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Modelling Electrostatic Dust Transport on Eros

Introduction: Images taken by the NEAR Shoemaker spacecraft of the surface of Eros in 2000 revealed that the surface of the asteroid is distinguished by a great deal of ejecta and debris as well as a fine dust layer that appears to form aprons around several of the larger boulders. This dust also forms many distinct, smooth ponds [1]. The ponds are located at the point of gravitational minima in craters and depressions and exhibit flat surfaces along gravitational, equipotential surfaces. Images indicate the regolith is smooth down to 1.2 cm per pixel resolution in some locations [2]. Furthermore, the pond spectra are slightly bluer than the background terrain. One explanation for this is simply one of grain size, suggesting that the material filling the ponds on Eros may be made up of micron scale particles. Robinson et al. [2] noted a correlation between these ponds and the latitude and gravity of the area. While ponds do appear at all latitudes, 91% of ponds over 30m in diameter lie within 30 degrees of the equator and are distinctly concentrated in areas of low surface gravity. One mechanism for transporting micron-sized particles into topographic lows is electrostatic transport of charged grains in the near-surface photoelectron layer or plasma sheath [3]. This mechanism may be particularly effective at the terminator, suggesting that the latitudinal dependence on the location of large ponds may be due in part to the high obliquity (89 degrees) of Eros. This extreme obliquity results in the equatorial region experiencing illumination near the terminator significantly more often than other regions of the asteroid [2]. Electrostatic Dust Transport Model: The work presented builds upon the numerical model of Colwell et al. [3] who simulated dust grain levitation in an electrostatic field. Because the presence and strength of the field depends upon the strength of the solar illumination striking the surface, no such field should exist over areas that lie in shadow. The result of this was found to be that levitated dust grains will tend to accumulate inside of craters and depressions where shadows have been cast by the crater rim. However, because of the 2-dimensional nature of the original model, seasonal effects and latitudinal variations could not be explored. We have expanded the model to three dimensions including the capability to study dust transport at arbitrary latitudes and to explore seasonal effects with non-zero obliquities. This allows first for the morphology of captured dust distributions to be explored based on differences in topography as shown in figure 1. Rather than simply a pile-up of dust due to a shadow-casting edge, we may now see variations in how dust is captured in depressions to form ponds, or settle near large boulders, possibly to build up the debris aprons seen in the NEAR Shoemaker images [1].