TY - JOUR A1 - Seiß, Martin A1 - Albers, Nicole A1 - Sremčević, Miodrag A1 - Schmidt, Jürgen A1 - Salo, Heikki A1 - Seiler, Michael A1 - Hoffmann, Holger A1 - Spahn, Frank T1 - Hydrodynamic Simulations of Moonlet-induced Propellers in Saturn's Rings BT - Application to Bleriot JF - The astronomical journal N2 - One of the biggest successes of the Cassini mission is the detection of small moons (moonlets) embedded in Saturns rings that cause S-shaped density structures in their close vicinity, called propellers. Here, we present isothermal hydrodynamic simulations of moonlet-induced propellers in Saturn's A ring that denote a further development of the original model. We find excellent agreement between these new hydrodynamic and corresponding N-body simulations. Furthermore, the hydrodynamic simulations confirm the predicted scaling laws and the analytical solution for the density in the propeller gaps. Finally, this mean field approach allows us to simulate the pattern of the giant propeller Blériot, which is too large to be modeled by direct N-body simulations. Our results are compared to two stellar occultation observations by the Cassini Ultraviolet Imaging Spectrometer (UVIS), which intersect the propeller Blériot. Best fits to the UVIS optical depth profiles are achieved for a Hill radius of 590 m, which implies a moonlet diameter of about 860 m. Furthermore, the model favors a kinematic shear viscosity of the surrounding ring material of ν0 = 340 cm2 s−1, a dispersion velocity in the range of 0.3 cm s−1 < c0 < 1.5 cm s−1, and a fairly high bulk viscosity 7 < ξ0/ν0 < 17. These large transport values might be overestimated by our isothermal ring model and should be reviewed by an extended model including thermal fluctuations. KW - diffusion KW - hydrodynamics KW - planets and satellites: rings Y1 - 2018 U6 - https://doi.org/10.3847/1538-3881/aaed44 SN - 0004-6256 SN - 1538-3881 VL - 157 IS - 1 PB - IOP Publishing Ltd. CY - Bristol ER - TY - JOUR A1 - Grätz, Fabio M. A1 - Seiß, Martin A1 - Schmidt, Jürgen A1 - Colwell, Joshua A1 - Spahn, Frank T1 - Sharp Gap Edges in Dense Planetary Rings BT - an Axisymmetric Diffusion Model JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - One of the most intriguing facets of Saturn's rings are the sharp edges of gaps in the rings where the surface density abruptly drops to zero. This is despite of the fact that the range over which a moon transfers angular momentum onto the ring material is much larger. Recent UVIS-scans of the edges of the Encke and Keeler gap show that this drop occurs over a range approximately equal to the rings' thickness. Borderies et al. show that this striking feature is likely related to the local reversal of the usually outward directed viscous transport of angular momentum in strongly perturbed regions. In this article we revise the Borderies et al. model using a granular flow model to define the shear and bulk viscosities, ν and ζ, and incorporate the angular momentum flux reversal effect into the axisymmetric diffusion model we developed for gaps in dense planetary rings. Finally, we apply our model to the Encke and Keeler division in order to estimate the shear and bulk viscosities in the vicinity of both gaps KW - celestial mechanics KW - diffusion KW - hydrodynamics KW - planets and satellites: rings KW - scattering Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/ab007e SN - 0004-637X SN - 1538-4357 VL - 872 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Hsu, Hsiang-Wen A1 - Schmidt, Jürgen A1 - Kempf, Sascha A1 - Postberg, Frank A1 - Moragas-Klostermeyer, Georg A1 - Seiss, Martin A1 - Hoffmann, Holger A1 - Burton, Marcia A1 - Ye, ShengYi A1 - Kurth, William S. A1 - Horanyi, Mihaly A1 - Khawaja, Nozair A1 - Spahn, Frank A1 - Schirdewahn, Daniel A1 - Moore, Luke A1 - Cuzzi, Jeff A1 - Jones, Geraint H. A1 - Srama, Ralf T1 - In situ collection of dust grains falling from Saturn’s rings into its atmosphere JF - Science N2 - Saturn’s main rings are composed of >95% water ice, and the nature of the remaining few percent has remained unclear. The Cassini spacecraft’s traversals between Saturn and its innermost D ring allowed its cosmic dust analyzer (CDA) to collect material released from the main rings and to characterize the ring material infall into Saturn. We report the direct in situ detection of material from Saturn’s dense rings by the CDA impact mass spectrometer. Most detected grains are a few tens of nanometers in size and dynamically associated with the previously inferred “ring rain.” Silicate and water-ice grains were identified, in proportions that vary with latitude. Silicate grains constitute up to 30% of infalling grains, a higher percentage than the bulk silicate content of the rings. Y1 - 2018 U6 - https://doi.org/10.1126/science.aat3185 SN - 0036-8075 SN - 1095-9203 VL - 362 IS - 6410 SP - 49 EP - + PB - American Assoc. for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Dzhanoev, Arsen R. A1 - Spahn, Frank A1 - Yaroshenko, Victoriya A1 - Lühr, Hermann A1 - Schmidt, Jürgen T1 - Secondary electron emission from surfaces with small structure JF - Physical review : B, Condensed matter and materials physics N2 - It is found that for objects possessing small surface structures with differing radii of curvature the secondary electron emission (SEE) yield may be significantly higher than for objects with smooth surfaces of the same material. The effect is highly pronounced for surface structures of nanometer scale, often providing a more than 100% increase of the SEE yield. The results also show that the SEE yield from surfaces with structure does not show a universal dependence on the energy of the primary, incident electrons as it is found for flat surfaces in experiments. We derive conditions for the applicability of the conventional formulation of SEE using the simplifying assumption of universal dependence. Our analysis provides a basis for studying low-energy electron emission from nanometer structured surfaces under a penetrating electron beam important in many technological applications. Y1 - 2015 U6 - https://doi.org/10.1103/PhysRevB.92.125430 SN - 1098-0121 SN - 1550-235X VL - 92 IS - 12 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Meier, Patrick A1 - Kriegel, Hendrik A1 - Motschmann, Uwe A1 - Schmidt, Jürgen A1 - Spahn, Frank A1 - Hill, Thomas W. A1 - Dong, Yaxue A1 - Jones, Geraint H. T1 - A model of the spatial and size distribution of Enceladus' dust plume JF - Planetary and space science KW - Enceladus KW - Plume KW - Nanograins KW - Cassini KW - Tail Y1 - 2014 U6 - https://doi.org/10.1016/j.pss.2014.09.016 SN - 0032-0633 VL - 104 SP - 216 EP - 233 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Tiscareno, Matthew S. A1 - Mitchell, Colin J. A1 - Murray, Carl D. A1 - Di Nino, Daiana A1 - Hedman, Matthew M. A1 - Schmidt, Jürgen A1 - Burns, Joseph A. A1 - Cuzzi, Jeffrey N. A1 - Porco, Carolyn C. A1 - Beurle, Kevin A1 - Evans, Michael W. T1 - Observations of Ejecta clouds produced by impacts onto Saturn's rings JF - Science N2 - We report observations of dusty clouds in Saturn's rings, which we interpret as resulting from impacts onto the rings that occurred between 1 and 50 hours before the clouds were observed. The largest of these clouds was observed twice; its brightness and cant angle evolved in a manner consistent with this hypothesis. Several arguments suggest that these clouds cannot be due to the primary impact of one solid meteoroid onto the rings, but rather are due to the impact of a compact stream of Saturn-orbiting material derived from previous breakup of a meteoroid. The responsible interplanetary meteoroids were initially between 1 centimeter and several meters in size, and their influx rate is consistent with the sparse prior knowledge of smaller meteoroids in the outer solar system. Y1 - 2013 U6 - https://doi.org/10.1126/science.1233524 SN - 0036-8075 VL - 340 IS - 6131 SP - 460 EP - 464 PB - American Assoc. for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Bodrova, Anna A1 - Schmidt, Jürgen A1 - Spahn, Frank A1 - Brilliantov, Nikolai V. T1 - Adhesion and collisional release of particles in dense planetary rings JF - Icarus : international journal of solar system studies N2 - We propose a simple theoretical model for aggregative and fragmentative collisions in Saturn's dense rings. In this model the ring matter consists of a bimodal size distribution: large (meter sized) boulders and a population of smaller particles (tens of centimeters down to dust). The small particles can adhesively stick to the boulders and can be released as debris in binary collisions of their carriers. To quantify the adhesion force we use the JKR theory (Johnson, K., Kendall, K., Roberts, A. [1971]. Proc. R. Soc. Lond. A 324, 301-313). The rates of release and adsorption of particles are calculated, depending on material parameters, sizes, and plausible velocity dispersions of carriers and debris particles. In steady state we obtain an expression for the amount of free debris relative to the fraction still attached to the carriers. In terms of this conceptually simple model a paucity of subcentimeter particles in Saturn's rings (French, R.G., Nicholson, P.D. [2000]. Icarus 145, 502-523; Marouf, E. et al. [2008]. Abstracts for "Saturn after Cassini-Huygens" Symposium, Imperial College London, UK, July 28 to August 1, p. 113) can be understood as a consequence of the increasing strength of adhesion (relative to inertial forces) for decreasing particle size. In this case particles smaller than a certain critical radius remain tightly attached to the surfaces of larger boulders, even when the boulders collide at their typical speed. Furthermore, we find that already a mildly increased velocity dispersion of the carrier-particles may significantly enhance the fraction of free debris particles, in this way increasing the optical depth of the system. KW - Planetary rings KW - Saturn, Rings KW - Collisional physics Y1 - 2012 U6 - https://doi.org/10.1016/j.icarus.2011.11.011 SN - 0019-1035 SN - 1090-2643 VL - 218 IS - 1 SP - 60 EP - 68 PB - Elsevier CY - San Diego ER - TY - JOUR A1 - Kempf, Sascha A1 - Srama, Ralf A1 - Grün, Eberhard A1 - Mocker, Anna A1 - Postberg, Frank A1 - Hillier, Jon K. A1 - Horanyi, Mihaly A1 - Sternovsky, Zoltan A1 - Abel, Bernd A1 - Beinsen, Alexander A1 - Thissen, Roland A1 - Schmidt, Jürgen A1 - Spahn, Frank A1 - Altobelli, Nicolas T1 - Linear high resolution dust mass spectrometer for a mission to the Galilean satellites JF - Planetary and space science N2 - The discovery of volcanic activity on Enceladus stands out amongst the long list of findings by the Cassini mission to Saturn. In particular the compositional analysis of Enceladus ice particles by Cassini's Cosmic Dust Analyser (CDA) (Srama et al., 2004) has proven to be a powerful technique for obtaining information about processes below the moon's ice crust. Small amounts of sodium salts embedded in the particles' ice matrices provide direct evidence for a subsurface liquid water reservoir, which is, or has been, in contact with the moon's rocky core (Postberg et al., 2009, 2011b). Jupiter's Galilean satellites Ganymede, Europa, and Callisto are also believed to have subsurface oceans and are therefore prime targets for future NASA and ESA outer Solar System missions. The Galilean moons are engulfed in tenuous dust clouds consisting of tiny pieces of the moons' surfaces (Kruger et al., 1999), released by hypervelocity impacts of micrometeoroids, which steadily bombard the surfaces of the moons. In situ chemical analysis of these grains by a high resolution dust spectrometer will provide spatially resolved mapping of the surface composition of Europa. Ganymede, and Callisto, meeting key scientific objectives of the planned missions. However, novel high-resolution reflectron-type dust mass spectrometers (Sternovsky et al., 2007; Srama et al., 2007) developed for dust astronomy missions (Gran et al., 2009) are probably not robust enough to be operated in the energetic radiation environment of the inner Jovian system. In contrast, CDA's linear spectrometer is much less affected by harsh radiation conditions because its ion detector is not directly facing out into space. The instrument has been continuously operated on Cassini for 11 years. In this paper we investigate the possibility of operating a CDA-like instrument as a high resolution impact mass spectrometer. We show that such an instrument is capable of reliably identifying traces of organic and inorganic materials in the ice matrix of ejecta expected to be generated from the surfaces of the Galilean moons. These measurements are complementary, and in some cases superior, compared to other traditional techniques such as infrared remote sensing or in situ ion or neutral mass spectrometers. KW - Europa KW - Ganymede KW - Callisto KW - Surface composition KW - Mass spectroscopy KW - Dust Y1 - 2012 U6 - https://doi.org/10.1016/j.pss.2011.12.019 SN - 0032-0633 VL - 65 IS - 1 SP - 10 EP - 20 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Postberg, Frank A1 - Grün, Eberhard A1 - Horanyi, Mihaly A1 - Kempf, Sascha A1 - Krueger, Harald A1 - Schmidt, Jürgen A1 - Spahn, Frank A1 - Srama, Ralf A1 - Sternovsky, Zoltan A1 - Trieloff, Mario T1 - Compositional mapping of planetary moons by mass spectrometry of dust ejecta JF - Planetary and space science N2 - Classical methods to analyze the surface composition of atmosphereless planetary objects from an orbiter are IR and gamma ray spectroscopy and neutron backscatter measurements. The idea to analyze surface properties with an in-situ instrument has been proposed by Johnson et al. (1998). There, it was suggested to analyze Europa's thin atmosphere with an ion and neutral gas spectrometer. Since the atmospheric components are released by sputtering of the moon's surface, they provide a link to surface composition. Here we present an improved, complementary method to analyze rocky or icy dust particles as samples of planetary objects from which they were ejected. Such particles, generated by the ambient meteoroid bombardment that erodes the surface, are naturally present on all atmosphereless moons and planets. The planetary bodies are enshrouded in clouds of ballistic dust particles, which are characteristic samples of their surfaces. In situ mass spectroscopic analysis of these dust particles impacting onto a detector of an orbiting spacecraft reveals their composition. Recent instrumental developments and tests allow the chemical characterization of ice and dust particles encountered at speeds as low as 1 km/s and an accurate reconstruction of their trajectories. Depending on the sampling altitude, a dust trajectory sensor can trace back the origin of each analyzed grain with about 10 km accuracy at the surface. Since the detection rates are of the order of thousand per orbit, a spatially resolved mapping of the surface composition can be achieved. Certain bodies (e.g., Europa) with particularly dense dust clouds, could provide impact statistics that allow for compositional mapping even on single flybys. Dust impact velocities are in general sufficiently high at orbiters about planetary objects with a radius > 1000 km and with only a thin or no atmosphere. In this work we focus on the scientific benefit of a dust spectrometer on a spacecraft orbiting Earth's Moon as well as Jupiter's Galilean satellites. This 'dust spectrometer' approach provides key chemical and isotopic constraints for varying provinces or geological formations on the surfaces, leading to better understanding of the body's geological evolution. KW - Moon KW - Europa KW - Ganymede KW - Dust KW - Surface composition KW - Spectrometry Y1 - 2011 U6 - https://doi.org/10.1016/j.pss.2011.05.001 SN - 0032-0633 VL - 59 IS - 14 SP - 1815 EP - 1825 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Schenk, Paul A1 - Hamilton, Douglas P. A1 - Johnson, Robert E. A1 - McKinnon, William B. A1 - Paranicas, Chris A1 - Schmidt, Jürgen A1 - Showalter, Mark R. T1 - Plasma, plumes and rings saturn system dynamics as recorded in global color patterns on its midsize icy satellites JF - Icarus : international journal of solar system studies N2 - 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 similar to 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 similar to 40 degrees 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. KW - Satellites, Surfaces KW - Saturn, Satellites KW - Saturn, Rings KW - Enceladus KW - Satellites, Composition KW - Magnetospheres Y1 - 2011 U6 - https://doi.org/10.1016/j.icarus.2010.08.016 SN - 0019-1035 VL - 211 IS - 1 SP - 740 EP - 757 PB - Elsevier CY - San Diego ER -