Experimental investigations on photoelectric and triboelectric charging of dust

Experiments are performed pertaining to the charging of single dust particles in space due to three effects: (1) photoemission, (2) the collection of electrons from a photoemissive surface, and (3) triboelectric charging. The particles tested are 90-106 mum 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 +/- 15 V) with a distribution approximately centered on zero. The nonconducting grains are weak photoemitters, and they attain a negative floating potential when dropped past a photoemitting surface. Our experimental results show that for silicate planetary regolith analogs, triboelectric charging map be the dominant charging process and will therefore play an important role in the subsequent behavior of dust grains released from planetary surfaces.