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Scientific findings and new discoveries from the international Cassini‐Huygens mission's exploration of the Saturn system are presented in this special issue of Geophysical Research Letters. In the mission's final year, Cassini dove through the gap between the rings and Saturn for the first time, returning new science in this previously unexplored region. Running low on fuel, Cassini's journey ended on 15 September 2017, as the spacecraft plunged into Saturn's atmosphere and vaporized. Just before the mission concluded, the Grand Finale orbits through the gap provided some of the highest resolution information on Saturn's interior and atmosphere, its rings and inner icy moons, and direct in situ sampling of this intriguing region. Initial results indicate that hydrocarbons appear to be descending into the atmosphere from the rings, that electric currents flow between the inner rings and Saturn, and that a new radiation belt exists in the gap and innermost ring. Plain Language Summary: The international Cassini‐Huygens mission explored Saturn, its magnificent rings, its astonishing array of moons, and magnetic environment for 13 years, returning a wealth of groundbreaking new science. Findings made by the Cassini spacecraft and Huygens probe significantly altered our views of this outer planet system and in some cases challenged long‐held theories. Cassini explored these worlds up close, returning images that are beautiful as well as data that are scientifically priceless. Cassini's last year exploring the Saturn system provided unprecedented new science, discussed in more detail in papers in this Cassini special issue. New discoveries examined in this issue include tiny ring particles with complex hydrocarbons streaming into Saturn's atmosphere, methane from the rings feeding Saturn's upper atmosphere, electric currents flowing between Saturn and its rings, and a new inner radiation belt. Saturn gravity and magnetic field measurements detected deep winds and differential rotation in its upper layers. Results from Cassini's final orbits turned out to be more interesting than we could have imagined. Understanding the interior of Saturn and the interplay between the rings and planet will provide insights into how our solar system formed and evolved and the role of gas giant planets in exoplanet systems. Key Points: Cassini's last year exploring the Saturn system provided unprecedented new scienceNew discoveries included ring particles with complex hydrocarbons, electric currents between Saturn and rings, and inner radiation beltSaturn gravity and magnetic field measurements detected deep winds and differential rotation in its upper layers [ABSTRACT FROM AUTHOR]

Abstract: Two and a half years after Saturn orbit insertion (SOI) the Cassini composite infrared spectrometer (CIRS) has acquired an extensive set of thermal measurements (including physical temperature and filling factor) of Saturn''s main rings for a number of different viewing geometries, most of which are not available from Earth. Thermal mapping of both the lit and unlit faces of the rings is being performed within a multidimensional observation space that includes solar phase angle, spacecraft elevation and solar elevation. Comprehensive thermal mapping is a key requirement for detailed modeling of ring thermal properties. To first order, the largest temperature changes on the lit face of the rings are driven by variations in phase angle while differences in temperature with changing spacecraft elevation are a secondary effect. Ring temperatures decrease with increasing phase angle suggesting a population of slowly rotating ring particles [Spilker, L.J., Pilorz, S.H., Wallis, B.D., Pearl, J.C., Cuzzi, J.N., Brooks, S.M., Altobelli, N., Edgington, S.G., Showalter, M., Michael Flasar, F., Ferrari, C., Leyrat, C. 2006. Cassini thermal observations of Saturn''s main rings: implications for particle rotation and vertical mixing. Planet. Space Sci. 54, 1167–1176, doi: 10.1016/j.pss.2006.05.033]. Both lit A and B rings show that temperature decreases with decreasing rings solar elevation while temperature changes in the C ring and Cassini Division are more muted. Variations in the geometrical filling factor, , are primarily driven by changes in spacecraft elevation. For the optically thinnest region of the C ring, variations are found to be nearly exclusively determined by spacecraft elevation. Both a multilayer and a monolayer model provide an excellent fit to the data in this region. In both cases, a ring infrared emissivity is required, together with a random and homogeneous distribution of the particles. The interparticle shadowing function required for the monolayer model is very well constrained by our data and matches experimental measurements performed by Froidevaux [1981a. Saturn''s rings: infrared brightness variation with solar elevation. Icarus 46, 4–17]. [Copyright &y& Elsevier]

Abstract: In late 2004 and 2005 the Cassini composite infrared spectrometer (CIRS) obtained spatially resolved thermal infrared radial scans of Saturn''s main rings (A, B and C, and Cassini Division) that show ring temperatures decreasing with increasing solar phase angle, α, on both the lit and unlit faces of the ring plane. These temperature differences suggest that Saturn''s main rings include a population of ring particles that spin slowly, with a spin period greater than 3.6h, given their low thermal inertia. The A ring shows the smallest temperature variation with α, and this variation decreases with distance from the planet. This suggests an increasing number of smaller, and/or more rapidly rotating ring particles with more uniform temperatures, resulting perhaps from stirring by the density waves in the outer A ring and/or self-gravity wakes. The temperatures of the A and B rings are correlated with their optical depth, τ, when viewed from the lit face, and anti-correlated when viewed from the unlit face. On the unlit face of the B ring, not only do the lowest temperatures correlate with the largest τ, these temperatures are also the same at both low and high α, suggesting that little sunlight is penetrating these regions. The temperature differential from the lit to the unlit side of the rings is a strong, nearly linear, function of optical depth. This is consistent with the expectation that little sunlight penetrates to the dark side of the densest rings, but also suggests that little vertical mixing of ring particles is taking place in the A and B rings. [Copyright &y& Elsevier]

We have undertaken an analysis of the Voyager photopolarimeter (PPS) stellar occultation data of Saturn''s A ring. The Voyager PPS observed the bright star δ Scorpii as it was occulted by Saturn''s main rings during the spacecraft flyby of the Saturn system in 1981. The occultation measurement produced a ring profile with radial resolution of approximately 100 m, and radial structure is evident in the profile down to the resolution limit. We have applied an autoregressive technique to the data for estimating the power spectrum as a function of radius, which has allowed us to identify 40 spiral density waves in Saturn''s A ring, associated with the strongest torques due to forcing from the moons. The majority of the detected waves are observed to disperse linearly in regions beginning a few kilometers from the resonance location. We have used the dispersion behavior for those waves to calculate local surface mass densities in the vicinity of each wave. We find that the inner three-quarters of the A ring (up to the beginning of the Encke gap) has an average surface mass density of 43.8±7.9 g cm-2, while the outer region has an average surface mass density of 28.3±10.8 g cm-2. The two regions have different mean surface mass densities with a significance of approximately 0.999993, as estimated with a T-statistic, which corresponds to about 4.5σ. While the mean optical depth of the A ring increases slightly with increasing distance from Saturn, we find that it is not significantly correlated with the surface mass density; the two quantities having a linear Pearson''s correlation coefficient of rcorr≈-0.03. The variation of mass density, independent of PPS optical depth, is consistent with previous conjectures that the particle size distribution and composition are not constant across the entire A ring, particularly in the very outer portion. We estimate the mass of Saturn''s A ring from our surface mass density estimates as 4.9×1021 gm, or 8.61×10-9 of the mass of Saturn, roughly equivalent to the mass of a 110-km diameter icy satellite. This mass is almost 25% smaller than estimates from previous studies, but is well within the expected errors of the derived mass densities. We also identified three previously unstudied features which exhibit linear dispersion. The strongest of these features is tentatively identified as the Janus 13:11 density wave. The other two features do not fall near any known satellite resonances and may represent density waves created by previously undetected satellites. [Copyright &y& Elsevier]

We present two numerical models designed to reproduce the temperatures of the illuminated Saturn rings as measured by the CASSINI-CIRS instrument. Our models are constrained by all available temperature measurements performed on the illuminated rings since SOI. Both models reproduce well the variations of temperature under any illumination and observation geometry. One model is derived from a purely numerical data mining approach, relying on the implementation of a Neural Network that treats the data set globally. This model is used as a test of coverage completeness of the observational parameter space, driving our ability to characterize the rings thermal response. The second (analytical) model is derived using simple physical considerations, by treating the rings as a surface rather than as a collection of individual particles, combined with an empirical anisotropy function to describe the temperature resulting from the Sun's and Saturn's heating. The thermal response of this ring-surface is parameterized by its Bond albedo and emissivity, thermal relaxation time and a set of geometrical parameters quantifying the anisotropy of the temperature measurements depending on azimuth and elevation of the observer with respect to the ring plane, as well as on the solar elevation. Both models provide formulae to predict the ring temperature, that will ease the benchmarking of future physical models against data. The physical model is applied to fit the temperature of tens of different radial slices, allowing us to constrain the combined emissivity and albedo, thermal relaxation time and anisotropy parameters of the ring slabs with the highest radial resolution achieved so far with CIRS. Using for the first time all observation geometries available for illuminated rings, we are confident that our values are as unbiased as possible against observation geometry. The thermal relaxation time appears to be short, a few tens of minutes, and independent of the radial distance across the whole ring. A study of the temperature anisotropy suggests inter-particle shadowing is important in the B ring and in the outer A ring regions. [ABSTRACT FROM AUTHOR]

Abstract: Saturn''s C ring thermal emission has been observed in mid-infrared wavelengths, at three different epochs and solar phase angles, using ground based instruments (CFHT in 1999 and VLT/ESO in 2005) and the Infrared Radiometer Instrument Spectrometer (IRIS) onboard the Voyager 1 spacecraft in 1980. Azimuthal variations of temperature in the C ring''s inner region, observed at several phase angles, have been analyzed using our new standard thermal model [Ferrari, C., Leyrat, C., 2006. Astron. Astrophys. 447, 745–760]. This model provides predicted ring temperatures for a monolayer ring composed of spinning icy spherical particles. We confirm the very low thermal inertia (on the order of 10 ) found previously by Ferrari et al. [Ferrari, C., Galdemard, P., Lagage, P.O., Pantin E., Quoirin, C., 2005. Astron. Astrophys. 441, 379–389] that reveals the very porous regolith at the surface of ring particles. We are able to explain both azimuthal variations of temperature and the strong asymmetry of the emission function between low and high phase angles. We show that large particles spinning almost synchronously might be present in the C ring to explain differences of temperature observed between low and high phase angle. Their cross section might represent about 45% of the total cross section. However, their numerical fraction is estimated to only ∼0.1% of all particles. Thermal behavior of other particles can be modeled as isothermal behavior. This work provides an indirect estimation of the particle''s rotation rate in Saturn''s rings from observations. [Copyright &y& Elsevier]