Mihaly Horanyi

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 mm 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 615 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 may be the dominant charging process and will therefore play an important role in the subsequent behavior of dust grains released from planetary surfaces.

Contact charging of lunar and Martian dust simulants

micron dust grain is typically more than 10 5 elementary charges and varies linearly with dust size. The measured contact charge of a dust particle increases with repeated agitation of the surface. The average contact charge also varies linearly with the work function of the contacting surface. The contact charging with oxidized metal surfaces is found to be independent of the metal’s work function. The effective work functions of the planetary analogs are determined by extrapolation to be 5.8 eVand 5.6 eV for the lunar and Martian dust simulants, respectively. INDEX TERMS: 3914 Mineral Physics: Electrical properties; 3947 Mineral Physics: Surfaces and interfaces; 5470 Planetology: Solid Surface Planets: Surface materials and properties; 6225 Planetology: Solar System Objects: Mars; 6250 Planetology: Solar System Objects: Moon (1221); KEYWORDS: Dust, contact charging, lunar dust, Martian dust, work function

Gas dynamic heating of chondrule precursor grains in the solar nebula

Abstract Conditions under which gas dynamic processes operating within the solar nebula could have been responsible for melting of chondrule precursor grains are investigated. Gas-grain energy and momentum transfer are treated in the free molecular flow approximation, including both drag heating due to grain relative motion and heating due to collisions with gas molecules in thermal motion. The effect of thermally emitted radiation on grain heating and cooling is also considered. It is found that melting of chondrulesized grains is most easily achieved in optically thick, dust-rich zones where radiative cooling of a grain is balanced by thermal radiation from surrounding grains. In this circumstance, grain melting can occur for relatively small gas-grain velocities (4 km sec −1 ) and gas temperatures (1500 K). The thermal buffering effect of a dust-rich zone would also assist in increasing the cooling time scale, allowing time for outgassing and collapse of vesicles to be more consistent with observed chondrule properties. It may also help to narrow the range of thermal effects on chondrules as compared to those expected for single grains. Nebular shock fronts are suggested as one means of providing the necessary relative velocities and heated gas regions. Shocks of the needed strength and scale size could be generated by several mechanisms including nonaxisymmetric structures in an evolving nebula and interaction of volatile-rich planetesimals in eccentric orbits with the nebula.

Sublimation and reformation of icy grains in the primitive solar nebula

Abstract We examine the frictional heating, sublimation, and recondensation of grains free-falling into the solar nebula from a surrounding interstellar cloud. The amount of water ice sublimated varies over a wide range—from over 90% of the grain mass at 30 AU from the nebular center to less than 10% beyond 100 AU. We further conclude that essentially all of the water sublimated eventually recondenses, because the cold nebular gas beyond 10 AU is able to hold only a small fraction as vapor. The expansion of the sublimating gas from the grain surface and abundance of cold grains implies that most of the gas returns to the solid phase near nebular ambient temperatures (∼50 K). Such a process could lead to at least two populations of grains: (1) essentially unaltered interstellar grains which did not sublimate due to drag or accretion shock heating and (2) a component comprised of water ice cocondensed, after heating, with more volatile gases at nebular ambient temperatures, yielding volatile-rich amorphous phases. Component (2) may be by far the most abundant in the portion of the outer solar nebula where Triton and Pluto formed; component (1) may be much more important for comets.

High-velocity streams of dust originating from Saturn

High-velocity submicrometre-sized dust particles expelled from the jovian system have been identified by dust detectors on board several spacecraft. On the basis of periodicities in the dust impact rate, Jupiters moon Io was found to be the dominant source of the streams. The grains become positively charged within the plasma environment of Jupiters magnetosphere, and gain energy from its co-rotational electric field. Outside the magnetosphere, the dynamics of the grains are governed by the interaction with the interplanetary magnetic field that eventually forms the streams. A similar process was suggested for Saturn. Here we report the discovery by the Cassini spacecraft of bursts of high-velocity dust particles (≥ 100 km s-1) within ∼70 million kilometres of Saturn. Most of the particles detected at large distances appear to originate from the outskirts of Saturns outermost main ring. All bursts of dust impacts detected within 150 Saturn radii are characterized by impact directions markedly different from those measured between the bursts, and they clearly coincide with the spacecrafts traversals through streams of compressed solar wind.

Io as a source of the jovian dust streams

Streams of dust emerging from the direction of Jupiter were discovered in 1992 during the flyby of the Ulysses spacecraft, but their precise origin within the jovian system remained unclear. Further data collected by the Galileo spacecraft, which has been orbiting Jupiter since December 1995, identified the possible sources of dust as Jupiters main ring, its gossamer ring, comet Shoemaker–Levy 9 (ref. 8) and Io. All but Jupiters gossamer ring and Io have since been ruled out. Here we find that the dominant source of the jovian dust streams is Io, on the basis of periodicities in the dust impact signal. Ios volcanoes, rather than impact ejecta, are the dust sources.

Measurement of the charging of individual dust grains in a plasma

An experiment is described for investigating the charging of dust grains in a plasma. The apparatus is a double plasma device into which single dust grains are dropped from the top. The dust charge is detected and measured by a sensitive electrometer attached to a Faraday cup on the bottom. Experiments with electrons from the emissive filaments but without plasma indicate that the grains charge to approximately the filament potential for filament bias voltages smaller in absolute value than /spl minus/70 V. The charge is of order 10/sup 6/ electrons for SiC grains 30-150 /spl mu/m in diameter. At higher bias voltage the charge is reduced due to secondary emission. The charge on grains increases with grain size and is nearly independent of the filament emission current. With plasma in the device, the grains charge both positively and negatively. >

Electrostatic charging properties of Apollo 17 lunar dust

We report on our experimental studies of the electrostatic charging properties of an Apollo 17 soil sample and two lunar simulants, Minnesota Lunar Simulant (MLS-I) and Johnson Space Center (JSC-1). We have measured their charge after exposing individual grains to a beam of fast electrons with energies in the range of 20≤E≤90 eV. Our measurements indicate that the secondary electron emission yield of the Apollo 17 sample is intermediate between MLS-1 and JSC-1, closer to that of MLS-1.

Coagulation of dust particles in a plasma

The electrostatic charge of small dust grains in a plasma in which the temperature varies in time is discussed, pointing out that secondary electron emission might introduce charge separation. If the sign of the charge on small grains is opposite to that on big ones, enhanced coagulation can occur which will affect the size distribution of grains in a plasma. Two scenarios where this process might be relevant are considered: a hot plasma environment with temperature fluctuations and a cold plasma environment with transient heating events. The importance of the enhanced coagulation is uncertain, because the plasma parameters in grain-producing environments such as a molecular cloud or a protoplanetary disk are not known. It is possible, however, that this process is the most efficient mechanism for the growth of grains in the size range of 0.1-500 microns. 9 refs.

A permanent, asymmetric dust cloud around the Moon

Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged and neutral gas clouds, as well as secondary ejecta dust particles. Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter and Saturn, but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a ‘horizon glow’ indicated a putative population of high-density small dust particles near the lunar terminators, although later orbital observations yielded upper limits on the abundance of such particles that were a factor of about 104 lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. The density of the lunar ejecta cloud increases during the annual meteor showers, especially the Geminids, because the lunar surface is exposed to the same stream of interplanetary dust particles. We expect all airless planetary objects to be immersed in similar tenuous clouds of dust.