicted flows from models of either axisymmetric currents driven by the Solar Wind or outer magnetosphere, or the planetary periodic currents associated with Saturn's variable rotation rate. The average of the ion wind flows across all the nights reveals a single narrow and focused approximately 0.3 km s−1 flow on the dawn side and broader and more extensive 1–2 km s−1 sub-corotation, spilt into multiple arcs, on the dusk side. The temperature maps reveal sharp gradients in ionospheric temperatures, varying between 300 and 600 K across the auroral region. These temperature changes are localized, resulting in hot and cold spots across the auroral region. These appear to be somewhat stable over several nights, but change significantly over longer periods. The position of these temperature extremes is not well organized by the planetary period and there is no evidence for a thermospheric driver of the planetary period current system. Since no past magnetospheric or thermospheric models explain the rich complexity observed here, these measurements represent a fantastic new resource, revealing the complexity of the interaction between Saturn's thermosphere, ionosphere and magnetosphere. This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H3+, H5+ and beyond’.

Cassini's last look at Saturn's rings During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science , this issue p. 1046 , p. eaat2965 , p. eaat2349 , p. eaau1017 ; see also p. 1028

International audience; We have analyzed the complete Visual and Infrared Mapping Spectrometer (VIMS) data archive of Titan. Our objective is to build global surface cartographic products, by combining all the data gathered during the 127 targeted flybys of Titan into synthetic global maps interpolated on a grid at 32 pixels per degree (∼1.4 km/pixel at the equator), in seven infrared spectral atmospheric windows. Multispectral summary images have been computed for each single VIMS cube in order to rapidly identify their scientific content and assess their quality. These summary images are made available to the community on a public website (vims.univ-nantes.fr). The global mapping work faced several challenges due to the strong absorbing and scattering effects of the atmosphere coupled to the changing observing conditions linked to the orbital tour of the Cassini mission. We determined a surface photometric function which accounts for variations in incidence, emergence and phase angles, and which is able to mitigate brightness variations linked to the viewing geometry of the flybys. The atmospheric contribution has been reduced using the subtraction of the methane absorption band wings, considered as proxies for atmospheric haze scattering. We present a new global three color composite map of band ratios (red: 1.59/1.27 µm; green: 2.03/1.27 µm; blue: 1.27/1.08 µm), which has also been empirically corrected from an airmass (the solar photon path length through the atmosphere) dependence. This map provides a detailed global color view of Titan's surface partially corrected from the atmosphere and gives a global insight of the spectral variability, with the equatorial dunes fields appearing in brownish tones, and several occurrences of bluish tones localized in areas such as Sinlap, Menvra and Selk craters. This kind of spectral map can serve as a basis for further regional studies and comparisons with radiative transfer outputs, such as surface albedos, and other additional data sets acquired by the Cassini Radar (RADAR) and Imaging Science Subsystem (ISS) instruments.

International audience; Methane rain on Saturn's moon Titan makes it the only place, other than Earth, where rain interacts with the surface. When and where that rain wets the surface changes seasonally in ways that remain poorly understood. Here we report the discovery of a bright ephemeral feature covering an area of 120,000 km 2 near Titan's north pole in observations from Cassini's near-infrared instrument, Visual and Infrared Mapping Spectrometer on 7 June 2016. Based on the overall brightness, spectral characteristics, and geologic context, we attribute this new feature to specular reflections from a rain-wetted solid surface like those off of a sunlit wet sidewalk. The reported observation is the first documented rainfall event at Titan's north pole and heralds the arrival of the northern summer (through climatic evidence), which has been delayed relative to model predictions. This detection helps constrain Titan's seasonal change and shows that the "wet-sidewalk effect can be used to identify other rain events." Plain Language Summary Cassini arrived in the Saturnian system in the southern summers of 2004. As expected, the Cassini team observed cloud cover, storms, and precipitation on the south pole. Like Earth, Titan has an axial tilt (27 •) and its seasons vary over its year (30 Earth years). Ever since this shift in season began, the Cassini team eagerly waited for observations indicating cloud cover and precipitation that went missing from the northern latitudes. Our rainfall observation at the north pole is a major finding for two important reasons. First, this discovery observation heralds the much awaited arrival of the north polar summer rainstorms on Titan. This atmospheric phenomenon has been delayed compared to the theoretical predictions and was perplexing Titan researchers and climate modelers especially because the north pole hosts most of Titan's lakes and seas. Second, it is extremely difficult to detect rainfall events on Titan due to its thick atmospheric haze and very limited opportunities to view the surface (and its changes). We have used a novel phenomenon-the smoothening of a previously dry, rough surface by a thin layer of fluid after rainfall, similar to a wet sidewalk-as evidence for rainfall events on the surface of Titan.

Cassini's last look at Saturn's rings During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science , this issue p. 1046 , p. eaat2965 , p. eaat2349 , p. eaau1017 ; see also p. 1028

著者人数: 11名, Accepted: 2019-06-19, 資料番号: SA1190089000

Saturn’s moon Titan has a dense nitrogen-rich atmosphere, with methane as its primary volatile. Titan’s atmosphere experiences an active chemistry that produces a haze of organic aerosols that settle to the surface and a dynamic climate in which hydrocarbons are cycled between clouds, rain and seas. Titan displays particularly energetic meteorology at equinox in equatorial regions, including sporadic and large methane storms. In 2009 and 2010, near Titan’s northern spring equinox, the Cassini spacecraft observed three distinctive and short-lived spectral brightenings close to the equator. Here, we show from analyses of Cassini spectral data, radiative transfer modelling and atmospheric simulations that the brightenings originate in the atmosphere and are consistent with formation from dust storms composed of micrometre-sized solid organic particles mobilized from underlying dune fields. Although the Huygens lander found evidence that dust can be kicked up locally from Titan’s surface, our findings suggest that dust can be suspended in Titan’s atmosphere at much larger spatial scale. Mobilization of dust and injection into the atmosphere would require dry conditions and unusually strong near-surface winds (about five times more than estimated ambient winds). Such strong winds are expected to occur in downbursts during rare equinoctial methane storms—consistent with the timing of the observed brightenings. Our findings imply that Titan—like Earth and Mars—has an active dust cycle, which suggests that Titan’s dune fields are actively evolving by aeolian processes. Saturn’s moon Titan may have an active dust cycle in equatorial regions driven by storm winds, Cassini observations consistent with dust suspension in Titan’s atmosphere suggest.

We use observations from the Imaging Science Subsystem on Cassini to create maps of Saturn's Northern Hemisphere (NH) from 2008 to 2015, a time period including a seasonal transition (i.e., Spring Equinox in 2009) and the 2010 giant storm. The processed maps are used to investigate vortices in the NH during the period of 2008-2015. All recorded vortices have diameters (east-west) smaller than 6000 km except for the largest vortex that developed from the 2010 giant storm. The largest vortex decreased its diameter from ~ 11000 km in 2011 to ~ 5000 km in 2015, and its average diameter is ~ 6500 km during the period of 2011-2015. The largest vortex lasts at least 4 years, which is much longer than the lifetimes of most vortices (less than 1 year). The largest vortex drifts to north, which can be explained by the beta drift effect. The number of vortices displays varying behaviors in the meridional direction, in which the 2010 giant storm significantly affects the generation and development of vortices in the middle latitudes (25°-45°N). In the higher latitudes (45°-90°N), the number of vortices also displays strong temporal variations. The solar flux and the internal heat do not directly contribute to the vortex activities, leaving the temporal variations of vortices in the higher latitudes (45°-90°N) unexplained.

The high altitude of Jupiter's Great Red Spot (GRS) may enhance the upward flux of gaseous ammonia (NH3) into the high troposphere, where NH3 molecules can be photodissociated and initiate a chain of chemical reactions with downwelling acetylene molecules (C2H2). These reactions, experimentally studied earlier by (Ferris and Ishikawa [1987] Nature 326, 777-778) and (Ferris and Ishikawa [1988] J. Amer. Chem. Soc. 110, 4306-4312), produce chromophores that absorb in the visible and ultraviolet regions. In this work we photolyzed mixtures of NH3 and C2H2 using ultraviolet radiation with a wavelength of 214 nm and measured the spectral transmission of the deposited films in the visible region (400-740 nm). From these transmission data we estimated the imaginary indices of refraction. Assuming that ammonia grains at the top of the GRS clouds are coated with this material, we performed layered sphere and radiative transfer calculations to predict GRS reflection spectra. Comparison of those results with observed and previously unreported Cassini visible spectra and with true-color images of the GRS show that the unknown GRS chromophore is spectrally consistent with the coupled NH3-C2H2 photochemical products produced in our laboratory experiments. Using high-resolution mass spectrometry and infrared spectroscopy we infer that the chromophore-containing residue is composed of aliphatic azine, azo, and diazo compounds.

The Saturnian satellite Tethys exhibits geological and spectral properties, whose appearance, nature and spatial distribution partly mirror those identified on the neighboring satellites Dione and Rhea or fit to the picture how spectral surface properties are expected to change from one satellite to the other within the inner Saturnian system. However, we also identified spectral variations that are unique in the Saturnian system. Whereas geologically young surface features are characterized by pure H2O-ice composition with relatively large particles, which match the particle sizes measured for fresh surface features also on Dione and Rhea, geologically old weathered regions are dominated by submicron-sized ice particles. Our investigations confirm that the Odysseus impact event did not cause the formation of Tethys' extended graben system Ithaca Chasma. On the contrary, Odysseus might be responsible for the N-S trending 'icy' bands that mark Tethys' surface in the center of its leading and trailing hemisphere.

The Saturn auroral campaign carried out in the spring of 2013 used multiple Earth-based observations, remote-sensing observations from Cassini, and in situ-observations from Cassini to further our understanding of auroras at Saturn. Most of the remote sensing and Earth-based measurements are, by nature, not continuous. And, even the in situ measurements, while continuously obtained, are not always obtained in regions relevant to the study of the aurora. Saturn kilometric radiation, however, is remotely monitored nearly continuously by the Radio and Plasma Wave Science instrument on Cassini. This radio emission, produced by the cyclotron maser instability, is tightly tied to auroral processes at Saturn as are auroral radio emissions at other planets, most notably Jupiter and Earth. This paper provides the time history of the intensity of the radio emissions through the auroral campaign as a means of understanding the temporal relationships between the sometimes widely spaced observations of the auroral activity. While beaming characteristics of the radio emissions are known to prevent single spacecraft observations of this emission from being a perfect auroral activity indicator, we demonstrate a good correlation between the radio emission intensity and the level of UV auroral activity, when both measurements are available.

On 21–22 April 2013, during a coordinated auroral observing campaign, instruments onboard Cassini and the Hubble Space Telescope observed Saturn’s aurora while Cassini traversed Saturn’s high latitude auroral field lines. Signatures of upward and downward field-aligned currents were detected in the nightside magnetosphere in the magnetic field and plasma measurements. The location of the upward current corresponded to the bright ultraviolet auroral arc seen in the auroral images, and the downward current region was located poleward of the upward current in an aurorally dark region. Within the polar cap magnetic field and plasma fluctuations were identified with periods of ∼20 and ∼60 min. The northern and southern auroral ovals were observed to rock in latitude in phase with the respective northern and southern planetary period oscillations. A solar wind compression impacted Saturn’s magnetosphere at the start of 22 April 2013, identified by an intensification and extension to lower frequencies of the Saturn kilometric radiation, with the following sequence of effects: (1) intensification of the auroral field-aligned currents; (2) appearance of a localised, intense bulge in the dawnside (04–06 LT) aurora while the midnight sector aurora remained fainter and narrow; and (3) latitudinal broadening and poleward contraction of the nightside aurora, where the poleward motion in this sector is opposite to that expected from a model of the auroral oval’s usual oscillation. These observations are interpreted as the response to tail reconnection events, initially involving Vasyliunas-type reconnection of closed mass-loaded magnetotail field lines, and then proceeding onto open lobe field lines, causing the contraction of the polar cap region on the night side.

On 19–21 April 2013, the ground-based 10-m W.M. Keck II telescope was used to simultaneously measure View the MathML sourceH3+ emissions from four regions of Saturn’s auroral ionosphere: (1) the northern noon region of the main auroral oval; (2) the northern midnight main oval; (3) the northern polar cap and (4) the southern noon main oval. The View the MathML sourceH3+ emission from these regions was captured in the form of high resolution spectral images as the planet rotated. The results herein contain twenty-three View the MathML sourceH3+ temperatures, column densities and total emissions located in the aforementioned regions – ninety-two data points in total, spread over timescales of both hours and days. Thermospheric temperatures in the spring-time northern main oval are found to be cooler than their autumn-time southern counterparts by tens of K, consistent with the hypothesis that the total thermospheric heating rate is inversely proportional to magnetic field strength. The main oval View the MathML sourceH3+ density and emission is lower at northern midnight than it is at noon, in agreement with a nearby peak in the electron influx in the post-dawn sector and a minimum flux at midnight. Finally, when arranging the northern main oval View the MathML sourceH3+ parameters as a function of the oscillation period seen in Saturn’s magnetic field – the planetary period oscillation (PPO) phase – we see a large peak in View the MathML sourceH3+ density and emission at ∼115° northern phase, with a full-width at half-maximum (FWHM) of ∼44°. This seems to indicate that the influx of electrons associated with the PPO phase at 90° is responsible at least in part for the behavior of all View the MathML sourceH3+ parameters. A combination of the View the MathML sourceH3+ production and loss timescales and the ±10° uncertainty in the location of a given PPO phase are likely, at least in part, to be responsible for the observed peaks in View the MathML sourceH3+ density and emission occurring at a later time than the peak precipitation expected at 90° PPO phase.

We identify and document the instances of bright ephemeral features (BEF)—bright areas that appear, disappear, and shift from flyby to flyby on Titan’s north pole, using the Cassini Visual and Infrared Mapping Spectrometer data set, thereby developing a sense of their spatial distribution and temporal frequency. We find that BEFs have differing geographic location and spatial extents. However, they have similar observation geometries and orders of surface area coverage and are mostly accompanied by specular reflections. We find the BEFs to represent either broad specular reflection off of a recently wetted surface on the north pole of Titan or a near-surface fog—both owing to probable recent rainfalls. Our surface model constrains the surface roughness to be of 9°–15° indicating the approximate vertical relief of the region to be that of cobbles. We also find that within less than two Titan days the BEF (if on the surface) might infiltrate into the subsurface. We hypothesize the parts of BEFs that extend into the maria to be precipitation fog.

The radiant energy budget and internal heat are fundamental properties of giant planets, but precise determination of these properties remains a challenge. Here, we report measurements of Jupiter’s radiant energy budget and internal heat based on Cassini multi-instrument observations. Our findings reveal that Jupiter’s Bond albedo and internal heat, 0.503 ± 0.012 and 7.485 ± 0.160 W m−2 respectively, are significantly larger than 0.343 ± 0.032 and 5.444 ± 0.425 Wm−2, the previous best estimates. The new results help constrain and improve the current evolutionary theories and models for Jupiter. Furthermore, the significant wavelength dependency of Jupiter’s albedo implies that the radiant energy budgets and internal heat of the other giant planets in our solar system should be re-examined. Finally, the data sets of Jupiter’s characteristics of reflective solar spectral irradiance provide an observational basis for the models of giant exoplanets.

We have analyzed the complete archive of the Visual and Infrared Mapping Spectrometer (VIMS) data in order to monitor and analyze the evolution of the clouds and haze coverage at both poles of Titan during the entire Cassini mission. Our objective is to give a cartographic synopsis from a VIMS perspective, to provide a global view of the seasonal evolution of Titan's atmosphere over the poles. We leave the detailed comparison with the Imaging Science Subsystem (ISS) and the Composite Infrared Spectrometer (CIRS) data sets to further studies. We have computed global hyperspectral mosaics for each of the 127 targeted flybys of Titan to produce synthetic color maps emphasizing the main atmospheric features. The north pole appears fully covered by a huge cloud as soon as the first observations in 2004 and up to the equinox in 2009 (Le Mou\'elic et al. 2012). The northern skies then became progressively clearer, after the circulation turnover in 2009, revealing the underlying lakes and seas to the optical instruments up to 2017. The reverse situation is observed over the south pole, which was mostly clear of such a high obscuring cloud during the first years of the mission, but started to develop a polar cloud in 2012. This feature grew up month after month until the end of the mission in 2017, with a poleward latitudinal extent of 75$^\circ$S in 2013 up to 58$^\circ$S in April 2017. Thanks to the spectral capabilities of VIMS, we have detected HCN spectral signatures over the north pole in almost all flybys between 2004 and 2008. These HCN signatures started then to show up over the south pole in almost all flybys between 2012 and 2017, so perfectly matching the timing and spatial extent of the northern and southern polar atmospheric features.

We use observations from the Imaging Science Subsystem on Cassini to create maps of Saturn’s Northern Hemisphere (NH) from 2008 to 2015, a time period including a seasonal transition (i.e., Spring Equinox in 2009) and the 2010 giant storm. The processed maps are used to investigate vortices in the NH during the period of 2008–2015. All recorded vortices have diameters (east-west) smaller than 6000 km except for the largest vortex that developed from the 2010 giant storm. The largest vortex decreased its diameter from ~11000 km in 2011 to ~5000 km in 2015, and its average diameter is ~6500 km during the period of 2011–2015. The largest vortex lasts at least 4 years, which is much longer than the lifetimes of most vortices (less than 1 year). The largest vortex drifts to north, which can be explained by the beta drift effect. The number of vortices displays varying behaviors in the meridional direction, in which the 2010 giant storm significantly affects the generation and development of vortices in the middle latitudes (25–45°N). In the higher latitudes (45–90°N), the number of vortices also displays strong temporal variations. The solar flux and the internal heat do not directly contribute to the vortex activities, leaving the temporal variations of vortices in the higher latitudes (45–90°N) unexplained.

James O’Donoghue’s research was supported by an appointment to the National Aeronautics and Space Administration (NASA) Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Universities Space Research Association under contract with NASA. This material is based upon work supported by NASA under grants NNX14AG72G and NNX17AF14G issued through the SSO Planetary Astronomy Program. Henrik Melin and Tom Stallard were supported by the Science and Technology Facilities Council under grant ST/K001000/1. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and the data in the form of fits files are available from the Keck archive at https://www2.keck.hawaii.edu/koa/ public/koa.php. We are grateful to the staff at the Keck Observatory. The authors wish to recognize the significant cultural role and reverence that the summit of Mauna Kea has within the indigenous Hawaiian community: we are fortunate to have the opportunity to conduct observations from this mountain.