Io’s Salty Secret

Volcanic activity on the Jovian moon Io stains the surface with red and yellow allotropes of sulfur. Recent observations show that the moon's frequent outpourings are seasoned with a little salt.
Courtesy University of Arizona and NASA.
It might not seem surprising that Io, the Jovian moon famous for its vigorous and continuous volcanism, should have an atmosphere permeated with noxious gases belched from its interior. But ever since in the mid-1970s, when telescopic observations revealed sodium atoms hovering over Io’s surface and a doughnut-shaped ring of sodium ions girding its orbit, geochemists have struggled to identify this element's source.

The sodium, it turns out, apparently comes from common table salt (NaCl). As detailed in Nature for January 2nd, a team led by Emmanuel Lellouch (Paris Observatory) discovered salt vapor in Io’s atmosphere early last year. The observers used the 30-meter dish at IRAM (Institut de Radio-Astronomie Millimétrique) in Spain to identify telltale emissions at the radio wavelengths of 1.3 and 2.1 millimeters.

Volcanic crater on Io
NASA's Galileo spacecraft took this close up view of Jupiter's volcanically active moon Io on October 31, 2000. The seemingly smoldering crater, called Tupan Patera, is about 75 kilometers across and is surrounded by cliffs 900 meters high. Courtesy
NASA/JPL/University of Arizona
The seasoning of Io’s atmosphere did not come as a total surprise. Other spectroscopists had identified chlorine ions on this moon a couple years ago, and subsequent models suggested that NaCl should be the most common sodium- and chlorine-bearing molecule in Io’s volcanic gases. Lellouch's team finds that the vapor has a patchy distribution over the surface, implying that it is being disgorged from a few dozen erupting volcanoes.

Still, it was a challenging observation to make: once it emerges from Io's interior, a salt molecule typically lasts only a few hours before sunlight breaks it down into its constituent atoms. In order to have enough of it to be detected, Lellouch estimates that Io must continuously pump roughly a ton of salt per second onto its surface.

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