The processes that lead to charging of dust grains in a plasma are briefly reviewed. Whereas for single grains the results have been long known, the reduction of the average charge on a grain by 'Debye screening' has only recently been discovered. This reduction can be important in the Jovian ring and in the rings of Uranus. The emerging field of gravitoelectrodynamics which deals with the motion of charged grains in a planetary magnetosphere is then reviewed. Important mechanisms for distributing grains in radial distance are due to stochastic fluctuations of the grain charge and a systematic variation due to motion through plasma gradients. The electrostatic levitation model for the formation of spokes is discussed, and it is shown that the radial transport of dust contained in the spokes may be responsible for the rich radial structure in Saturn's rings. Finally, collective effects in dusty plasmas are discussed which affect various waves, such as density waves in planetary rings and low-frequency plasma waves. The possibility of charged grains forming a Coulomb lattice is briefly described.

Dusty plasmas in the solar system

Cassini has identified a geologically active province at the south pole of Saturn's moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at ∼55°S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturn's E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.

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Cassini Observes the Active South Pole of Enceladus

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Supporting Online Material for Cassini Observes the Active South Pole of Enceladus

Voyager 2 photography has complemented that of Voyager I in revealing many additional characteristics of Saturn and its satellites and rings. Saturn's atmosphere contains persistent oval cloud features reminiscent of features on Jupiter. Smaller irregular features track out a pattern of zonal winds that is symmetric about Saturn's equator and appears to extend to great depth. Winds are predominantly eastward and reach 500 meters per second at the equator. Titan has several haze layers with significantly varying optical properties and a northern polar "collar" that is dark at short wavelengths. Several satellites have been photographed at substantially improved resolution. Enceladus' surface ranges from old, densely cratered terrain to relatively young, uncratered plains crossed by grooves and faults. Tethys has a crater 400 kilometers in diameter whose floor has domed to match Tethys' surface curvature and a deep trench that extends at least 270° around Tethys' circumference. Hyperion is cratered and irregular in shape. Iapetus' bright, trailing hemisphere includes several dark-floored craters, and Phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours. Within Saturn's rings, the "birth" of a spoke has been observed, and surprising azimuthal and time variability is found in the ringlet structure of the outer B ring. These observations lead to speculations about Saturn's internal structure and about the collisional and thermal history of the rings and satellites.

A new look at the saturn system: the voyager 2 images.

Optical constants of organic tholins produced in a simulated Titanian atmosphere: From soft x-ray to microwave frequencies

As Voyager 1 flew through the Saturn system it returned photographs revealing many new and surprising characteristics of this complicated community of bodies. Saturn's atmosphere has numerous, low-contrast, discrete cloud features and a pattern of circulation significantly different from that of Jupiter. Titan is shrouded in a haze layer that varies in thickness and appearance. Among the icy satellites there is considerable variety in density, albedo, and surface morphology and substantial evidence for endogenic surface modification. Trends in density and crater characteristics are quite unlike those of the Galilean satellites. Small inner satellites, three of which were discovered in Voyager images, interact gravitationally with one another and with the ring particles in ways not observed elsewhere in the solar system. Saturn's broad A, B, and C rings contain hundreds of "ringlets," and in the densest portion of the B ring there are numerous nonaxisymmetric features. The narrow F ring has three components which, in at least one instance, are kinked and crisscrossed. Two rings are observed beyond the F ring, and material is seen between the C ring and the planet.

Encounter with Saturn: Voyager 1 imaging science results

MRO's High Resolution Imaging Science Experiment (HiRISE)

Nine eruption plumes were observed over a period of 6 1/2 days during the Voyager 1 encounter with Io. During the Voyager 2 encounter, 4 months later, eight of these eruptions were still active. The largest plume viewed by Voyager 1 became inactive sometime between the two encounters. However, a major eruption occurred at Surt sometime between the two encounters and deposited an ejecta blanket comparable in size to those associated with the largest plumes. The plumes range in height from about 60 to over 300 km with corresponding ejection velocities of about 0.5 to 1.0 km/s. Where topographic information exists (plumes 1 and 8), the plume source is located on level plains rather than topographic highs and consists of either fissures or calderas. With the exception of plume 1 (Pele), the brightness distribution monotonically decreases from the core to the top of the plume. The shape and brightness distribution together with the pattern of the surface deposit of at least plume 3 (Prometheus) can be simulated by a ballistic model in which the ejection velocity is constant (0.5 km/s) and ejection angles vary from vertical (0°) to 55°. The brightness distribution of plume 1 is probably better explained by a shock front near the top of the plume. Numerous surface deposits similar to those associated with active plumes probably mark the sites of recent eruptions. The distribution of active and recent eruptions appears to be concentrated in the equatorial regions and indicates the volcanic activity is more frequent and intense in the equatorial regions than the polar regions. This suggests that the depositional rate is greater and the surface age younger in the equatorial regions, which may account for the darker polar regions. The geologic setting of certain plume sources and the very large reservoirs of volatiles required for the active eruptions suggests that sulphur volcanism rather than silicate volcanism is the most likely driving mechanism for the eruption plumes.

Volcanic Eruptions on IO

Introduction: The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter has been used to image seasonal processes on Mars since 2006. Spring on Mars is a time of active change at latitudes covered by seasonal CO2 ice. This is the third northern spring that we have collected images as the seasonal CO2 cap sublimates. In the first spring observed our emphasis was to characterize the types of seasonal activity extant in northern spring. With images from the second Mars year we identified morphological changes on the dunes of the north polar erg, including new alcoves and debris aprons, caused by seasonal processes [1]. In this third year we are systematically investigating the factors influencing the efficacy of seasonal activity such as slipface orientation and dune morphology. Types of Seasonal Activity: The vast north polar erg manifests numerous seasonal phenomena. Seasonal activity shows up as dark blotches and fans on top of the relatively bright seasonal CO2 ice layer as shown in Figure 1. Dark material often comes from cracks in the ice or the crests of the dunes. Our working hypothesis in the north polar region is that, like in the south, seasonal CO2 ice is at times translucent, allowing sunlight to penetrate and warm the ground below (the “Kieffer model” [2]). The ice layer then sublimates from the bottom, trapping gas, which flows to the nearest opening. The gas mobilizes sand, carrying it out to the top of the ice layer. When this sand is blown downwind or slides downhill it appears as a dark fan, otherwise just as a localized dark blotch.

Candice Hansen

Papers

Encounter with Saturn: Voyager 1 imaging science results

MRO's High Resolution Imaging Science Experiment (HiRISE)

Volcanic Eruptions on IO

Year 3 HiRISE Observations of Sublimation of the Northern Seasonal Polar Cap on Mars

Orbiting Observatory for Studying Hydrologically Active Regions on Mars