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During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn's D ring and the denser layers of the planet's atmosphere, revealing the presence of an unanticipated low‐latitude current system via the associated azimuthal perturbation field peaking typically at ~10–30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field‐aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field‐aligned current densities above the ionosphere are typically ~5–10 nA/m2, more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field‐aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north‐south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller‐scale structures, possibly due to rapid temporal or local time variations. Key Points: We calculate ionospheric and field‐aligned currents associated with the intra‐D ring azimuthal fields observed on Cassini's proximal passesIonospheric currents are 0.5‐1.5 MA/rad similar to auroral values, while current densities are 5‐10 nA/m2 more than an order smallerComplex current patterns often show close north‐south conjugacy on larger spatial scales consistent with a variable interhemispheric system [ABSTRACT FROM AUTHOR]

Cassini's final orbits in 2016 and 2017 provided unprecedented spatial resolution of Saturn's polar regions from near‐polar spacecraft viewing geometries. Long‐wavelength channels of Cassini's Ultraviolet Imaging Spectrograph instrument detected Saturn's UV‐dark north polar hexagon near 180 nm at planetocentric latitudes near 75°N. The dark polar hexagon is surrounded by a larger, less UV‐dark collar poleward of planetocentric latitude 65°N associated with the dark north polar region seen in ground‐based images. The hexagon is closely surrounded by the main arc of Saturn's UV aurora. The UV‐dark material was locally darkest on one occasion (23 January 2017) at the boundary of the hexagon; in most Ultraviolet Imaging Spectrograph images the dark material more uniformly fills the hexagon. The observed UV‐dark stratospheric material may be a hydrocarbon haze produced by auroral ion‐neutral chemistry at submicrobar pressure levels. Ultraviolet Imaging Spectrograph polar observations are sensitive to UV‐absorbing haze particles at pressures lower than about 10–20 mbar. Key Points: Saturn's polar hexagon is visible in Cassini UVIS dataSaturn's polar hexagon is adjacent to Saturn's auroral emissionsIt is likely that auroral ion chemistry provides a source of dark material for Saturn's polar hexagon [ABSTRACT FROM AUTHOR]

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]

We have conducted an extensive survey of whistler mode hiss emission at Saturn using nearly the complete data set from the Cassini Radio and Plasma Wave Science investigation. We distinguish the hiss emission by frequency and intensity threshold criteria. The survey includes all longitudes and latitudes >25° for r < 10 RS, limited to the availability of plasma density data. The results indicate that hiss intensity is highest at high latitude, and the level of intensity is larger than chorus by at least an order of magnitude. After a rapid increase in intensity starting near L = 13, the hiss intensity remains relatively constant extending to L > 100. The intensity of the hiss as a function of frequency has a shallow peak near 35 Hz but remains always in the range ~5 × 10−5 nT2 < I < ~10−3 nT2. These survey results will be important in modeling the scattering interactions of whistler mode emissions with electrons in the Saturn magnetosphere. Based on recent particle and field observations, sources of Saturn hiss occur on and near the footprints of field‐aligned currents, plasma injections, and reconnected field lines. Key Points: Hiss occurrence and intensity are highest at high latitude and organized in L‐shell, with shallow peak at 35 HzRegions of hiss intensity can identify field lines containing plasma injections, field‐aligned currents, and SKR sourcesIntense hiss at low frequencies suggests a likely strong source of electron scattering at high energies [ABSTRACT FROM AUTHOR]

The inner boundary of Saturn's electron radiation belts, near the planet's A ring (∼2.27 Rs), is studied using Cassini's proximal orbit measurements. We find that variable convective flows transport energetic electrons to the A ring, which absorbs them instantaneously, forming the inner belt boundary. These flows are also responsible for a variable and longitudinally asymmetric boundary configuration. Prenoon, the boundary oscillates toward and away from the A ring with a 2‐week period. Postnoon, it maps persistently near the F ring (∼2.32 Rs) and coexists with localized MeV electron intensity enhancements (microbelts). We propose that the microbelts contain electrons in drift resonance with corotation, trapped in local time‐confined trajectories, which result from the aforementioned convective flows. The microbelts' collocation with the F ring implies either a local, secondary electron production due to galactic cosmic ray collisions with F ring dust or an enhanced resonant electron trapping due to an electrodynamic interaction between the F ring and Saturn's magnetosphere. Plain Language Summary: The material density of Saturn's main rings is large enough to instantaneously remove even the highest energy particles of the planet's radiation belts that come in contact with them. Even though this interaction does not sound trivial, Cassini observations indicate the opposite. The transition between Saturn's electron radiation belts and the Saturn's A ring, the outermost component of the main rings, was crossed 45 times during Cassini's final orbits. In all cases, the expected signature of electron depletion by the rings was observed but only twice this depletion mapped on the outer edge of the A ring. All other times, it was found several thousand kilometers away from its expected location. On certain areas in the afternoon sector of the radiation belts, the dropout was preceded by an unexpected, sharp increase of high‐energy electron intensities, which we named microbelts. Both the microbelts and the electron depletions map closer to Saturn's F ring, a highly structured, narrow ring of Saturn. The study investigates the topological properties of the microbelts and the electron belt depletion to demonstrate that the ring belts transition likely combines complex electron flows and a local F ring‐driven electron source that make its structure much more complex than ever anticipated. Key Points: The radial transport of energetic electrons onto Saturn's rings is dominated by convectionThe F ring contains regions of MeV electron pileup evolving separately from Saturn's electron beltsThe F ring may be both a source and a sink of energetic electrons [ABSTRACT FROM AUTHOR]

Highlights • Low-latitude H 3 + temperatures and densities were derived at Saturn for the first time. • Saturn's rings flow into the planetary ionosphere along magnetic field lines. • The flow rate to the planet is 432 - 2870 kg per second - mostly to the south. • Assuming this continued rate of loss, the rings will be gone in 292 million years. • The results suggest ring rain falls and/or reacts primarily in the deep ionosphere. Abstract 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. [ABSTRACT FROM AUTHOR]

In this paper whistler mode waves have been investigated in magnetosphere of Saturn. The derivation for perturbed distribution function, dispersion relation and growth rate have been determined by using the method of characteristic and kinetic approach. Analytical expressions for growth rate and real frequency of whistlers propagating oblique to magnetic field direction are attained. Calculations have been performed at 6 radial distances in plasma sheet region of Saturn’s magnetosphere as per data provided by Cassini. Work has been extended for bi-Maxwellian as well as Loss-cone distribution function. Parametric analysis show that temperature anisotropy, increase in number density, energy density and angle of propagation increases the growth rate of whistler waves along with significant shift in wave number. In case of Loss-cone distribution, increase in growth rate of whistlers is significantly more than for bi-Maxwellian distribution function. Generation of second harmonics can also be seen in the graphs plotted. It is concluded that parallel DC field stabilizes the wave and temperature anisotropy, angle of propagation, number density and energy density of electrons enhances the growth rate. Thus the results are of importance in analyzing observed VLF emissions over wide spectrum of frequency range in Saturnian magnetosphere. The analytical model developed can also be used to study various types of instabilities in planetary magnetospheres.

The story of Troilus and Criseyde - whether in Chaucer’s or Henryson’s renditions - is not a story about a new beginning, but a story about an end: the end of love, of hope, and finally - the end of life: Troilus’s life in Chaucer’s poem and Cresseid’s life in Henryson’s. The Scottish version of the story, however, not only evokes the end of an individual life, but also the end of the world. The purpose of this paper is to situate Henryson’s poem in the context of apocalyptic fiction - fiction which is concerned with loss, decay and the finality of things. My contention that the poem belongs to the apocalyptic genre is based on a number of its features, such as the elegiac mood and imagery, the contrast between the past and the present, as well as the pattern of sin-redemption-preparation for death, which applies to Cresseid’s life, but also invites reflection on our own.

Theoretical models play an important role in the Planetary Space Weather Services (PSWS) of the European Planetary Network (“Europlanet”), due to their ability to predict the physical response of magnetospheric environments to compressions or rarefactions in the upstream solar wind flow. We illustrate this aspect by presenting examples of some calculations done with the UCL Magnetodisc Model in both “Jupiter” and “Saturn” mode. Similar model outputs can now be provided via the PSWS MAGNETODISC service. For each planet’s space environment, we present example model outputs showing the effect of compressions and rarefactions on the global magnetic field, plasma pressure and azimuthal current density. As a simple illustration of the physics underlying these reference models, we quantify solar wind effects by comparing the “compressed” and “expanded” outputs to a nominal “average-state” model, reflecting more typical solar wind dynamic pressures. We also describe the implementation of the corresponding PSWS MAGNETODISC Service, through which similar outputs may be obtained by potential users.

Titan has an organic-rich atmosphere and surface with a subsurface liquid water ocean that may represent a habitable environment. In this work, we determined the amount of organic material that can be delivered from Titan's surface to its ocean through impact cratering. We assumed that Titan's craters produce impact melt deposits composed of liquid water that can founder in its lower-density ice crust and estimated the amount of organic molecules that could be incorporated into these melt lenses. We used known yields for HCN and Titan haze hydrolysis to determine the amount of glycine produced in the melt lenses and found a range of possible flux rates of glycine from the surface to the subsurface ocean. These ranged from 0 to 10 11 mol/Gyr for HCN hydrolysis and from 0 to 10 14 mol/Gyr for haze hydrolysis. These fluxes suggest an upper limit for biomass productivity of ∼10 3 kgC/year from a glycine fermentation metabolism. This upper limit is significantly less than recent estimates of the hypothetical biomass production supported by Enceladus's subsurface ocean. Unless biologically available compounds can be sourced from Titan's interior, or be delivered from the surface by other mechanisms, our calculations suggest that even the most organic-rich ocean world in the Solar System may not be able to support a large biosphere.

The 2-week, virtual Future of the Search for Life science and engineering workshop brought together more than 100 scientists, engineers, and technologists in March and April 2022 to provide their expert opinion on the interconnections between life-detection science and technology. Participants identified the advances in measurement and sampling technologies they believed to be necessary to perform in situ searches for life elsewhere in our Solar System, 20 years or more in the future. Among suggested measurements for these searches, those pertaining to three potential indicators of life termed "dynamic disequilibrium," "catalysis," and "informational polymers" were identified as particularly promising avenues for further exploration. For these three indicators, small breakout groups of participants identified measurement needs and knowledge gaps, along with corresponding constraints on sample handling (acquisition and processing) approaches for a variety of environments on Enceladus, Europa, Mars, and Titan. Despite the diversity of these environments, sample processing approaches all tend to be more complex than those that have been implemented on missions or envisioned for mission concepts to date. The approaches considered by workshop breakout groups progress from nondestructive to destructive measurement techniques, and most involve the need for fluid (especially liquid) sample processing. Sample processing needs were identified as technology gaps. These gaps include technology and associated sampling strategies that allow the preservation of the thermal, mechanical, and chemical integrity of the samples upon acquisition; and to optimize the sample information obtained by operating suites of instruments on common samples. Crucially, the interplay between science-driven life-detection strategies and their technological implementation highlights the need for an unprecedented level of payload integration and extensive collaboration between scientists and engineers, starting from concept formulation through mission deployment of life-detection instruments and sample processing systems.

Due to considerable tilting of the equator to the orbital plane, presence of rings and elongation of the orbit around the Sun, Saturn is strongly affected by seasonal changes in the solar radiation influx. Such change of the mode of irradiation of Saturn's atmosphere should affect physical properties and optical characteristics of the cloud cover and/or haze above the clouds of the planet, as well as on the vertical structure of the atmosphere at different latitudes. Once in 14,7 terrestrial years we have the equality of conditions of insolation for both hemispheres, when the planet's rings are visible edge-on to a terrestrial observer. Therefore, these moments are interest for comparative study of possible differences in atmospheric characteristics of the northern and southern hemispheres. In recent years, possibilities for such observations were in 1966, 1980, 1995 and 2009. Using the observational data on the distribution of reflective characteristics of the atmosphere on the visible planetary disk, we have compared the results of calculations in the framework of a two-layer model of Saturn's atmosphere for above moments of equinoxes. It was found that in moments of equinox in 1966, 1980 and 1995 for spring hemisphere, which had just emerged from the rings shadow, the planetary latitudinal belts differ significantly from the other zones by the values of scattering component of the optical thickness τ_c of gas layer above the clouds, the volume concentration of aerosol cloud n, and imaginary part of refractive index n_i of cloud particles. But at the time of the equinox in 2009, in the spring hemisphere the expected changes of reflective and absorptive characteristics – did not happen. There methane absorption remained at the same high level and does not form any high-altitude haze and/or rarefied cloud layer. Because they have the photochemical nature, we can assume an insufficient amount of incoming energy into the atmosphere for the possibility of the formation a photochemical aerosol layer in the lower stratosphere and/or upper troposphere of Saturn.

Methane and ammonia in the atmosphere of Saturn are in the form of impurities at the level of less than tenths of a percentage. They take part in photochemical processes, the main products of which are hydrocarbons and ammonia NH_3. Polyacetylenes absorb sunlight almost to 400 nm, and hydrocarbons

Between 26 April and 15 September 2017, Cassini executed 23 highly inclined Grand Finale orbits through a new frontier for space exploration, the narrow region between Saturn and the D Ring, providing the first opportunity for obtaining in situ ionospheric measurements. During the Grand Finale orbits, the Radio and Plasma Wave Science instrument observed broadband whistler mode emissions and narrowband upper hybrid frequency emissions. Using known wave propagation characteristics of these two plasma wave modes, the electron density is derived over a broad range of ionospheric latitudes and altitudes. A two‐part exponential scale height model is fitted to the electron density measurements. The model yields a double‐layered ionosphere with plasma scale heights of 545/575 km for the northern/southern hemispheres below 4,500 km and plasma scale heights of 4,780/2,360 km for the northern/southern hemispheres above 4,500 km. The interpretation of these layers involves the interaction between the rings and the ionosphere. Plain Language Summary: For the final 5 months of the Cassini mission in 2017, the spacecraft executed 23 orbits through a new frontier for space exploration, the narrow region between Saturn and the innermost of Saturn's main rings, the D Ring. For the first time in the history of space exploration, the Cassini instruments were able to take measurements inside Saturn's ionosphere. This paper provides the density distribution of Saturn's ionospheric electrons, derived from plasma waves detected by the Radio and Plasma Wave Science instrument. The electron density distributions with altitude and latitude show that the ionospheric electron densities peak at 10,000 particles per cubic centimeter at low altitudes in the equatorial region and drop below 100 particles per cubic centimeter at higher altitudes and latitudes. Two simple ionospheric scale height density models for the northern and southern hemispheres are presented. Key Points: We present the first in situ measurements of the electron density in the low to middle latitudes of Saturn's ionosphereThe distribution of electron density measurements with altitude shows evidence of a two‐layered ionospheric electron density distribution up to an altitude of 15,000 kmWe present a scale height electron density model for a double‐layered ionosphere for both the northern and southern hemispheres [ABSTRACT FROM AUTHOR]

Atmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere). Waves may be generated from instabilities, convective storms or other atmospheric phenomena. The inferred wave amplitudes change little with height within the sampled region, raising the possibility of the waves being damped, which in turn may enhance the eddy friction within the thermosphere. Using our Saturn Thermosphere Ionosphere General Circulation Model, we explore the parameter space of how an enhanced Rayleigh drag in different latitude regimes would affect the global circulation pattern within the thermosphere and, in turn, its global thermal structure. We find that Rayleigh drag of sufficient magnitude at midlatitudes may reduce the otherwise dominant Coriolis forces and enhance equatorward winds to transport energy from poles toward the equator, raising the temperatures there to observed values. Without this Rayleigh drag, energy supplied into the polar upper atmosphere by magnetosphere‐atmosphere coupling processes remains trapped at high latitudes and causes low‐latitude thermosphere temperatures to remain well below the observed levels. Our simulations thus suggest that giant planet upper atmosphere global circulation models need to include additional Rayleigh drag in order to capture the effects of physical processes otherwise not resolved by the codes. Plain Language Summary: Atmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere) with typical vertical wavelengths ranging from 100–200 km and density amplitudes reaching around 10%. Amplitudes are roughly constant over this height range, implying that wave damping occurs, which in turn is expected to enhance eddy friction (Rayleigh drag) within the thermosphere. Using the Saturn Thermosphere Ionosphere General Circulation Model (STIM), we explore the response of Saturn's thermosphere to a range of possible Rayleigh drag profiles. We find that the introduction of momentum dissipation equatorward of ±60° latitude will slow down the zonal winds on Saturn sufficiently to enhance equatorward winds and thereby allow energy propagation from the poles toward the equator. Under the assumption that sufficiently strong Rayleigh drag is present in Saturn's thermosphere, large temperatures at low latitudes may result from wind‐driven global redistribution of energy from the polar regions. Key Points: Atmospheric waves have been detected in situ in Saturn's thermosphereRayleigh drag possibly associated with observed gravity waves has the potential to alter the global winds and thereby the temperaturesEnergy from magnetosphere‐atmosphere coupling is transported equatorward and explains the observed high temperatures at low latitudes [ABSTRACT FROM AUTHOR]

Highlights • We derived vertically-resolved spectral structures of Saturnian polar haze between 375 and 925 km altitude. • The polar haze is dominated by aromatic hydrocarbons. • The haze at lower latitudes mostly consists of aliphatic hydrocarbons. • There is a possibility that the aging process of haze can occur during both horizontal and vertical migrations. Abstract We analyzed the 3-µm emission spectra of Saturn's southern auroral atmosphere observed by the Visual Infrared Mapping Spectrometer (VIMS) on-board Cassini. Extracting adjacent 3-µm H 3 + , CH 4 , and C 2 H 6 emissions, we derived vertically-resolved spectral structures of polar haze between 375 and 925 km altitude. A comparison of the derived polar haze spectra with those of the relatively low-altitudinal (257–520 km) and low-latitudinal haze reported by Kim et al. (2012) indicates that the polar haze is dominated by aromatic hydrocarbons while the haze at lower latitudes mostly consists of aliphatic hydrocarbons. This spectral and chemical variation in the different latitudinal regions suggests a possibility that newly created haze particles at the high altitudes in the auroral regions of Saturn undergo an aging process during latitudinal advection and diffusion from the polar atmosphere to the low altitude and low latitudinal regions, while in the atmosphere of Titan, the aging process occurs during the vertical precipitation process of haze (Courtin et al., 2015a). However, there is a possibility that the aging process can occur during both the horizontal and vertical migrations. Thus, further investigation of the 3-µm spectral structure of the high-altitude haze (>520 km) at low latitudes is needed. [ABSTRACT FROM AUTHOR]

Highlights • Deuterium and carbon 13 are detected in the rings and on satellite surfaces in the Saturn system. • New methods are presented for deriving isotopic ratios from reflectance spectra of solids. • The D/H ratio of the water in Saturn's rings and icy satellites except Phoebe is close to terrestrial bulk Earth values. • Phoebe's D/H is the highest value yet measured in the Solar system implying an origin in the cold outer Solar System beyond Saturn. • Phoebe also has a high 13C/12C much greater than terrestrial, consistent with an origin in the cold outer Solar System. Abstract Isotopic ratios have long been used to learn about physical processes acting over a wide range of geological environments, and in constraining the origin and/or evolution of planetary bodies. We report the spectroscopic detection of deuterium in Saturn's rings and satellites, and use these measurements to determine the (D/H) ratios in their near-surface regions. Saturn's moons, Phoebe and Iapetus, show a strong signature of CO 2 and the 13C component of this molecule is detected and quantified. Large averages of spectra obtained by the Cassini Visual and Infrared Mapping Spectrometer, VIMS, were computed for the rings and icy satellites. The observed intensities of the infrared absorptions in H 2 O and CO 2 and their isotopes were calibrated using laboratory data and radiative transfer models to derive the D/H and 13C/12C ratios. We find that the D/H in Saturn's rings and satellites is close to the Vienna Standard Mean Ocean Water (VSMOW) and bulk Earth (4% lower than VSMOW) value except for Phoebe, which is 8.3 times the VSMOW value. This is the highest value for any Solar-System surface yet measured, and suggests that Phoebe formed from material with a different D/H ratio than the other satellites in the Saturn system. Phoebe's 13C/12C ratio is also unusual: 4.7 times greater than terrestrial, and greater than values measured for the interstellar medium and the galactic center. The high 13C abundance in the CO 2 suggests that Phoebe was never warm enough for the large D/H ratio in its surface to have originated by evaporative fractionation of its water ice (e.g., from heating in the inner Solar System before its eventual capture by Saturn). We also report the detection of a probable O-D stretch absorption due to OD in minerals on Phoebe at 3.62 μm. This absorption is not detected on other Saturnian satellites. Stronger signatures of bound water absorptions are found in the dark material of Iapetus and we report a new detection of bound water at 1.9 μm. The position of this absorption matches that seen in spectra of hydrated iron oxides but does not match absorptions seen in spectra of tholins. Despite the strong bound water signature in the Iapetus dark material, no 3.62 μm OD absorption is seen in the spectra, further indicating the high deuterium level on Phoebe is unusual. As such, it is likely that Phoebe originated in a colder part of the outer Solar System, relative to the prevailing temperatures at Saturn's distance from the Sun. [ABSTRACT FROM AUTHOR]

Highlights • Formation of Saturn's rings and Mimas from the collision of a comet with an icy moon. • Saturn ring formation mechanism produces nearly pure ice rings recently. • Ring parent moon initially trapped in mean motion resonance heats Enceladus. Abstract The disruption of an icy moon in a collision with an interloping comet a few hundred million years ago is a simple way to create Saturn's rings. A ring parent moon with a mass comparable to Mimas could be trapped in mean motion resonance with Enceladus and Dione in an orbit near the current outer edge of the rings just beyond the Roche zone. I present collisional N-body simulations of cometary impacts that lead to the partial disruption of a differentiated moon with a rocky core and icy mantle. The core can survive largely intact while the debris from the mantle settles into a ring of predominantly ice particles straddling the orbital radius of the parent moon. The nascent ring spreads radially due to collisional viscosity while mass re-accretes onto the remnant rocky core to form a new moon that can be identified as Mimas. The icy debris that migrates into the Roche zone evolves into Saturn's ring system. Torques from tidal interaction with Saturn and resonant interactions with the rings push the recently formed Mimas outward to its current position on the same timescale of a few hundred million years. This scenario accounts for the high ice fraction observed in Saturn's rings and explains why the ring mass is comparable to the mass of Mimas. The prior existence of a ring parent moon in mean motion resonance results in a tidal heating rate for Enceladus in the recent past that is significantly larger than the current rate. [ABSTRACT FROM AUTHOR]

In this paper, I publish and analyse a compilation of observations of the synodic phenomena of Saturn from Babylon, which can be dated to the early sixth century B.C. By comparing these observations with records found in other early compilations of planetary observations it is possible to trace the development of observational practice in Babylonia. The compilation contains references to several stars that are previously unattested in cuneiform texts. An attempt is made to identify these stars. [ABSTRACT FROM AUTHOR]

We compare periods and phases of Saturn planetary period oscillations determined from Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014. The mean absolute differences between the magnetic and SKR periods are ~0.0036 hr (~13 s), consistent with estimated magnetic measurement uncertainties, while the overall mean difference is less than 0.001 hr (~2–3 s), at the limit of resolution. The relative phasing between magnetic and SKR modulations is correspondingly near constant and such that the equatorial planetary period oscillation fields of the northern/southern systems point radially outward near‐oppositely at ~14.3/2.5 hr local time at corresponding SKR maxima, with upward planetary period oscillation currents located ~2 hr postdawn for both systems, consistent with previous intervals having dawnside spacecraft apoapsides. Southern SKR emissions are found to be significantly dual modulated at both southern and northern periods in data limited to lie well within the southern shadow zone of the northern sources. These northern period modulations are shown to be approximately in phase with those in the northern emissions, consistent with a recent suggestion that bidirectional auroral electron acceleration may generate in phase SKR emissions in both hemispheres. Key Points: Magnetic and SKR periods are near constant at 10.79/10.68 hr (±0.01 hr) for northern/southern PPOs during 2016–2017, agreeing to within ~ ±15 sNorthern/southern upward PPO currents are centered postdawn at related SKR maxima as for other mission intervals with dawnside apoapsidesSouthern hemisphere SKR emissions are dual modulated at the northern period approximately in phase with the northern emissions [ABSTRACT FROM AUTHOR]

How deep do Saturn's zonal winds penetrate below the cloud level has been a decades‐long question, with important implications not only for the atmospheric dynamics but also for the interior density structure, composition, magnetic field, and core mass. The Cassini Grand Finale gravity experiment enables answering this question for the first time, with the premise that the planet's gravity harmonics are affected not only by the rigid body density structure but also by its flow field. Using a wide range of rigid body interior models and an adjoint based optimization for the flow field using thermal wind balance, we calculate the flow structure below the cloud level and its depth. We find that with a wind profile, largely consistent with the observed winds, when extended to a depth of around 8,800 km, all the gravity harmonics measured by Cassini are explained. This solution is in agreement with considerations of angular momentum conservation and is consistent with magnetohydrodynamics constraints. Plain Language Summary: Observations show strong east‐west flows at the cloud level of Saturn. These winds are strongest at the equatorial regions, reaching up to 400 m/s, about 4 times stronger than tornado strength winds on Earth. Yet until now we had no knowledge on how deep these winds penetrate into the interior of the gas giant. The gravity experiment executed during the Grand Finale stage (May–August 2017) of the NASA Cassini mission helps answering this question. It is well established that any large‐scale motion of the fluid would have a signature in the density distribution and therefore in the planet gravity field. If we can estimate the internal structure and shape of the planet, we might be able to decipher the depth of the winds from its signal in the gravity measurements. Moreover, the rigid‐body and flow contribution to gravity field are entangled together, therefore it is necessary to use a wide range of rigid‐body models in order to define the wind‐induced gravity signal. In this work we propose a solution to the problem. We find that the gravity measurements can be explained with a flow pattern, similar to that observed at the cloud level, penetrating to depths of more than 8,000 km into the planet interior. This has important implications not only for the atmospheric dynamics but also for the interior density structure, composition, magnetic field, and core mass. Key Points: Cassini gravity measurements point to deep differential flows within SaturnUsing a wide range of rigid body internal structure models, the required wind‐induced gravity signal is definedWith a conservatively modified cloud‐level wind and an optimized vertical profile, extended to a depth of 8,800km, all gravity measurements are explained [ABSTRACT FROM AUTHOR]

Highlights • How Saturnian mid-sized moons avoid resonant capture during orbital evolution? • The tidal orbital evolution of these moons was investigated by N-body simulations. • The ring torque is a key to reproduce the current orbits of the mid-sized moons. • The stored heat in Enceladus during the evolution may explain the current heat flow. Abstract The formation and orbital evolution of Saturn's inner mid-sized moons – Rhea, Dione, Tethys, Enceladus, and Mimas – are still debated. The most puzzling aspects are 1) how the Tethys–Dione pair and the Mimas–Enceladus pair passed through their strong 3:2 mean-motion resonances during the tidal orbital evolution, and 2) the current strong heat flow from Enceladus, which is a few orders of magnitude higher than the tidal energy dissipation caused by the present orbital eccentricity of Enceladus. Here we perform N-body simulations of the moons' orbital evolution from various initial conditions – assuming that the moons were formed from Saturn's hypothetical massive ring – and investigate possible paths to solve the above difficulties. If the moons remain on nearly circular orbits and the influence of the rings is neglected, we find that the Tethys–Dione pair cannot avoid becoming trapped in the 2:1 and 3:2 mean-motion resonances as they recede from Saturn, and that the Tethys–Enceladus pair cannot avoid collisions after the resonance trapping, in case Saturn's quality factor is smaller than 15,000. These findings are inconsistent with the current orbital configuration. However, taking into account both the eccentricity excitation and the orbital expansion caused by the ring torque, we find that these resonance captures are avoided. With the relatively high eccentricity pumped up by the torque, Enceladus passes through all the mean-motion resonances with Tethys, and the Dione–Tethys pair passes through their 2:1 resonance and possibly the 3:2 resonance as well. After Enceladus resides beyond the 2:1 resonance with the outer ring edge, the eccentricity can be tidally damped. While this is a promising path of evolution, in most runs, Enceladus collides with Tethys by the excited eccentricity. There is a hint that a ring mass decrease (possibly due to Mimas formation) could avoid the collision between Enceladus and Tethys. The parameter survey taking into account detailed ring evolution and Mimas is left for future study. The heat that was tidally dissipated due to the eccentricity excitation by the ring torque in the past is stored in the moons and slowly radiated away through conductive transfer. The stored heat in Enceladus may account for the current anomalously high heat flow. [ABSTRACT FROM AUTHOR]

Highlights • Ring spectrophotometric behavior is investigated on VIMS-Cassini radial mosaics. • Light scattering models are used to infer ring composition and physical properties. • A mixture of water ice, tholins and a broad-band absorber(s) provides the best-fit. • The distribution of the broad-band absorber(s) is compatible with an exogenous origin. • Tholins distribution suggests an intrinsic origin. Abstract This work is the second in a series of manuscripts devoted to the investigation of the spectrophotometric properties of Saturn's rings from Cassini-VIMS (Visible and Infrared Mapping Spectrometer) observations. The dataset used for this analysis is represented by ten radial spectrograms of the rings which have been derived in Filacchione et al. (2014) by radial mosaics produced by VIMS. Spectrograms report the measured radiance factor I / F of the main rings of Saturn as a function of both radial distance (from 73500 to 141375 km) and wavelength (0.35–5.1 µm) for different observation geometries (phase angle ranging in the 2°–132° interval). We take advantage of a Monte Carlo ray-tracing routine (Ciarniello et al., 2014) to characterize the photometric behavior of the rings at each wavelength and derive the spectral Bond albedo of ring particles. This quantity is used to infer the composition of the regolith covering ring particles by applying Hapke's theory. Four different regions, characterized by different optical depths, and respectively located in the C ring, inner B ring, mid B ring and A ring, have been investigated. Results from photometric modeling indicate that, in the VIS-NIR spectral range, B ring particles are intrinsically brighter than A and C ring particles, with the latter having the lowest albedo, while the single particle phase function of the ring's particles is compatible with an Europa-like or Callisto-like formulation, depending on the investigated region. Spectral modeling of the inferred Bond albedo indicates that ring spectrum can be reproduced by water ice grains with inclusion of organic materials (tholin) as a UV absorber intimately mixed with variable amounts of other compounds in pure form (carbon, silicates) or embedded in water ice grains (nanophase hydrated iron oxides, carbon, silicates, crystalline hematite, metallic iron, troilite). The abundance of tholin decreases with radial distance from C ring (0.2–0.6%) to A ring (0.06%) for the selected regions. Its distribution is compatible with an intrinsic origin and is possibly related to the different plasma environment of the different ring regions. The identification of the other absorber(s) and its absolute volumetric abundance is uncertain, depending on the adopted grain size and mixing modality (intraparticle or intimate). However, assuming a common composition of the other absorber in the ring plane, we find that its abundance anti-correlates with the optical depth of the investigated regions, being maximum in the thinnest C ring and minimum in the thickest mid B ring. In the case of the C ring, an additional population of low-albedo grains is required to match the positive spectral slope of the continuum in the 0.55–2.2 µm interval, represented by an intraparticle mixture of water ice and a spectrum similar to troilite or metallic iron. The distribution of the darkening compounds is interpreted as the result of a contamination by exogenous material, which is more effective in the less dense regions of the rings because of their lower surface mass density of pure water ice. [ABSTRACT FROM AUTHOR]

Highlights • In situ cryogenic trap in a plasma discharge enables quantification of gas-phase precursors in Titan-like conditions • C 2 H 4 is a major hydrocarbon precursor to tholin formation • HCN and NH 3 are also important volatile products • NH 3 is the most abundant product formed with a 1% CH 4 mixing ratio Abstract The Cassini Mission has greatly improved our understanding of the dynamics and chemical processes occurring in Titan's atmosphere. It has also provided us with more insight into the formation of the aerosols in the upper atmospheric layers. However, the chemical composition and mechanisms leading to their formation were out of reach for the instruments onboard Cassini. In this context, it is deemed necessary to apply and exploit laboratory simulations to better understand the chemical reactivity occurring in the gas phase of Titan-like conditions. In the present work, we report gas phase results obtained from a plasma discharge simulating the chemical processes in Titan's ionosphere. We use the PAMPRE cold dusty plasma experiment with an N 2 CH 4 gaseous mixture under controlled pressure and gas influx. An internal cryogenic trap has been developed to accumulate the gas products during their production and facilitate their detection. The cryogenic trap condenses the gas-phase precursors while they are forming, so that aerosols are no longer observed during the 2 h plasma discharge. We focus mainly on neutral products NH 3 , HCN, C 2 H 2 and C 2 H 4. The latter are identified and quantified by in situ mass spectrometry and infrared spectroscopy. We present here results from this experiment with mixing ratios of 90–10% and 99–1% N 2 CH 4 , covering the range of methane concentrations encountered in Titan's ionosphere. We also detect in situ heavy molecules (C 7). In particular, we show the role of ethylene and other volatiles as key solid-phase precursors. [ABSTRACT FROM AUTHOR]

A sophisticated interplanetary spacecraft, Cassini-Huygens was launched on 15 October 1997. After achieving orbit at Saturn in 2004, Cassini collected science data throughout its four-year prime mission (2004-2008) and nine-year extended mission (2008-2017). The Cassini Attitude and Articulation Control Subsystem (AACS) is perhaps the spacecraft subsystem that must satisfy the most mission and science pointing requirements. The performance of the Cassini AACS design was superb from launch through end of mission. All key mission and science requirements were met with significant margins. Overviews of the Cassini attitude control system design and flight performance (in 1997-2017) are given in this paper. Processes taken by the attitude control operations team to guard against human errors are also outlined. Cassini flight experience and many lessons learned are equally applicable to the safe operations of other interplanetary missions. [ABSTRACT FROM AUTHOR]

Communication with spacecraft is essential to every space mission. However, when the estimation or control of the motion of a spacecraft cannot be performed precisely, it is extremely difficult to communicate with the spacecraft in real-time operations. To enable communication in this situation, this paper proposes an off-line signal-processing method to detect the weak signal transmitted from a spacecraft under poor communication conditions. This is achieved by the following process. First, the Doppler shift effect caused by the motion of the spacecraft is removed from the received signal. The signal power is then enhanced by integrating the signal spectrum over a long time period. Moreover, the estimated frequency model, which contains the Doppler shift information, can be used for orbit and attitude determination as well. A coarse-ranging technique based on the detected weak signal is then introduced as a method of improving the orbit-estimation accuracy. The validity and effectiveness of the proposed method are demonstrated by applying these algorithms to actual radio waves transmitted from the IKAROS. [ABSTRACT FROM AUTHOR]

The magnetosphere of Saturn is known to be warped into a bowl shape, and the severity of the warping varies with the seasonal tilt of Saturn's spin axis in its orbital plane. This peculiar geometry complicates the analysis of many magnetospheric phenomena, because they are symmetric about the warped magnetodisk equator rather than the spin equator. To reorganize the Cassini data, a new coordinate system is proposed wherein a zA coordinate represents the perpendicular distance above or below the warped surface, and a ρA coordinate represents the arc length along the surface from the origin to where the perpendicular intersects the surface. Using the Cassini spacecraft trajectory, examples show how these disk coordinates (ρA, zA) significantly deviate from the standard spin‐equatorial coordinates (ρ, z), especially outside the orbit of Titan. Another example indicates that fluxes of energetic electrons are well organized in this new system. Supporting information tabulates the disk coordinates as a function of time during the Cassini mission. Plain Language Summary: The sundry phenomena within Saturn's magnetosphere are typically organized in a "flat" coordinate system with Cartesian (x,y,z) locations. However, Saturn's magnetosphere is known to have a "bowl" shape that reflects a curved magnetodisk. The curvature of the bowl depends on season, so that the bowl is more curved near the solstices and has no curvature at the equinoxes. A new curvilinear coordinate system is proposed that is based on the bowl shape and which varies with season. An example shows how energetic electron fluxes are well‐organized in the new coordinate system. Key Points: Normal spin‐equator coordinates are inadequate to organize phenomena that are symmetric about a warped, bowl‐shaped magnetodiskA new curvilinear system is proposed that is aligned with the curved disk and changes with seasonal warping, as at SaturnThe new system differs from the spin‐equator system, especially at the solstices, but consistently reorganizes magnetospheric phenomena [ABSTRACT FROM AUTHOR]

We overview the properties of the azimuthal magnetic fields observed during the periapsis passes of the final 23 orbits of the Cassini spacecraft, including the partial orbit at end of mission, on near equatorial field lines passing inside of Saturn's D ring. The signatures are variable in form and amplitude, though generally approximately symmetric about the point where the spacecraft trajectory lies tangent to a flux shell, corresponding to where the ionospheric field line feet map closest to the equator, consistent with the effect of interhemispheric field‐aligned currents. The perturbations usually begin and end near symmetrically at some point on field lines threading the D ring and extend into the interior region, but in no case do they clearly extend outward onto field lines passing through the C ring. About 35% of cases display a ~20‐40 nT single positive central field peak indicative of southward field‐aligned current flow, while a further ~30% display two or three weaker ~10‐20 nT positive peaks indicative of multiple sheets of northward and southward current. Significant smaller‐scale >5 nT peak‐to‐peak field fluctuations are commonly superposed. A further ~20% of cases exhibit unique profiles within the data set, including two with ~20‐30 nT negative fields and two with only <10 nT fluctuating fields. The variable nature of the signatures is not connected with the pass altitude, local time, planetary period oscillation phase, or D68 ringlet phase but may relate to variable structured thermospheric winds and/or ionospheric conductivities that suggest a significant dynamical role for D ring‐atmosphere interactions. Key Points: Azimuthal magnetic fields observed on Saturn's intra‐D ring (IDR) field lines exhibit a range of profiles on the 23 Cassini proximal passesOne, two, or three near‐symmetrical positive peaks ~10‐40 nT are typical, but two passes have ~20‐30 nT negative peaks and two <10 nT fieldsVariable IDR field signatures are not organized by periapsis radius, local time, planetary period oscillation phase, or D68 ringlet phase [ABSTRACT FROM AUTHOR]

The megaelectron volt proton radiation belts of Saturn are isolated from the middle and outer magnetosphere, and the source of these high‐energy protons is thought to be linked to the access of galactic cosmic rays (GCRs) in the system. To validate this hypothesis, it is first of all necessary to determine the realistic spectrum of GCRs at Saturn. Previously, only analytical attempts were performed in order to calculate the GCR spectra. In this letter we provide for the first time the numerical solution for the determination of the GCR access to the upper atmosphere and rings of Saturn. The proposed method is based on the charged particle tracing technique. As a result the GCRs access energies to the atmosphere and to the equatorial plane of the magnetosphere as a function of radial distance to the planet were obtained and the GCRs spectra were reconstructed. Dependencies of the spectral parameters such as the time or the incidence direction were also obtained offering all necessary information for simulating the interaction of GCRs with the Saturnian system during different phases of the Cassini mission. Plain Language Summary: In this research letter we propose the numerical solution for the calculation of the galactic cosmic rays (GCRs) access energies to Saturn and its magnetosphere. Previously, only analytical approach was used to address this problem in the Saturnian system providing uncertain results. The new numerical approach on the basis of particle tracing brings much more accurate access energies, GCR spectra, and integrated GCR flux to the planet and across the magnetosphere and allows to estimate the Cosmic Ray Albedo Neutron Decay process on Saturn, calculate its contribution to the Saturnian radiation belts, and evaluate the impact of GCRs on the atmosphere, rings and moons of Saturn. Key Points: We provide GCR access energies to the Saturnian atmosphere and across the magnetosphereGCR spectra and integrated flux to Saturn and to the equatorial plane of the magnetosphere are calculatedAdvantages of a particle tracing approach over the analytical solution are demonstrated [ABSTRACT FROM AUTHOR]

We conduct a statistical analysis of 2,094 reconnection events in Saturn's near‐equatorial magnetotail previously identified in Cassini magnetometer data from intervals during 2006 and 2009/2010. These consist of tailward propagating plasmoids and planetward propagating dipolarizations, with approximately twice as many plasmoids as dipolarizations. We organize these by three related planetary period oscillation (PPO) phase systems, the northern and southern PPO phases relative to noon, the same phases retarded by a radial propagation delay, and the local retarded phases that take account of the azimuth (local time) of the observation. Clear PPO modulation is found for both plasmoid and dipolarization events, with local retarded phases best organizing the event data with the modulation in event frequency propagating across the tail as the PPO systems rotate. This indicates that the events are localized in azimuth, rather than simultaneously affecting much of the tail width. Overall, events occur preferentially by factors of ~3 at northern and southern phases where the tail current sheet is expected locally to be thinnest in the PPO cycle, with field lines contracting back from their maximum radial displacement, compared with the antiphase conditions. Separating the events into those representing the start of independent reconnection episodes, occurring at least 3 hr after the last, and events in subsequent clusters, shows that the above phases are predominantly characteristic of the majority cluster events. The phases at the start of independent reconnection episodes are typically ~60° earlier. Key Points: We analyze a catalogue of 2,094 reconnection events in Saturn's magnetotail and organize them according to northern and southern PPO phasesEvents are best organized by local PPO phases rather than global phases, implying that reconnection is more locally than globally triggeredEvents are best organized by phases where the tail current sheet should be locally thin and the field lines near maximum radial displacement [ABSTRACT FROM AUTHOR]

We surveyed energetic charged particle measurements by Cassini between 2016 and the end of the mission in order to identify transients of solar energetic particles and galactic cosmic rays. Such transients offer hints about the state of the heliosphere and valuable context for interpreting space weather phenomena in Saturn's magnetosphere. The period studied includes the mission's Ring‐Grazing and Proximal orbits, which due to their week‐long periods are ideal for capturing short timescale dynamics in the planet's magnetosphere, including solar periodicities. We find that Saturn was exposed to corotating interaction regions for nearly all the 21 final months of Cassini and encountered just two interplanetary coronal mass ejections. Several independent studies report solar periodicities and storm‐time conditions in the magnetosphere at times, which coincide with the transients that we identify here. Plain Language Summary: Saturn's magnetosphere is embedded in the solar wind, with which it continuously interacts. Understanding the influence of the solar wind in shaping the Saturnian magnetosphere with measurements by the Cassini orbiter is challenging, due to the lack of a solar wind monitor that could inform us about the conditions that the planet is exposed to when Cassini is within the magnetospheric boundaries. To overcome this limitation, we use a method that relies on the measurement of solar energetic particles and galactic cosmic rays, the intensity of which is modulated by transient solar wind conditions. These particles can penetrate easily from the solar wind into Saturn's magnetosphere and be monitored continuously. We describe the results of this monitoring for the period between 2016 and the end of the Cassini mission (September 2017), and we provide valuable context for interpreting magnetospheric observations especially during the last two phases of the mission (Ring‐Grazing and Proximal orbits). During those phases, Cassini was scanning the magnetosphere once per week and had the opportunity to capture solar wind‐induced periodicities in the magnetosphere (∼13 or ∼26 days) or the immediate aftermath of the impact of an interplanetary coronal mass ejection on the system. Key Points: We provide heliospheric context to interpret magnetospheric measurements at Saturn after 2016After 2016 Saturn was nearly continuously exposed to corotating interaction regions (CIRs)Two intervals of ICMEs are identified, with the strongest on the final days of the Cassini mission [ABSTRACT FROM AUTHOR]

Planetary magnetospheres receive plasma and energy from the Sun or moons of planets and consequently stretch magnetic field lines. The process may last for varied timescales at different planets. From time to time, energy is rapidly released in the magnetosphere and subsequently precipitated into the ionosphere and upper atmosphere. Usually, this energy dissipation is associated with magnetic dipolarization in the magnetosphere.This process is accompanied by plasma acceleration and field‐aligned current formation, and subsequently auroral emissions are often significantly enhanced. Using measurements from multiple instruments on board the Cassini spacecraft, we reveal that magnetic dipolarization events at Saturn could reoccur after one planetary rotation and name them as recurrent dipolarizations. Three events are presented, including one from the dayside magnetosphere, which has no known precedent with terrestrial magnetospheric observations. During these events, recurrent energizations of plasma (electrons or ions) were also detected, which clearly demonstrate that these processes shall not be simply attributed to modulation of planetary periodic oscillation, although we do not exclude the possibility that the planetary periodic oscillation may modulate other processes (e.g., magnetic reconnection) which energizes particles. We discuss the potential physical mechanisms for generating the recurrent dipolarization process in a comprehensive view, including aurora and energetic neutral atom emissions. Plain Language Summary: Using measurements from the Cassini spacecraft, we reveal a new feature of magnetic dipolarization at Saturn, that is, the magnetic signature repeat after one planetary rotation, which is named recurrent dipolarization. Up to hundreds of kiloelectron volt electrons and ions are identified for the recurrent dipolarization events, suggesting that these particles have experienced efficient acceleration and cannot be purely due to planetary modulation. It remains a mystery why the magnetic dipolarization process associated with energetic ions and electrons could reoccur after one planetary rotation. Moreover, dipolarization process in Saturn's dayside magnetosphere is reported for the first time at Saturn, which has no known precedent with terrestrial or other planetary magnetospheric observations. The results demonstrate that magnetosphere‐ionosphere coupling dynamics at Saturn and Earth have fundamental similarities and differences. Key Points: We report a recurrent type of magnetic dipolarization at SaturnThe associated processes could efficiently accelerate electrons and ions to tens to hundreds of kiloelectron voltsCorotation of magnetosphere and planetary periodic oscillation are suggested as possible mechanisms [ABSTRACT FROM AUTHOR]

Periodic variations have been observed in many field and particle properties in Saturn's magnetosphere, modulated at a period close to the planetary rotation rate. Magnetic field observations by Cassini's magnetometer instrument suggest that in the outer magnetosphere (beyond ∼12 Saturn radii) Saturn's current sheet is periodically displaced with respect to the rotational equator, to a first approximation acting as a rotating, tilted disk. This manifests as a "flapping" mode when observed by the spacecraft. Recent studies suggest the magnetosphere also has a "breathing" mode, expanding and contracting with a period close to the planetary rotation rate. We model these two modes in tandem by combining a global, geometrical model of a tilted and rippled current sheet with a local, force‐balance model of Saturn's magnetodisk, accounting for the magnetospheric size and hot plasma content. We simulate the breathing behavior by introducing an azimuthal dependence of the system size. We fit Cassini magnetometer data acquired on equatorial orbits from 23 October to 17 December 2009 (Revs 120–122), close to Saturn equinox, in order that seasonal effects on the current sheet are minimized. We find that our model characterizes well the amplitude and phase of the oscillations in the data, for those passes that show clear periodic signatures in the field. In particular, the Bθ (meridional) component can only be characterized when the breathing mode is included. This study introduces calculations for an oscillating boundary, which provide a basis for understanding the complex relationship between current sheet dynamics and the periodic field perturbations. Key Points: Saturn's equatorial current sheet displays periodic dynamical behavior at roughly 10.7 hrWe model the magnetic field with a geometric current sheet model and force‐balance magnetodisk modelIncluding both displacement and thickness modulation of current sheet improves agreement with data [ABSTRACT FROM AUTHOR]

Moments of the charged particle distribution function provide a compact way of studying the transport, acceleration, and interactions of plasma and energetic particles in the magnetosphere. We employ κ‐distributions to describe the energy spectra of H+ and O+, based on >20 keV measurements by the three detectors of Cassini's Magnetospheric Imaging Instrument, covering the time period from DOY 183/2004 to 016/2016, 5 < L < 20. From the analytical spectra we calculate the equatorial distributions of energetic ion moments inside Saturn's magnetosphere and then focus on the distributions of the characteristic energy (Ec=IE/In), temperature, and κ‐index of these ions. A semiempirical model is utilized to simulate the equatorial ion moments in both local time and L‐shell, allowing the derivation of the polytropic index (Γ) for both H+ and O+. Primary results are as follows: (a) The ∼9 < L < 20 region corresponds to a local equatorial acceleration region, where subadiabatic transport of H+ (Γ∼1.25) and quasi‐isothermal behavior of O+ (Γ∼0.95) dominate the ion energetics; (b) energetic ions are heavily depleted in the inner magnetospheric regions, and their behavior appears to be quasi‐isothermal (Γ<1); (c) the (quasi‐) periodic energetic ion injections in the outer parts of Saturn's magnetosphere (especially beyond 17–18 RS) produce durable signatures in the energetic ion moments; (d) the plasma sheet does not seem to have a ground thermodynamic state, but the extended neutral gas distribution at Saturn provides an effective cooling mechanism that does not allow the plasma sheet to behave adiabatically. Key Points: Derivation of energetic ion moments, κ‐index, characteristic energy, temperature, and polytropic index in Saturn's magnetospherePresentation of a semiempirical analytical model for the 20 keV energetic ion Pressure, density, and temperatureThe neutral gas at Saturn provides an effective cooling mechanism and does not allow the plasma sheet to behave adiabatically [ABSTRACT FROM AUTHOR]

Throughout 2017, the Hubble Space Telescope (HST) observed the northern far‐ultraviolet aurorae of Saturn at northern solstice, during the Cassini Grand Finale. These conditions provided a complete viewing of the northern auroral region from Earth and a maximal solar illumination, expected to maximize the ionosphere‐magnetosphere coupling. We analyze 24 HST images concurrently with Cassini measurements of Saturn's kilometric radiation and solar wind parameters predicted by two magnetohydrodynamic models. The aurorae reveal highly variable components, down to timescales of minutes, radiating 7 to 124 GW. They include a nightside‐shifted main oval, unexpectedly frequent and bright cusp emissions, and a dayside low‐latitude component. On average, these emissions display a strong local time dependence with two maxima at dawn and premidnight, the latter being newly observed and attributed to nightside injections possibly associated with solstice conditions. These results provide a reference frame to analyze Cassini in situ measurements, whether simultaneous or not. Plain Language Summary: In 2017, the Hubble Space Telescope regularly observed the northern ultraviolet aurorae of Saturn in coordination with Cassini in situ measurements obtained during the Grand Finale, when the spacecraft flew across magnetic field lines connected to the aurorae. Hubble imaged Saturn's aurorae at 24 occasions spread over 7 months during northern solstice, when the northern auroral region was both fully visible from Earth and permanently illuminated by the Sun. The observed aurorae display a variety of components observed poleward of 68° latitude with different properties, some of which were unreported before. These emissions strongly vary with time, down to a few minutes, and radiate from 7 to 124 GW. On average, the auroral intensity also strongly varies with local time (a Sun‐referenced frame) and peaks at dawn, as previously observed, and also premidnight, pointing to a recurrent nightside activity of the magnetosphere. These results provide a reference basis to analyze Cassini in situ measurements. Key Points: Saturn's northern UV aurorae at solstice were sampled from HST observations coordinated with Cassini's Grand FinaleThe observed aurorae are highly variable with powerful events, radiating up to 124 GW, controlled by solar wind and planetary rotationThe average auroral brightness strongly varies with LT with two maxima at dawn (previously known) and premidnight (newly identified) [ABSTRACT FROM AUTHOR]

We present new models of Saturn's equatorial ionosphere based on the first in situ measurements of its upper atmosphere. The neutral spectrum measured by Cassini's Ion and Neutral Mass Spectrometer, which includes substantial methane, ammonia, and organics in addition to the anticipated molecular hydrogen, helium, and water, serves as input for unexpectedly complex ionospheric chemistry. Heavy molecular ions are found to dominate Saturn's equatorial low‐altitude ionosphere, with a mean ion mass of 11 Da. Key molecular ions include H3O+ and HCO+; other abundant heavy ions depend upon the makeup of the mass 28 neutral species, which cannot be uniquely determined. Ion and Neutral Mass Spectrometer neutral species lead to generally good agreement between modeled and observed plasma densities, though poor reproduction of measured H+ and H3+ variability and an overabundance of modeled H3+ potentially hint at missing physical processes in the model, including a loss process that affects H3+ but not H+. Plain Language Summary: Cassini's Grand Finale enabled the first‐ever direct measurements of Saturn's upper atmosphere. Here we use Cassini's unique measurements to construct new models of the plasma in this important boundary region that separates the dense lower atmosphere from space. Based on the complex array of observed gases, we find that heavy molecular ions are dominant near Saturn's equator. This surprising result demonstrates that the chemistry in Saturn's equatorial upper atmosphere is substantially more complex than anticipated. The presence of these unexpected ions potentially represents a new method of monitoring Saturn's ionosphere remotely. Furthermore, as other Cassini measurements indicate that the complex chemistry is likely driven by an influx of ring‐derived material, such observations may even help to track the evolution of Saturn's rings as they lose mass to its atmosphere. Key Points: Unexpectedly complex influx of ring material is found in Saturn's equatorial upper atmosphere, including organics, water, and nanograinsRing influx leads to reduction in major ions (H+ and H3+); heavier molecular ions dominate Saturn's low‐altitude equatorial ionosphereMajor molecular ions at low‐altitude are still uncertain but are likely to include H3O+ and HCO+, and the mean modeled ion mass is 11 Da [ABSTRACT FROM AUTHOR]

Abstract: On rare occasions Titan has been observed in the magnetosheath, where the solar wind interferes with the regular magnetic field generated by Saturn. This particular orbital position allows for a deeper investigation of Titan's upper atmosphere response to its highly energetic magnetospheric environment. Although solar extreme ultraviolet radiation is known to be the main source of ionization in Titan's upper atmosphere, magnetospheric particle precipitation can also account for about 10% of the ionization process. We are reporting here on ultraviolet observations of Titan airglow enhancements occurring while the spacecraft and Titan are known to have been exposed to the magnetosheath environment. Using remote sensing Cassini Ultraviolet Imaging Spectrograph observations of Titan in correlation with in situ Cassini Plasma Spectrometer measurements, we present evidence of Titan's upper atmosphere response to an electron burst crossing the magnetosheath at the time of the T32 flyby, on 13 June 2007. [ABSTRACT FROM AUTHOR]