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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]

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]

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]

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]

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]

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]

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]

Abstract: We present plasma data from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP), mapping the ion density and velocity of Saturn''s inner plasma torus. Data from 129 orbits, recorded during the period from the 1st of February 2005 to the 27th of June 2010, are used to map the extension of the inner plasma torus. The dominant part of the plasma torus is shown to be located in between 2.5 and 8 Saturn radii (1RS=60,268km) from the planet, with a north-southward extension of . The plasma disk ion density shows a broad maximum in between the orbits of Enceladus and Tethys. Ion density values vary between 20 and 125cm−3 at the location of the density maximum, indicating considerable dynamics of the plasma disk. The equatorial density structure, , shows a slower decrease away from the planet than towards. The outward decrease, from 5RS, is well described by the relation . The plume of the moon Enceladus is clearly visible as an ion density maximum of 105 cm−3, only present at the south side of the ring plane. A less prominent density peak, of 115cm−3, is also detected at the orbit of Tethys, at . No density peaks are recorded at the orbits of the moons Mimas, Dione, and Rhea. The presented ion velocity shows a clear general trend in the region between 3 and 7RS, described by . The average starts to deviate from corotation at around 3RS, reaching of corotation close to 5RS. [Copyright &y& Elsevier]

Abstract: We have processed all images of Titan''s surface acquired by the Visual and Infrared Mapping Spectrometer between 2004 and 2010, with the objective of producing seamless global mosaics of the surface in the six infrared atmospheric windows at 1.08, 1.27, 1.59, 2.03, 2.6–2.7 and 5μm. A systematic study of the photometry at 5μm, where haze scattering can be neglected, shows that the surface behaves to first order like a Lambert surface. The results at 5μm are generalized to lower wavelengths, adding an empirical correction accounting for scattering from atmospheric aerosols, using methane band wings as a proxy for the calculation of the scattering additive term. Mosaics incorporating this empirical correction for the geometry and haze scattering show significantly less seams than any previous maps integrating data acquired over such a wide range of observing geometries, and may therefore be used for the study of surface properties. We provide several suggestions for further studies aimed at improving the global mapping of the surface of Titan. With the considered thresholds limits on the acquisition parameters, we found that 13% of Titan''s surface has been mapped at an instrument resolution better than 10km/pixel, 56% of the surface was seen at a resolution between 10 and 20km/pixel, and 24% of the coverage falls in the range 20–50km/pixel. [Copyright &y& Elsevier]

Abstract: Cassini radar images of Titan''s surface reveal numerous dark circular features in the equatorial region. These may be related to methane sublimation and have been interpreted as pits. In the T8 equatorial swath, we identify 199 individual pits and estimate their diameters. We analyze the observed spatial distribution and size to characterize these features and understand their formation. Chi-square analysis confirms a significant deviation from a random distribution and shows clustering over the entire swath. However, analysis of the densest cluster of pits, a group of 50, shows a more random distribution. Fractal analysis and comparison with a same-sized random set find only a hint of linearity. A Poisson distribution fits the observed pit-sizes, although resolution limits size determination for the smallest pits, those less than 1km in diameter. Models for random pit generation and evolution simulate the observed Poisson distribution of pit sizes, with larger pits forming by the coalescing of smaller, overlapping ones. Pits, estimated to cover 0.5% of the equatorial T8 swath, could be an erosion mechanism that significantly contributes to the negative skew that has been observed in Titan''s hypsography. [Copyright &y& Elsevier]

Abstract: The Cassini Imaging Science Subsystem (ISS) acquired 370 high-resolution images (<500m/pixel) of Rhea during two close flybys and 9 non-targeted flybys between 2004 and 2010. We combined these images with lower-resolution Cassini images and others taken by the Voyager cameras to produce a high-resolution semi-controlled mosaic of Rhea. This global mosaic is the baseline for a high-resolution Rhea atlas. The nomenclature used in this atlas was proposed by the Cassini imaging team and approved by the International Astronomical Union (IAU). The atlas is available to the public through the Imaging Team''s website 〈http://ciclops.org/maps〉 and the Planetary Data System 〈http://pds.jpl.nasa.gov〉. This atlas completes the series of the atlases of the Saturnian medium-sized satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus. [Copyright &y& Elsevier]

Abstract: Saturn''s Moon Titan has a thick atmosphere with a meteorological cycle. We report on the evolution of the giant cloud system covering its north pole using observations acquired by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft. A radiative transfer model in spherical geometry shows that the clouds are found at an altitude between 30 and 65km. We also show that the polar cloud system vanished progressively as Titan approached equinox in August 2009, revealing at optical wavelengths the underlying sea known as Kraken Mare. This decrease of activity suggests that the north-polar downwelling has begun to shut off. Such a scenario is compared with the Titan global circulation model of , which predicts a decrease of cloud coverage in northern latitudes at the same period of time. [Copyright &y& Elsevier]

Abstract: Titan''s fluvial channels have been investigated based on data obtained by the Synthetic Aperture Radar (SAR) instrument and the Visible and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft. In this paper, a database of fluvial features is created based on radar-SAR data aiming to unveil the distribution and the morphologic and spectral characteristics of valleys on Titan on a global scale. It will also study the spatial relations between fluvial valleys and Titan''s geologic units and spectral surface units which have become accessible thanks to Cassini-VIMS data. Several distinct morphologic types of fluvial valleys can be discerned by SAR-images. Dendritic valley networks appear to have much in common with terrestrial dendritic systems owing to a hierarchical and tree-shaped arrangement of the tributaries which is indicative of an origin from precipitation. Dry valleys constitute another class of valleys resembling terrestrial wadis, an indication of episodic and strong flow events. Other valley types, such as putative canyons, cannot be correlated with rainfall based on their morphology alone, since it cannot be ruled out that they may have originated from volcanic/tectonic action or groundwater sapping. Highly developed and complex fluvial networks with channel lengths of up to 1200km and widths of up to 10km are concentrated only at a few locations whereas single valleys are scattered over all latitudes. Fluvial valleys are frequently found in mountainous areas. Some terrains, such as equatorial dune fields and undifferentiated plains at mid-latitudes, are almost entirely free of valleys. Spectrally, fluvial terrains are often characterized by a high reflectance in each of Titan''s atmospheric windows, as most of them are located on Titan''s bright ‘continents’. Nevertheless, valleys are spatially associated with a surface unit appearing blue due to its higher reflection at in a VIMS false color RGB composite with R: , G: , and B: ; the channels either dissect pure bluish surface units or they are carved into terrain with a mixed spectral signature between bright and bluish surface materials. The global picture of fluvial flows clearly indicates a high diversity of parameters controlling fluvial erosion, such as climatic processes, as well as surface and bedrock types. Recent fluvial activity is very likely in the north polar region in contrast to more arid conditions at lower latitudes and at the south pole of Titan. This divergence is probably an indication of seasonal climatic asymmetries between the hemispheres. However, traces of previous fluvial activity are scattered over all latitudes of Titan, which is indicative of previous climatic conditions with at least episodic rainfall. [Copyright &y& Elsevier]

Abstract: The helium resonance line at 584Å has been observed with the UltraViolet Imaging Spectrograph (UVIS) Extreme Ultraviolet channel during the flyby of Venus by Cassini at a period of high solar activity. The brightness was measured along the disk from the morning terminator up to the bright limb near local noon. The mean disk intensity was ∼320R, reaching ∼700R at the bright limb. These values are slightly higher than those determined from previous observations. The sensitivity of the 584Å intensity to the helium abundance is analyzed using recent cross-sections and solar irradiance measurements at 584Å. The intensity distribution along the UVIS footprint on the disk is best reproduced using the EUVAC solar flux model and the helium density distribution from the VTS3 empirical model. It corresponds to a helium density of 8×106 cm−3 at the level of where the CO2 is 2×1010 cm−3. [Copyright &y& Elsevier]

Abstract: During the final three of the five consecutive and similar Cassini Titan flybys T55–T59 we observe a region characterized by high plasma densities (electron densities of 1–8cm−3) in the tail/nightside of Titan. This region is observed progressively farther downtail from pass to pass and is interpreted as a plume of ionospheric plasma escaping Titan, which appears steady in both location and time. The ions in this plasma plume are moving in the direction away from Titan and are a mixture of both light and heavy ions with composition revealing that their origin are in Titan''s ionosphere, while the electrons are more isotropically distributed. Magnetic field measurements indicate the presence of a current sheet at the inner edge of this region. We discuss the mechanisms behind this outflow, and suggest that it could be caused by ambipolar diffusion, magnetic moment pumping or dispersive Alfvén waves. [Copyright &y& Elsevier]

Abstract: We present RPWS Langmuir probe data from the third Enceladus flyby (E3) showing the presence of dusty plasma near Enceladus’ South Pole. There is a sharp rise in both the electron and ion number densities when the spacecraft traverses through Enceladus plume. The ion density near Enceladus is found to increase abruptly from about 102 cm−3 before the closest approach to 105 cm−3 just 30s after the closest approach, an amount two orders of magnitude higher than the electron density. Assuming that the inconsistency between the electron and ion number densities is due to the presence of dust particles that are collecting the missing electron charges, we present dusty plasma characteristics down to sub-micron particle sizes. By assuming a differential dust number density for a range in dust sizes and by making use of Langmuir probe data, the dust densities for certain lower limits in dust size distribution were estimated. In order to achieve the dust densities of micrometer and larger sized grains comparable to the ones reported in the literature, we show that the power law size distribution must hold down to at least 0.03μm such that the total differential number density is dominated by the smallest sub-micron sized grains. The total dust number density in Enceladus’ plume is of the order of 102 cm−3 reducing to 1cm−3 in the E-ring. The dust density for micrometer and larger sized grains is estimated to be about 10−4 cm−3 in the plume while it is about 10−6–10−7 cm−3 in the E-ring. Dust charge for micron sized grains is estimated to be about eight thousand electron charges reducing to below one hundred electron charges for 0.03μm sized grains. The effective dusty plasma Debye length is estimated and compared with inter-grain distance as well as the electron Debye length. The maximum dust charging time of 1.4h is found for 0.03μm sized grains just 1min before the closest approach. The charging time decreases substantially in the plume where it is only a fraction of a second for 1μm sized grains, 1s for 0.1μm sized grains and about 10s for 0.03μm sized grains. [Copyright &y& Elsevier]

Abstract: The MIMI experiment (Magnetosphere Imaging Instrument) onboard Cassini is dedicated to the study of energetic particles, with in particular LEMMS analyzing charged particles, or the INCA detector which can image the Energetic Neutral Atoms produced by charge exchange collisions between cold neutrals and energetic ions. The MIMI experiment is thus well adapted to the study of the interaction between the Titan nitrogen rich atmosphere and the energetic Saturnian magnetospheric plasma. We analyze here the energetic protons at the Titan orbit crossings before January 2008 (MIMI-LEMMS data; 27–255keV), which are very dynamic, with tri-modal flux spectra and probably quasi-isotropic pitch angle distributions. We provide statistical parameters for the proton fluxes, leading to estimates of the average energy deposition into Titan''s atmosphere, before we discuss the possible influence of Titan on the magnetopause. We then analyze the H ENA images (24–55keV) during the Titan flybys before June 2006 to obtain a better diagnostic of the Titan interaction: the ENAs variability is mostly related to the magnetospheric variability (the exosphere being roughly stable) or the distance from the moon, the ENAs halo around Titan is very stable (corresponding to a lower limit for ENAs emission at the exobase), and strong asymmetries are observed, due to finite gyroradii effects for the parent ions. [Copyright &y& Elsevier]

Abstract: We present a hybrid simulation study (kinetic ions, fluid electrons) of Titan''s plasma interaction during an excursion of this moon from Saturn''s magnetosphere into its magnetosheath, as observed for the first time during Cassini''s T32 flyby on 13 June 2007. In contrast to earlier simulations of Titan''s plasma environment under non-stationary upstream conditions, our model considers a difference in the flow directions of magnetospheric and magnetosheath plasma. Two complementary scenarios are investigated, with the flow directions of the impinging magnetospheric/magnetosheath plasmas being (A) antiparallel and (B) parallel. In both cases, our simulations show that due to the drastically reduced convection speed in the slow and dense heavy ion plasma near Titan, the satellite carries a bundle of “fossilized” magnetic field lines from the magnetosphere in the magnetosheath. Furthermore, the passage through Saturn''s magnetopause goes along with a disruption of Titan''s pick-up tail. Although the tail is not detached from the satellite, large clouds of heavy ion plasma are stripped of its outer flank, featuring a wave-like pattern. Whereas in case (B) under parallel flow conditions there is only a small retardation of about 5min between the passage of Titan through the magnetopause and the reconfiguration of the pick-up tail, the tail reconfiguration in the case (A) scenario is completed not until 25min after the magnetopause passage. The lifetime of fossil fields in the moon''s ionosphere is approximately 25min, regardless of whether parallel or antiparallel flow conditions are applied. [Copyright &y& Elsevier]

Abstract: We present new results concerning electron temperature (T e ) mapping in the inner magnetosphere of Saturn (<7RS). The data are obtained by the Langmuir Probe (LP) instrument of the Radio and Plasma Wave Science (RPWS) investigation on board Cassini. The study is based on the fourteen first orbits around Saturn, and focus is on cold (<10eV) and dense (>5cm−3) electron populations in the inner plasma torus (plasma disc). Apart from a general increase with distance from Saturn and away from the equatorial plane (latitude), we identify a possibly anisotropic electron temperature in the plasma disc, as well as the presence of photoelectrons from the spacecraft or E-ring dust particles. A polytropic index of γ≈+0.4 is determined from the observations, which can be used in theoretical models of the plasma disc. We also infer local variations in T e near the equatorial plane, where the E-ring of dusty plasma and enhancements of water rich neutral gas exist, and determine constraints on the energy source required to heat the electrons in the plasma disc. We suggest that Joule heating or heating by low-frequency plasma waves as well as cooling by neutral gas and E-ring dust are involved in the energy balance, together with Coulomb collision heating by co-rotating ions. [Copyright &y& Elsevier]

Abstract: We present the two-stage method used to calibrate the electron spectrometer (ELS), part of the plasma spectrometer (CAPS) on board the Cassini spacecraft currently in orbit around Saturn. The CAPS-ELS is a top-hat electrostatic analyser designed to measure electron fluxes between 0.5eV and 26keV. The on-ground calibration method described here includes the production of photoelectrons, which are energised and passed into the CAPS-ELS in a purpose designed calibration facility. Knowledge of the intensity of these incident electrons and the subsequent instrument output provides an on-ground calibrated geometric factor. Comparative studies of physical quantities such as plasma density and electron differential flux calculated using on-ground calibration factor with the quantities deduced from the wave experiment and high energy electron detector provide in-flight calibration. The results of this are presented together with a comparison of the experimentally calibrated values with simulated calibration values. [Copyright &y& Elsevier]

Abstract: We study the interaction of Saturn''s small, icy moon Enceladus and its plume with the corotating magnetospheric plasma by means of a 3D hybrid simulation model, which treats the ions as individual particles and the electrons as a massless, charge-neutralizing fluid. We analyze systematically how Enceladus’ internal conductivity and plasma absorption at the surface as well as charge exchange and pick-up in the plume contribute to the overall structure of the interaction region. Furthermore, we provide a comparison of our simulation results to data obtained by the Cassini magnetometer instrument. The major findings of this study are: (1) the magnetic field diffuses through the solid body of Enceladus almost unaffected, whereas plasma absorption gives rise to a symmetric depletion wake downstream of the moon; (2) due to the small gyroradii of the newly generated plume ions, the pick-up tail possesses a 2D structure; (3) the magnetic field lines drape around the plume, which triggers an Alfvén wing system that dominates the structure of Enceladus’ plasma environment. Inside the plume itself, a magnetic cavity is formed; (4) besides the reproduction of the key features of the observed magnetic field signatures, evidence for variability in the locations of the active jets and in the total gas content of the plume are shown. [Copyright &y& Elsevier]

Abstract: In this paper Saturn''s nightside and pre-dawn electron (0.5eV–28keV) plasma sheet is studied using Cassini plasma electron and magnetic field data from 2006. Case studies are presented which exemplify the typical and atypical states of the plasma sheet, and are complemented by a statistical study of the plasma sheet. It will be shown that Saturn''s nightside and pre-dawn electron plasma sheet exists in two states: a quiescent state with a steady electron temperature of and where the electron distribution functions are best characterised by Kappa distributions, and a disturbed state where the electrons are hot and often seen in alternating layers between warm and hot populations. Evidence is also presented for bimodal cold/warm (both quiet and disturbed states) and warm/hot distributions (disturbed states). The disturbed states are qualitatively similar to electron distributions from Earth''s magnetotail during intervals of reconnection and we argue that these disturbed states also result from periods of tail reconnection. We present statistics of electron number density, temperature, partial electron beta, and pressure, and show that large values of partial beta are necessary but not sufficient to uniquely identify the central plasma sheet. Finally the thermodynamic properties of the electron plasma sheet are studied and we show that the electrons behave isothermally. These results are important for modelling and theoretical analyses, and for use in studies which examine dynamics in Saturn''s magnetosphere. [Copyright &y& Elsevier]

Abstract: This is a data paper designed to facilitate the use of and comparisons to Cassini/visual and infrared mapping spectrometer (VIMS) spectral mapping data of Saturn''s moon Titan. We present thumbnail orthographic projections of flyby mosaics from each Titan encounter during the Cassini prime mission, 2004 July 1 through 2008 June 30. For each flyby we also describe the encounter geometry, and we discuss the studies that have previously been published using the VIMS dataset. The resulting compliation of metadata provides a complementary big-picture overview of the VIMS data in the public archive, and should be a useful reference for future Titan studies. [Copyright &y& Elsevier]

Abstract: The data obtained by the Cassini Radio and Plasma Wave Science (RPWS) instrument during the shallow (17.02.2005) and the steep (14.07.2005) crossings of the E-ring revealed a considerable electron depletion in proximity to Enceladus''s orbit (the difference between the ion and electron densities can reach ). Assuming that this depletion is a signature of the presence of charged dust particles, the main characteristics of dust down to submicron sized particles are derived. The differential size distribution is found to be well described by a power law with an index for the lower size limit and for . The calculated average integral dust number density is weakly affected by values of and . For , both flybys gave the maximum dust density about in the vicinity of Enceladus. Our results imply that the dust structure near Enceladus is characterized by approximately the same vertical length scale of 8000km and reaches a maximum at the same radial distance (displaced outward of the orbit of Enceladus) as found by Kempf et al. [2008. The E-ring in the vicinity of Enceladus. Spatial distribution and properties of the ring particles. Icarus 193, 420–437], from the dust impact data. [Copyright &y& Elsevier]

Abstract: During its tour Cassini has observed numerous plasma injection events in Saturn''s inner magnetosphere. Here, we present a case study of one “young” plasma bubble observed when Cassini was in the equatorial plane. The bubble was observed in the equatorial plane at ∼7 Saturn radii from Saturn and had a maximum azimuthal extent of ∼0.25Rs (Rs=Saturn radius ∼60330km). We show that the electron density inside the event is lower by a factor ∼3 and the electron temperature higher by over an order of magnitude compared to its surroundings. The injection contains slightly increased magnetic field magnitude of 49nT compared with a background field of 46nT. Modelling of pitch angle distributions inside the plasma bubble and measurements of plasma drift provide a novel way to estimate that the bubble originated between 9

Abstract: We present observational evidence that the ionosphere of Titan below an altitude of 1150km is a significant source of heavy (>100amu) molecular organic species. This study is based on measurements by five instruments (RPWS/LP, RPWS/E, INMS, CAPS/ELS, CAPS/IBS) onboard the Cassini spacecraft during three flybys (T17, T18, T32) of Titan. The ionospheric peaks encountered at altitudes of 950–1300km had densities in the range 900–3000cm−3. Below these peaks the number densities of heavy positively charged ions reached 100–2000cm−3 and approached 50–70% of the total ionospheric density with an increasing trend toward lowest measured altitudes. Simultaneously measured negatively charged ion densities were in the range 50–150cm−3. These results imply that ~105–106 heavy positively charged ions/m3/s are continuously recombining into heavy neutrals and supply the atmosphere of Titan. The ionosphere may in this way produce 0.1–1Mt/yr of heavy organic compounds and is therefore a sizable source for aerosol formation. We also predict that Titan''s ionosphere is dominated by heavy (>100amu) molecular ions below 950km. [Copyright &y& Elsevier]

Abstract: We present the first results of ionospheric conductivities at Titan based on measurements during 17 Titan flybys from the Cassini spacecraft. We identify an ionospheric region ranging from (approximately the location of the exobase) to approximately 1000km where electrical currents perpendicular to the magnetic field may become important. In this region the ionosphere is highly conductive with peak Pedersen conductivities of 0.002–0.05S/m and peak Hall conductivities of 0.01–0.3S/m depending on Solar illumination and magnetospheric conditions. Ionospheric conductivities are found to be typically higher on the sunlit side of Titan. However, Hall and Pedersen conductivities depend strongly on the magnetic field magnitude which is highly variable, both in altitude and with respect to the draping configuration of Saturn''s magnetic field around Titan. Furthermore, a consistent double peak nature is found in the altitude profile of the Pedersen conductivity. A high altitude peak is found to be located between 1300 and 1400km. A second and typically more conductive region is observed below 1000km, where the magnetic field strength drops sharply while the electron density still remains high. This nature of the Pedersen conductivity profile may give rise to complicated ionospheric–atmospheric dynamics and may be expected also at other unmagnetized objects with a substantial atmosphere, such as e.g. Mars and Venus. Estimates of the total Pedersen conductance are found to range between 1300 and 22,000S. The Pedersen conductance is always higher than the local Alfvén conductance but the difference varies by two orders of magnitude (from a factor 4 to 100). Thus, the conditions for reflection or absorption of Alfvén waves in Titans ionosphere are highly variable and depends strongly on the magnetic field strength. [Copyright &y& Elsevier]

Abstract: In this study we present data from 17 Titan flybys showing that solar photons are the main ionisation source of Titan''s dayside atmosphere. This is the first comprehensive study of Solar Zenith Angle (SZA) dependence of the electron number density and electron temperature at the ionospheric peak. The results show on average four times more plasma on the dayside compared to the nightside, with typical dayside electron densities of around 2500– and corresponding nightside densities of around 400–. We identify a broad transition region between SZA and , where the ionosphere of Titan changes from being entirely sunlit to being in the shadow of the moon. For SZA the ionisation peak altitude increases with increasing SZA, whereas the transition region and the nightside show more scattered ionospheric peak altitudes. Typical electron temperatures at the ionospheric peak are 0.03–0.06eV (–700K) for both dayside and nightside. [Copyright &y& Elsevier]

Abstract: We present several independent in-situ measurements, which provide evidence that charged dust in the E-ring interacts collectively with the dense surrounding plasma disk of Saturn, i.e., form a system of dust-plasma interaction. The results are based on data sampled by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini, which allows for interferometry of plasma density inhomogeneities (δn/n) with two antenna elements and a Langmuir probe sensor. The interferometer experiment detects two ion populations; one co-rotating with the planetary magnetic field and another moving with near Keplerian speed around Saturn. The full range of RPWS measurements indicates that the Keplerian population consists of colder ions (T i

Abstract: The intensity of H Ly (2P–1S) and OI () is mapped in the magnetosphere of Saturn using the ultraviolet imaging spectrograph (UVIS) [Esposito, L.W., Barth, C.A., Colwell, J.E., Lawrence, G.M., McClintock, W.E., Stewart, A.I.F., Keller, H.U., Korth, A., Lauche, H., Festou, M.C., Lane, A.L., Hansen, C.J., Maki, J.N., West, R.A., Jahn, H., Reulke, R., Warlich, K., Shemansky, D.E., Yung, Y.L., 2004. The Cassini ultraviolet imaging spectrograph investigation. Space Science Reviews 115, 299–361] onboard Cassini. Spatial coverage is built up by stepping the slit sequentially across the system (system scan). Data are obtained at a large range of space-craft–Saturn distances. The observed atomic hydrogen distribution is very broad, extending beyond in the equatorial plane, with the intensity increasing with decreasing distances to Saturn. The distribution displays persistent local-time asymmetries, and is seen connecting continuously to the upper atmosphere of the planet at sub-solar latitudes located well outside of the equatorial (ring) plane. This is consistent with the source of the atomic hydrogen being located at the top of the atmosphere on the sun-lit side of the planet on the southern hemisphere. In addition there are a number of temporally persistent features in the intensity distribution, indicating a complex hydrogen energy distribution. The emission from OI is generally distributed as a broad torus centered around although the position of the peak intensity can vary by as much as . There is significant intensity present out to . HST observations of hydroxyl (OH) are re-analyzed and display a distribution half as broad as that of oxygen, also centered at . The observed atomic oxygen distribution requires a sourcing of against loss due to charge capture with the plasma. Using the ion partitioning of Schippers et al. [2008. Multi-instrument analysis of electron populations in Saturn''s magnetosphere. Journal of Geophysical Research (Space Physics) 113 (A12) 7208–+] then recombination of and will account for about a quarter of the mass-loss in the inner magnetosphere, with charge capture of accounting for the rest. The oxygen loss rate is seen to vary by over periods of weeks. [Copyright &y& Elsevier]

Abstract: Saturn''s largest moon, Titan, provides an interesting opportunity to study how dense atmospheres interact with the surrounding plasma environment. Without an intrinsic magnetic field, this satellite''s nitrogen-rich atmosphere is relatively unprotected from plasma interactions. Therefore, the energy-deposition rate is important for understanding chemistry and dynamics in Titan''s atmosphere. Since the plasma environment can vary significantly we focus here on the T18 Titan encounter using in-situ data from instruments on board the Cassini spacecraft. These instruments cannot provide in-situ information below the spacecraft closest approach altitude (∼>960km) so we use the Cassini magnetospheric imaging instrument (MIMI) ion-neutral camera (INCA) to remotely image energetic hydrogen particle fluxes (20–80keV) at altitudes below Titan closest approach. We also use the MIMI low-energy magnetosphere measurements system (LEMMS) to measure the incident ion fluxes as the spacecraft approaches Titan and combine these data sets with an atmospheric model to first reproduce INCA images. We then use this model to calculate the energy-deposition profiles for the observed incident proton flux. Our model is able to reproduce the INCA observations and give the energy density deposited vs. altitude in Titan''s atmosphere; however, we find that the incident fluxes and energy-deposition profiles vary significantly during the encounter. [Copyright &y& Elsevier]

Abstract: The Chemical Analyser subsystem of the Cosmic Dust Analyser (CDA) aboard the Cassini spacecraft performs in situ measurements of the chemical composition of dust in space. The instrument records time-of-flight mass spectra of cations, extracted from the impact cloud that is created by high-velocity particle impacts onto the detector target. Thus, the spectra not only show signals of particle components but also of ions from the target material and target contamination. The aim of this work is to determine which non-particle ions are to be expected in the spectra obtained in space operation at Saturn. We present an analysis of the contamination state of the instrument''s impact target. Beside investigations of the purity of the rhodium target surface, spectra from CDA calibration experiments at the dust accelerator facility are evaluated with regard to contamination signatures. Furthermore, contamination mass lines in spectra obtained by impacts of Jovian and Saturnian dust stream particles are analysed. Due to their small size and high speed, stream particle impacts predominantly produce ions from the target material and therefore the spectra are excellent probes of the contamination state of the target operating in space. With the exception of adsorbed hydrogen and carbon, the level of contamination is very low. Implications for CDA spectra of Saturnian E ring particle impacts are derived. The findings confirm the published interpretations. The low level of alkali metal contamination implies a significant sodium contribution in the composition of E ring ice particles. Additionally, ionisation thresholds for the occurrence of contamination mass lines can be utilised to set limits for the impact velocity. [Copyright &y& Elsevier]

Abstract: Data from the Cassini plasma spectrometer (CAPS) electron spectrometer (ELS) have been found to be contaminated with an energy-independent background count rate which has been associated with radiation sources on Cassini. In this paper we describe this background radiation and quantitatively assess its impact on numerically integrated electron moments. The general properties of such a background and its effects on numerical moments are derived. The properties of the ELS background are described and a model for the background presented. A model to generate synthetic ELS spectra is presented and used to evaluate the density and temperature of pure noise and then extended to include ambient distributions. It is shown that the presence of noise produces a saturation of the electron density and temperature at quasi-constant values when the instrument is at background, but that these noise level moments are dependent on the floating spacecraft potential and the orientation of the ELS instrument with respect to the spacecraft. When the ambient distribution has a poor signal-to-noise ratio (SNR) the noise determines the density and temperature; however, as the SNR increases (increasing primarily with density) the density and temperature tend to those of the ambient distribution. It is also shown that these noise effects produce highly artificial density–temperature inverse correlations. A method to subtract this noise is presented and shown to correct for the presence of the noise. Simulated error estimates for the density and temperature are also presented. The analysis described in this paper not only applies to weak background noise, but also to more significant penetrating backgrounds such as those in radiation belt regions of planetary magnetospheres. [Copyright &y& Elsevier]

Abstract: The Cassini Imaging Science Subsystem (ISS) acquired 282, 258, and 513 high-resolution images (<800m/pixel) of Mimas, Tethys, and Iapetus, respectively, during two close flyby of Tethys and Iapetus and eight non-targeted flybys between 2004 and 2007. We combined these images with lower-resolution Cassini images and others taken by Voyager cameras to produce high-resolution semi-controlled mosaics of Mimas, Tethys, and Iapetus. These global mosaics are the baseline for high-resolution Mimas and Iapetus maps and a Tethys atlas. The nomenclature used in these maps was proposed by the Cassini imaging team and was approved by the International Astronomical Union (IAU). The two maps and the atlas are available to the public through the Imaging Team''s website [http://ciclops.org/maps] and the Planetary Data System [http://pds.jpl.nasa.gov]. [Copyright &y& Elsevier]

Abstract: In the inner magnetosphere of Saturn, the plasma density and drift velocity are high enough, and the photoelectron current low enough, for a Langmuir probe to produce useful data on ion parameters. Plasma density and velocity are found by analyzing the current due to collected ions and emitted photoelectrons for a negative probe potential. In order to correctly analyze the data, the current caused by photoelectrons emitted from the probe must be known. For a spherical probe at negative bias this should be a constant current, but for Cassini''s probe it varies with attitude. A likely cause of this is a leakage current from the stub to the probe. The plasma drift velocities derived from Langmuir probe measurements did not agree with those found by the Cassini plasma spectrometer in the inner magnetosphere, but did so elsewhere. A possible solution to this is a two-component plasma where the components have different drift velocities. [Copyright &y& Elsevier]

Abstract: The Cassini imaging science subsystem (ISS) acquired 449 high-resolution images (<800m/pixel) during one close flyby of Dione in 2005 and three non-targeted flybys in 2004, 2006, and 2007. We combined these images with lower-resolution Cassini images and one other taken by Voyager cameras to produce a high-resolution semi-controlled mosaic of Dione. This global mosaic is the baseline for a high-resolution Dione atlas that consists of 15 tiles mapped at a scale of 1:1,000,000. The nomenclature used in this atlas was proposed by the Cassini imaging team and was approved by the International Astronomical Union (IAU). The whole atlas is available to the public through the Imaging Team''s website [http://ciclops.org/maps]. [Copyright &y& Elsevier]

Abstract: In this paper we present two methods to derive electron fluid parameters from the CAPS–ELS spectrometer on board the Cassini spacecraft currently in orbit around Saturn. In the first part of the paper we give a basic overview of the instrument and describe the challenges inherent in the derivation of density and temperature values using these techniques. We then describe a method to calculate electron moments by integrating the particle distribution function. We also describe a second technique in which we fit the electron energy spectrum with a Gaussian curve and use the peak energy of this curve to derive density and temperature values. We then compare the two methods with particular emphasis on their application to Cassini SOI observations in the saturnian environment and point out the limitations of the two techniques. We will show that results from the two very different methods are in agreement when the physical properties of the environment and of the observed electron populations have been inferred from inspection of the raw data. Finally we will suggest future developments that will remove these limitations. [Copyright &y& Elsevier]

Abstract: The Cassini Imaging Science Subsystem (ISS) acquired 377 high-resolution images (<1km/pixel) during three close flybys of Enceladus in 2005 [Porco, C.C., et al., 2006. Cassini observes the active south pole of Enceladus. Science 311, 1393–1401.]. We combined these images with lower resolution Cassini images and four others taken by Voyager cameras to produce a high-resolution global controlled mosaic of Enceladus. This global mosaic is the baseline for a high-resolution Enceladus atlas that consists of 15 tiles mapped at a scale of 1:500,000. The nomenclature used in this atlas was proposed by the Cassini imaging team and was approved by the International Astronomical Union (IAU). The whole atlas is available to the public through the Imaging Team''s website (http://ciclops.org/maps). [Copyright &y& Elsevier]

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

Abstract: Titan''s nitrogen-rich atmosphere is directly bombarded by energetic ions, due to its lack of a significant intrinsic magnetic field. Singly charged energetic ions from Saturn magnetosphere undergo charge exchange collisions with neutral atoms in Titan''s exosphere, being transformed into energetic neutral atoms (ENAs). The ion and neutral camera (INCA), one of the three sensors that comprise the magnetosphere imaging instrument on the Cassini/Huygens mission to Saturn and Titan, images the ENA emissions from various ion/gas interaction regions in the Saturnian magnetosphere. During Cassinis second orbit around Saturn the spacecraft performed the Ta Titan flyby (26 October 2004), at an altitude of only 1174km. INCA data acquired during this targeted close flyby not only confirm model predictions of dominant finite ion gyroradii effects, but also reveal a much more complex interaction. These observations are analyzed and compared to simulations. A new Titan exosphere model is then proposed. [Copyright &y& Elsevier]

Abstract: The Visual and Infrared Mapping Spectrometer (VIMS) instrument on the Cassini Saturn Orbiter returned spectral imaging data as the spacecraft undertook six close encounters with Titan beginning 7 July, 2004. Three of these flybys each produced overlapping coverage of two distinct regions of Titan''s surface. Twenty-four points were selected on approximately opposite hemispheres to serve as photometric controls. Six points were selected in each of four reflectance classes. On one hemisphere each control point was observed at three distinct phase angles. From the derived phase coefficients, preliminary normal reflectances were derived for each reflectance class. The normal reflectance of Titan''s surface units at 2.0178μm ranged from 0.079 to 0.185 for the most absorbing to the most reflective units assuming no contribution from absorbing haze. When a modest haze contribution of τ =0.1 is considered these numbers increase to 0.089–0.215. We find that the lowest three reflectance classes have comparable normal reflectance on either hemisphere. However, for the highest brightness class the normal reflectance is higher on the hemisphere encompassing longitude 14–65° compared to the same high brightness class for the hemisphere encompassing 122–156° longitude. We conclude that an albedo dichotomy observed in continental sized units on Titan is due not only to one unit having more areal coverage of reflective material than the other but the material on the brighter unit is intrinsically more reflective than the most reflective material on the other unit. This suggests that surface renewal processes are more widespread on Titan''s more reflective units than on its less reflective units. We note that one of our photometric control points has increased in reflectance by 12% relative to the surrounding terrain from July of 2004 to April and May of 2005. Possible causes of this effect include atmospheric processes such as ground fog or orographic clouds; the suggestion of active volcanism cannot be ruled out. Several interesting circular features which resembled impact craters were identified on Titan''s surface at the time of the initial Titan flyby in July of 2004. We traced photometric profiles through two of these candidate craters and attempted to fit these profiles to the photometric properties expected from model depressions. We find that the best-fit attempt to model these features as craters requires that they be unrealistically deep, approximately 70km deep. We conclude that despite their appearance, these circular features are not craters, however, the possibility that they are palimpsests cannot be ruled out. We used two methods to test for the presence of vast expanses of liquids on Titan''s surface that had been suggested to resemble oceans. Specular reflection of sunlight would be indicative of widespread liquids on the surface; we found no evidence of this. A large liquid body should also show uniformity in photometric profile; we found the profiles to be highly variable. The lack of specular reflection and the high photometric variability in the profiles across candidate oceans is inconsistent with the presence of vast expanses of flat-lying liquids on Titan''s surface. While liquid accumulation may be present as small, sub-pixel-sized bodies, or in areas of the surface which still remain to be observed by VIMS, the presence of large ocean-sized accumulations of liquids can be ruled out. The Cassini orbital tour offers the opportunity for VIMS to image the same parts of Titan''s surface repeatedly at many different illumination and observation geometries. This creates the possibility of understanding the properties of Titan''s atmosphere and haze by iteratively adapting models to create a best fit to the surface reflectance properties. [Copyright &y& Elsevier]

Abstract: We present a quantitative analysis of CO thermal emissions discovered on the nightside of Titan by Baines et al. [2005. The atmospheres of Saturn and Titan in the near-infrared: First results of Cassini/VIMS. Earth, Moon, and Planets, 96, 119–147]. in Cassini/VIMS spectral imagery. We identify these emission features as the P and R branches of the 1-0 vibrational band of carbon monoxide (CO) near 4.65μm. For CH3D, the prominent Q branch of the ν2 fundamental band of CH3D near 4.55μm is apparent. CO2 emissions from the strong v 3 vibrational band are virtually absent, indicating a CO2 abundance several orders of magnitude less than CO, in agreement with previous investigations. Analysis of CO emission spectra obtained over a variety of altitudes on Titan''s nightside limb indicates that the stratospheric abundance of CO is 32±15ppm, and together with other recent determinations, suggests a vertical distribution of CO nearly constant at this value from the surface throughout the troposphere to at least the stratopause near 300km altitude. The corresponding total atmospheric content of CO in Titan is ∼2.9±1.5×1014 kg. Given the long lifetime of CO in the oxygen-poor Titan atmosphere (∼0.5–1.0Gyr), we find a mean CO atmospheric production rate of 6±3×105 kgyr−1. Given the lack of primordial heavy noble gases observed by Huygens [Niemann et al., 2005. The abundances of constituents of Titan''s atmosphere from the GCMS on the Huygens probe. Nature, 438, 779–784], the primary source of atmospheric CO is likely surface emissions. The implied CO/CH4 mixing ratio of near-surface material is 1.8±0.9×10−4, based on an average methane surface emission rate over the past 0.5Gyr of 1.3×10−13 gmcm−2 s−1 as required to balance hydrocarbon haze production via methane photolysis [Wilson and Atreya, 2004. Current state of modeling the photochemistry of Titan''s mutually dependent atmosphere and ionosphere. J. Geophys. Res. 109, E06002 Doi:10.1029/2003JE002181]. This low CO/CH4 ratio is much lower than expected for the sub-nebular formation region of Titan and supports the hypothesis [e.g., Atreya et al., 2005. Methane on Titan: photochemical-meteorological-hydrogeochemical cycle. Bull. Am. Astron. Soc. 37, 735] that the conversion of primordial CO and other carbon-bearing materials into CH4-enriched clathrate-hydrates occurs within the deep interior of Titan via the release of hydrogen through the serpentinization process followed by Fischer–Tropsch catalysis. The time-averaged predicted emission rate of methane-rich surface materials is ∼0.02km3 yr−1, a value significantly lower than the rate of silicate lava production for the Earth and Venus, but nonetheless indicative of significant active geological processes reshaping the surface of Titan. [Copyright &y& Elsevier]

Abstract: Titan''s bulk density along with Solar System formation models indicates considerable water as well as silicates as its major constituents. This satellite''s dense atmosphere of nitrogen with methane is unique. Deposits or even oceans of organic compounds have been suggested to exist on Titan''s solid surface due to UV-induced photochemistry in the atmosphere. Thus, the composition of the surface is a major piece of evidence needed to determine Titan''s history. However, studies of the surface are hindered by the thick, absorbing, hazy and in some places cloudy atmosphere. Ground-based telescope investigations of the integral disk of Titan attempted to observe the surface albedo in spectral windows between methane absorptions by calculating and removing the haze effects. Their results were reported to be consistent with water ice on the surface that is contaminated with a small amount of dark material, perhaps organic material like tholin. We analyze here the recent Cassini Mission''s visual and infrared mapping spectrometer (VIMS) observations that resolve regions on Titan. VIMS is able to see surface features and shows that there are spectral and therefore likely compositional units. By several methods, spectral albedo estimates within methane absorption windows between 0.75 and 5μm were obtained for different surface units using VIMS image cubes from the Cassini-Huygens Titan Ta encounter. Of the spots studied, there appears to be two compositional classes present that are associated with the lower albedo and the higher albedo materials, with some variety among the brighter regions. These were compared with spectra of several different candidate materials. Our results show that the spectrum of water ice contaminated with a darker material matches the reflectance of the lower albedo Titan regions if the spectral slope from 2.71 to 2.79μm in the poorly understood 2.8-μm methane window is ignored. The spectra for brighter regions are not matched by the spectrum of water ice or unoxidized tholin, in pure form or in mixtures with sufficient ice or tholin present to allow the water ice or tholin spectral features to be discerned. We find that the 2.8-μm methane absorption window is complex and seems to consist of two weak subwindows at 2.7 and 2.8μm that have unknown opacities. A ratio image at these two wavelengths reveals an anomalous region on Titan that has a reflectance unlike any material so far identified, but it is unclear how much the reflectances in these two subwindows pertain to the surface. [Copyright &y& Elsevier]