Charging and Discharging of Amorphous Solid Water Ice: The effects of Cracking and Implications for E- ring Grain Surface Potential

Introduction: Water-ice is ubiquitous on many planetary satellites, rings, and dust grains in the solar system. Within Saturn’s E ring, resident ice and dust grains (0.3–3 μm) are coated by water-vapor coreleased with ice grains from geyser-like eruptions from south polar region of Enceladus [1]. These water-coated particles are subsequently processed by Saturn’s inner magnetospheric plasma (dominated by the cold electrons and ions; Te ≈ 0.5–8 eV, Ti < 100 eV) [2], which changes the charge characteristics (and composition) of the grains and alters the electric fields within the E-ring. The grain surface potentials can affect the flux of incoming charged particles; however, published models [3] for the charging of the E-ring grains rarely discussed the effects of the surface potential resulting from water-ice coatings [4]. In this study we performed a series of laboratory experiments to investigate the surface potential of water ice coatings using amorphous solid water (ASW) films as laboratory analogues, providing ground-truth data for new studies of the charging of E-ring grains. Experiments: Experiments were performed in an ultra-high vacuum system (base pressure: ~2×10 Torr). ASW films were deposited by directing collimated vapor beams onto a liquid-helium cooled, goldcoated quartz crystal microbalance (QCM). The column density (in units of monolayers, ML, 1 ML = 1×10 molecules/cm) of the film was measured by the QCM, and the thickness (in units of μm) was measured using a UV-visible interferometry. The density (ρ) of the ice film was calculated from the ratio of the column density to the thickness. Surface potential (Vs, in units of volt, V) of the film was measured using a Kelvin probe. Surface microstructures of the film, cracks larger than a few microns, were imaged using an optical microscope. During growth, the deposition rate was controlled at 0.7 ML/s. The surface potential was monitored as a function of the thickness, and the deposition was stopped when the film cracked. Films with different porosities (defined as Φ = 1 – ρ/ρc, ρc = 0.94 g/cm) were prepared by changing the deposition temperature or the incidence angle of the vapor beam [5]. After the growth, the cracked films were warmed from the deposition temperature to 200 K at a rate of 4.8 K/min, while measuring the surface potential to study the effects of thermal cycling. To simulate the positive charging of ice-coated grains in the E-ring, the cracked films were charged electrostatically with 500 eV He at normal incidence for 270 seconds to a fluence of (0.9 ± 0.1) × 10/(cm s) prior to warming,. Results: During Growth. The magnitude of the native negative surface potential (|Vs|) of an ASW film increases linearly with thickness for films of less than 0.5 μm [5]. Extending the experimental thickness to a few microns, the |Vs| decreases abruptly above a critical thickness (Fig. 1(a)), compared the corresponding extrapolated value using the straight-line fit (open circles in Fig. 1(a)). At this critical thickness, cracks appear in telescopic images of the films (Fig. 1(b)).