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The narrow rings of Uranus and Neptune exist in a system of observed and hypothesized small moons. Catastrophic fragmentation of these moons by comet impact has been proposed as the mode of origin of those rings, and earlier efforts to model the process showed that small moons are destroyed by impact on short timescales, leading to rapid collisional erosion of any primordial satellite system (Colwell and Esposito, 1992). We reexamine the question of impact fragmentation of small satellites in the light of new observational data on the population of Kuiper Belt and Centaur objects that produce the impacting flux and new theoretical and computational studies of catastrophic fragmentation. We find that the impacting flux used by Colwell and Esposito (1992) is consistent with the new observations of Kuiper Belt objects and calculations of their transport into the solar system. However, new fragmentation criteria from modeling of the asteroid belt and hydrocode simulations lengthen the model collisional lifetimes of satellite systems. The observed distribution of rings, dust bands, and moons at Uranus and Neptune suggest a catastrophic disruption model with a relatively weak dependence on target radius.

The satellite Enceladus is obviously the main source of Saturn's E ring. Up to now, different mechanisms of how particles are delivered from this satellite have been suggested. In this paper, we try to answer the question of whether these different launch processes can be distinguished by the cosmic dust analyzer (CDA) aboard the Cassini spacecraft. To this aim, the dynamics of dust particles just launched from the surface of Enceladus is studied numerically. We have integrated the equations of motion for a wide range of initial conditions including ejecta from interplanetary and E ring impactors onto Enceladus. According to our simulations, Cassini will encounter a significant dust stream about the time of closest approach to Enceladus. The duration and intensity of this expected enhanced impact rate onto the CDA depends on the way the particles are ejected from the satellite. The counting rate yields information about the distribution of ejecta sources on the surface of Enceladus. For instance, an anisotropy of the ejecta between the leading and the trailing hemispheres of Enceladus should be detectable, and impactors of different origin should be distinguishable. Furthermore, the vertical component of the ejecta velocities can explain the vertical extent of the E ring.

The planets, and their moons and rings, are bombarded by a population of interplanetary and interstellar micrometeoroids. This bombardment affects the evolution of planetary rings and electron densities in ionospheres. It also generates and overturns regolith on planetary satellites and ring particles. The impacting flux is enhanced by the planet's gravity, but the magnetosphere can also affect the flux for micrometeoroids near 1 μm in radius. We present calculations for micrometeoroids hitting Jupiter which show the magnetosphere shields the planet from impactors smaller than about 0.1 μm in radius, and enhances the flux of impactors by a factor of up to about 4 for grains near 0.5 μm in radius. Thus, in addition to the gravitational focusing factor, there is a size-dependent electromagnetic focusing factor active up to about 1.0 μm. In addition, we find that after 1 (Earth) year, significant numbers of small particles still orbit Jupiter. The fate of these particles may represent an additional enhancement of the flux of micrometeoroids onto the planet.

Catastrophic fragmentation of the ring moons of Uranus and Neptune occurs in approximately 10 exp 8 years. The fate of the debris following a fragmenting impact is central to understanding the evolution of these satellites and the hypothesized origin of rings from their debris. In this paper the possible effects of the velocity distribution of fragments following a catastrophic fragmentation on satellite diminution via a collisional cascade is examined. Fragment velocities are critical in the evolution of the collisional cascade because of the possibility of reaccretion following disruption. The fragment velocity distribution is used to calculate the initial phase space distribution of the new ring particles. This provides a physically realistic initial condition for simulations of the collisional evolution of planetary rings.

micron dust grain is typically more than 10 5 elementary charges and varies linearly with dust size. The measured contact charge of a dust particle increases with repeated agitation of the surface. The average contact charge also varies linearly with the work function of the contacting surface. The contact charging with oxidized metal surfaces is found to be independent of the metal’s work function. The effective work functions of the planetary analogs are determined by extrapolation to be 5.8 eVand 5.6 eV for the lunar and Martian dust simulants, respectively. INDEX TERMS: 3914 Mineral Physics: Electrical properties; 3947 Mineral Physics: Surfaces and interfaces; 5470 Planetology: Solid Surface Planets: Surface materials and properties; 6225 Planetology: Solar System Objects: Mars; 6250 Planetology: Solar System Objects: Moon (1221); KEYWORDS: Dust, contact charging, lunar dust, Martian dust, work function

[1] We use a thermal model with large-scale surface roughness to study the effects of topography and roughness on thermal infrared spectra of planetary surfaces. We study a range of roughness values, illumination geometries, and viewing geometries and find that surface roughness can strongly alter the slope of a thermal infrared spectrum, especially for large solar incidence angles. This roughness-induced slope in the spectrum can shift the location of the Christiansen frequency and should be removed in order to study the mineralogy and other surface properties. The effect is minimized for viewing near zero phase angle. This makes it possible to measure large-scale roughness by comparing thermal infrared spectra taken near zero phase angle with those taken at a large solar incidence angle and nonzero phase angle. Changes in apparent emissivity with wavelength in the thermal infrared due to roughness are calculated for Clementine longwave-infrared images of the Moon. These roughness-induced spectral slopes observed for the Moon are 5% between 5 and 8 microns for regions of moderate topography.