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Planetary geology
Io’s salty atmosphere

Io, a moon the size of our own, orbits Jupiter through an electrically charged plasma haze. Two years ago, astronomers identified trace amounts of chlorine in this plasma torus, as the ionized gas cloud is called. The chlorine raised hopes of finding a salty answer to another strange feature known since 1974: Closer to the satellite’s surface, neutral clouds of sodium form a tenuous atmosphere. But understanding just how the sodium clouds formed has challenged astronomers. Not until they found chlorine in the plasma torus did they know for certain what type of salt they should look for in the atmosphere. Now, after two years of searching Io’s atmosphere for a trace of sodium chloride (NaCl), scientists have identified some using a millimeter-wavelength radio telescope in Spain. In the Jan. 2 issue of Nature, Emmanuel Lellouch of the Paris Observatory in France and colleagues reported their observations, taken during nightly views of Io.

The source of the salt, however, is still unclear. Lellouch’s findings support the idea that Io’s active volcanic emissions are spewing the salt into the atmosphere. But he found less NaCl than previous calculations had predicted based on assumptions of Io’s erupting magma. “This disagreement between observations and expectations may require a revision of our understanding of Io’s lithospheric composition.” He allows, however, for alternative solutions, including a loss of NaCl through possible condensation at the vent sites. Also, he says, the data, taken during five nights of observing Io, represent a snapshot of what might over longer times be a variable amount of the NaCl in the atmosphere.

Christina Reed

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