Impact-Generated Dust Clouds Surrounding the Galilean Moons

Tenuous dust clouds of Jupiter's Galilean moons Io, Europa, Ganymede and Callisto have been detected with the in-situ dust detector on board the Galileo spacecraft. The majority of the dust particles have been sensed at altitudes below five radii of these lunar-sized satellites. We identify the particles in the dust clouds surrounding the moons by their impact direction, impact velocity, and mass distribution. Average particle sizes are 0.5 to $\rm 1 \mu m$, just above the detector threshold, indicating a size distribution with decreasing numbers towards bigger particles. Our results imply that the particles have been kicked up by hypervelocity impacts of micrometeoroids onto the satellites' surfaces. The measured radial dust density profiles are consistent with predictions by dynamical modeling for satellite ejecta produced by interplanetary impactors (Krivov et al., PSS, 2003, 51, 251--269), assuming yield, mass and velocity distributions of the ejecta from laboratory measurements. The dust clouds of the three outer Galilean moons have very similar properties and are in good agreement with the model predictions for solid ice-silicate surfaces. The dust density in the vicinity of Io, however, is more than an order of magnitude lower than expected from theory. This may be due to a softer, fluffier surface of Io (volcanic deposits) as compared to the other moons. The log-log slope of the dust number density in the clouds vs. distance from the satellite center ranges between --1.6 and --2.8. Appreciable variations of number densities obtained from individual flybys with varying geometry, especially at Callisto, might be indicative of leading-trailing asymmetries of the clouds due to the motion of the moons with respect to the field of impactors.Comment: Icarus, in press, 46 pages, 16 figures, 5 table

Physical characteristics and non-keplerian orbital motion of "propeller" moons embedded in Saturn's rings

We report the discovery of several large "propeller" moons in the outer part of Saturn's A ring, objects large enough to be followed over the 5-year duration of the Cassini mission. These are the first objects ever discovered that can be tracked as individual moons, but do not orbit in empty space. We infer sizes up to 1--2 km for the unseen moonlets at the center of the propeller-shaped structures, though many structural and photometric properties of propeller structures remain unclear. Finally, we demonstrate that some propellers undergo sustained non-keplerian orbit motion. (Note: This arXiv version of the paper contains supplementary tables that were left out of the ApJL version due to lack of space).Comment: 9 pages, 4 figures; Published in ApJ

Collisional Velocities and Rates in Resonant Planetesimal Belts

We consider a belt of small bodies around a star, captured in one of the external or 1:1 mean-motion resonances with a massive perturber. The objects in the belt collide with each other. Combining methods of celestial mechanics and statistical physics, we calculate mean collisional velocities and collisional rates, averaged over the belt. The results are compared to collisional velocities and rates in a similar, but non-resonant belt, as predicted by the particle-in-a-box method. It is found that the effect of the resonant lock on the velocities is rather small, while on the rates more substantial. The collisional rates between objects in an external resonance are by about a factor of two higher than those in a similar belt of objects not locked in a resonance. For Trojans under the same conditions, the collisional rates may be enhanced by up to an order of magnitude. Our results imply, in particular, shorter collisional lifetimes of resonant Kuiper belt objects in the solar system and higher efficiency of dust production by resonant planetesimals in debris disks around other stars.Comment: 31 pages, 11 figures (some of them heavily compressed to fit into arxiv-maximum filesize), accepted for publication at "Celestial Mechanics and Dynamical Astronomy

Saturn\u27S F Ring As Seen By Cassini Uvis: Kinematics And Statistics

We present a new orbital model of Saturn\u27s F ring core based on 93 occultations by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Voyager radio and stellar occultations. We demonstrate that the core, despite its intrinsic variability, is well-described as an inclined, freely precessing ellipse. We find that post-fit residuals with a root-mean-square of 24. km are genuine, representing the well-known non-Keplerian features observed in the ring. Over the nearly 4. years of UVIS observations we find the residual variance to increase, coincident with the apse anti-alignment of Prometheus and F ring core in December 2009. This increase in dynamical F ring core temperature most likely reflects the ever-stronger perturbations by Prometheus. Our results are in good agreement with Earth-based and HST observations as well as Voyager imaging.Cassini UVIS stellar occultations resolve the F ring at unprecedented resolutions of a few meters and we identify the F ring core and inner and outer strands. We infer their normal optical depth and full width at half maximum (FWHM) and show that core and strands form distinct morphological groups. Typically, a strand is about ten times wider than the core (average FWHM is ~10. km) while having a ten times smaller optical depth. Unlike in pre-Cassini occultations the F ring core displays significant optical depth with in some cases \u3e3. In many cases we find a narrow optically thick component (~ few km and τ\u3e 0.5) embedded in the F ring core. Entertaining the possibility that this is the actual, true F ring core then UVIS results suggest that this true core is highly non-continuous. In addition, we report the detection of a previously unknown structure - dubbed the secondary as it visually resembles the F ring core. Its morphology is similar to that of the core in optical depth and FWHM and it displays individual opaque features. Despite its core-like appearance, we show that its kinematics is consistent with that of strands. We conclude that it is the most prominent strand seen to date. It represents a striking example of strand creation resulting in what could be called a morphological small-scale version of the F ring core. This extraordinary object should be one of the prime targets of future F ring studies. © 2011 Elsevier Inc

Waves in Cassini UVIS stellar occultations. 2. The C ring

International audienceWe performed a complete wavelet analysis of Saturn's C ring on 62 stellar occultation profiles. These profiles were obtained by Cassini's Ultraviolet Imaging Spectrograph High Speed Photometer. We used a WWZ wavelet power transform to analyze them. With a co-adding process, we found evidence of 40 wavelike structures, 18 of which are reported here for the first time. Seventeen of these appear to be propagating waves (wavelength changing systematically with distance from Saturn). The longest new wavetrain in the C ring is a 52-km-long wave in a plateau at 86,397 km. We produced a complete map of resonances with external satellites and possible structures rotating with Saturn's rotation period up to the eighth order, allowing us to associate a previously observed wave with the Atlas 2:1 inner Lindblad resonance (ILR) and newly detected waves with the Mimas 6:2 ILR and the Pandora 4:2 ILR. We derived surface mass densities and mass extinction coefficients, finding r = 0.22(±0.03) g cm À2 for the Atlas 2:1 ILR, r = 1.31(±0.20) g cm À2 for the Mimas 6:2 ILR, and r = 1.42(±0.21) g cm À2 for the Pandora 4:2 ILR. We determined a range of mass extinction coefficients (j = s/r) for the waves associated with resonances with j = 0.13 (±0.03) to 0.28(±0.06) cm 2 g À1 , where s is the optical depth. These values are higher than the reported values for the A ring (0.01-0.02 cm 2 g À1) and the Cassini Division (0.07-0.12 cm 2 g À1 from Colwell et al. (Colwell, J.E., Cooney, J.H., Esposito, L.W., Sremčević , M. [2009]. Icarus 200, 574-580)). We also note that the mass extinction coefficient is probably not constant across the C ring (in contrast to the A ring and the Cassini Division): it is systematically higher in the plateaus than elsewhere, suggesting smaller particles in the plateaus. We present the results of our analysis of these waves in the C ring and estimate the mass of the C ring to be between3.7(±0.9) Â 10 16 kg and 7.9(±2.0) Â 10 16 kg (equivalent to an icy satellite of radius between 28.0(±2.3) km and 36.2(±3.0) km with a density of 400 kg m À3 , close to that of Pan or Atlas). Using the ring viscosity derived from the wave damping length, we also estimate the vertical thickness of the C ring between 1.9(±0.4) m and 5.6(±1.4) m, comparable to the vertical thickness of the Cassini Division

Waves In Cassini Uvis Stellar Occultations. 2. The C Ring

We performed a complete wavelet analysis of Saturn\u27s C ring on 62 stellar occultation profiles. These profiles were obtained by Cassini\u27s Ultraviolet Imaging Spectrograph High Speed Photometer. We used a WWZ wavelet power transform to analyze them. With a co-adding process, we found evidence of 40 wavelike structures, 18 of which are reported here for the first time. Seventeen of these appear to be propagating waves (wavelength changing systematically with distance from Saturn).The longest new wavetrain in the C ring is a 52-km-long wave in a plateau at 86,397km. We produced a complete map of resonances with external satellites and possible structures rotating with Saturn\u27s rotation period up to the eighth order, allowing us to associate a previously observed wave with the Atlas 2:1 inner Lindblad resonance (ILR) and newly detected waves with the Mimas 6:2 ILR and the Pandora 4:2 ILR. We derived surface mass densities and mass extinction coefficients, finding σ=0.22(±0.03)gcm-2 for the Atlas 2:1 ILR, σ=1.31(±0.20)gcm-2 for the Mimas 6:2 ILR, and σ=1.42(±0.21)gcm-2 for the Pandora 4:2 ILR. We determined a range of mass extinction coefficients (κ=τ/σ) for the waves associated with resonances with κ=0.13 (±0.03) to 0.28(±0.06)cm2g-1, where τ is the optical depth. These values are higher than the reported values for the A ring (0.01-0.02cm2g-1) and the Cassini Division (0.07-0.12cm2g-1 from Colwell et al.(Colwell, J.E., Cooney, J.H., Esposito, L.W., Sremčević, M. [2009].Icarus 200, 574-580)).We also note that the mass extinction coefficient is probably not constant across the C ring (in contrast to the A ring and the Cassini Division): it is systematically higher in the plateaus than elsewhere, suggesting smaller particles in the plateaus. We present the results of our analysis of these waves in the C ring and estimate the mass of the C ring to be between3.7(±0.9)×1016kg and 7.9(±2.0)×1016kg (equivalent to an icy satellite of radius between 28.0(±2.3)km and 36.2(±3.0)km with a density of 400kgm-3, close to that of Pan or Atlas). Using the ring viscosity derived from the wave damping length, we also estimate the vertical thickness of the C ring between 1.9(±0.4)m and 5.6(±1.4)m, comparable to the vertical thickness of the Cassini Division. © 2011

A Predator-Prey Model For Moon-Triggered Clumping In Saturn\u27S Rings

UVIS occultation data show clumping in Saturn\u27s F ring and at the B ring outer edge, indicating aggregation and disaggregation at these locations that are perturbed by Prometheus and by Mimas. The inferred timescales range from hours to months. Occultation profiles of the edge show wide variability, indicating perturbations by local mass aggregations. Structure near the B ring edge is seen in power spectral analysis at scales 200-2000. m. Similar structure is also seen at the strongest density waves, with significance increasing with resonance strength. For the B ring outer edge, the strongest structure is seen at longitudes 90° and 270° relative to Mimas. This indicates a direct relation between the moon and the ring clumping. We propose that the collective behavior of the ring particles resembles a predator-prey system: the mean aggregate size is the prey, which feeds the velocity dispersion; conversely, increasing dispersion breaks up the aggregates. Moons may trigger clumping by streamline crowding, which reduces the relative velocity, leading to more aggregation and more clumping. Disaggregation may follow from disruptive collisions or tidal shedding as the clumps stir the relative velocity. For realistic values of the parameters this yields a limit cycle behavior, as for the ecology of foxes and hares or the boom-bust economic cycle. Solving for the long-term behavior of this forced system gives a periodic response at the perturbing frequency, with a phase lag roughly consistent with the UVIS occultation measurements. We conclude that the agitation by the moons in the F ring and at the B ring outer edge drives aggregation and disaggregation in the forcing frame. This agitation of the ring material may also allow fortuitous formation of solid objects from the temporary clumps, via stochastic processes like compaction, adhesion, sintering or reorganization that drives the denser parts of the aggregate to the center or ejects the lighter elements. Any of these more persistent objects would then orbit at the Kepler rate. We would also expect the formation of clumps and some more permanent objects at the other perturbed regions in the rings... including satellite resonances, shepherded ring edges, and near embedded objects like Pan and Daphnis (where the aggregation/disaggregation cycles are forced similar to Prometheus forcing of the F ring). © 2011 Elsevier Inc