IMPACT Laboratory investigations of electrostatic dust lofting on comet and asteroid (airless bodies) surfaces Xu Wang Joseph Schwan, Hsiang-Wen Hsu, Mihály Horányi Laboratory for Atmospheric and Space Physics (LASP), NASA/SSERVI’s Institute for Modeling Plasma, Atmospheres and Cosmic Dust (IMPACT) University of Colorado – Boulder 2015 EPSC October 2, 2015

Surface Charging of Airless Bodies IMPACT Courtesy: NASA Dust particles on the regolith of airless bodies are charged and may be transported and lofted due to electrostatic forces.

In-situ Observations IMPACT (Electrostatic dust transport could play a role) IMPACT The Spokes in Saturn’s B ring (Mitchell et al., Science, 2006) Lunar horizon glow (Rennilson and Criswell, 1974) Dust particles from comet 67P collected by Rosetta (Schulz et al., Nature, 2015) Dust pond on asteroid Eros (Renno and Kok, 2008)

Previous Laboratory Experiments IMPACT (A couple of examples) IMPACT Primary e- (75 eV) and Plasma Dust particles < 25 μm High terrain Laser Light Scatter (LLS) Wang et al., 2010 Previous lab experiments did show dust moving and lofting in plasmas. Then, the question is how this works. Flanagana and Goree, 2006

Examination of Current Charge Models IMPACT Shared charge model (uniform surface charge density) Case I (Wang et al., 2010) E = 100 V/cm r = 12.5 μm Fe= 1.7e-12 N Fc = 4.3e-13 N Fg= 1.5e-10 N Fe+Fc ≈ 10-2 Fg Case II (?) (Lunar case) E = 10 V/m r = 5 μm Fe= 2.8e-19 N Fc= 6.9e-20 N Fg= 2.5e-12 N Fe+Fc ≈ 10-7 Fg Q = ε0EA = 4πε0r2E Fe = QE Fc = 1/4πε0 (Q/2r)2 Fg = mg *Cohesion force is not yet considered Charge fluctuation theory (due to discrete electron and ion fluxes to the surface) Case I dQrms / Q = 807 / 1085= 0.74 Qmax ≈ 2Q, small enhancement. (Sheridan and Hayes, 2011) Charge induced by plasma is too small for dust particles to be lifted off.

Plasma and primary e – (~ 140 eV) New Dust Experiments IMPACT UV (172 nm) Plasma and primary e – (~ 140 eV) Sorry for not being able to upload this dust movie. Dust particles (Mars simulants, 38 < d < 48 μm) in a crater 1.9 cm in diameter and 0.1 cm deep.

Trajectories of Dust Particles IMPACT Plasma UV For the first time, we see dust transport in solely UV illumination.

Size Distributions of Lofting Dust IMPACT Filtered image of dust particles landing on the surface Size composition Small residues on single particles; Single particles; Aggregates due to the cohesion between particles. More small residues were lofted in the UV illumination than in the plasma.

A New Patched Charge Model (PCM) IMPACT Electrons or photons Ions Plasma charging, Qp (-) SE or photoelectron charging, Qse or Qphe (-) Micro-cavity SEs or photoelectrons are absorbed inside the micro-cavity and collected on the surface patches (light blue). Light-blue patches are also shielded from incoming electrons/ions or photons. These surface patches (light blue) can go to a very negative potential, where emitted electrons with higher energies (the tail in the Maxwellian energy distribution) are stopped from reaching the patches.

Experimental Verification of the PCM IMPACT Ji: Ions Je: Plasma electrons Jphoton: Photons Jse: SEs Jphe: Photoelectrons Plasma or UV Ji Je or Jphoton Insulator Jse or Jphe Plate C The potentials on the two probes (green) are measured. The fluxes to/from the probes: Top: Ji, Je and Jse or Jphe (probe itself) Bottom: Jse or Jphe (plate) V

Experimental Verification of the PCM (continued…) IMPACT Top probe potential is slightly more positive than Vplate. Bottom probe potential is as negative as - 12V (plasma) or -2.5V (UV) relative to Vplate. 45μm particles (plasma) Qse ~ 3e-14 C >> Qp ~ 6e-17 C Fc ~ 4e-9 N >> Fe ~ 3e-11 N Fc > Fg = 9e-10 N 20μm particles (UV) Qphe ~ 3e-15 C >> Qp ~ 1e-17 C Fc ~ 2e-10 N >> Fe ~ 3e-12 N Fc > Fg = 8e-11 N Dust particles that form the micro-cavities are charged largely negatively due to the recollection of SEs or photoelectrons. Coulomb force (repulsion) is a dominant electrostatic force to lift dust off. Cohesive force may also be a significant ‘negative’ force to be overcome. However, it largely depends on surface morphology and can vary in orders of magnitude.

Implications to dust charging on airless bodies IMPACT On the dayside surface, photoelectrons play the role. Due to much shorter UV wavelengths in space than in our laboratory, higher energy photoelectrons are expected, leading to even more negative charge on dust particles that form micro-cavities. Recalculation for the lunar case with the new patched charge model: E = 10 V/m, r = 5 μm, Fg= 2.5e-12 N Fc= 1.7e-10 N (V = -2.5V) Fc= 2.8e-9 N (V = -10V) Now, Fc > Fg Fco= CS2r = 2.2e-9 N On the nightside surface, secondary electrons play the role. Due to tenuous plasma density, dust activity could be quiet. In the case that the moons enter their parent planets’ magnetosphere, dust activity can be enhanced due to the presence of hot electrons, which produce more secondary electrons.

Summary and Conclusions IMPACT Dust particles were lofted in both plasma and UV illumination conditions. A new “patched charge model” with the structure of micro-cavities formed in-between neighboring dust particles was proposed. The recollection of secondary or photo- electrons leads to large negative charge on the neighboring dust particles. Coulomb force (repulsion) between dust particles is likely a dominant electrostatic force to lift dust off the surface. The cohesive force, surface morphology, and shape of dust particles are all the factors to determine the dust transport. Our experimental results suggest that dust particles on the surfaces of airless bodies are likely to become mobilized and lofted by electrostatic forces.