Jupiter's
volcanic moon Io is veiled by a thin atmosphere, but how much its volcanoes and
chunks of frozen gas contribute to its atmosphere has puzzled scientists for
decades.
The New
Horizons spacecraft recently documented the moon's glowing aurora, however,
giving researchers a chance to solve the atmospheric mystery.
Io is the most
volcanically active object in the solar system. The moon's pockmarked and
colorful appearance is not unlike a pepperoni pizza.
"Io
is volcanically active, and that volcanism ultimately is the source material
for Io's sulfur-dioxide atmosphere," said Kurt Retherford, a space
scientist at the Southwest Research Institute in San Antonio. "But the
relative contributions of volcanic plumes and sublimation of frosts deposited
near the plumes have remained a question for almost 30 years."
Io's
volcanoes spew out sulfur dioxide, which is a gas that stinks of freshly lit
matches and almost entirely makes up the moon's atmosphere. As Io rotates from
daylight into darkness, chilling the yellowish rock down to -226 F (-143 C),
the gas freezes into a solid, much like dry ice (frozen carbon dioxide gas).
"The
atmosphere at that point collapses down so that all that is left supplying the
atmosphere are the volcanoes," Retherford said.
Because
Io's volcanic gas stays warm enough not to freeze and creates glowing
auroras, scientists were able to find out how much the volcanoes supply
Io's atmosphere by measuring the moon's nightside aurora.
About 1 to
3 percent of Io's dayside atmosphere, it turns out, is created by the
volcanoes. The rest is generated from frozen sulfur dioxide turning directly
into gas which, over eons, has accumulated on Io's surface.
New
Horizon's used its Alice ultraviolet spectrograph to capture images of Io's
auroras on the
spacecraft's way to Pluto, which mission scientists expect to reach in
2015. Retherford and his colleagues' findings based on the Alice data are
detailed in a recent issue of the journal Science.
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