Sputtering products of sodium sulfate: Implications for Io's surface and for sodium-bearing molecules in the Io torus

Abstract

The composition of the sodium-bearing molecular ion cloud in the vicinity of Io may yield clues to Io's geochemistry. Likely Na-bearing source minerals are Na-sulfides and Na_2SO_4, with the relative composition depending on the distribution of thermal environments in the upper crust. These materials will be sputtered by co-rotating torus ions either from the surface or from atmospheric aerosols. Using laser postionization, we investigated the ion-sputtered neutral products of Na_2SO_4 to determine whether NaO in the Io torus might be a diagnostic indicator for sodium sulfate on Io's surface or in aerosols. With an ArF excimer laser (λ = 6.4 eV), single-photon saturation of the ionization step was achieved for NaS, Na_2O, and the combination Na_2S + Na_2O_2 at mass 78, while other species, including NaO, were close to saturation. Photofragmentation during the ionization step was minimal by all indications. The results predict sputtering ratios NaO/NaS > 8, Na_2O/NaS ∼8, Na_2/NaS ≥ 6, and Na/NaS ≥ 100. Multiphoton ionization was also attempted using a frequency-doubled (3.2-eV) Ti:sapphire femtosecond laser at high intensities, but saturation was not achieved for the major species, and there was evidence of photofragmentation. Velocity distributions of the sputter products show for the first time that significant fractions of sodium-bearing molecules can be sputtered into unbound trajectories from Io's surface, with escape fractions > 50% in the case of NaO. Combining relative sputter yields and velocity distributions gives escape ratios of NaO/NaS > 10, Na_2/NaS ∼ 6, and Na_2O/NaS ∼2.8 from Na_2SO_4. While absolute sputtering and escape ratios need to be compared for Na-sulfides versus Na_2SO_4, this work shows that, in the absence of significant modification by atmospheric chemical reactions, measurement of the neutral NaO/NaS ratio in the vicinity of Io could identify the major Na-bearing minerals.

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