Visual And Infrared Mapping Spectrometer Research Articles

Detailed chemical composition analysis of the Soi crater region on Titan

The Soi crater region (0° to 60°N, 180°W to −110°W), which includes the well-preserved Soi crater in its center, spans a region from Titan's aeolian-dominated equatorial regions to fluvially-dominated high northern latitudes. This provides a rich diversity of landscapes, one that is also representative of the diversity encountered across Titan. Schoenfeld et al. (2023) mapped this region at 1:800,000 scale and produced a geomorphological map showing that the area consists of 22 types of geomorphological units. The Visual and Infrared Mapping Spectrometer (VIMS) coverage of the region enabled the detailed analysis of spectra of 261 different locations using a radiative transfer technique and a mixing model, yielding compositional constraints on Titan's optical surface layer. Additional constraints on composition on the near-surface substrate were obtained from microwave emissivity. We have derived combinations of top surface materials between dark materials, tholins, water-ice, and methane suggesting that dark mobile organic material at equatorial and high latitudes indicates “young” terrains and compositions, while tholin/water-ice mixtures that dominate areas around latitude 35°N show a material that is older plains deposits that we interpret to be the end stage of aeolian and fluvial transport and deposition. We found no spectral evidence of CO2, HC3N, and NH3 ice. We use the stratigraphic relations between the various mapping units and the relation between the geomorphology and the composition of the surface layers to build hypotheses on the origin and evolution of the regional geology. We suggest that sedimentary deposits, likely aeolian, are dominant in the region with fluvial activity and leaching changing the nature of the top surfaces of the midlatitude areas of the Soi crater region.

Constraining Time Variations in Enceladus’s Water-vapor Plume with Near-infrared Spectra from Cassini’s Visual and Infrared Mapping Spectrometer

Water vapor produces a series of diagnostic emission lines in the near-infrared between 2.60 and 2.75 μm. The Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft detected this emission signal from Enceladus’s plume, and so VIMS observations provide information about the variability of the plume’s water-vapor content. Using a data set of 249 spectral cubes with relatively high signal-to-noise ratios, we confirmed the strength of this water-vapor emission feature corresponds to a line-of-sight column density of order 1020 molecules m−2, which is consistent with previous measurements from Cassini’s Ultraviolet Imaging Spectrograph. Comparing observations made at different times indicates that the water-vapor flux is unlikely to vary systematically with Enceladus’s orbital phase, unlike the particle flux, which does vary with orbital phase. However, variations in the column density on longer and shorter timescales cannot be ruled out and merit further investigation.

The history and processes of Titan's equator from the geospatial-topology of spectrally distinct units

We use hyperspectral-imaging observations from the Visual and Infrared Mapping Spectrometer (VIMS) to identify and explain processes of Titan's equator through the qualitative spatial relationships between geographic features (i.e. geospatial-topology). Our geographic features are defined by their spectra and geomorphology. We use tens of millions of VIMS pixels between 30°S and 30°N with incidence and emission angles <75°, and a pixel spatial scale of 200 km or less. Our dataset is several orders of magnitude larger than previous studies. This is possible through our use of novel techniques to reduce scattering and improve inter-flyby comparisons. We validate the dataset produced by these techniques by reproducing the results of previous studies. We use vector quantization, dimension reduction, and the Monte Carlo method to identify 14–16 spectrally and spatially distinct units within our dataset, a priori maps or images. These spectral units occur in distinct sequences, indicating that there is a discrete number of spectral pathways to describe the transitions across the surface. Using the same methodology used to identify the spectral units, we determine the spectral transition between units can be explained by five spectroclines. We define a spectrocline as the geographic expression of change in spectra between two spectrally distinct units; it is the spectral equivalent of an ecocline. We compare the spectra of the five spectroclines to the USGS spectral library to identify candidates for the change in compositions across the equator. We find evidence among the spectra of the spectroclines for changes in abundance of water-ice, acetylene, benzene, and alkane species. With the help of the geomorphological units identified in Cassini RADAR images, we discuss the significance of the spectral units and the spectroclines based on their geospatial-topology including their distribution, frequency, size, patterns, and sequences. From the correlations we identify between the spectra and geomorphology, we propose mechanisms for the formation and evolution of Titan's equatorial surface features.

Updated Radiative Transfer Model for Titan in the Near-infrared Wavelength Range: Validation against Huygens Atmospheric and Surface Measurements and Application to the Cassini/VIMS Observations of the Dragonfly Landing Area

We present an analysis of Titan data acquired by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) at the landing site of the Dragonfly mission, using a new version of our radiative transfer model for Titan, with significant updates for the spectroscopic parameters of atmospheric gases and photochemical aerosols. Our updated radiative transfer model is validated against the in situ spectroscopic measurements of the Huygens probe during its descent and once landed. We confirm that aerosols with a fractal dimension of 2.3–2.4 provide the best fit to the observations. We apply our radiative transfer model to four VIMS data cubes over the Selk crater region including the Dragonfly landing and exploration areas, further validating our model by producing consistent aerosol population and surface albedo maps. These infrared albedo maps, further corrected from the photometry, enable us to study the Selk crater region in terms of surface composition, landscape formation, and evolution. Our results suggest that the Selk crater is in an intermediate state of degradation and that the mountainous terrains of the area (including the crater rim and ejecta) are likely to be dominated by fine grains of tholin-like sediment. This organic sediment would be transported to the lowlands (crater floor and surrounding plains), possibly with water ice particles, by rivers, and further deposited and processed to form the sand particles that feed the neighboring dune fields. These results provide information for the operational and scientific preparation of the Dragonfly mission, paving the way for future exploration of Titan’s surface composition and geology.

Packed media radiative-transfer modeling with Gaussian particles: Application to spectra of icy regolith of Saturnian satellites

The radiative transfer theory with packed media correction (RTT-PM) is a balanced, physically rigorous, and practical light scattering model that is suitable for modeling reflectance and similar spectra of densely packed particulate media. The static structure factor correction to the classical radiative transfer solution and actualization of fundamental particles of media with spheres or aggregates are key properties of this model that ensure both physical rigor and practical efficiency. We improved upon the assumptions in the RTT-PM method by incorporating irregularly shaped Gaussian particles into its scheme. This incorporation of Gaussian particles is a notable advancement for applications of the RTT-PM method to planetary surfaces that often are layers of irregularly shaped particles. With the Gaussian particle RTT-PM method, we modeled spectra of Saturnian moons Dione, Rhea, and Tethys observed with the Cassini Visual and Infrared Mapping Spectrometer (VIMS), assuming pure water ice composition. For Rhea and Tethys, the Gaussian particle RTT-PM technique modeled VIMS spectra better than models with spherical or aggregate particles, strengthening prior suggestion that particles on Rhea and Tethys are solid, non-spherical particles. Dione's spectra were best modeled not with Gaussian particles but rather with an aggregate of 128 monomers. This makes Dione's icy regolith different from that on Rhea and Tethys. Prior studies have indicated that the cause for the difference might arise from the presence of surface macrostructures or absorbing materials on Dione, but according to our modeling results, increased multiple scattering from small fluffy aggregate particles is an alternative explanation.

The surface of (4) Vesta in visible light as seen by Dawn/VIR

Aims.We analyzed the surface of Vesta at visible wavelengths, using the data of the Visible and InfraRed mapping spectrometer (VIR) on board the Dawn spacecraft. We mapped the variations of various spectral parameters on the entire surface of the asteroid, and also derived a map of the lithology.Methods.We took advantage of the recent corrected VIR visible data to map the radiance factor at 550 nm, three color composites, two spectral slopes, and a band area parameter relative to the 930 nm crystal field signature in pyroxene. Using the howardite-eucrite-diogenite meteorites data as a reference, we derived the lithology of Vesta using the variations of the 930 and 506 nm (spin-forbidden) band centers observed in the VIR dataset.Results.Our spectral parameters highlight a significant spectral diversity at the surface of Vesta. This diversity is mainly evidenced by impact craters and illustrates the heterogeneous subsurface and upper crust of Vesta. Impact craters also participate directly in this spectral diversity by bringing dark exogenous material to an almost entire hemisphere. Our derived lithology agrees with previous results obtained using a combination of infrared and visible data. We therefore demonstrate that it is possible to obtain crucial mineralogical information from visible wavelengths alone. In addition to the 506 nm band, we identified the 550 nm spin-forbidden one. As reported by a laboratory study for synthetic pyroxenes, we also do not observe any shift of the band center of this feature across the surface of Vesta, and thus across different mineralogies, preventing use of the 550 nm spin-forbidden band for the lithology derivation. Finally, the largest previously identified olivine rich-spot shows a peculiar behavior in two color composites but not in the other spectral parameters.

Laboratory Investigations Coupled to VIR/Dawn Observations to Quantify the Large Concentrations of Organic Matter on Ceres

Organic matter directly observed at the surface of an inner planetary body is quite infrequent due to the usual low abundance of such matter and the limitation of the infrared technique. Fortuitously, the Dawn mission has revealed, thanks to the Visible and InfraRed mapping spectrometer (VIR), large areas rich in organic matter at the surface of Ceres, near Ernutet crater. The origin of the organic matter and its abundance in association with minerals, as indicated by the low altitude VIR data, remains unclear, but multiple lines of evidence support an endogenous origin. Here, we report an experimental investigation to determine the abundance of the aliphatic carbon signature observed on Ceres. We produced relevant analogues containing ammoniated-phyllosilicates, carbonates, aliphatic carbons (coals), and magnetite or amorphous carbon as darkening agents, and measured their reflectance by infrared spectroscopy. Measurements of these organic-rich analogues were directly compared to the VIR spectra taken from different locations around Ernutet crater. We found that the absolute reflectance of our analogues is at least two orders of magnitude higher than Ceres, but the depths of absorption bands match nicely the ones of the organic-rich Ceres spectra. The choices of the different components are discussed in comparison with VIR data. Relative abundances of the components are extrapolated from the spectra and mixture composition, considering that the differences in reflectance level is mainly due to optical effects. Absorption bands of Ceres’ organic-rich spectra are best reproduced by around 20 wt.% of carbon (a third being aliphatic carbons), in association with around 20 wt.% of carbonates, 15 wt.% of ammoniated-phyllosilicate, 20 wt.% of Mg-phyllosilicates, and 25 wt.% of darkening agent. Results also highlight the pertinence to use laboratory analogues in addition to models for planetary surface characterization. Such large quantities of organic materials near Ernutet crater, in addition to the amorphous carbon suspected on a global scale, requires a concentration mechanism whose nature is still unknown but that could potentially be relevant to other large volatile-rich bodies.

Geomorphological map of the South Belet Region of Titan

We mapped in detail Titan's South Belet region which spans from longitude 60°E to 120°E and from latitude 60°S to 0°, encompassing both equatorial and southern mid-latitude regions. We used Cassini RADAR in its Synthetic Aperture Radar (SAR) mode data as our basemap, which covers 31.8% of the region, supplemented with data from the RADAR's radiometry mode, the Imagining Science Subsystem (ISS), the Visual and Infrared Mapping Spectrometer (VIMS), and topographic data. This mapping work is a continuation of the detailed global mapping effort introduced in Malaska et al. (2016a) and continued in Lopes et al. (2020). We followed the mapping procedure described in Malaska et al. (2016a) for the Afekan Crater region and identified four major terrain classes in South Belet: craters, hummocky/mountainous, plains, and dunes. Each terrain class was subdivided into terrain units by characteristic morphology, including border shape, texture, general appearance, and radar backscatter. There are two terrain units that were not included in previous studies but were identified in our mapping of South Belet: “bright alluvial plains” and “pitted hummocky”. Similar to the Afekan Crater region, we find that plains dominate the surface make-up of South Belet, comprising ~47% of the mapped area. Unlike Afekan, the areal extent of the dunes closely rivals the dominance of plains, making up 43% of the mapped area. The next most widespread unit by area in the region following the dunes are the mountains/hummocky terrains (10%), and finally, crater terrains (0.01%). The introduction of two new units, “bright alluvial plains” and “pitted hummocky”, are necessary to capture the full range of morphologies seen in South Belet and expands our understanding of processes typical of Titan's equatorial and mid-latitude regions. For example, the presence of alluvial fans indicates a period in Titan's past where discharges and slopes were such that sediment could be mobilized and deposited. Similarly, the pits associated with the “pitted hummocky” may represent an important erosional feature, with implications for the removal of volatiles from Titan's crust. However, analysis of our geomorphological mapping results suggests the geology of South Belet is consistent with the narrative of organics dominating the equatorial and mid-latitudes. This is similar to the conclusion we arrived at through our mapping and analysis of the Afekan region. Lastly, the applicability of the terrain units from our mapping of the Afekan region, which bears a similar latitude but in the northern hemisphere, to our mapping of South Belet suggests latitudinal symmetry in Titan's surface processes and their evolution.

Lower Surface Temperature at Bright Ephemeral Feature Site on Titan's North Pole

AbstractWe report a temperature difference in one of the regions on Titan that is, documented as a bright ephemeral feature (BEF). Spectra were recorded by Cassini's Composite Infrared Spectrometer. BEFs are 5 µm bright areas that appear, disappear, and shift from flyby to flyby at Titan's North Pole observed in Visual and Infrared Mapping Spectrometer (VIMS) data. They are inferred to be broad‐specular reflections caused by a smooth surface on Titan or a near surface fog—both of which indicate that the atmosphere interacted with the surface. We used 24 spectra for “on the BEF” and 51 spectra for “off the BEF” region to determine the temperature difference during the T121 flyby (July 25, 2016). At the 410–560 cm−1 window where surface emissions are perceptible we see a decrease in temperature by 1.2 ± 0.2 K. This decrease in temperature might indicate precipitative cooling after a fresh rainfall on Titan's land/sea surface.

Spectral Modeling Using Radiative Transfer Theory with Packing Density Correction: Demonstration for Saturnian Icy Satellites

Abstract We demonstrate the capabilities of the radiative transfer theory with packed media correction (RTT-PM) in analyzing spectral data of planetary surfaces by modeling to first order the shape and band depths of spectra of icy satellites of Saturn acquired by Cassini Visual and Infrared Mapping Spectrometer (VIMS). The RTT-PM is an efficient and physically strict numerical method that employs a packing density correction, the static structure factor, to single-scattering properties of particles to simulate the light scattering by densely packed media. Originally created for layers formed by spherical homogeneous particles, the RTT-PM method has been recently updated to treat particles of arbitrary shapes and structures, including aggregates. We apply the RTT-PM method to roughly model Cassini VIMS spectra from Dione, Rhea, and Tethys as layers of spherical particles versus aggregates. The shape and structure of particles strongly affect the modeled spectra; the best model comparisons to the VIMS spectra were obtained when the surface icy particles were assumed to be small aggregates consisting of micron-sized monomers, which may imply rather compact, irregular particles. Our results suggest that presenting the icy regolith as a dense layer of nonspherical particles may noticeably affect the modeling results and bring a better understanding of the satellite surface structure and composition. The RTT-PM demonstrated itself to be a powerful tool for such studies: we computed a reflectance for 22 wavelengths within minutes using a regular desktop computer. The combination of such high efficiency and physical strictness makes the RTT-PM method advantageous for analyzing large spaceborne instrument data sets.

The surface of (1) Ceres in visible light as seen by Dawn/VIR

Aims.We study the surface of Ceres at visible wavelengths, as observed by the Visible and InfraRed mapping spectrometer (VIR) onboard the Dawn spacecraft, and analyze the variations of various spectral parameters across the whole surface. We also focus on several noteworthy areas of the surface of this dwarf planet.Methods.We made use of the newly corrected VIR visible data to build global maps of a calibrated radiance factor at 550 nm, with two color composites and three spectral slopes between 400 and 950 nm. We have made these maps available for the community via the Aladin Desktop software.Results.Ceres’ surface shows diverse spectral behaviors in the visible range. The color composite and the spectral slope between 480 and 800 nm highlight fresh impact craters and young geologic formations of endogenous origin, which appear bluer than the rest of the surface. The steep slope before 465 nm displays very distinct variations and may be a proxy for the absorptions caused by the O2− → Fe3+or the 2Fe3+→ Fe2++ Fe4+charge transfers, if the latter are found to be responsible for the drop in this spectral range. We notice several similarities between the spectral slopes and the abundance of phyllosilicates detected in the infrared by the VIR, whereas no correlation can be clearly established with carbonate species. The region of the Dantu impact crater presents a peculiar spectral behavior – especially through the color and the spectral slope before 465 nm – suggesting a change in composition or in the surface physical properties that is not observed elsewhere on Ceres.

The chemical composition of impact craters on Titan

We investigate the spectral behavior of nine Titan impact craters in order to constrain their composition. Past studies that have examined the chemical composition of impact craters on Titan have either used qualitative comparisons between craters or combined all craters into a single unit, rather than separating them by geographic location and/or degradation state. Here, we use Visual and Infrared Mapping Spectrometer (VIMS) data and a radiative transfer code to estimate the atmospheric contribution to the data, extract the surface albedos of the impact craters, and constrain their composition by using a library of candidate Titan materials, including essentially water ice, tholin, a dark component, and other possible ices at different grain sizes. Following a general characterization of the impact craters, we study two impact crater subunits, the “crater floor” and the “ejecta blanket”. The results show that the equatorial dune craters – Selk, Ksa, Guabonito, and the crater on Santorini Facula – appear to be purely composed of organic material (mainly an unknown dark component). Titan’s midlatitude plain craters – Afekan, Soi, and Forseti – along with Menrva and Sinlap, are enriched in water ice within an organic-based mixture. This follows the geographic pattern observed in our previous work with VIMS data, where the uppermost layers of the midlatitude alluvial fans, undifferentiated plains, and labyrinth terrains were found to consist of a mixture of organics and water ice, while the equatorial plains, hummocky terrains, and dunes were found to consist of a mixture of dark material and tholins. Furthermore, we found that the addition of some form of ice improves the fit in the ejecta spectra of Afekan and Sinlap craters. We find no indication for the presence of either NH3 or CO2 ice. Our main results agree with an existing Titan surface evolution scenario, wherein the impact cratering process produces a mixture of organic material and water ice, which is later “cleaned” through fluvial erosion in the midlatitude plains. This cleaning process does not appear to operate in the equatorial regions, which are quickly covered by a thin layer of sand sediment (with the exception of the freshest crater on Titan, Sinlap). Thus, it appears that active processes are working to shape the surface of Titan, and it remains a dynamic world in the present day.

Tidal Currents Detected in Kraken Mare Straits from Cassini VIMS Sun Glitter Observations

Abstract We present Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations of sun glitter—wave-induced reflections from a liquid surface offset from a specular point—on Kraken Mare. Sun glitter reveals rough sea surfaces around Kraken Mare, namely the coasts and narrow straits. The sun glitter observations indicate wave activity driven by the winds and tidal currents in Kraken Mare during northern summer. T104 Cassini VIMS observations show three sun glitter features in Bayta Fretum indicative of variegated wave fields. We cannot uniquely determine one source for the coastal Bayta waves, but we lean toward the interpretation of surface winds, because tidal currents should be too weak to generate capillary–gravity waves in Bayta Fretum. T105 and T110 observations reveal wave fields in the straits of Seldon Fretum, Lulworth Sinus, and Tunu Sinus that likely originate from the constriction of tidal currents. Coastlines of Bermoothes and Hufaidh Insulae adjoin rough sea surfaces, suggesting a complex interplay of wind-roughened seas and localized tidal currents. Bermoothes and Hufaidh Insulae may share characteristics of either the Torres Strait off Australia or the Åland region of Finland, summarized as an island-dense strait with shallow bathymetry that hosts complex surface circulation patterns. Hufaidh Insulae could host seafloor bedforms formed by tidal currents with an abundant sediment supply, similar to the Torres Strait. The coastlines of Hufaidh and Bermoothes Insulae likely host ria or flooded coastal inlets, suggesting that the Insulae may be local peaks of primordial crust isolated by an episode of sea-level rise or tectonic uplift.

Photometrically-corrected global infrared mosaics of Enceladus: New implications for its spectral diversity and geological activity

Between 2004 and 2017, spectral observations have been gathered by the Visual and Infrared Mapping Spectrometer (VIMS) on-board Cassini (Brown et al., 2004) during 23 Enceladus close encounters, in addition to more distant surveys. The objective of the present study is to produce a global hyperspectral mosaic of the complete VIMS data set of Enceladus in order to highlight spectral variations among the different geological units. This requires the selection of the best observations in terms of spatial resolution and illumination conditions. We have carried out a detailed investigation of the photometric behavior at several key wavelengths (1.35, 1.5, 1.65, 1.8, 2.0, 2.25, 2.55 and 3.6 μm), characteristics of the infrared spectra of water ice. We propose a new photometric function, based on the model of Shkuratov et al. (2011). When combined, corrected mosaics at different wavelengths reveal heterogeneous areas, in particular in the terrains surrounding the Tiger Stripes on the South Pole and in the northern hemisphere around 30°N, 90°W. Those areas appear mainly correlated to tectonized units, indicating an endogenous origin, potentially driven by seafloor hotspots.

Merging spatial and spectral datasets to place olivine in stratigraphic context at Arruntia crater, a rare window into Vesta's northern hemispheric crust

A major goal of the Dawn mission to Vesta was to test and refine current models of the asteroid's formation by characterizing the distribution of mineral components on its surface. Detection of the mineral olivine by the Visible and Infrared Mapping spectrometer (VIR) and the Framing Camera (FC) onboard Dawn was a key milestone in this effort, and was expected to help resolve a debate regarding the dominant mode of Vesta's petrologic evolution (i.e., magma ocean vs. serial magmatism). However, the subtleties of the olivine spectral signature combined with the small size of individual olivine-rich exposures (tens of meters) prohibits detailed mapping of this petrologically significant mineral component when the data from either instrument are evaluated independently. As a result, the particular role of these olivine exposures in Vesta's geologic history remains largely unresolved.Here, we fully characterize the type locality of Vesta's olivine-rich materials, the northern hemisphere craters Bellicia and Arruntia, using a novel data fusion approach for linking the VIR and FC datasets. Specifically, we have leveraged a spectral mixture analysis framework to create a new dataset that maps, or “extrapolates”, full resolution spectra, derived from VIR data, onto the higher spatial resolution of FC. When used as an exploratory tool in conjunction with the original FC and VIR data, the new “extrapolated” dataset reveals important new details about the distribution of olivine around Bellicia and Arruntia craters.There are clear compositional distinctions between the exposures at the two craters, with a much stronger olivine component in the walls of Bellicia than Arruntia. The proximal Arruntia ejecta appear more consistent with high proportions of an evolved high‑calcium pyroxene (as in eucrites) than with an enhanced olivine component. Examples of diogenite-rich howardite also occur in the region. Further, we leverage Arruntia's role as the freshest large crater in the northern hemisphere, and thus a rare window into the Vesta's northern hemispheric crust, to suggest an overall stratigraphy for the region: a thin veneer of howardite overlies a sequence of diogenite-rich (near surface), olivine-rich (intermediate), and eucrite-rich materials (at depth). The manner of exposure of these various components in a marked stratification point to a plutonic model for emplacing these mineralogically unique components in the near surface of Vesta.

Correction of the VIR-visible data set from the Dawn mission.

Data acquired at Ceres by the visible channel of the Visible and InfraRed mapping spectrometer (VIR) on board the NASA Dawn spacecraft are affected by the temperatures of both the visible (VIS) and the infrared (IR) sensors, which are, respectively, a charged coupled device and a HgCdTe array. The variations of the visible channel temperatures measured during the sessions of acquisitions are correlated with the variations in the spectral slope and shape for all the mission phases. The IR channel temperature is more stable during the acquisitions; nonetheless, it is characterized by a bimodal distribution whether the cryocooler (and, therefore, the IR channel) is used or not during the visible channel operations. When the infrared channel temperature is high (175 K, i.e., not in use and with the cryocooler off), an additional negative slope and a distortion are observed in the spectra of the visible channel. We developed an empirical correction based on a reference spectrum for the whole dataset; it is designed to correct the two issues related to the sensor temperatures that we have identified. The reference spectrum is calculated to be representative of the global Ceres' surface. It is also made of the data acquired when the visible and infrared channel temperatures are equal to the ones measured during an observation of the Arcturus star by VIR, which is consistent with several ground-based observations. The developed correction allows reliable analysis and mapping to be performed by minimizing the artifacts induced by fluctuations of the VIS temperature. Thanks to this correction, a direct comparison between different mission phases during which the VIR experienced different visible and infrared channel temperatures is now possible.

Saturn's south polar cloud composition and structure inferred from 2006 Cassini/VIMS spectra and ISS images

We used 0.85–5.1 μm 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 evidence for ammonia ice clouds without lightning. Finding ammonia ice spectral signatures in polar regions is surprising because over most of Saturn an overlying haze layer of unknown composition but significant optical depth completely obscures it, unless penetrated by significant convection, confirmed by lightning, as in the Great Storm of 2010–2011 (Sromovsky et al. 2013, Icarus 226, 402–418), and in the storms of Storm Alley region (Baines et al. 2009, Planet. & Space Sci. 57, 1650–1658). This is clarified by our radiation transfer modeling of VIMS spectra of the south polar background and discrete features, using 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 NH3 ice particles near 900 mbar, and (4) extending from 5 bars up to 2–4 bars, an assumed optically thick layer where NH4SH and H2O are likely condensables. The ammonia layer is the main modulator of pseudo-continuum I/F in reflected sunlight. That layer has about one optical depth in background clouds, but about double that in the brightest clouds, and about half that in discrete dark clouds. What makes the 3-μm absorption unexpectedly apparent in these polar clouds is the relatively low optical depth of the top tropospheric cloud layer, which can be an order of magnitude less than in non-polar regions on Saturn, perhaps because of polar downwelling and/or lower photochemical production rates. We found changes in the PH3 vertical profile and AsH3 mixing ratio that support the existence of downwelling within 2° of the pole. We also found evidence for step-wise decreases in optical depth of the stratospheric haze near 87.9° S and in the putative diphosphine layer near 88.9° S, and evidence against the idea that deep convective eyewalls are responsible for the shadows observed near the same latitudes. In 752-nm Cassini images we identified moderately bright features extending from shadow-producing boundaries when those boundaries rotated to the opposite side of Saturn's pole. Under those observing conditions an illuminated eyewall should produce a bright feature extending towards the pole. Instead, the features extend away from the pole, as expected for what we call antishadows, which are bright features produced by light illuminating a translucent layer from below. This provides strong qualitative evidence that both shadows and antishadows are produced by small step changes in the optical depth of the overlying translucent aerosol layers.

Occultation observations of Saturn's rings with Cassini VIMS

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 (Hedman and Nicholson, 2019b).

Spectral and emissivity analysis of the raised ramparts around Titan's northern lakes

Some of Titan's small northern hemisphere lakes show raised rampart features (which are distinct from raised rims), and appear as SAR-bright mound-like annuli extending away from the lake for up to tens of kilometers from the shoreline. We investigate the infrared and microwave characteristics of these features using Cassini Visual and Infrared Mapping Spectrometer (VIMS) and RADAR data. A spectral comparative analysis is performed among the lakes, their ramparts, and the surrounding regions. We overcome the profound difference in spatial resolution between VIMS and SAR data by using a method that provides overlays between the spectral images and SAR, thus enabling the correct selection of VIMS pixels. The surface properties of the selected areas are obtained using a radiative transfer analysis on the selected VIMS pixels, in addition to emissivity obtained from the RADAR in radiometry mode. Analysis of these combined and co-registered data provides constraints for the formation mechanism(s) of raised ramparts. The results show that the emissivity of the raised ramparts is close to that of Titan's labyrinthic terrains and to that of empty lake floors in the northern polar regions. This is confirmed by the VIMS analysis that also shows that the infrared spectral response of the raised ramparts is very similar to that of some empty lake floors. This suggests that both areas are made from or are covered by a similar material. In addition, two out of the eight lakes with raised ramparts show spectral differences at three specific wavelengths, 1.6, 2.0, and 5.0 μm, between the ramparts and the surrounding terrain. We hypothesize that this could be due to some component, or mixture of components in the ramparts that is less absorbent at these specific wavelengths, or it could be an effect of different grain sizes. These observations provide first insights into the possible mechanisms leading to the formation of the raised ramparts that are discussed here.

Climatology of CH4, HCN and C2H2 in Titan's upper atmosphere from Cassini/VIMS observations

The Visual and Infrared Mapping Spectrometer (VIMS) measurements of non-Local Thermodinamic Equilibrium (non-LTE) emissions of CH4, HCN and C2H2 in the near-infrared represent a dataset with unique coverage to study Titan's upper atmosphere in the altitude range from 500 to 1000 km. This region is the key to a better understanding of the middle atmosphere circulation. The assessment of the latitudinal and seasonal variations in the distributions of HCN and C2H2 could give important hints about it and complement the Composite InfraRed Spectrometer (CIRS) observations, that target the atmosphere below 500 km. The measurement of CH4 and HCN above 900 km can also help in understanding the Ion and Neutral Mass Spectrometer (INMS) observations near and above 1000 km. We have analysed the VIMS daytime spectra in the 2.9–3.4 μm region measured from 2004 to 2012 that span from the mid-northern winter through mid-northern spring Titan seasons. The measurements exhibit very strong non-LTE emissions of CH4, HCN and C2H2. Non-LTE vibrational temperatures have been calculated for the emitting levels of the three molecules, and have been used in the inversion of their abundances in the 500–1100 km region. The retrieved abundances have been averaged in latitudinal bins and in two seasons: north hemisphere winter (2004–2009) and north hemisphere early spring (2010−2012). The C2H2 average profile, retrieved for the first time from VIMS measurements, shows good agreement with UltraViolet Imaging Spectrograph (UVIS) measurements below 850 km, while higher values are found above that altitude. The HCN volume mixing ratio (VMR) shows an enhancement at the north pole during winter at all altitudes (from ∼700 km up to near 1000 km) and at the north and south poles during spring below 900 km. Also, the average C2H2 abundance is larger at both poles during spring below 800 km, while no significant variation is seen during winter. For CH4 we found enhancements above 800 km at the north pole during winter and at both poles during spring. Our results suggest that the middle atmosphere circulation on Titan extends well above the 500 km level, up to 750–800 km or even higher if we assume that the CH4 concentration above 800 km is mainly controlled by the circulation. The behavior of CH4 at the winter pole above 800 km is in line with the strong zonal winds suggested to explain INMS observations, although it can also be explained by the slower photo-dissociation occurring during polar winter. Despite the poor statistic at the polar regions, our results represent a unique dataset that could help to improve our understanding of Titan's atmosphere above 500 km.