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A. A. Sickafoose, J. E. Colwell, M. Horanyi (LASP, University of Colorado), S. Robertson (Dept. of Physics, University of Colorado)
Dust particles in the regoliths of objects with no atmosphere and low surface gravity may be levitated and transported vertically and horizontally by electric fields in the near-surface photoelectron layer. This phenomenon has been observed on the Moon and may affect the size distribution and spatial distribution of regolith on asteroids and small planetary satellits. We have performed experiments on the charging of single dust particles due to photoemission, collection of electrons from a photoemissive surface, and triboelectric charging.
The particles tested are ~100 microns in diameter and include JSC-1 (lunar regolith simulant), and JSC-Mars-1 (martian regolith simulant). Isolated conducting grains (Zn, Cu, and graphite) illuminated by ultraviolet light reach a positive equilibrium floating potential (a few volts) that depends upon the work function of the particle. Conducting grains dropped past a photoemitting surface attain a negative floating potential for which the sum of the emitted and collected currents is zero. Nonconducting grains (glass, SiC, and the regolith simulants) have a large initial triboelectric charging potential (up to + 10 V) with a distribution approximately centered on zero. The nonconducting grains are weak photoemitters and attain a negative floating potential when dropped past a photoemitting surface.
Our experimental results show that for realistic silicate planetary regolith analogs, triboelectric charging may be the dominant charging process and will therefore play an important role in the subsequent dynamical behavior of grains released from planetary regoliths. New experiments and numerical simulations are under way to study the levitation and dynamics of charged dust grains near surfaces in space.
This research supported by NASA Microgravity Fluid Physics Program (NAG3-2136).