A Possible Dust Origin for an Unusual Feature in Io’s Sodium Neutral Clouds

Abstract : We report the results of model simulations performed to explain the nature of a sodium emission feature in Io Neutral Clouds. The feature was detected via high-resolution spectroscopic observations from the 3.6 m Italian telescope TNG. The emission feature is blueshifted compared to the main emission (the banana-shaped Neutral Cloud of Io) by a few tens of km s⁻¹, and it is most prominent when Io is a few tens of degrees before eclipse behind Jupiters shadow. The feature's morphology changes with time, indicative of a geometrical effect. We constrained its direction, velocity, and column density with a model of sodium atom trajectories under the influence of Io's and Jupiter's gravity, and solar radiation pressure. The model that best explains this emission feature has the atoms injected into the exosphere from the leading/sub-Jovian hemisphere of Io (45°–68° west longitude), with velocities from 50 to 90 km s⁻¹ relative to Io. These trajectories are consistent with those of negatively charged dust grains (radius ∼10 nm) accelerated by the corotational electric field of Jupiters magnetosphere. We propose that sputtering of sodium atoms from Na-bearing molecules (NaCl and Na2SO4) in these nanodust grains is the process responsible for our emission feature. Both modeling and observational constraints provide an order-of-magnitude estimate of the sodium production rate of ∼10²⁶ s⁻¹. Our work provides another method to monitor the amount of material that Io is supplying to its Neutral Clouds and plasma torus.