Illumination of a space-borne object by solar ultraviolet light produces photoelectrons which form a layer, or sheath, near its surface. Typical sheath dimensions are tens of centimeters to one meter at distances of one AU. A vertical electric field, having a sign that returns electrons to the surface, is generated by the emission of photoelectrons into this sheath. Differential charging in a photoelectron layer over small spatial scales, such as along the terminator on a rough surface, leads to horizontal electric fields in addition to the vertical field. Dust grains on a planetary surface can enter the photoelectron layer by levitation or as a result of external disturbances. Levitation can occur when grains in the regolith become electrostatically charged such that the Lorentz force overcomes gravity and adhesive forces. Particles ejected from the surface by external disturbances, such as micrometeorite impacts or human/spacecraft activity, can become charged by photoemission and electron collection when immersed in a plasma. Therefore, particles levitated or ejected from the surface can be trapped in the photoelectron sheath and transported vertically and horizontally above the surface by the electric fields.
Dust grains suspended above the lunar surface have been observed on multiple
occasions. A horizon glow tens of centimeters above the surface of the Moon was detected by
Surveyors 5, 6, and 7 [1]. At spacecraft sunrise, Apollo astronauts observed
high altitude streaks due to light scattered off particles extending from the
lunar surface to above the spacecraft [2]. More recently, a horizon glow 10 to 20 km above the lunar surface was detected by the
Clementine spacecraft [3]. Furthermore, the Lunar Ejecta and Meteorite Experiment (LEAM)
deployed by Apollo 17 detected evidence for horizontal dust transport on the
surface of the Moon at terminator crossings [4].
Dust transport is also important in planetary ring systems, as confirmed by
Voyager's discovery of spokes in Saturn's B-ring in 1981. These features are
most likely clouds of particles electrostatically levitated off the surfaces of
larger bodies in the ring and into a plasma cloud created by meteoritic impacts.
Many small, airless bodies in the solar system are coated with a dusty regolith;
therefore, dust levitation and transport may occur on Mars, Mercury, planetary
satellites, asteroids, comets, and even planetesimals [5,6,7,8]. Understanding
dust charging and dynamics is thus crucial to interpreting observations of
planetary bodies. In addition, surface activity can agitate dust and inject
particles into the photoelectron layer, possibly causing contamination of
instruments on planetary surfaces. Therefore, the charging and dynamics of dust
near planetary surfaces is a necessary component of future manned and unmanned
exploration of the solar system.
We have begun the investigation of dust charging and dynamics near surfaces in
space by performing experiments in which dust grains are dropped through a beam of UV and
dropped past a UV illuminated surface having a photoelectron sheath. The experiments are
performed in vacuum, with illumination from a 1 kW Hg-Xe arc lamp having a
spectrum extending to ~ 200nm ( ~ 6.2 eV). The photoemitter is a 12-cm diameter
zirconium plate, which also acts as a Langmuir probe to verify the properties of
the photoelectron sheath. We have examined [9] photoelectric charging for particles 90-106 mm in diameter composed of zinc,
copper, graphite, glass, SiC, lunar regolith simulant (JSC-1), and martian regolith simulant (JSC Mars-1). We find that the photoelectric charging
properties of the metallic materials are consistent with charges calculated from
the work function of the materials, the energy of incoming photons or photoelectrons, and the capacitance of the grains. Dust dropped through UV
illumination loses electrons due to photoemission, while dust dropped past an
illuminated surface gains electrons from the photoelectron sheath. The photoelectric charging properties of the non-conducting materials are difficult
to interpret due to large amounts of triboelectric charging. However, the results suggest that the JSC Mars-1 is more susceptible to photoelectric
charging than the JSC-1.
In a second experiment, a flat surface is coated in dust (JSC-1 or JSC-Mars-1)
and a photoelectron layer is generated above it. Plasma sheath characteristics are
determined through Langmuir probe and floating potential probe sweeps. A window
in the top of the chamber allows investigation of dust levitation and charging
properties as a function of ultraviolet illumination. An agitator under the
surface provides external disturbances to eject dust into the photoelectron
layer. The charges of individual dust particles in the sheath can be measured by
a Faraday cup, and tungsten filaments are used for ambient plasma production.
The topography of the surface can be easily altered to simulate a terminator
region, in order to create strong horizontal electric fields within the sheath.
Dust levitation and transport in the sheath is observed by a CCD camera.
These ground-based studies of dust dynamics near surfaces in space provide
information on the transport of micron-sized particles. Particles of this size
are in the transitional range between gravitational and electromagnetic control
on a planetary surface. A microgravity environment will allow continuation of
the experiments to smaller particle sizes. In microgravity, the ratio of
electrostatic forces to gravitational forces is high, so the environment will
also be more like that of small, dusty planetary bodies. Thus, this research
provides a set of experimental techniques upon which subsequent microgravity
experiments will be based.
Robertson, S., Sickafoose, A., Colwell, J., Horanyi, M., Dusty Plasma Dynamics Near Surfaces in Space, Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 987-1005, August 9, 2000.