Dynamics of Charged Dust Near Surfaces in Space

Details Dynamics of Charged Dust Near Surfaces in Space Dynamics of Charged Dust Near Surfaces in Space

Nov 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002mfpt....1..395c&link_type=abstract

Sixth Microgravity Fluid Physics and Transport Phenomena Conference, vol. 1, p. 395-416

Physics

Space Plasmas, Planetary Surfaces, Dust, Electric Fields, Lunar Surface, Saturn Rings, Plasma Sheaths

Scientific paper

Objects in plasma, such as planetary bodies in the solar wind, charge to a floating potential determined by the balance between charging currents in the local plasma environment. In cases where secondary electron emission and photoemission are weak, objects will become negatively charged due to electron collection and will be surrounded by a plasma sheath. Solar ultraviolet radiation can produce a photoelectron sheath above the sunlit surface of airless planetary bodies. In both cases an electric field is present near the surface that can accelerate charged dust particles near the surface. Dust may be stably levitated if the electric force balances the gravitational force. Experiments in a plasma sheath have shown that particles can be stably levitated with surface potentials consistent with those expected on planetary surfaces. Our experiments have also shown that particles can be lifted off the surface by the electric field without any additional disturbance. This may explain the separation of dust from the surfaces of larger particles in Saturn's rings observed as 'spokes' by local plasma presumably generated by an impact. Observations of smooth deposits of regolith in crater bottoms on the asteroid 433 Eros by the NEAR spacecraft suggest a transport mechanism for regolith. Levitation of charged dust and transport in an inhomogeneous electric field is a possible explanation for the distribution of regolith on Eros and other asteroids. More generally, acceleration of charged dust in the near-surface sheath can lead to loss of fine-grained particles from objects with weak gravitational accelerations. We have carried out experiments on charging, levitation, and transport of dust in plasma and photoelectron sheaths. A tungsten filament beneath the surface plate creates the primary electrons that ionize gas in the chamber. When the surface is biased to a sufficiently high voltage (-40V to -80V), dust particles are lifted off the surface. Some of these particles, depending on size and mass, become stably levitated a few cm above the surface. On objects with weaker surface gravity the particle size-dependent levitation height is higher. In addition to levitation our experimental studies show that dust can be transported horizontally over a uniform surface in a uniform plasma sheath. We conducted experiments where a surface is partially covered by dust grains and partially clean. After cycling the plasma on and off, net transport of dust to the initially dust-free portions of the surface is observed. Further experiments will include topographical features on the surface simulating craters and rocks, and we will quantify the horizontal transport of dust in the sheath as a function of grain properties and plasma properties. Simple numerical simulations also show a net transport of dust when inhomogeneities in the sheath are introduced. We calculate the trajectories of individual dust grains using a numerical integrator that simultaneously solves for the charge on the particle. The forces on the particle are gravity downward and the electric force resulting from the grain charge and the electric field normal to the model surface that is produced by the sheath. Although there are no horizontal forces in our initial simulations, horizontal transport occurs as a result of a combination of initial horizontal velocity components for dust lifted off the surface, the effects of topography, and discontinuities in the photoelectron sheath at the terminator. When dust particles enter shadow the sheath vanishes in this model, and the particle falls to the surface under the effects of gravity. In our simulations this leads to an accumulation of dust at the terminator. Dust also accumulates at the borders of topographical features such as blocks and craters.

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