How the Enceladus dust plume feeds Saturn’s E ring

Pre-Cassini models of Saturn’s E ring [Horányi, M., Burns, J., Hamilton, D., 1992. Icarus 97, 248-259; Juhász, A., Horányi, M., 2002. J. Geophys. Res. 107, 1-10] failed to reproduce its peculiar vertical structure inferred from Earth-bound observations [de Pater, I., Martin, S.C., Showalter, M.R., 2004. Icarus 172, 446-454]. After the discovery of an active ice-volcanism of Saturn’s icy moon Enceladus the relevance of the directed injection of particles for the vertical ring structure of the E ring was swiftly recognised [Juhász, A., Horányi, M., Morfill, G.E., 2007. Geophys. Res. Lett. 34, L09104; Kempf, S., Beckmann, U., Moragas-Klostermeyer, G., Postberg, F., Srama, R., Economou, T., Schmidt, J., Spahn, F., Grün, E., 2008. Icarus 193, 420-437]. However, simple models for the delivery of particles from the plume to the ring predict a too small vertical ring thickness and overestimate the amount of the injected dust.Here we report on numerical simulations of grains leaving the plume and populating the dust torus of Enceladus. We run a large number of dynamical simulations including gravity and Lorentz force to investigate the earliest phase of the ring particle life span. The evolution of the electrostatic charge carried by the initially uncharged grains is treated selfconsistently. Freshly ejected plume particles are moving in almost circular orbits because the Enceladus orbital speed exceeds the particles’ ejection speeds by far. Only a small fraction of grains that leave the Hill sphere of Enceladus survive the next encounter with the moon. Thus, the flux and size distribution of the surviving grains, replenishing the ring particle reservoir, differs significantly from the flux and size distribution of the particles freshly ejected from the plume. Our numerical simulations reproduce the vertical ring profile measured by the Cassini Cosmic Dust Analyzer (CDA) [Kempf, S., Beckmann, U., Moragas-Klostermeyer, G., Postberg, F., Srama, R., EconoDmou, T., Smchmidt, J., Spahn, F., Grün, E., 2008. Icarus 193, 420-437]. From our simulations we calculate the deposition rates of plume particles hitting Enceladus’ surface. We find that at a distance of 100 m from a jet a 10 m sized ice boulder should be covered by plume particles in 10⁵-10⁶ years.     

Collisional spreading of Enceladus’ neutral cloud

We describe a direct simulation Monte Carlo (DSMC) model of Enceladus’ neutral cloud and compare its results to observations of OH and O orbiting Saturn. The OH and O are observed far from Enceladus (at 3.95 R_S), as far out as 25 R_S for O. Previous DSMC models attributed this breadth primarily to ion/neutral scattering (including charge exchange) and molecular dissociation. However, the newly reported O observations and a reinterpretation of the OH observations (Melin, H., Shemansky, D.E., Liu, X. [2009] Planet. Space Sci., 57, 1743-1753, PS&S) showed that the cloud is broader than previously thought. We conclude that the addition of neutral/neutral scattering (Farmer, A.J. [2009] Icarus, 202, 280-286), which was underestimated by previous models, brings the model results in line with the new observations. Neutral/neutral collisions primarily happen in the densest part of the cloud, near Enceladus’ orbit, but contribute to the spreading by pumping up orbital eccentricity. Based on the cloud model presented here Enceladus maybe the ultimate source of oxygen for the upper atmospheres of Titan and Saturn. We also predict that large quantities of OH, O and H₂O bombard Saturn’s icy satellites.     

The ultraviolet reflectance of Enceladus: Implications for surface composition

The reflectance of Saturn’s moon Enceladus has been measured at far ultraviolet (FUV) wavelengths (115-190 nm) by Cassini’s Ultraviolet Imaging Spectrograph (UVIS). At visible and near infrared (VNIR) wavelengths Enceladus’ reflectance spectrum is very bright, consistent with a surface composed primarily of H₂O ice. At FUV wavelengths, however, Enceladus is surprisingly dark - darker than would be expected for pure water ice. Previous analyses have focused on the VNIR spectrum, comparing it to pure water ice (Cruikshank, D.P., Owen, T.C., Dalle Ore, C., Geballe, T.R., Roush, T.L., de Bergh, C., Sandford, S.A., Poulet, F., Benedix, G.K., Emery, J.P. [2005] Icarus, 175, 268-283) or pure water ice plus a small amount of NH₃ (Emery, J.P., Burr, D.M., Cruikshank, D.P., Brown, R.H., Dalton, J.B. [2005] Astron. Astrophys., 435, 353-362) or NH₃ hydrate (Verbiscer, A.J., Peterson, D.E., Skrutskie, M.F., Cushing, M., Helfenstein, P., Nelson, M.J., Smith, J.D., Wilson, J.C. [2006] Icarus, 182, 211-223). We compare Enceladus’ FUV spectrum to existing laboratory measurements of the reflectance spectra of candidate species, and to spectral models. We find that the low FUV reflectance of Enceladus can be explained by the presence of a small amount of NH₃ and a small amount of a tholin in addition to H₂O ice on the surface. The presence of these three species (H₂O, NH₃, and a tholin) appears to satisfy not only the low FUV reflectance and spectral shape, but also the middle-ultraviolet to visible wavelength brightness and spectral shape. We expect that ammonia in the Enceladus plume is transported across the surface to provide a global coating.     

Release of N2, CH4, CO2, and H2O from surface ices on Enceladus

We vapor deposit at 20 K a mixture of gases with the specific Enceladus plume composition measured in situ by the Cassini INMS [Waite, J.H., Combi, M.R., Ip, W.H., Cravens, T.E., McNutt, R.L., Kasprzak, W., Yelle, R., Luhmann, J., Niemann, H., Gell, D., Magee, B., Fletcher, G., Lunine, J., Tseng, W.L., 2006. Science 311, 1419-1422] to form a mixed molecular ice. As the sample is slowly warmed, we monitor the escaping gas quantity and composition with a mass spectrometer. Pioneering studies [Schmitt, B., Klinger, J., 1987. Different trapping mechanisms of gases by water ice and their relevance for comet nuclei. In: Rolfe, E.J., Battrick, B. (Eds.), Diversity and Similarity of Comets. SP-278. ESA, Noordwijk, The Netherlands, pp. 613-619; Bar-Nun, A., Kleinfeld, I., Kochavi, E., 1988. Phys. Rev. B 38, 7749-7754; Bar-Nun, A., Kleinfeld, I., 1989. Icarus 80, 243-253] have shown that significant quantities of volatile gases can be trapped in a water ice matrix well above the temperature at which the pure volatile ice would sublime. For our Enceladus ice mixture, a composition of escaping gases similar to that detected by Cassini in the Enceladus plume can be generated by the sublimation of the H₂O:CO₂:CH₄:N₂ mixture at temperatures between 135 and 155 K, comparable to the high temperatures inferred from the CIRS measurements [Spencer, J.R., Pearl, J.C., Segura, M., Flasar, F.M., Mamoutkine, A., Romani, P., Buratti, B.J., Hendrix, A.R., Spilker, L.J., Lopes, R.M.C., 2006. Science 311, 1401-1405] of the Enceladus “tiger stripes.” This suggests that the gas escape phenomena that we measure in our experiments are an important process contributing to the gases emitted from Enceladus. A similar experiment for ice deposited at 70 K shows that both the processes of volatile trapping and release are temperature dependent over the temperature range relevant to Enceladus.     

Obliquity tides do not significantly heat Enceladus

Recently, Tyler [Tyler, R.H., 2009. Geophys. Res. Lett. 36, L15205; Tyler, R., 2011. Icarus, 211, 770-779] proposed that the tide due to an obliquity of greater than 0.1° might drive resonant flow in a liquid ocean at Enceladus, and that dissipation of the ocean’s kinetic energy may be an alternate source for the observed global heat flux. While there is currently no measurement of Enceladus’ obliquity, dissipation is expected to drive the spin pole to a Cassini state. Under this assumption, we find that Enceladus should occupy Cassini state 1 and that the obliquity of Enceladus should be less than 0.0015° for values of the degree-2 gravity coefficient C_2,2 between 1.0 × 10⁻³ and 2.5 × 10⁻³. Unless there is a significant free obliquity or the gravity coefficient C_2,2 has been significantly overestimated, it is unlikely that obliquity-driven flow in a subsurface ocean is the source of the extreme heat on Enceladus.     

Thermal convection in ice-I shells of Titan and Enceladus

Cassini-Huygens observations have shown that Titan and Enceladus are geologically active icy satellites. Mitri and Showman [Mitri, G., Showman, A.P., 2005. Icarus 177, 447-460] and McKinnon [McKinnon, W.B., 2006. Icarus 183, 435-450] investigated the dynamics of an ice shell overlying a pure liquid-water ocean and showed that transitions from a conductive state to a convective state have major implications for the surface tectonics. We extend this analysis to the case of ice shells overlying ammonia-water oceans. We explore the thermal state of Titan and Enceladus ice-I shells, and also we investigate the consequences of the ice-I shell conductive-convective switch for the geology. We show that thermal convection can occur, under a range of conditions, in the ice-I shells of Titan and Enceladus. Because the Rayleigh number Ra scales with δ³/ηb, where δ is the thickness of the ice shell and ηb is the viscosity at the base of the ice-I shell, and because ammonia in the liquid layer (if any) strongly depresses the melting temperature of the water ice, Ra equals its critical value for two ice-I shell thicknesses: for relatively thin ice shell with warm, low-viscosity base (Onset I) and for thick ice shell with cold, high-viscosity base (Onset II). At Onset I, for a range of heat fluxes, two equilibrium states—corresponding to a thin, conductive shell and a thick, convective shell—exist for a given heat flux. Switches between these states can cause large, rapid changes in the ice-shell thickness. For Enceladus, we demonstrate that an Onset I transition can produce tectonic stress of ∼500 bars and fractures of several tens of km depth. At Onset II, in contrast, we demonstrate that zero equilibrium states exist for a range of heat fluxes. For a mean heat flux within this range, the satellite experiences oscillations in surface heat flux and satellite volume with periods of ∼50-800 Myr even when the interior heat production is constant or monotonically declining in time; these oscillations in the thermal state of the ice-I shell would cause repeated episodes of extensional and compressional tectonism.     

Rebuttal to “Comment on the papers “Production of hot nitrogen atoms in the martian thermosphere” by F. Bakalian (2006), and “Monte Carlo computations of the escape of atomic nitrogen from Mars” by F. Bakalian and R.E. Hartle (2006)"

This response is to address some of the comments made by Dr. Fox and also to clarify some points in the Bakalian [2006. Icarus 183, 69-73] and Bakalian and Hartle [2006. Icarus 183, 55-68] papers. It is regrettable that some of the statements in the Bakalian and Bakalian and Hartle papers were misinterpreted.     

Vertical cloud structure of Uranus from UKIRT/UIST observations and changes seen during Uranus’ northern spring equinox from 2006 to 2008

Long-slit spectroscopy observations of Uranus by the United Kingdom Infrared Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus’ vertical and latitudinal cloud structure through the planet’s northern spring equinox in December 2007.The observed reflectance spectra in the Long J (1.17-1.31 μm) and H (1.45-1.65 μm) bands, obtained with the slit aligned along Uranus’ central meridian, have been fitted with an optimal estimation retrieval model to determine the vertical cloud profile from 0.1 to 6-8 bar over a wide range of latitudes. Context images in a number of spectral bands were used to discriminate general zonal cloud structural changes from passing discrete clouds. From 2006 to 2007 reflection from deep clouds at pressures between 2 and 6-8 bar increased at all latitudes, although there is some systematic uncertainty in the absolute pressure levels resulting from extrapolating the methane coefficients of Irwin et al. (Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Teanby, N.A., Bowles, N., Calcutt, S.B., Remedios, J.J. [2006] Icarus, 181, 309-319) at pressures greater than 1 bar, as noted by Tomasko et al. and Karkoschka and Tomasko (Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E. [2008] Planet. Space Sci., 56, 624-647; Karkoschka, E., Tomasko, M. [2009] Icarus). However, from 2007 to 2008 reflection from these clouds throughout the southern hemisphere and from both northern and southern mid-latitudes (30° N,S) diminished. As a result, the southern polar collar at 45°S has diminished in brightness relative to mid-latitudes, a similar collar at 45°N has become more prominent (e.g. Rages, K.A., Hammel, H.B., Sromovsky, L. [2007] Bull. Am. Astron. Soc., 39, 425; Sromovsky, L.A., Fry, P.M., Ahue, W.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M.A. [2008] vol. 40 of AAS/Division for Planetary Sciences Meeting Abstracts, pp. 488-489; Sromovsky, L.A., Ahue, W.K.M., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R. [2009] Icarus), and the lowering reflectivity from mid-latitudes has left a noticeable brighter cloud zone at the equator (e.g. Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557;Karkoschka, E., Tomasko, M. [2009] Icarus). For such substantial cloud changes to have occurred in just two years suggests that the circulation of Uranus’ atmosphere is much more vigorous and/or efficient than is commonly thought. The composition of the main observed cloud decks between 2 and 6-8 bar is unclear, but the absence of the expected methane cloud at 1.2-1.3 bar (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987] J. Geophys. Res., 92, 14987-15001) is striking (as previously noted by, among others, Sromovsky, L.A., Irwin, P.G.J., Fry, P.M. [2006] Icarus, 182, 577-593; Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557; Sromovsky, L.A., Fry, P.M. [2008] Icarus, 193, 252-266; Karkoschka, E., Tomasko, M. [2009] Icarus) and suggests that cloud particles may be considerably different from pure condensates and may be linked with stratospheric haze particles drizzling down from above, or that tropospheric hazes are generated near the methane condensation level and then drizzle down to deep pressures as suggested by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus).The retrieved cloud structures were also tested for different assumptions of the deep methane mole fraction, which Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus) find may vary from ∼1-2% in polar regions to perhaps as much as 4% equatorwards of 45°N,S. We found that such variations did not significantly affect our conclusions.     

Solid tidal friction above a liquid water reservoir as the origin of the south pole hotspot on Enceladus

Earth, Jupiter's moon Io and Saturn's tiny moon Enceladus are the only solid objects in the Solar System to be sufficiently geologically active for their internal heat to be detected by remote sensing. Interestingly, the endogenic activity on Enceladus is only located on a specific region at the south pole, from which jets of water vapor and ice particles have been observed [Spencer, J.R., and 9 colleagues, 2006. Science 311, 1401-1405; Porco, C.C., and 24 colleagues, 2006. Science 311, 1393-1401]. The current polar location of the thermal anomaly can possibly be explained by diapir-induced reorientation of the satellite [Nimmo, F., Pappalardo, R.T., 2006. Nature 441, 614-616], but the thermal anomaly triggering and the heat power required to sustain it over geological timescales remain problematic. Using a three-dimensional viscoelastic numerical model simulating the response of Enceladus to tidal forcing, we explore the effect of a low-viscosity anomaly in the ice shell, localized to the south polar region, on the tidal dissipation patterns. We demonstrate that only interior models with a liquid water layer at depth can explain the observed magnitude of dissipation rate and its particular location at the south pole. Moreover, we show that tidally-induced heat must be generated over a relatively broad region in the southern hemisphere, and it is then transferred toward the south pole where it is episodically released during relatively short resurfacing events. As large tidal dissipation and internal melting cannot be induced in the south polar region in the absence of a pre-existing liquid decoupling layer, we propose that liquid water must have been present in the interior for a very long period of time, and possibly since the satellite formation. Owing to the orbital equilibrium requirement [Meyer, J., Wisdom, J., 2007. Icarus 188, 535-539], sustaining some liquid water at depth is impossible if heat is continuously emitted at a rate of 4-8 GW at the south pole. Based on that requirement, we propose that the current thermal emission is not in equilibrium with the heat production, and that the thermal emission rate is abnormally high at present time. Alternatively, continuous dissipation at a rate of 1-2 GW in the ice shell at the south pole should be sufficient to induce internal melting and it could sustain a layer of liquid water at depth over geologic timescales.     

UV spectrum of Enceladus

We present a far ultraviolet (FUV) spectrum of Saturn’s moon Enceladus from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). We have put upper limits on emission from C, N, and O lines in Enceladus’ atmosphere and column densities for the C lines assuming solar resonance scattering. We find these upper limits to be relatively low—on the order of tens to thousands of Rayleighs and with C column densities on the order of 10⁸–10¹⁵ cm^?2, depending on the assumed source size. We also present a segment of a reflectance spectrum in the FUV from ～1900–2130 Å. This region was sensitive to the different ice mixtures in the model spectra reported by Hendrix et al. (Hendrix, A.R., Hansen, C.J., Holsclaw, G.M. [2010]. Icarus, 206, 608). We find the spectrum brightens quickly longward of ～1900 Å, constraining the absorption band observed by Hendrix et al. from ～170 to 190 nm. We find our data is consistent with the suggestion of Hendrix et al. of the presence of ammonia ice (or ammonia hydrate) to darken that region, and also possibly tholins to darken the mid-UV, as reported by Verbiscer et al. (Verbiscer, A.J., French, R.G., McGhee, C.A. [2005]. Icarus, 173, 66).     

The orbit and size distribution of small Solar System objects orbiting the Sun interior to the Earth's orbit

We present the first observational measurement of the orbit and size distribution of small Solar System objects whose orbits are wholly interior to the Earth's (Inner Earth Objects, IEOs, with aphelion <0.983 AU). We show that we are able to model the detections of near-Earth objects (NEO) by the Catalina Sky Survey (CSS) using a detailed parameterization of the CSS survey cadence and detection efficiencies as implemented within the Jedicke et al. [Jedicke, R., Morbidelli, A., Spahr, T., Petit, J.M., Bottke, W.F., 2003. Icarus 161, 17-33] survey simulator and utilizing the Bottke et al. [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H.F., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399-433] model of the NEO population's size and orbit distribution. We then show that the CSS detections of 4 IEOs are consistent with the Bottke et al. [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H.F., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399-433] IEO model. Observational selection effects for the IEOs discovered by the CSS were then determined using the survey simulator in order to calculate the corrected number and H distribution of the IEOs. The actual number of IEOs with H<18 (21) is 36±26 (530±240) and the slope of the H magnitude distribution (∝¹⁰αH) for the IEOs is . The slope is consistent with previous measurements for the NEO population of αNEO=0.35±0.02 [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H.F., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399-433] and αNEO=0.39±0.013 [Stuart, J.S., Binzel, R.P., 2004. Icarus 170, 295-311]. Based on the agreement between the predicted and observed IEO orbit and absolute magnitude distributions there is no indication of any non-gravitational effects (e.g. Yarkovsky, tidal disruption) affecting the known IEO population.     

Craters on asteroids: Reconciling diverse impact records with a common impacting population

O'Brien and Greenberg [O'Brien, D.P., Greenberg, R., 2005. Icarus 178, 179-212] developed a self-consistent numerical model of the collisional and dynamical evolution of the main-belt and NEA populations that was tested against a diverse range of observational and theoretical constraints. In this paper, we use those results to update the asteroid cratering model of Greenberg et al. [Greenberg, R., Nolan, M.C., Bottke, W.F., Kolvoord, R.A., Veverka, J., 1994. Icarus 107, 84-97; Greenberg, R., Bottke, W.F., Nolan, M., Geissler, P., Petit, J., Durda, D.D., Asphaug, E., Head, J., 1996. Icarus 120, 106-118], and show that the main-belt asteroid population from the O'Brien and Greenberg collisional/dynamical evolution modeling is consistent with the crater records on Gaspra, Ida, Mathilde, and Eros, the four asteroids that have been observed by spacecraft.     

Dynamical passage to approximate equilibrium shapes for spinning, gravitating rubble asteroids

Many asteroids are thought to be particle aggregates held together principally by self-gravity. Here we study — for static and dynamical situations — the equilibrium shapes of spinning asteroids that are permitted for rubble piles. As in the case of spinning fluid masses, not all shapes are compatible with a granular rheology. We take the asteroid to always be an ellipsoid with an interior modeled as a rigid-plastic, cohesion-less material with a Drucker-Prager yield criterion. Using an approximate volume-averaged procedure, based on the classical method of moments, we investigate the dynamical process by which such objects may achieve equilibrium. We first collapse our dynamical approach to its statical limit to derive regions in spin-shape parameter space that allow equilibrium solutions to exist. At present, only a graphical illustration of these solutions for a prolate ellipsoid following the Drucker-Prager failure law is available [Sharma, I., Jenkins, J.T., Burns, J.A., 2005a. Bull. Am. Astron. Soc. 37, 643; Sharma, I., Jenkins, J.T., Burns, J.A., 2005b. Equilibrium shapes of ellipsoidal soil asteroids. In: García-Rojo, R., Hermann, H.J., McNamara, S. (Eds.), Proceedings of the 5th International Conference on Micromechanics of Granular Media, vol. 1. A.A. Balkema, UK; Holsapple, K.A., 2007. Icarus 187, 500-509]. Here, we obtain the equilibrium landscapes for general triaxial ellipsoids, as well as provide the requisite governing formulae. In addition, we demonstrate that it may be possible to better interpret the results of Richardson et al. [Richardson, D.C., Elankumaran, P., Sanderson, R.E., 2005. Icarus 173, 349-361] within the context of a Drucker-Prager material. The graphical result for prolate ellipsoids in the static limit is the same as those of Holsapple [Holsapple, K.A., 2007. Icarus 187, 500-509] because, when worked out, his final equations will match ours. This is because, though the formalisms to reach these expressions differ, in statics, at the lowest level of approximation, volume-averaging and the approach of Holsapple [Holsapple, K.A., 2007. Icarus 187, 500-509] coincide. We note that the approach applied here was obtained independently [Sharma, I., Jenkins, J.T., Burns, J.A., 2003. Bull. Am. Astron. Soc. 35, 1034; Sharma, I., 2004. Rotational Dynamics of Deformable Ellipsoids with Applications to Asteroids. Ph.D. thesis, Cornell University] and it provides a general, though approximate, framework that is amenable to systematic improvements and is flexible enough to incorporate the dynamical effects of a changing shape, different rheologies and complex rotational histories. To demonstrate our technique, we investigate the non-equilibrium dynamics of rigid-plastic, spinning, prolate asteroids to examine the simultaneous histories of shape and spin rate for rubble piles. We have succeeded in recovering most results of Richardson et al. [Richardson, D.C., Elankumaran, P., Sanderson, R.E., 2005. Icarus 173, 349-361], who obtained equilibrium shapes by studying numerically the passage into equilibrium of aggregates containing discrete, interacting, frictionless, spherical particles. Our mainly analytical approach aids in understanding and quantifying previous numerical simulations.     

Comment on the papers “Production of hot nitrogen atoms in the martian thermosphere” by F. Bakalian and “Monte Carlo computations of the escape of atomic nitrogen from Mars” by F. Bakalian and R.E. Hartle

In recent articles published in Icarus, Bakalian [2006. Icarus 183, 69-78] discusses and computes the production rates of hot nitrogen atoms in the martian thermosphere due to N₂ photodissociation and N⁺₂ dissociative recombination, and Bakalian and Hartle [2006. Icarus 183, 55-68] use a Monte Carlo code to compute the escape rates of nitrogen atoms from Mars due to photodissociation of N₂, dissociative recombination of N⁺₂, and pickup ion escape due to photoionization of N atoms above the ionopause. Bakalian concludes that “photodissociation of N₂ is the dominant escape mechanism in the martian atmosphere.” We will show that this conclusion is not supportable. In addition, both papers contain scientific errors, misrepresentations, inaccurate referencing, lack of proper attribution, and they fail to place these investigations into the existing extensive body of work on this subject.     

Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites

New global maps of the five inner midsize icy saturnian satellites, Mimas, Enceladus, Tethys, Dione, and Rhea, have been constructed in three colors (UV, Green and near-IR) at resolutions of 1 km/pixel. The maps reveal prominent global patterns common to several of these satellites but also three major color features unique to specific satellites or satellite subgroups. The most common features among the group are first-order global asymmetries in color properties. This pattern, expressed on Tethys, Dione and Rhea, takes the form of a ∼1.4-1.8 times enhancement in redness (expressed as IR/UV ratio) of the surface at the center of the trailing hemisphere of motion, and a similar though significantly weaker IR/UV enhancement at the center of the leading hemisphere. The peak in redness on the trailing hemisphere also corresponds to a known decrease in albedo. These double hemispheric asymmetries are attributable to plasma and E-ring grain bombardment on the trailing and leading hemispheres, respectively, for the outer three satellites Tethys, Dione and Rhea, whereas as E-ring bombardment may be focused on the trailing hemisphere of Mimas due to its orbital location interior to Enceladus. The maps also reveal three major deviations from these basic global patterns. We observe the previously known dark bluish leading hemisphere equatorial band on Tethys but have also discovered a similar band on Mimas. Similar in shape, both features match the surface patterns expected for irradiation of the surface by incident MeV electrons that drift in a direction opposite to the plasma flow. The global asymmetry on Enceladus is offset ∼40° to the west compared to the other satellites. We do not consider Enceladus in detail here, but the global distribution of bluish material can be shown to match the deposition pattern predicted for plume fallback onto the surface (Kempf, S., Beckmann, U., Schmidt, S. [2010]. Icarus 206, 446-457. doi:10.1016/j.icarus.2009.09.016). E-ring deposition on Enceladus thus appears to mask or prevent the formation of the lenses and hemispheric asymmetries we see on the other satellites. Finally, we observe a chain of discrete bluish splotches along the equator of Rhea. Unlike the equatorial bands of Tethys and Mimas, these splotches form a very narrow great circle ?10-km wide (north-to-south) and appear to be related to surface disruption, exposing fresh, bluish ice on older crater rims. This feature is unique to Rhea and may have formed by impact onto its surface of orbiting material.     

A proof that tidal heating in a synchronous rotation is always larger than in an asymptotic nonsynchronous rotation state

In a recent paper, Wisdom [Wisdom, J., 2008. Icarus, 193, 637-640] derived concise expressions for the rate of tidal dissipation in a synchronously rotating body for arbitrary orbital eccentricity and obliquity. He provided numerical evidence than the derived rate is always larger than in an asymptotic nonsynchronous rotation state at any obliquity and eccentricity. Here, I present a simple mathematical proof of this conclusion and show that this result still holds for any spin-orbit resonance.     

Enceladus: The likely dominant nitrogen source in Saturn's magnetosphere

The spatial distribution of N⁺ in Saturn's magnetosphere obtained from Cassini Plasma Spectrometer (CAPS) data can be used to determine the spatial distribution and relative importance of the nitrogen sources for Saturn's magnetosphere. We first summarize CAPS data from 15 orbits showing the spatial and energy distribution of the nitrogen component of the plasma. This analysis re-enforces our earlier discovery [Smith, H.T., Shappirio, M., Sittler, E.C., Reisenfeld, D., Johnson, R.E., Baragiola, R.A., Crary, F.J., McComas, D.J., Young, D.T., 2005. Geophys. Res. Lett. 32 (14). L14S03] that Enceladus is likely the dominant nitrogen source for Saturn's inner magnetosphere. We also find a sharp enhancement in the nitrogen ion to water ion ratio near the orbit of Enceladus which, we show, is consistent with the presence of a narrow Enceladus torus as described in [Johnson, R.E., Liu, M., Sittler Jr., E.C., 2005. Geophys. Res. Lett. 32. L24201]. The CAPS data and the model described below indicate that N⁺ ions are a significant fraction of the plasma in this narrow torus. We then simulated the combined Enceladus and Titan nitrogen sources using the CAPS data as a constraint. This simulation is an extension of the model we employed earlier to describe the neutral tori produced by the loss of nitrogen from Titan [Smith, H.T., Johnson, R.E., Shematovich, V.I., 2004. Geophys. Res. Lett. 31 (16). L16804]. We show that Enceladus is the principal nitrogen source in the inner magnetosphere but Titan might account for a fraction of the observed nitrogen ions at the largest distances discussed. We also show that the CAPS data is consistent with Enceladus being a molecular nitrogen source with a nitrogen to water ratio roughly consistent with INMS [Waite, J.H., and 13 colleagues, 2006. Science 311 (5766), 1419-1422], but out-gassing of other nitrogen-containing species, such as ammonia, cannot be ruled out.     

A model for the temperature-dependence of tidal dissipation in convective plumes on icy satellites: Implications for Europa and Enceladus

To explain the formation of surface features on Europa, Enceladus, and other satellites, many authors have postulated the spatial localization of tidal heating within convective plumes. However, the concept that enhanced tidal heating can occur within a convective plume has not been rigorously tested. Most models of this phenomenon adopt a tidal heating with a temperature-dependence derived for an incompressible, homogeneous (zero-dimensional) Maxwell material, but it is unclear whether this formulation is relevant to the heterogeneous situation of a warm plume surrounded by cold ice. To determine whether concentrated dissipation can occur in convective plumes, we develop a two-dimensional model to compute the volumetric dissipation rate for an idealized, vertically oriented, isolated convective plume obeying a Maxwellian viscoelastic compressible rheology. We apply the model to the Europa and Enceladus ice shells, and we investigate the consequences for partial melting and resurfacing processes on these bodies. We find that the tidal heating is strongly temperature dependent in a convective ice plume and could produce elevated temperatures and local partial melting in the ice shells of Europa and Enceladus. Our calculation provides the first quantitative verification of the hypothesis by Sotin et al. [Sotin, C., Head, J.W., Tobie, G., 2002. Geophys. Res. Lett. 29. 74-1] and others that the tidal dissipation rate is a strong function of temperature inside a convective plume. On Europa, such localized heating could help allow the formation of domes and chaos terrains by convection. On Enceladus, localized tidal heating in a thermal plume could explain the concentrated activity at the south pole and its associated heat transport of 2-7 GW.     

Stratigraphical and morphological evidence for pingo genesis in the Cerberus plains

‘Rootless’ debris cones (or pseudocraters) occur in platy, patterned ground throughout the Cerberus plains of Mars and are thought to represent the products of explosive magma-ice interaction [Lanagan et al., 2001. Geophys. Res. Lett. 28, 2365-2368; Fagents et al., 2002. In: Smellie, J.L., Chapman, M.G. (Eds.), Volcano-Ice Interaction on Earth and Mars. In: Geol. Soc. Spec. Publ., vol. 202, pp. 295-317]. Requiring lava and water interspersed, they are central to theories of multiple magmatic and aqueous flood events [Burr et al., 2002. Icarus 159, 53-73; Berman, D.C., Hartmann, W.K., 2002. Icarus 159, 1-17] and widespread sheet volcanism [Keszthelyi et al., 2000. J. Geophys. Res. 105, 15027-15049] in the region during the late Amazonian (a region reported to have been occupied by water bodies ranging from lakes to oceans [Scott et al., 1995. Map of Mars showing channels and possible paleolake basins. USGS Miscellaneous Investigations Series, Map I-2461 (1:30,000,000)]). The nature of the platy substrate is the subject of debate, with evidence given for lava [Keszthelyi et al., 2000. J. Geophys. Res. 105, 15027-15049; Plescia, J.B., 2003. Icarus 164, 79-95] and ice [Brakenridge, G.R., 1993. Lunar Planet. Sci. XXIV (Part 1), 175-176; Rice et al., 2002. Lunar Planet. Sci. XXXIII. Abstract #2026; Murray et al., 2005. Nature 434, 352-355]. The superposition relationships of cones and platy deposits in the channels of the Athabasca Valles precludes a magmatic origin, indicating later formation as permafrost mounds (or ‘pingos’), with implications for geologically recent flood volcanism, age constraints on young surfaces and recent climate change on Mars.