Icy Satellites Of Saturn Research Articles

Microwaving Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus and Phoebe: Insights into the regolith properties and geological history of Saturn's icy satellites

While Saturn's main airless moons are all composed largely of water ice, their respective thermal histories and near environments have led to different regolith compositions and structures. Part of this history is recorded in their subsurface which can be probed by microwaves. Using a combined thermal and radiative transfer model, we here investigate all distant observations acquired in the passive mode of the RADAR on board the Cassini spacecraft (2004–2017) at 2.2-cm wavelength. The joint analysis of the derived disk-integrated emissivities and published radar albedos provides new insights into the purity and maturity of the regolith of Saturn's icy moons. We find that satellite-to-satellite variations and large-scale regional anomalies in microwave signatures primarily reflect different degrees of contamination of the regolith by non-ice compounds. To a lesser extent, they may also point to different concentrations of scatterers in the subsurface; these scatterers must be made of ice and/or void rather than of non-ice contaminants. Enceladus appears to have the cleanest regolith likely due to the geological youth of its surface. Observations also suggest that the current heat flux emanating from this moon is not confined to the South Pole Terrain. In the inner system, the degree of purity of the satellites' regoliths decreases from Enceladus outward likely due to the decrease of the E-ring influx. In the outer system, Phoebe's ring mantles Iapetus' leading hemisphere with a decimetric layer of optically-dark and microwave-absorbent dust. Dione is surprisingly less radar-bright and more emissive than expected from both the observed general trend and the current understanding of its geological history. Another question remains outstanding: why are Saturnian moons, and to a lesser extent Jovian moons, so radar-bright at centimetric wavelengths? Current models assuming purely-random scattering in their subsurface fail to simultaneously reproduce active and passive microwave observations, especially for Saturn's inner moons. This may be due the presence of organized and especially efficient backscattering structures in their subsurface. The challenge is now to identify structures that are geologically plausible.

Compositional control on impact crater formation on mid-sized planetary bodies: Dawn at Ceres and Vesta, Cassini at Saturn

High-resolution mapping of Ceres, Vesta and the icy satellites of Saturn, Uranus and Pluto reveals a rich variety of well-preserved impact crater morphologies on these low gravity bodies. These objects provide a natural laboratory to study effects of composition on crater formation processes under similar surface gravity conditions (though mean impact velocities vary by several factors). Simple craters occur on all these bodies but subtle differences in morphology on Ceres and Vesta are recognized. Immature complex craters (with large floor mounds but not terraces or conical central peaks) occur on Vesta and while smaller than predicted are consistent with its silicate composition. Asymmetric simple craters (with incomplete scarp development) on all bodies are likely related to differential overburden stresses in the rim, and their occurrence is consistent with lower crustal strength on icy bodies including Ceres. Immature and mature complex craters exhibit increasing degrees of complexity, including spiral floor deformation patterns (related to failure in converging floor material), central peaks, and impact melt. Cerean crater morphologic types and simple-complex transition diameters are smaller than on Vesta but similar to those on icy satellites, indicating a much weaker rheology for Ceres' outer layers under impact conditions. These are consistent with geophysical indications of a low-density water ice and probably clathrate rich outer shell. Fluidized floor deposits (impact melt or melt-solid mixtures) are significant in craters >25 km across on Ceres but absent on Saturn satellites. Central pit craters are common on Ceres (at diameters of ~75 to 150 km consistent with gravity scaling from the larger Galilean satellites) but are absent on Saturnian satellites and Charon. The contrasting impact melt and central pit behaviors on Ceres and Saturn's moons is contrary to expectation given the higher impact velocities at Saturn but might be related to lower internal temperatures, or the higher fraction of non-ice material on Ceres. The correlation or scaling of transition diameters to surface gravity is near −0.65 rather than −1, perhaps due to increased porosity on lower gravity bodies. The fundamental similarity of crater morphologies on Ceres and icy satellites, however, indicates that the weaker rheology of water ice results in similar craters even if the non-(ice+clathrate) components are as high as ~30 vol%.

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

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

End-of-mission calibration of the Cassini Imaging Science Subsystem

We provide an end-of-mission update to the in-flight calibration of the Imaging Science Subsystem cameras on the Cassini spacecraft, resolving discrepancies in our previous analyses from 2004 to 2010, incorporating data through the end of the mission, adding a correction for the weak but measurable camera sensitivity decline over time, and providing new transmission values for polarizing filters. In the new analysis, we add full-disk photometry of Jupiter and the Saturnian icy satellites Rhea, Dione and Enceladus to the previous measurements, which had consisted primarily of standard star targets, while the stellar photometry is updated to include a correction for light lost to the extended wings of the instrumental point spread function. The resulting absolute flux corrections differ considerably from those previously reported, shifting measured radiance by close to 10% in most broadband filters, and bringing agreement between star and satellite targets to within 3%. We also detail improvements to the narrow-angle camera flat field correction, and provide updates to the filter-by-filter point spread functions, hot pixel correction, and polarizer calibration. For the latter, we present new measurements of Titan’s polarization from near-UV to near-IR, including three methane bands and nearby spectral windows.

Thermal Properties of Icy Surfaces in the Outer Solar System

The thermal properties of airless icy surfaces are providing a wealth of information on their regolith structure after eons of space weathering. Numerous observations of the thermal cycles of Jupiter and Saturn icy satellites or Centaurs and TNOs have been acquired in the latest decades thanks to the Galileo and Cassini missions and to the Spitzer and Herschel telescopes. These observations and the latest developments on thermophysical modeling which have been achieved to link the thermal inertia to the regolith structure are reviewed here. Measured thermal inertias of these surfaces covered with water ice are very low, roughly between about 1 and 100 J/m2/K/s1/2. Often interpreted as due to unconsolidated or highly porous regoliths, these low values may result from a composition of amorphous ice or from the roughness of grains defacing contacts in a regolith of normal compaction. Taken together, thermal inertias appear to increase with probed depth and to decrease with heliocentric distance. This latter effect can be easily reproduced if heat transfer is dominated by radiation in pores, despite low temperatures, because the conduction through grains is limited, either due to the presence of amorphous ice or because of the roughness of grains.

Final Mimas and Enceladus atlases derived from Cassini-ISS images

The Imaging Science Subsystem (ISS) on-board Cassini took a few high-resolution images of the icy Saturnian satellites Mimas and Enceladus over the last seven years of the Cassini mission during non-targeted flybys. We used the new Mimas images to improve the existing semi-controlled mosaic of Mimas. A new controlled Enceladus mosaic was published recently (Bland et al., 2015; Bland et al. in prep.). Both new mosaics are the baseline for improved atlases of Mimas in three tiles with a map scale of 1:1,000,000 and Enceladus in 15 tiles with a map scale of 1:400,000. The nomenclature for both satellites was proposed by the Cassini-ISS team and approved by the IAU.

The Far‐UV Albedo of Europa From HST Observations

AbstractWe present an analysis of Europa's far‐UV spectral albedo using observations during the 1999–2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk‐integrated observations show that the far‐UV spectrum in the ~130 to 170‐nm range is relatively flat or slightly blue (increasing albedo with decreasing wavelength) for the studied hemispheres: the leading, trailing, and anti‐Jovian hemispheres. At Lyman‐α (121.6 nm), the albedo of the trailing hemisphere continues the blue trend, but it reddens for the leading hemisphere. Also at this wavelength, the albedo of the leading hemisphere, which is higher than the trailing hemisphere at near‐UV and visible wavelengths, is lower than the trailing hemisphere, exhibiting spectral inversion. We find no evidence of a sharp water‐ice absorption edge at 165 nm on any hemisphere of Europa, which is intriguing since such an absorption feature has been observed on the icy Saturnian satellites.

One‐Dimensional Convective Thermal Evolution Calculation Using a Modified Mixing Length Theory: Application to Saturnian Icy Satellites

AbstractSolid‐state thermal convection plays a major role in the thermal evolution of solid planetary bodies. Solving the equation system for thermal evolution considering convection requires 2‐D or 3‐D modeling, resulting in large calculation costs. A 1‐D calculation scheme based on mixing length theory (MLT) requires a much lower calculation cost and is suitable for parameter studies. A major concern for the MLT scheme is its accuracy due to a lack of detailed comparisons with higher dimensional schemes. In this study, I quantify its accuracy via comparisons of thermal profiles obtained by 1‐D MLT and 3‐D numerical schemes. To improve the accuracy, I propose a new definition of the mixing length (l), which is a parameter controlling the efficiency of heat transportation due to convection, for a bottom‐heated convective layer. Adopting this new definition of l, I investigate the thermal evolution of Saturnian icy satellites, Dione and Enceladus, under a wide variety of parameter conditions. Calculation results indicate that each satellite requires several tens of GW of heat to possess a thick global subsurface ocean suggested from geophysical analyses. Dynamical tides may be able to account for such an amount of heat, though the reference viscosity of Dione's ice and the ammonia content of Dione's ocean need to be very high. Otherwise, a thick global ocean in Dione cannot be maintained, implying that its shell is not in a minimum stress state.

Icy Saturnian satellites: Disk-integrated UV-IR characteristics and links to exogenic processes

Combined Cassini observations obtained at similar observing geometries in the ultraviolet through infrared spectral range, along with additional ultraviolet (UV) data from Hubble Space Telescope where available, are used to study system-wide trends in spectral albedos of the inner icy Saturnian satellites (Mimas, Enceladus, Tethys, Dione, Rhea). We derive UV and visible geometric albedos and UV absorption strengths of the leading and trailing hemispheres and compare with E ring grain flux and charged particle intensities (electrons and ions of varying energies) to those hemispheres. We find that the UV absorption strength on the leading and trailing hemispheres is anti-correlated with E ring grain flux. On the trailing hemispheres, the UV absorption strength is correlated with intensity of electrons in the tens of keV range. We suggest that these relationships could imply links with the organic component of the E ring. Radiolytic processing of organics causes the products to become spectrally redder, increasing the UV absorption strength. Such processing occurs while organic-rich grains are in the E ring, and increases with exposure time in the E ring, such that grains interacting with Rhea are redder (more processed) than those impacting moons closer to Enceladus. Further processing (and associated darkening/reddening) occurs on the trailing hemispheres of the satellites, via radiolysis by electrons in the tens of keV range. Silicates and salts also redden with weathering; however because organics are present in the E ring in significantly greater abundance than salts or silicates, we suggest here that weathering of organics dominates the coloring of the inner Saturnian moons.

IR SPECTRAL MAPPING OF THE MARTIAN SOUTH POLAR RESIDUAL CAP USING CRISM

Polycyclic aromatic hydrocarbons (PAHs) are considered to be important in theories of abiogenesis (Allamandola, 2011) . There is evidence that PAHs have been detected on two icy Saturnian satellites using the Visual and Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft (Cruikshank et al., 2007). The hypothesised presence of PAHs in Mars south polar cap has not been systematically examined even though the Mars south polar cap may allow the preservation of organic molecules that are typically destroyed at the Martian surface by UV radiation (Dartnell et al. 2012). This hypothesis is supported by recent analyses of South Polar Residual Cap (SPRC) structural evolution (Thomas et al., 2009) that suggest the possibility that seasonal and long term sublimation may excavate dust particles from within the polar ice. Periodic sublimation is believed to be responsible for the formation of so-called “Swiss Cheese Terrain”, a unique surface feature found only in the Martian south polar residual cap consisting of flat floored, circular depressions (Byrne, 2009). We show the first examples of work towards the detection of PAHs in Swiss Cheese Terrain, using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), on board NASA’s Mars Reconnaissance Orbiter (MRO). CRISM is designed to search for mineralogical indications of past and present water, thus providing extensive coverage of the south polar cap. In this work, we discuss whether CRISM infrared spectra can be used to detect PAHs in Swiss Cheese Terrain and demonstrate a number of maps showing shifts in spectral profiles over the SPRC.

IR SPECTRAL MAPPING OF THE MARTIAN SOUTH POLAR RESIDUAL CAP USING CRISM

Abstract. Polycyclic aromatic hydrocarbons (PAHs) are considered to be important in theories of abiogenesis (Allamandola, 2011) . There is evidence that PAHs have been detected on two icy Saturnian satellites using the Visual and Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft (Cruikshank et al., 2007). The hypothesised presence of PAHs in Mars south polar cap has not been systematically examined even though the Mars south polar cap may allow the preservation of organic molecules that are typically destroyed at the Martian surface by UV radiation (Dartnell et al. 2012). This hypothesis is supported by recent analyses of South Polar Residual Cap (SPRC) structural evolution (Thomas et al., 2009) that suggest the possibility that seasonal and long term sublimation may excavate dust particles from within the polar ice. Periodic sublimation is believed to be responsible for the formation of so-called “Swiss Cheese Terrain”, a unique surface feature found only in the Martian south polar residual cap consisting of flat floored, circular depressions (Byrne, 2009). We show the first examples of work towards the detection of PAHs in Swiss Cheese Terrain, using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), on board NASA’s Mars Reconnaissance Orbiter (MRO). CRISM is designed to search for mineralogical indications of past and present water, thus providing extensive coverage of the south polar cap. In this work, we discuss whether CRISM infrared spectra can be used to detect PAHs in Swiss Cheese Terrain and demonstrate a number of maps showing shifts in spectral profiles over the SPRC.

Polygonal impact craters on Dione: Evidence for tectonic structures outside the wispy terrain

Plan-view impact crater geometries can be indicative of pre-impact structures within the target material. Impact events that occur on a pre-fractured surface generate craters exhibiting one or more straight rim segments, termed polygonal impact craters (PICs). Impact craters that appear to be PICs are abundant on the surface of Saturn's icy satellite, Dione, both within the wispy terrain, a region with large visible fractures, and also outside the wispy terrain (the ‘non-wispy terrain’), where less evidence for fracturing has been observed. In the non-wispy terrain, subtle lineaments are hypothesized to be NE–SW, NW–SE, and E–W trending fractures, suggesting that tectonism may have been an important process in this terrain.Results of previous studies have shown that PIC straight rim segment azimuths (‘PIC azimuths’) commonly parallel pre-impact fracture azimuths, although disagreements about this relationship exist in the literature. We investigated the hypothesis that fractures, either subtle or nonvisible with available spacecraft images, are present within Dione's non-wispy terrain. Our first step was to assess the relationship between PICs and pre-existing fracture azimuths in the wispy terrain. Our results from this initial assessment show a parallel relationship between PIC azimuths and fracture azimuths. Based on this correlation in the wispy terrain, we find it likely that this relationship would hold true in the non-wispy terrain if PICs are present.We tested for PICs using crater rim azimuth data collected from randomly distributed study locations throughout the non-wispy terrain. From these data, we identified widespread PICs in this terrain, which supports the hypothesis that subtle fractures are also present. Analysis of the PIC azimuth data yield a pattern for these inferred fractures across Dione's surface that is consistent with the hypothesized global deformation that would result from a combination of satellite despinning and volume expansion. Our results provide evidence for these previously hypothesized events in Dione's history and demonstrate that mapping PICs and their azimuths is a useful tool for investigating subtle fractures on Solar System bodies.

Shallow normal fault slopes on Saturnian icy satellites

AbstractFault dips are a function of the coefficient of internal friction, μi, of the lithospheric material. Laboratory deformation experiments of H2O ice at conditions applicable to icy bodies yield 0 ≤ μi ≤ 0.55 such that normal faults dip between 45° and 59°. We tested the hypothesis that normal faults on icy bodies reflect these values by using digital elevation models to examine geometries of large extensional systems on three Saturnian satellites. Analyzed faults within Ithaca Chasma on Tethys and Avaiki Chasmata on Rhea all exhibit shallower‐than‐predicted topographic slopes across the fault scarp, which we term “fault slopes.” A scarp of Padua Chasmata within Dione's Wispy Terrain also has a shallow fault slope, although three others that make up Palatine Chasmata exhibit steeper slopes as predicted. We infer that viscous relaxation is the most viable explanation for these shallow fault slopes, and we model the potential role of viscous relaxation in creating shallow slopes. Our modeling results support formation of these normal faults with steep dips consistent with deformation experiments, followed by their relaxation due to lithospheric heating events related to radionuclide decay. The steepest fault slopes in this terrain yield 0 ≤ μi ≤ 0.73 for Dione's lithospheric ice, which overlaps the dip range predicted from experiments. Results of this work suggest that viscous relaxation substantially affected fault slopes on Tethys, Rhea, and Dione. By implication, these processes may have also affected fault geometries on other icy satellites.

Titan’s surface spectra at the Huygens landing site and Shangri-La

Titan is an icy satellite of Saturn with a dense atmosphere and covered by a global photochemical organic haze. Ground based observations and the Huygens descent probe allowed to retrieve the main spectral signature of the water ice (Griffith, C.A. et al. [2003]. Science 300(5619), 628–630; Coustenis, A. et al. [2005]. Icarus 177, 89–105) at the surface, possibly covered by a layer of sedimented organic material (Tomasko, M.G. et al. [2005]. Nature 438(7069), 765–778). However, the spectrum of the surface is not yet understood. In this study, we find that the surface reflectivity at the Huygens Landing Site (HLS) is well modeled by a layer of water ice grains overlaid by a moist layer of weakly compacted photochemical aggregated aerosols. Moist soils have spectra shifted toward short wavelengths relatively to spectra of dry soils. Cassini observations of Shangri-La region from orbit also show a very dark surface with a reflectivity peak shifted toward short wavelengths in respect to the reflectivity peak of bright surfaces, revealing a dichotomy between terrains based to their spectra in visible.

First far-ultraviolet disk-integrated phase curve analysis of Mimas, Tethys and Dione from the Cassini-UVIS data sets

We perform an analysis of the photometric properties of the icy saturnian satellites at 180nm, based on the first far-UV disk-integrated phase curves of Mimas, Tethys and Dione. Their interactions with the environment (the E-ring and the magnetosphere) are investigated, leading to a better understanding of the effects of exogenic processes on the system of Saturn. We find that Tethys and Dione have a leading hemisphere brighter than their trailing hemisphere at far-UV wavelengths, while Mimas exhibits a quite uniform reflectance on its surface. No asymmetry is observed between the saturnian and anti-saturnian hemispheres of those satellites, indicating that exogenic processes are important primarily on the leading and trailing hemispheres. Tethys shows a narrower opposition effect, suggesting a more porous regolith on its surface than on Dione and Mimas. This could be the consequence of more significant bombardment by the E-ring grains at the orbit of Tethys. Dione’s photometric properties reveal a more absorbing surface, which could be explained by a lower amount of E-ring grain bombardment and/or by the deposit of a darkening agent mainly on its trailing side.

Remarks on the Iapetus’ bulge and ridge

Iapetus is a medium sized icy satellite of Saturn. It has two spectacular features: the equatorial ridge (ER) and the abnormally large flattening. The flattening is usually explained in terms of large non-hydrostatic fossil equatorial bulge (EB) supported by a thick lithosphere. Here we show, building on the principle of isostasy, that EB and ER could be a result of low density roots underlying the lithosphere below the equator. The low density matter formed the layer over the core of the satellite. Such situation was unstable. The instability led to origin of axially symmetric plumes that formed equatorial bulge and equatorial ridge. So, we explain both: EB and ER in the frame of one hypothesis.

Laboratory infrared reflection spectrum of carbon dioxide clathrate hydrates for astrophysical remote sensing applications

We present 1–5μm IR reflectance spectra of CO2 clathrate hydrates acquired under temperature and pressure conditions representative of the icy moons’ surfaces. The IR reflectance spectrum of the CO2 clathrate hydrates is similar to the water ice IR reflectance spectrum except for two main absorption bands corresponding to the CO2 guest molecule at 2.71 and 4.28μm (3693 and 2334cm−1). The specific configuration of the clathrate hydrate structure is identified through the ν3 absorption band splitting which produces a band at 4.26μm (2347cm−1) for molecules trapped in small cages and a band at 4.28μm (2334cm−1) for molecules trapped in large cages. In general, the reflection spectra are similar to spectra obtained in transmission spectroscopy. But, it appears that the aspect of the ν3 absorption band is strongly influenced by physical (roughness, thickness, mixing properties) and optical (n and k) characteristics of the sample. A qualitative discussion of the effects of these sample properties on near-IR signatures of clathrate hydrates is proposed. Finally, a comparison between the absorption bands of CO2 clathrate hydrates obtained in this work and CO2 absorption bands as detected by VIMS on the icy satellites of Saturn is achieved. The experimental near-IR reflection spectra, made in pressure–temperature (P–T) conditions close to those of the icy surfaces, confirm that VIMS data are not consistent with the presence of structure I CO2 clathrate hydrates on the surface of the icy moons.

Analysis of Cassini magnetic field observations over the poles of Rhea

We analyze Cassini magnetic field observations from the only two polar flybys of Saturn's largest icy satellite Rhea (R2 on 02 March 2010 and R3 on 11 January 2011) which are scheduled between Saturn Orbit Insertion and the end of the mission in 2017. For the interpretation of these data, we apply estimations from simple analytical models as well as results from numerical hybrid simulations (kinetic ions, fluid electrons) of Rhea's interaction with the incident magnetospheric plasma. In‐situ observations of exospheric neutral gas and pick‐up ions suggest Rhea to be embedded in a tenuous gas envelope. However, the interaction of this gas with the magnetospheric flow does not make any measurable contributions to the magnetic field perturbations detected above the poles of the moon. Instead, the field perturbations observed in these regions mainly arise from the absorption of magnetospheric particles with large field‐aligned velocities, impinging on the north and south polar surface of Rhea. In addition to numerous interaction features known from preceding Cassini flybys of Saturn's plasma‐absorbing moons, the magnetic field data acquired above Rhea's poles reveal perturbations of the flow‐aligned field component, corresponding to a draping/Alfvén wing pattern. Based on our hybrid simulations, we suggest that these signatures arise from the finite extension of Rhea's wakeside plasma void along the corotational flow direction, yielding a density gradient in corotation direction, and thereby generating a diamagnetic current from the Saturn‐facing into the Saturn‐averted hemisphere of the moon. This transverse current is responsible for generating a weak Alfvén wing pattern at Rhea which has been detected by the Cassini spacecraft during the R2 and R3 flybys. Due to the large gyroradii of the incident magnetospheric ions, this structure features a pronounced asymmetry with respect to the direction of the convective electric field. Results of our simulation, considering only plasma absorption on the moon, are in good agreement with Cassini magnetometer data from both flybys. At Saturn's icy satellites Tethys and Dione, the low value of the magnetospheric plasma beta most likely prevents the formation of similar currents and measurable flow‐aligned magnetic field distortions.

Modeling of electron fluxes in the Enceladus plume

Observations by instruments onboard the Cassini spacecraft of Saturn's icy satellite Enceladus have shown that a plume containing water vapor and ice grains is present in the southern hemisphere. Energy distributions of electrons in this plume were measured by the electron sensor part of the Cassini plasma spectrometer (CAPS – ELS). A significant suprathermal electron population was detected. The nature of the electron population is important for understanding the composition and chemistry of the plume plasma because the electron‐ion recombination rate depends on the energy distribution and because ionizing collisions by energetic electrons creates new plasma. We present the results of a two‐stream electron transport model for plume electrons that includes neutral densities that agree with Cassini Ion and Neutral Mass Spectrometer (INMS) data. Electron production within the plume due to photoionization by solar radiation and by electron impact ionization was included, as were energy losses due to electron‐neutral collisions. Model cases were considered that both included and did not include electron inputs from outside the plume. Comparisons are made of model fluxes with measured fluxes by CAPS – ELS on October 9, 2008. The model‐data comparisons indicate that photoelectrons (10 eV–70 eV energies) locally produced within the plume can explain the data. The possible role of electron‐grain collisions was also explored and it was determined that nanograin densities in excess of 106 cm−3 would be needed to affect the electron distribution.

Recent improvements of the Saturnian satellites atlases: Mimas, Enceladus, and Dione

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).