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The Cassini spacecraft performed several flybys of Saturn's largest moon, Titan, collecting valuable data. During several passes, altimetric data were acquired. Here, we focus on altimetric measurements collected by Cassini's radar when flying over the same region at different epochs in order to correlate such measurements (crossovers) and investigate differences in altimetry. In our study, we assess altimetric errors associated with three distinct methods for extracting topography from Cassini's radar data: the maximum likelihood estimator (MLE), the threshold method, and the first moment technique. Focusing on crossover events, during which Cassini revisited specific areas of Titan's surface, we conduct a detailed examination of the consistency and accuracy of these three topography extraction methods. The proposed analysis involves closely examining altimetric data collected at different epochs over identical geographical regions, allowing us to investigate potential errors due to the variations in off-nadir angle, relative impact, uncertainties, and systematic errors inherent in the application of these methodologies. Our findings reveal that the correction applied for the off-nadir angle to the threshold and first moment methods significantly reduces the dispersion in the delta difference at the crossover, resulting in a dispersion of the order of 60 m, even lower than what is achieved with the MLE (~70 m). Additionally, an effort is made to assess the potential of Cassini for estimating the tidal signal on Titan. Considering the altimetric errors identified in our study and the relatively low number of crossovers performed by Cassini, our assessment indicates that it is not feasible to accurately measure the tidal signal on Titan using the currently available standard altimetry data from Cassini. Our assessment regarding the accuracy of the Cassini altimeter provides valuable insights for future planetary exploration endeavors. Our study advances the understanding of Titan's complex landscape and contributes to refining topographical models derived from Cassini's altimetry observations. These insights not only enhance our knowledge of Saturn's largest moon but also open prospects for Titan surface and interior exploration using radar systems. [ABSTRACT FROM AUTHOR]

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. •.At Titan's equator, variance in the spectra can be explained by 14–16 units and in the spectral gradient by 5 spectroclines. • Compositional candidatesfor changing spectra are identified including water-ice, acetylene, benzene, and alkanes. • Hummock rich plains, like Xanadu, show evidence of being more organic rich than the undifferentiated plains on the surface. • Windward vs. leeward spectral topology shows aeolian transport effects on equatorial evolution. [ABSTRACT FROM AUTHOR]

The exploration of interplanetary space and our solar bubble, the heliosphere, has made a big leap over the past two decades, due to the path-breaking observations of the two Voyager spacecraft, launched more than 44 years ago. Their in-situ particle and fields measurements were complemented by remote observations of 5.2 to 55 keV Energetic Neutral Atoms (ENA) from the Cassini mission (Ion and Neutral Camera-INCA), revealing a number of previously unanticipated heliospheric structures such as the "Belt", a region of enhanced particle pressure inside the heliosheath. The Suprathermal Time Of Flight (HSTOF) instrument on the Solar and Heliospheric Observatory (SOHO) also provided information of 58–88 keV ENAs from the heliosphere. In this chapter we provide a brief discussion for the contribution of the Voyager 1 and 2 Low Energy Charged Particle (LECP) observations that provided "ground truth" to the ENA images from Cassini/INCA towards addressing fundamental questions for the heliosphere's interaction with the Very Local Interstellar Medium. [ABSTRACT FROM AUTHOR]

Gravity wave (GW) signatures have been derived from temperature profiles observed by Cassini/Ultraviolet Imaging Spectrograph in the Saturnian thermosphere during the Grand Finale campaign. They demonstrate upward propagation of GW packets, their saturation, and breaking. We determined wave amplitudes, potential energy, and momentum fluxes and estimated the associated wave drag imposed by dissipating harmonics on the ambient flow. The data set of 18 profiles covers the middle and high latitudes of both hemispheres, which allows for exploring the global impact of waves. The diagnostics based on the Transformed Eulerian Mean and modified geostrophy approach reveal that the GW drag induces an equatorward flow in both hemispheres, facilitating transport of heat away from the auroral zones and redistributing energy across latitudes. Like all the outer planets, Saturn's thermosphere is hundreds of degrees hotter than what follows from radiative balance and these results help to explain the observed temperatures at all latitudes. Plain Language Summary: Gravity waves are small‐scale fluctuations of air density, temperature, and other atmospheric variables, which are dynamically important in the upper atmospheres. During Cassini's Grand Finale, observations with the Ultraviolet Imaging Spectrograph delivered a set of density profiles in the Saturnian thermosphere with a resolution sufficient to detect such waves. We derived various characteristics and estimated the forcing imposed by dissipating waves on the global circulation. It turned out that this "gravity wave drag" causes enhanced flow toward the equator in both hemispheres, transporting heat away from the auroral sources in high latitudes and distributing it over Saturn's thermosphere. Previous studies suggested that such redistribution would be prevented by fast westward jets (the "energy crisis"). We show that the waves observed on Saturn can successfully help winds to overcome this barrier. A similar mechanism can exist on other outer planets, whose thermospheres are also much hotter than what follows from radiative balance. Key Points: We present evidence for the omnipresence of gravity waves in the Saturnian thermosphere from Cassini/Ultraviolet Imaging Spectrograph Grand Finale measurementsWe determine wave amplitudes, energy, momentum fluxes, and forcing imposed on the mean flow along with their spatial distributionsGravity wave drag enhances pole‐to‐equator flow in both hemispheres, which helps to redistribute energy across latitudes [ABSTRACT FROM AUTHOR]

Before Cassini, scientists viewed Saturn’s unique features only from Earth and from three spacecraft flying by. During more than a decade orbiting the gas giant, Cassini studied the planet from its interior to the top of the atmosphere. It observed the changing seasons, provided up-close observations of Saturn’s exotic storms and jet streams, and heard Saturn’s lightning, which cannot be detected from Earth. During the Grand Finale orbits, it dove through the gap between the planet and its rings and gathered valuable data on Saturn’s interior structure and rotation. Key discoveries and events include: watching the eruption of a planet-encircling storm, which is a 20- or 30-year event, detection of gravity perturbations from winds 9000 km below the tops of the clouds, demonstration that eddies are supplying energy to the zonal jets, which are remarkably steady over the 25-year interval since the Voyager encounters, re-discovery of the north polar hexagon after 25 years, determination of elemental abundance ratios He/H, C/H, N/H, P/H, and As/H, which are clues to planet formation and evolution, characterization of the semiannual oscillation of the equatorial stratosphere, documentation of the mysteriously high temperatures of the thermosphere outside the auroral zone, and seeing the strange intermittency of lightning, which typically ceases to exist on the planet between outbursts every 1–2 years. These results and results from the Jupiter flyby are all discussed in this review. [ABSTRACT FROM AUTHOR]

Background: The objective of this study was to compare the central corneal curvature and astigmatism measurements obtained with three different systems in healthy eyes and to assess the level of interchangeability between them.Methods: This was a comparative study examining 30 healthy eyes of 30 patients (age 15-53 years). A complete eye examination was performed in all cases including analysis of anterior corneal curvature and astigmatism with three devices: the colour-LED topography system Cassini (i-Optics) (CAS), the Scheimpflug-based system Pentacam (Oculus Optikgeräte) (PTC) and the optical biometer IOL-Master 500 (Carl Zeiss Meditec) (IOLM). Differences between devices in terms of curvature in the flattest (flat K) and steepest meridians (steep K) as well as in the magnitude (AST) and power vector components of astigmatism (J0 and J45 ) were evaluated. The interchangeability between devices was evaluated with the Bland-Altman method.Results: Statistically significant differences between devices were found in steep K and flat K (p < 0.001). No statistically significant differences between devices were found in AST (p = 0.057) and J0 power vector (p = 0.185). However, differences between devices in J45 did reach statistical significance (p = 0.039). Ranges of agreement for curvature measures ranged from 0.123 (flat K, CAS-PTC) to 0.165 mm (steep K, CAS-PTC). Ranges for the magnitude of astigmatism were 0.868, 1.059 and 0.739 D in CAS-IOLM, PTC-IOLM and CAS-PTC comparisons, respectively. For J0 and J45 , ranges of agreement were below 0.522 D.Conclusions: Measurements of central corneal curvature and astigmatism obtained with the three devices evaluated cannot be used interchangeably. [ABSTRACT FROM AUTHOR]

Doppler tracking of Solar System probes is used for spacecraft navigation, planetary geodesy, and tests of the theory of General Relativity. The spacecraft radial velocity is measured by observing the Doppler shift of a radio signal transmitted from an Earth station to the spacecraft and then re-transmitted back, while preserving phase coherence, to the same station (two-way link) or to a different station (three-way link). Specialized orbit determination software is then used to reconstruct the spacecraft trajectory and estimate planetary gravity field coefficients or relativistic parameters. The measurement noise is a crucial element for the accuracy of the final estimates, thus considerable effort has been devoted to improve the range rate accuracy by adopting higher frequency links to reduce the dispersive noise from interplanetary and ionospheric plasmas, and by calibrating the tropospheric path delays with microwave radiometers. While Ka-band radio links (32–34 GHz) allowed a successful suppression of plasma noise, reducing tropospheric noise and ground antenna mechanical noise has been more challenging. The Time-Delay Mechanical noise Cancellation (TDMC) technique is a promising method to reduce mechanical and tropospheric noises and to improve further the accuracy of Doppler measurements. The TDMC is a linear combination of simultaneous Doppler data from a main antenna providing the two-way link and a three-way antenna (generally smaller and stiffer). If the listen-only, three-way antenna is also located in a particularly dry site, the TDMC can considerably reduce both tropospheric and antenna mechanical noises, which are the leading disturbances in two-way Ka-band radio links. For an operational test of this method, we applied the TDMC to Doppler data at X-band (7.2–8.4 GHz) from the Cassini spacecraft acquired during the Saturn tour phase of the mission. Although X-band links are generally dominated by the highly-variable interplanetary plasma noise and are not suitable for the TDMC, we found that, when local noises are particularly large at the two-way antenna, this technique may still lead to up to a factor-of-three noise reduction (at 60-s integration time) with respect to the two-way link. The TDMC can maximize the data quality during unique events, mainly planetary or satellite flybys (such as those considered in the Europa Clipper and JUICE missions), where the scientific results could be severely hampered by adverse conditions at the tracking station. • Reducing noise in Doppler links improves the accuracy of the estimated parameters. • With favorable conditions, TDMC technique reduces two-way antenna's local noises. • Simultaneous two-way and three-way links are required to apply the TDMC. • We present examples of successful two-way noise reduction with Cassini Doppler data. [ABSTRACT FROM AUTHOR]

The nature and structure of the observed east-west flows on Jupiter and Saturn have been a long-standing mystery in planetary science. This mystery has been recently unraveled by the accurate gravity measurements provided by the Juno mission to Jupiter and the Grand Finale of the Cassini mission to Saturn. These two experiments, which coincidentally happened around the same time, allowed the determination of the overall vertical and meridional profiles of the zonal flows on both planets. This paper reviews the topic of zonal jets on the gas giants in light of the new data from these two experiments. The gravity measurements not only allow the depth of the jets to be constrained, yielding the inference that the jets extend to roughly 3000 and 9000 km below the observed clouds on Jupiter and Saturn, respectively, but also provide insights into the mechanisms controlling these zonal flows. Specifically, for both planets this depth corresponds to the depth where electrical conductivity is within an order of magnitude of 1 S m−1, implying that the magnetic field likely plays a key role in damping the zonal flows. An intrinsic characteristic of any gravity inversion, as discussed here, is that the solutions might not be unique. We analyze the robustness of the solutions and present several independent lines of evidence supporting the results presented here. [ABSTRACT FROM AUTHOR]

On 10 January 2001, Cassini briefly entered into the magnetosphere of Jupiter, en route to Saturn. During this excursion into the Jovian magnetosphere, the Cassini Magnetosphere Imaging Instrument/Charge‐Energy‐Mass Spectrometer detected oxygen and sulfur ions. While Charge‐Energy‐Mass Spectrometer can distinguish between oxygen and sulfur charge states directly, only 95.9 ± 2.9 keV/e ions were sampled during this interval, allowing for a long time integration of the tenuous outer magnetospheric (~200 RJ) plasma at one energy. For this brief interval for the 95.9 keV/e ions, 96% of oxygen ions were O+, with the other 4% as O2+, while 25% of the energetic sulfur ions were S+, 42% S2+, and 33% S3+. The S2+/O+ flux ratio was observed to be 0.35 (±0.06 Poisson error). Plain Language Summary: While en route to Saturn, Cassini received a gravity assist from Jupiter. During this Jupiter flyby, Cassini briefly entered into the Jovian magnetosphere, due to an especially extended period of low solar wind pressure. In this paper, we focus on observations of the relative flux abundance of energetic (95.9 keV/e) heavy ions (oxygen and sulfur) by ion charge state (i.e., On+ and Sn+). These heavy ions would have largely originated from the moon Io, located 5.9 Jupiter radii (RJ) from Jupiter, while the observations from Cassini are near 200 RJ. By comparing the relative charge state zabundances of these heavy ions in the outer magnetosphere to previous observations closer to Io, we can study the evolution of plasma as it is transported away from Jupiter. These observations also serve to constrain modeling efforts and provide context to instruments incapable of independent ion mass and charge state determination. Key Points: Cassini measured the relative charge state abundances of 95.9 keV/e oxygen and sulfur in the outer magnetosphere (~200 RJ) of JupiterThe flux of 95.9 keV/e O+ was higher than that of S2+, with S2+ being the most abundant charge state among sulfur ionsThe relative abundances of 95.9 keV/e heavy ions are compared to thermal ions in the inner to middle magnetosphere [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the intrasession repeatability of anterior corneal topographic and aberrometric measurements provided by a color-LED topographer as well as their interchangeability with those provided by a Scheimpflug-based system in healthy eyes. Methods: Thirty-five healthy eyes of 35 patients (age, 16–66 years) were enrolled. A complete eye examination was performed in all cases including a complete corneal analysis with the Scheimpflug-based system Pentacam (Oculus Optikgeräte) (one measurement) and the Cassini system (i-Optics) (three consecutive measurements). Intrasession repeatability of the Cassini measurements was assessed with the within-subject standard deviation (Sw) and the intraclass correlation coefficient (ICC). The Bland-Altman analysis was used to evaluate the agreement between both devices. Results: Mean Sw for keratometric readings was 0.02 mm (ICC ≥ 0.992), ranging between 0.16 and 0.05 D (ICC 0.930–0.978) for anterior and total astigmatic measurements. Mean Sw for asphericity and corneal diameter were 0.06 (ICC 0.926) and 0.03 mm (IC 0.997), respectively. Aberrometric parameters showed ICCs ≥ 0.816, except for Z42 (ICC 0.741) and Z44 (ICC 0.544). When comparing devices, statistically significant differences were found for most of topographic and aberrometric data (p ≤ 0.044). Likewise, ranges of agreement between devices were clinically relevant (keratometry > 0.06 mm; total astigmatic components > 0.69 D; asphericity 0.35; second-, third-, and fourth-order Zernike terms, more than 0.20, 0.13, and 0.01 μm, respectively). Conclusions: Consistent anterior corneal topographic, total corneal astigmatic, and aberrometric measurements are obtained with color-LED topography in healthy eyes, which are not interchangeable with those provided by the Scheimpflug-based topography. [ABSTRACT FROM AUTHOR]

Exploration of the heliosphere and the far reaches of our solar system by the Voyagers and New Horizons and near-Earth observations of the Kuiper Belt and exoplanetary systems have all profoundly changed how we view our own home in space. An Interstellar Probe escaping beyond the solar-system boundaries with new observational techniques would be a bold move in space exploration, enabling detailed, new understanding of the global heliosphere in the context of other astrospheres, further discoveries of unexplored Kuiper Belt Objects, and the first observations of our circumsolar dust disk. These would offer insight into the evolution of the solar system and our understanding of exoplanetary systems. With the power supplies on the Voyagers nearing their end of life, the rapid pace of discovery of exoplanets in other stellar systems, and the evolution in our own understanding brought about by results from the Voyager, Cassini, and Interstellar Boundary Explorer (IBEX) spacecraft, the time is right for looking at the next steps we can make into our stellar neighborhood. The question facing us today is what the appropriate next step - a true first step - is in negotiating the transition from science fiction to engineering reality. The scientific imperatives associated with reaching the near interstellar medium in order to understand our origins and our own current locale in the Milky Way trace to the beginnings of the Space Age, and they have been debated and refined since that time and into the current epoch. The subjects of interstellar travel, interstellar probes, and interstellar "precursor" missions are not new but have lacked traction with policy makers and the scientific community at large because of the states of both scientific knowledge and engineering realities. The next step in reaching to the stars will require the recognition of engineering limits, scientific trades, and scientific compromises, but this is new neither in science nor exploration. Such a step would be an "Interstellar Probe." The time for that step has come. • An "Interstellar Probe" has been discussed in depth for almost 60 years. • NASA requested a study of this mission to prepare for the upcoming Decadal Surveys. • Traditional science goals are re-examined, updated, and expanded. • Approaches to required payload, spacecraft, and analysis methodology are identified. • Three mission approaches using NASA's large new booster and studied and assessed. [ABSTRACT FROM AUTHOR]

The Cassini RADAR altimeter has provided broad-scale surface topography data for Saturn’s largest moon Titan. Herein, we adapt the delay/Doppler algorithm to take into account Cassini geometries and antenna mispointing usually occurring during hyperbolic Titan flybys. The proposed algorithm allows up to tenfold improvement in the along-track resolution. Preliminary results are provided that show how the improved topography presented herein can advance our understanding of Titan’s surface characteristics. [ABSTRACT FROM AUTHOR]

We report "ground truth," 28‐ to 3,500‐keV in situ ion and 5.2‐ to 55‐keV remotely sensed ENA measurements from Voyager 2/Low Energy Charged Particle detector and Cassini/Ion and Neutral Camera, respectively, that assess the components of the ion pressure in the heliosheath. In this process, we predict an interstellar neutral hydrogen density of ∼0.12 cm−3 and an interstellar magnetic field strength of ∼0.5‐nT upstream of the heliopause in the direction of V2, that is, consistent with the measured magnetic field and neutral density measurements at Voyager 1 from August 2012, when the spacecraft entered interstellar space, to date. Further, this analysis results in an estimated heliopause crossing by V2 of ∼119 AU, as observed, suggesting that the parameters deduced from the pressure analysis are valid. The shape of the >5.2‐keV ion energy spectra play a critical role toward determining the pressure balance and acceleration mechanisms inside the heliosheath. Plain Language Summary: The Voyager missions, together with Cassini, provide the only combination of spacecraft to date that can establish "ground truth" at ∼100 AU and beyond and have recently settled the long‐standing issue on the dual heliosphere models, showing that the heliosphere behaves as rough diamagnetic bubble. Leveraging from the synergy between remote‐sensed ENAs and in situ measured ions, we estimate (accurately) the recently reported V2 heliopause crossing at ∼119 AU, showing that the shape of the ion energy spectra plays a critical role toward determining the pressure balance and acceleration mechanisms inside the heliosheath. In anticipation of measurements from V2 after it crossed the heliopause, the normalization of ENA and ion intensities provides an important insight on the properties of the Local Interstellar Medium, showing an interstellar neutral hydrogen density of nH∼ 0.12 cm−3 and a magnetic field upstream of the heliopause of BISMF∼ 0.5 nT. Key Points: "Ground truth" Energetic Neutral Atom and ion data in the heliosphere estimate the reported Heliopause crossing by V2 at ∼119 AUThe normalization of Energetic Neutral Atom and ion intensities yields an interstellar neutral hydrogen density of nH∼ 0.12 cm−3The 5.2‐ to 24‐keV H+ pressures dominate the 5.2‐ to 3,500‐keV distribution, whereas pressure balance implies that BISMF∼ 0.5 nT [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]

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]

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: We report on unusual dynamics in Titan's ionosphere as a significant difference in ionospheric electron density is observed between the T118 and T119 Cassini nightside flybys. Two distinct nightside electron density peaks were present during T118, at 1,150 and 1,200 km, and the lowest density ever observed in Titan's ionosphere at altitudes 1,000–1,350 km was during T118. These flybys were quite similar in geometry, Saturn local time, neutral density, extreme ultraviolet flux, and ambient magnetic field conditions. Despite this, the Radio and Plasma Waves/Langmuir Probe measured a density difference up to a factor of 6 between the passes. The overall difference was present and similar during both inbound and outbound legs. By ruling out other factors, we suggest that an exceptionally low rate of particle impact ionization in combination with dynamics in the ionosphere is the explanation for the observations. [ABSTRACT FROM AUTHOR]

Abstract: We report on a set of clear and abrupt decreases in the high‐frequency boundary of whistler mode emissions detected by Cassini at high latitudes (about ±40°) during the low‐altitude proximal flybys of Saturn. These abrupt decreases or dropouts have start and stop locations that correspond to L shells at the edges of the A and B rings. Langmuir probe measurements can confirm, in some cases, that the abrupt decrease in the high‐frequency whistler mode boundary is associated with a corresponding abrupt electron density dropout over evacuated field lines connected to the A and B rings. Wideband data also reveal electron plasma oscillations and whistler mode cutoffs consistent with a low‐density plasma in the region. The observation of the electron density dropout along ring‐connecting field lines suggests that strong ambipolar forces are operating, drawing cold ionospheric ions outward to fill the flux tubes. There is an analog with the refilling of flux tubes in the terrestrial plasmasphere. We suggest that the ring‐connected electron density dropouts observed between 1.1 and 1.3 Rs are connected to the low‐density ring plasma cavity observed overtop the A and B rings during the 2004 Saturn orbital insertion pass. [ABSTRACT FROM AUTHOR]

Abstract: We present images of Saturn from the final phases of the Cassini mission, including images with 0.5 km per pixel resolution, as high as any Saturn images ever taken. Notable features are puffy clouds resembling terrestrial cumulus, shadows indicating cloud height, dome and bowl shaped cloud structures indicating upwelling and downwelling in anticyclones and cyclones respectively, and filaments, which are thread‐like clouds that remain coherent over distances of 20,000 km. From the coherence of the filaments, we give upper bounds on the diffusivity and kinetic energy dissipation. A radiative transfer analysis by Sanz‐Requena et al. (2018) indicates that methane‐band imagery is most useful in determining cloud and haze properties in the 60–250 mbar pressure range. Our methane‐band imagery finds haze in this pressure range covering 64°‐74°planetocentric latitude. Filaments lie within the haze, and cumulus clouds lie below it, but pressure levels are uncertain below the 250 mbar level. [ABSTRACT FROM AUTHOR]

Abstract: Using images from the Cassini spacecraft, we analyzed three ribbon waves in Saturn's 42°N eastward jet at 45°N, 42°N, and 39°N planetocentric latitudes. In this report, we demonstrate that the morphology, wavelength, and propagation of the ribbon waves are consistent with barotropic Rossby waves with a smaller baroclinic component. We report on the appearance and disappearance of these waves during Cassini's mission. We suggest that the temporal evolution of these waves are related to the great Saturn storm of 2010–2011. [ABSTRACT FROM AUTHOR]

Cassini has recently completed its 13-year mission at Saturn leaving a vast data set. A large interest among the scientific community is to investigate plasma waves and instabilities at Saturn. It is no longer feasible to manually search through Cassini’s vast data set to identify all such waves of interest. Thus, the feasibility of using artificial neural networks (ANNs) to identify plasma waves at Saturn is demonstrated using Cassini data. A convolutional neural network (CNN) was trained to identify low-frequency plasma waves that occur in the upstream region of Saturn using images constructed from the Cassini magnetometer time series data. By systematically varying the network architecture during training and validation, a CNN was obtained that can identify upstream waves with an accuracy of 94% ± 2%. The CNN’s high accuracy for wave identification demonstrates that it is, in fact, feasible to use ANNs to identify plasma waves at Saturn and by extension in other planetary and lunar plasma environments using spacecraft data. [ABSTRACT FROM AUTHOR]

Abstract: Between April and the end of its mission on 15 September, Cassini executed a series of 22 very similar 6.5‐day‐period proximal orbits, covering the mid‐latitude region of the nightside magnetosphere. These passes provided us with the opportunity to examine the variability of the nightside plasma sheet within this time scale for the first time. We use Cassini particle and magnetic field data to quantify the magnetospheric dynamics along these orbits, as reflected in the variability of certain relevant plasma parameters, including the energetic ion pressure and partial (hot) plasma beta. We use the University College London/Achilleos‐Guio‐Arridge magnetodisk model to map these quantities to the conjugate magnetospheric equator, thus providing an equivalent equatorial radial profile for these parameters. By quantifying the variation in the plasma parameters, we further identify the different states of the nightside ring current (quiescent and disturbed) in order to confirm and add to the context previously established by analogous studies based on long‐term, near‐equatorial measurements. [ABSTRACT FROM AUTHOR]

Abstract: During the Cassini mission the Saturnian moons Dione and Tethys showed intriguing and multiple clues suggesting residual geologic activity that might be detectable as an atmosphere, plume, or even heat signature. These clues included an atmospheric aura around Dione, injection of particles into Saturn's magnetosphere, mysterious red streaks on Tethys, and possible cryovolcanic features on Dione. A concerted effort that was strengthened in the latter stages of the mission included the acquisition of stellar occulations by the Ultraviolet Imaging Spectrograph (UVIS) of both Dione and Tethys and high solar phase observations of Dione by the Visible Infrared Mapping Spectrometer (VIMS) to detect forward scattering from grains in a plume. Analysis of these observations shows no evidence for even a low level of activity on either moon. In addition, infrared images at 2.65 μm obtained throughout the mission were scrutinized for the reappearance of an atmosphere‐like aura, with negative results. [ABSTRACT FROM AUTHOR]

Abstract: Near‐infrared spectral maps of Saturn's north polar vortex obtained at high spatial resolution provided by Cassini/Visual Infrared Mapping Spectrometer (VIMS) reveal localized ammonia clouds composed of unusually large particles exceeding 13 μm in radius, the largest cloud particles documented during the Cassini mission. Taken under near‐optimum direct polar lighting and viewing conditions within a month of the summer solstice, these small (~200 km in breadth) discrete clouds are located within the eye of the polar vortex, which otherwise is unusually clear of observable aerosols in reflected sunlight, with total 2‐μm opacity <0.04 versus >1.0 elsewhere on Saturn. The dichotomy of large‐particle condensate cloud features—indicative of convective upwelling—within a large (~ 2000‐km diameter) nearly aerosol‐free region of downwelling characteristic of the core of a polar vortex reveals surprising polar dynamics on Saturn. [ABSTRACT FROM AUTHOR]

Abstract: Cassini observations of Titan's weather patterns over >13 years, almost half a Saturnian year, provide insight into seasonal circulation patterns and the methane cycle. The Imaging Science Subsystem and the Visual and Infrared Mapping Spectrometer documented cloud locations, characteristics, morphologies, and behavior. Clouds were generally more prevalent in the summer hemisphere, but there were surprises in locations and timing of activity: Southern clouds were common at midlatitudes, northern clouds initially appeared much sooner than model predictions, and north polar summer convective systems did not appear before the mission ended. Differences from expectations constrain atmospheric circulation models, revealing factors that best match observations, including the roles of surface and subsurface reservoirs. The preference for clouds at mid‐northern latitudes rather than near the pole is consistent with models that include widespread polar near‐surface methane reservoirs in addition to the lakes and seas, suggesting a broader subsurface methane table is accessible to the atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: We have used data from the Cassini Composite Infrared Spectrometer to map the temperatures in Saturn's polar cyclones at the highest spatial resolution obtained during the Cassini mission. We find temperature contrasts of 7 K in the upper troposphere within 1.4° of both poles, roughly 50 percent larger than earlier measurements at lower spatial resolution. The polar hot spots weaken with depth, disappearing near 500 mbar. In the stratosphere, the polar hot spot becomes broader, extending 4° from the poles, and weakens with altitude disappearing near 1 mbar. A thermal relaxation model shows that the tropospheric hot spot is consistent with adiabatic heating from subsidence with a vertical velocity of about −0.05 mm/s above 500 mbar. The observed temperature gradients imply that the winds in the polar cyclone decay with increasing altitude over roughly three pressure scale heights above the 200‐mbar level. [ABSTRACT FROM AUTHOR]

We model the emission of energetic neutral atoms (ENAs) that are generated by the interaction between energetic ions from Saturn's magnetosphere and neutrals from the upper atmosphere of the giant planet's largest moon Titan. The trajectories of the parent ions and the resulting ENA emission morphology are highly sensitive to the electromagnetic field configuration near the moon. We therefore compare the ENA emission pattern for spatially homogeneous fields to the emission obtained from a magnetohydrodynamic (MHD) and a hybrid (kinetic ions, fluid electrons) model of Titan's magnetospheric interaction, by computing the trajectories of several billion energetic test particles. While the MHD model takes into account the draping of the magnetic field lines around Titan, the hybrid approach also considers the significant asymmetries in the electromagnetic fields due to the large gyroradii of pick-up ions from Titan's ionosphere. In all three models, the upstream parameters correspond to the conditions during Cassini's TA flyby of Titan. The shape, magnitude, and location of the ENA emission maxima vary considerably between these three field configurations. The magnetic pile-up region at Titan's ramside deflects a large number of the energetic parent ions, thereby reducing the ENA flux. However, the draped magnetic field lines in Titan's lobes rotate the gyration planes of the incident energetic ions, thereby facilitating the observable ENA production. Overall, the ENA flux calculated for the MHD model is weaker than the emission obtained for the electromagnetic fields from the hybrid code. In addition, we systematically investigate the dependency of the ENA emission morphology on the energy of the parent ions and on the upstream magnetic field strength. [ABSTRACT FROM AUTHOR]

Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼30 min. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images). [ABSTRACT FROM AUTHOR]

Based on magnetic field data from Cassini's high-inclination orbits of Saturn (radius R S = 60 , 268 km ), we analyze the latitudinal distribution of ion cyclotron waves in the giant planet's magnetosphere. Our survey takes into account magnetic field data from all high-inclination orbits between 2004 and 2015. We analyze the dependency of the occurrence rate and amplitude of the ion cyclotron waves on radial distance ρ to Saturn's rotation axis, vertical distance z to Saturn's equatorial plane, and magnetic latitude λ . The occurrence rate of ion cyclotron waves is approximately 100% in Saturn's equatorial plane between the orbits of Enceladus and Dione and decreases to 50% at altitudes of | z | ≈ 0.6 R S . Ion cyclotron waves were detected up to | z | = 2.0 R S . The occurrence rate displays strong, non-monotonic variations with respect to ρ , z , and λ . The vertical amplitude profile of the waves exhibits an M-like pattern with two distinct peaks near z = ± 0.3 R S and the central minimum at z =0. Compared to earlier observations, we find this M-like structure to be inflated in± z direction by a factor of three. The available magnetic field data provides only weak evidence for a local impact of Enceladus and Dione on the ion cyclotron wave field. Using the observed Doppler shift of the ion cyclotron wave frequency during Cassini's high-inclination orbits, we demonstrate the existence of a narrow band of bidirectional wave propagation. This band is centered around Saturn's equatorial plane and possesses a half-width of | z | = 0.15 R S , which agrees well with the vertical scale height of Saturn's neutral cloud. To the north of this band, all ion cyclotron waves propagate towards the north ( z > 0 ); and to the south, all waves propagate towards the south ( z < 0 ). In companion with our previous study (Meeks et al., 2016), this survey provides the complete three-dimensional picture of the ion cyclotron wave field between the orbits of Enceladus and Rhea during the Cassini era. [ABSTRACT FROM AUTHOR]

Enceladus has a dynamic plume that is emitting gas, including water vapor, and dust. The gas is ionized by solar EUV radiation, charge exchange, and electron impact and extends throughout the inner magnetosphere of Saturn. The charge exchange collisions alter the plasma composition. Ice grains (dust) escape from the vicinity of Enceladus and form the E ring, including a portion that is negatively charged by the local plasma. The inner magnetosphere within 10 R S (Saturn radii) contains a complex mixture of plasma, neutral gas, and dust that links back to Enceladus. In this paper we investigate the energy distributions, ion species and densities of water group ions in the plume of Enceladus using test particle and Monte Carlo methods that include collisional processes such as charge exchange and ion-neutral chemical reactions. Ion observations from the Cassini Ion and Neutral Mass Spectrometer (INMS) for E07 are presented for the first time. We use the modeling results to interpret observations made by the Cassini Plasma Spectrometer (CAPS) and the INMS. The low energy ions, as observed by CAPS, appear to be affected by a vertical electric field ( E Z =−10 µV/m) in the plume. The E Z field may be associated with the charged dust and/or the pressure gradient of plasma. The model results, along with the results of earlier models, show that H 3 O + ions created by chemistry are predominant in the plume, which agrees with INMS and CAPS data, but the INMS count rate in the plume for the model is several times greater than the data, which we do not fully understand. This composition and the total ion count found in the plume agree with INMS and CAPS data. On the other hand, the Cassini Langmuir Probe measured a maximum plume ion density more than 30,000 cm −3 , which is far larger than the maximum ion density from our model, 900 cm −3 . The model results also demonstrate that most of the ions in the plume are from the external magnetospheric flow and are not generated by local ionization. The origin of the ions in the plume was investigated using two different velocity models. Most ions were created by the interaction with background magnetospheric plasma and by photoionization. INMS and CAPS also detected water cluster ions. We will interpret these observations as a result of ion collisions with neutral water clusters, (H 2 O) n , originating in the tiger stripe vents as suggested by Tokar et al. (2009). We also estimated the process of generating cluster ions based on the INMS observations. We suggest that the most likely source is reaction of H 3 O + with neutral water clusters or dimers such as (H 2 O) 2 formed in the plume vents. [ABSTRACT FROM AUTHOR]

We presented a theory describing interaction between cold plasma and dust grain based on Cassini observations of Enceladus in Saturn's E ring. We assumed a finite-sized dust cloud with inside plasma to simulate the environment. The dust cloud that results from this scenario has a different potential than the surrounding plasma. Cassini's observations revealed the cold plasma characteristics of the Enceladus plume, which include a high plasma density for both electrons and ions at sufficient low temperature. Therefore, the electron fluid is incorporated in the cloud to form a quantum electron gas governed through Fermi-Dirac statistics. According to observations, more electrons bound to the dust grain results in a collective dust plasma interaction with surrounding cold plasma. Dust-plasma interaction predominantly occurred at negative sub-micrometer sized dust grain. • Using Cassini observations of Enceladus in Saturn's E ring as a groundwork, a theory is presented in this article. • The Enceladus plume has cold plasma characteristics. We considered ions as Boltzmannian distributed particles and Fermi-Dirac distribution is applicable for electrons. • We derived relations for the potential variation in the Enceladus plume region and the E-rings. We found that more electrons bound to the dust grain results in a collective dust plasma interaction. • Analytical analysis and the numerical results form the expressions for the given environment is presented. [ABSTRACT FROM AUTHOR]

• Packed media are modeled by radiative transfer with static structure factor, RTT-PM. • Gaussian particles in packed media scheme effectively model icy regolith spectra. • Parameters of the shape of Saturnian moon regolith particles have been found. • The model balances physical rigor with practicality and is promising for large data. 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. [ABSTRACT FROM AUTHOR]

We present a comprehensive analysis of ion cyclotron waves in the equatorial magnetosphere of Saturn, considering all magnetic field data collected during the Cassini era (totaling to over 4 years of data from the equatorial plane). This dataset includes eight targeted flybys of Enceladus, three targeted flybys of Dione, and three targeted flybys of Rhea. Because all remaining orbits of Cassini are high-inclination, our study provides the complete map of ion cyclotron waves in Saturn's equatorial magnetosphere during the Cassini era. We provide catalogs of the radial and longitudinal dependencies of the occurrence rate and amplitude of the ion cyclotron fundamental and first harmonic wave modes. The fundamental wave mode is omnipresent between the orbits of Enceladus and Dione and evenly distributed across all Local Times. The occurrence rate of the fundamental mode displays a Fermi–Dirac-like profile with respect to radial distance from Saturn. Detection of the first harmonic mode is a rare event occurring in only 0.49% of measurements taken and always in conjunction with the fundamental mode. We also search for a dependency of the ion cyclotron wave field on the orbital positions of the icy moons Enceladus, Dione, and Rhea. On magnetospheric length scales, the wave field is independent of the moons' orbital positions. For Enceladus, we analyze wave amplitude profiles of seven close flybys (E9, E12, E13, E14, E17, E18, and E19), which occurred during the studied trajectory segments, to look for any local effects of Enceladan plume variability on the wave field. We find that even in the close vicinity of Enceladus, the wave amplitudes display no discernible dependency on Enceladus' angular distance to its orbital apocenter. Thus, the correlation between plume activity and angular distance to apocenter proposed by Hedman et al. (2013) does not leave a clearly distinguishable imprint in the ion cyclotron wave field. [ABSTRACT FROM AUTHOR]

Analogous to Earth's water cycle, Titan's methane-based hydrologic cycle supports standing bodies of liquid and drives processes that result in common morphologic features including dunes, channels, lakes, and seas. Like lakes on Earth and early Mars, Titan's lakes and seas preserve a record of its climate and surface evolution. Unlike on Earth, the volume of liquid exposed on Titan's surface is only a small fraction of the atmospheric reservoir. The volume and bulk composition of the seas can constrain the age and nature of atmospheric methane, as well as its interaction with surface reservoirs. Similarly, the morphology of lacustrine basins chronicles the history of the polar landscape over multiple temporal and spatial scales. The distribution of trace species, such as noble gases and higher-order hydrocarbons and nitriles, can address Titan's origin and the potential for both prebiotic and biotic processes. Accordingly, Titan's lakes and seas represent a compelling target for exploration. [ABSTRACT FROM AUTHOR]

This study investigates the processes that lead to the detection of split signatures in ion density during several crossings of the Cassini spacecraft through Titan's mid-range plasma tail (T9, T63, and T75). During each of these flybys, the Cassini Plasma Spectrometer detected Titan's ionospheric ion population twice; i.e., the spacecraft passed through two spatially separated regions where cold ions were detected, with the regions also being dominated by ions of different masses in the case of T9. Whether this filamented tail structure is an omnipresent feature of Titan's plasma interaction or a result of non-stationary upstream conditions during specific flybys is still unclear. To explain these features, we apply the hybrid simulation code AIKEF (kinetic ions and fluid electrons). Our model includes chemical reactions as well as a realistic photoionization model for a sophisticated description of the ionospheric composition of Titan. Our simulations show that the filamentation of Titan's tail is indeed a common feature of the moon's plasma interaction. Light ionospheric species escape along draped magnetic field lines to form a parabolically shaped filament structure, which is mainly seen in planes that contain the upstream magnetospheric magnetic field and the upstream flow direction. In addition, transport of ions of all species from the ramside towards downstream produces a cone structure behind Titan, with a region of decreased density inside and filaments of 1–2 R T ( R T =2575 km) thickness and enhanced density at the surface of the cone. Spacecraft trajectories that penetrate these structures allow for the detection of split signatures in the tail. The orientation of the upstream magnetic field and plasma flow as well as local time effects (i.e., Titan's orbital position) influence the location of the filaments in the tail and can also cause asymmetries in their sizes and densities. The detection of the split signatures along a spacecraft trajectory may therefore be made possible or completely prevented by moving the narrow filaments in or out of the way of the spacecraft. Our results imply that the detections of split signatures during T9, T63 and T75 are consistent by Cassini penetrating through parts of these filament structures. [ABSTRACT FROM AUTHOR]

Cassini Ion Neutral Mass Spectrometer (INMS) measurements from roughly a hundred Titan encounters over the Cassini mission yield neutral and ion densities systematically lower, by factors approximately 2 to 3, than estimates from several other spacecraft systems, including the Attitude and Articulation Control System, and Navigation system. In this paper we present a new INMS instrument sensitivity model, obtained by re-analyzing (1) the capture and transmission of neutral gas through the instrument, and (2) the detector gain reduction during pre-launch testing. By correcting for an under-estimation of gas leakage out of the instrument into space by the original calibration model, and adjusting for the gain change, the new model brings INMS densities into much closer agreement with the other Cassini systems. Accordingly, the INMS ion densities are revised upward by a constant detector sensitivity correction factor of 1.55±21 %, while the neutral sensitivities have a complex instrument pointing direction dependence, due (mostly) to the effect of the INMS vent and antechamber-to-closed source tube. In the special case of on-ram pointing the neutral densities are revised upward by a constant factor of 2.2±23 %. The corrected neutral and ion sensitivities given here are applicable to all previously published INMS results at Titan, Enceladus and elsewhere in the Saturn system. The new model gives reliable densities at high ram angles, in some cases above 90 degrees, thereby expanding the list of Titan flybys from which INMS densities may be extracted. We apply the model to obtain accurate densities from several off-ram Titan flybys which gave unusual neutral density vs. altitude profiles, or unreasonably high densities, with the original calibration. [ABSTRACT FROM AUTHOR]

We present the impact rates of dust particles recorded by the Cosmic Dust Analyzer (CDA) aboard the Cassini spacecraft. The “dust counters” evaluate the quality of an impact and give rise to the apparent density of dust particles in space. The raw data is pre-selected and refined to a new structure that serves to a better investigation of densities, flows, and properties of interplanetary dust grains. Our data is corrected for the dead time of the instrument and corresponds to an assumed Kepler orbit (pointing of the sensitive area). The processed data are published on the website for the Magnetosphere and Plasma Science (MAPSview), where it can be correlated with other Cassini instruments. A sample is presented for the Titan flyby on DOY 250/2006. We find that the dust density peaks at two times, at least, in a void region between Titan and Rhea. Such features may point to extended clouds of small particles drifting slowly in space. These density clouds seem to be stable for as long as several months or few years before dispersing. [ABSTRACT FROM AUTHOR]

The structure of Enceladus׳ south polar plume of charged dust is studied by simulations of the dust grain dynamics. The model considers the Lorentz force and charging of the grains by the plasma environment within the plume. Simulated dust plumes are investigated by applying 10 selected sets of dust parameters that include variations of the grain production rate, the slope of the grain size distribution and the start conditions (velocity, direction) of the grains. The modeled dust plume profiles are in good agreement with nanograin data of Cassini Plasma Spectrometer (CAPS). Major results are (1) due to the local plasma environment the nanograins are accelerated by the Lorentz force and form a structured tail; (2) due to the finite charging time the peak dust charge density is located about 0.3 – 0.6 r E below Enceladus׳ south pole; (3) nanograins smaller than 10 nm are more than 99% of the produced dust; (4) CAPS data are best matched if the nanograins are launched with high, collimated start velocities; (5) the grain charging time is crucially affected by inhomogeneities in the local plasma environment. [ABSTRACT FROM AUTHOR]

Though Langmuir probes (LP) are designed to investigate cold plasma regions (e.g. ionospheres), a recent analysis revealed a strong sensitivity of the Cassini LP measurements to hundreds of eV electrons. These warm electrons impact the surface of the probe and generate a significant current of secondary electrons, that impacts both the DC level and the slope of the current–voltage curve of the LP (for negative potentials) through energetic contributions that may be modeled with a reasonable precision. We show here how to derive information about the incident warm electrons from the analysis of these energetic contributions, in the regions where the cold plasma component is small with an average temperature in the range ~ [ 100 – 500 ] eV . First, modeling the energetic contributions (based on the incident electron flux given by a single anode of the CAPS spectrometer) allows us to provide information about the pitch angle anisotropies of the incident hundreds of eV electrons. The modeling reveals indeed sometimes a large variability of the estimated maximum secondary electron yield (which is a constant for a surface material) needed to reproduce the observations. Such dispersions give evidence for strong pitch angle anisotropies of the incident electrons, and using a functional form of the pitch angle distribution even allows us to derive the real peak angle of the distribution. Second, rough estimates of the total electron temperature may be derived in the regions where the warm electrons are dominant and thus strongly influence the LP observations, i.e. when the average electron temperature is in the range ~ [ 100 – 500 ] eV . These regions may be identified from the LP observations through large positive values of the current–voltage slope at negative potentials. The estimated temperature may then be used to derive the electron density in the same region, with estimated densities between ~ 0.1 and a few particles / cm 3 (cc). The derived densities are in better agreement with the CAPS measurements than the values derived from the proxy technique ( Morooka et al., 2009 ) based on the floating potential of the LP. Both the electron temperature and the density estimates lie outside the classical capabilities of the LP, which are essentially n e > 5 cc and T e < 5 eV at Saturn. This approximate derivation technique may be used in the regions where the cold plasma component is small with an average temperature in the range ~ [ 100 – 500 ] eV , which occurs often in the L range 6.4–9.4 R S when Cassini is off the equator, but may occur anywhere in the magnetosphere. This technique may be all the more interesting since the CAPS instrument was shut down, and, though it cannot replace the CAPS instrument, the technique can provide useful information about the electron moments, with probably even better estimates than CAPS in some cases (when the plasma is strongly anisotropic). Finally, a simple modeling approach allows us to predict the impact of the energetic contributions on LP measurements in any plasma environment whose characteristics (density, temperature, etc.) are known. LP observations may thus be influenced by warm electrons in several planetary plasma regions in the solar system, and ambient magnetospheric electron density and temperature could be estimated in some of them (e.g. around several galilean satellites) through the use of Langmuir probes. [ABSTRACT FROM AUTHOR]

We provide a global view of ~20 to 800 keV electron pitch angle distributions (PADs) close to Saturn’s current sheet using observations from the Cassini MIMI/LEMMS instrument. Previous work indicated that the nature of pitch angle distributions in Saturn’s inner to middle magnetosphere changes near the radial distance of 10 R S . This work confirms the existence of a PAD transition region. Here we go further and develop a new technique to statistically quantify the spatial profile of butterfly PADs as well as present new spatial trends on the isotropic PAD. Additionally, we perform a case study analysis and show the PADs exhibit strong energy dependent features throughout this transition region. We also present a diffusion theory model based on adiabatic transport, Coulomb interactions with Saturn’s neutral gas torus, and an energy dependent radial diffusion coefficient. A data-model comparison reveals that adiabatic transport is the dominant transport mechanism between ~8 to 12 R S , however interactions with Saturn’s neutral gas torus become dominant inside ~7 R S and govern the flux level of ~20 to 800 keV electrons. We have also found that field-aligned fluxes were not well reproduced by our modeling approach. We suggest that wave–particle interactions and/or a polar source of the energetic particles needs further investigation. [ABSTRACT FROM AUTHOR]

Contrary to the permanent MeV ion belts which are relatively stable in intensity over both short and long time scales and are modulated by a single Galactic Cosmic Ray driven source, the electron belts of Saturn appear to be much more complex in both structure and temporal evolution. In order to understand the responses of this system to the different factors that may control it (internal or external/solar sources) we study its long-term, temporal evolution. We achieve that by tracking the equatorial distance of the belts׳ outer boundary, using MIMI/LEMMS energetic charged particle observations over a period of more than 7 years. This boundary is defined at the distance that a selected count rate level is measured in a LEMMS channel that has the properties of an omnidirectional, integral energy detector. Simulated solar wind moments, energetic neutral atom (ENA) observations and solar irradiance data are used to support the analysis. In many cases, correlations of the different datasets are weak, suggesting that the electron belts are modulated in time scales that are much shorter than the sampling of the electron belt boundary (typically every 10–30 days). Still, we find several cases of persistent, long term and strong perturbations in the system that appear to have corresponding disturbances in the extension of the electron belts, even on such long time scales. From the analysis of those intervals we believe that we have established a solid link with the planetary ring current as the primary source of the electron belts of Saturn. This is concluded mostly on the basis of an extended ring current decay in 2011 (inferred through ENA observations), coinciding with a similar, unusual drop in the electron belt extension (and intensity). This means that the electron belts should reflect also the modulation of the ring current. We suggest that possible sources of long term modulation are both the solar UV irradiance of the Saturnian thermosphere and the solar wind. The former appears to acquire large values and exhibit significant changes during the period of the electron belt/ring current decay, while an approximate 2-week periodicity (characteristic for the case of two solar wind streams per solar rotation) appears in the electron belts in the form of recurrent expansions or intensifications. The latter finding indicates that external forcing by the solar wind can have a considerable impact on the planet׳s electron belts, contrary to the expectations that Saturn׳s magnetospheric dynamics are rotationally dominated and internally driven. [ABSTRACT FROM AUTHOR]

A numerical study of the MIDACO optimization software on the well known GTOP benchmark set, published by the European Space Agency (ESA), is presented. The GTOP database provides trajectory models of real-world interplanetary space missions such as Cassini, Messenger or Rosetta. The trajectory models are formulated as constrained nonlinear optimization problems and are known to be difficult to solve. Here a comprehensive and rigorous numerical analysis of the MIDACO out-of-the-box performance on the GTOP benchmark set is presented and discussed. In the past, the putative best known solutions of these benchmarks often required several months and even years to be found. In this contribution it will be shown, that MIDACO is able to solve five out of seven of these benchmarks to their best known solution within minutes to hours. [ABSTRACT FROM AUTHOR]

Abstract: The United States has pioneered the use of nuclear power systems for outer planetary space probes since the 1970s. These systems have enabled the Viking landings to reach the surface of Mars and both Pioneers 10 and 11 and Voyagers 1 and 2 to travel to the limits of the solar system. Although the American public has long been concerned about safety of these systems, in the 1980s a reaction to nuclear accidents – especially the Soviet Cosmos 954 spacecraft destruction and the Three Mile Island nuclear power plant accidents – heightened awareness about the hazards of nuclear power and every spacecraft launch since that time has been contested by opponents of nuclear energy. This has led to a debate over the appropriateness of the use of nuclear power systems for spacecraft. It has also refocused attention on the need for strict systems of control and rigorous checks and balances to assure safety. This essay describes the history of space radioisotope power systems, the struggles to ensure safe operations, and the political confrontation over whether or not to allow the launch the Galileo and Cassini space probes to the outer planets. Effectively, these efforts have led to the successful flights of 12 deep space planetary probes, two-thirds of them operated since the accidents of Cosmos 954, Three Mile Island, and Chernobyl. [Copyright &y& Elsevier]

Abstract: Titan's atmosphere has been extensively studied during the Cassini–Huygens Mission. The polar environment undergoes considerable seasonal variation and is distinct from the more quiescent equatorial environment experienced by the Huygens probe in 2005. The thermal structure of the upper atmosphere is affected by interaction of Titan's atmosphere with the plasma and energetic particles in Saturn's magnetosphere or in the solar wind. Titan's polar regions, and its hydrocarbon lakes in particular, are of interest for future exploration as is the complex organic chemistry that takes place in the upper atmosphere. Thus, specific environmental models are required for future exploration. Furthermore, the models developed to support the design of Huygens had to accommodate wide uncertainties, requiring large design margins. The extensive observations by the Cassini spacecraft (and indeed Huygens itself) provide a basis for narrowing these uncertainties, notably in composition. We provide a description of these new data sets, their scientific basis, and present limitations. The empirically constrained neutral temperatures and densities presented here also have important implications for fundamental chemical modeling. The empirical modeling presented here provides constraints on these models, which will lead to improved estimations of chemical products. Therefore, the paper describes a new model of Titan's atmospheric structure to guide both scientific studies and future engineering studies of exploration of the Saturn system. Of particular interest are aero-capture and aero-braking missions that may enable future exploration of the Saturnian satellite system. [Copyright &y& Elsevier]

Abstract: The Cassini spacecraft Radio and Plasma Wave Science (RPWS) Langmuir Probe (LP) provides in-situ measurements of Titan's ionosphere. We present here data from 47 deep flybys in the time period October 2004–July 2012 of charge densities of positive and negative ions as well as electrons. These densities have been mapped with respect to altitude and solar zenith angle (SZA) in an altitude range of 880–1400km. The inferred electron number densities are consistent with earlier presented observational results. Negative ion charge densities exhibit a trend that exponentially increases towards lower altitudes within the covered altitude range. This is especially evident on the nightside of Titan (SZA >110°). The negative ion charge densities at the lowest traversed altitudes (near 960km) are inferred to be in the range 300–2500cm−3. The results show that very few free electrons (n e /n i ∼0.1–0.7) exist in the deepest regions (880–1050km) of Titan's nightside ionosphere. Instead the deep nightside part of Titan's ionosphere is dominated by both negatively and positively charged heavy (>100amu) organic ions. We therefore believe a dust/aerosol-ion plasma exists here, similar to what is found in noctilucent clouds in Earth's mesosphere. [Copyright &y& Elsevier]

Basic documents that are made according to measurements and topographic surveying for the purpose of creating various projects are plans and topographic maps. Achieving these documents must accurately represent the earth's surface with its representative elements. These documents are made by using cartographic projections that must be analyzed according to the quality of the representation in order to obtain plans and maps with high accuracy and superior quality. Regarding the elements that can be deformed within a projection, these are angles, distance or acres. Between the years 1855-1857, the first map of Wallachia was made (also known as "Satmari Map") using the scale 1:57 600. This map contains the historical provinces Wallachia and Oltenia, and it is the first map under the name of Romania. After the First World War, due to economic, political and administrative reasons, the necessity arose for a large-scale map (1:20 000, 1:10 000 or even 1:5 000) by reproducing and multiplying existing land originals. The new cartographic documents, known as "drawing master plans" were drawn up in a unique projection throughout the country, namely the Lambert projection. After the Second World War, the Romanian Cartography has strongly developed. Thus, in order to obtain plans and topographic maps in the shortest time and at the lowest possible price, the Photogrammetry Center was established in 1958, which later in 1970 became the Institute of Geodesy, Photogrammetry, Cartography and Land Management (IGFCOT), where plans were drawn up to the scales 1:2 000, 1:5 000 and 1:10 000 required either by national economy, or for land records, or to urban planning. The modern topographic map was also created to the scale 1:25 000 (1972-1981) using the Gauss-Krüger transverse cylindrical projection. In Romania, over time many cartographic projections were used to create basic maps, as follows: Muffling polyhedral projection introduced in 1873 - for the Austrian map covering also Transilvania; Cassini cylindrical projection introduced in 1876 based on Bessel ellipsoid; Bonne equivalent conical projection introduced in 1873, based on Clarke 1880 and Bessel 1841 ellipsoid; Lambert- Cholesky conformal conic projection - introduced in 1917, based on Clarke ellipsoid; Gauss Kruger transverse cylindrical conformal projection introduced in 1951, based on ellipsoid Krasovski 1940; Stereographic secant conformal perspective azimuthal projection 1970 plan introduced in 1970, based on ellipsoid Krasovski 1940;UTM conformal secant transversal cylindrical projection (Universal Transverse Mercator) introduced in 1947 based on WGS 84 ellipsoid. [ABSTRACT FROM AUTHOR]

Abstract: Even though the solar wind is highly supersonic, intense ultra-low frequency (ULF) wave activity has been detected in regions just upstream of the bow shocks of magnetized planets. This feature was first observed ahead of the Earth's bow shock, and the corresponding region was called the ULF wave foreshock, which is embedded within the planet's foreshock. The properties as well as the spatial distribution of ULF waves within the Earth's foreshock have been extensively studied over the last three decades and have been explained as a result of plasma instabilities triggered by solar wind ions backstreaming from the bow shock. Since July 2004, the Cassini spacecraft has characterized the Saturnian plasma environment including its upstream region. Since Cassini's Saturn orbit insertion (SOI) in June 2004 through August 2005, we conducted a detailed survey and analysis of observations made by the Vector Helium Magnetometer (VHM). The purpose of the present study is to characterize the properties of waves observed in Saturn's ULF wave foreshock and identify its boundary using single spacecraft techniques. The amplitude of these waves is usually comparable to the mean magnetic field intensity, while their frequencies in the spacecraft frame yields two clearly different types of waves: one with frequencies below the local proton cyclotron frequency ( ) and another with frequencies above . All the wave crossings described here, clearly show that these waves are associated to Saturn's foreshock. In particular, the presence of waves is associated with the change in to quasi-parallel geometries. Our results show the existence of a clear boundary for Saturn's ULF wave foreshock, compatible with surfaces. [Copyright &y& Elsevier]

Abstract: The Cassini Imaging Science Subsystem (ISS) acquired many high-resolution images (<1km/pixel) of the Icy Saturnian satellites during the nominal mission of the Cassini spacecraft between 2004 and 2008. These images were used to create high-resolution mosaics of these satellites. The Cassini mission is expected to continue till 2017 and high-resolution images of the first three years of the extension were used to improve the mosaics, especially in the Northern parts which were not illuminated during the nominal mission. These improved mosaics were the baseline for new versions of the atlases of Mimas, Enceladus, and Dione described in this paper. These new atlases supersede the previous versions from 2006 (Enceladus) and 2008 (Mimas and Dione), and include the official names of additional features, proposed by the Cassini imaging team, approved by the International Astronomical Union (IAU). The new atlases are available to the public through the Imaging Team''s website and the Planetary Data System (PDS). [Copyright &y& Elsevier]