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JETS of sodium are streaking away from Io, one of Jupiter鈥檚 moons, creating a
vast cloud around the gas giant planet. These observations from the Galileo
spacecraft also hint that Io鈥檚 atmosphere is mostly concentrated around the
moon鈥檚 equator, while the poles are barely covered at all.

Io, the largest, innermost moon of Jupiter, is constantly squeezed, stretched
and heated by Jupiter鈥檚 gravity. As a result, the moon鈥檚 tortured innards are
churning and volcanoes are erupting on its surface, enriching the atmosphere
with such elements as sodium.

Matthew Burger of the University of Colorado in Boulder, planned to use
Galileo鈥檚 observations of the greenish light sodium emits to image Io鈥檚
atmosphere. When he did the analysis, however, he was surprised. 鈥淭here was a
jet coming out鈥攏obody expected it,鈥� he says. 鈥淚t wasn鈥檛 what I was looking
蹿辞谤.鈥�

The observations suggest that electric fields generated on Io as it sweeps
through Jupiter鈥檚 magnetic field accelerate sodium ions in the moon鈥檚 atmosphere
away from its equator. The ions are neutralised by neutral atoms before escaping
from the moon in high-velocity jets. The sodium atoms then form a cloud around
Jupiter. 鈥淓verything鈥檚 escaping,鈥� says Burger. 鈥淭here鈥檚 a large sodium
苍别产耻濒补.鈥�

The fact that the sodium jets come from the equatorial regions implies that
Io鈥檚 atmosphere itself is localised to a band near the equator. That may be
because there are more volcanoes there, spewing more material into the sky.
These jets should help astronomers work out the orientation and strength of Io鈥檚
magnetic field, which will soon be measured during a Galileo flyby. 鈥淚t鈥檚 going
to be really interesting,鈥� says Burger.