Volcanically emitted sodium chloride as a source for Io's neutral clouds and plasma torus

Abstract

The atmosphere of Jupiter's satellite Io is extremely tenuous, time variable and spatially heterogeneous. Only a few molecules—SO2, SO and S2—have previously been identified as constituents of this atmosphere, and possible sources1,2,3,4 include frost sublimation, surface sputtering and active volcanism. Io has been known5,6 for almost 30 years to be surrounded by a cloud of Na, which requires an as yet unidentified atmospheric source of sodium. Sodium chloride has been recently proposed as an important atmospheric constituent, based on the detection of chlorine in Io's plasma torus7,8 and models of Io's volcanic gases9 . Here we report the detection of NaCl in Io's atmosphere; it constitutes only 0.3% when averaged over the entire disk, but is probably restricted to smaller regions than SO2 because of its rapid photolysis and surface condensation10. Although the inferred abundance of NaCl in volcanic gases is lower than predicted9, those volcanic emissions provide an important source of Na and Cl in Io's neutral clouds and plasma torus.

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Figure 1: Overview of the NaCl observations.
Figure 2: The SO2 and NaCl 15 and 17 January observations, compared to various models.

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Acknowledgements

We thank W. Thuillot for providing us with the topocentric velocities of Io necessary for the data reduction.

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Correspondence to E. Lellouch.

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Lellouch, E., Paubert, G., Moses, J. et al. Volcanically emitted sodium chloride as a source for Io's neutral clouds and plasma torus. Nature 421, 45–47 (2003). https://doi.org/10.1038/nature01292

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