'Atmosphere on Jupiter's moon Io collapses, repairs everyday'

Jupiter's volcanic moon Io has a thin atmosphere that freezes in the shadow of the giant planet during daily eclipses, a new study has found for the first time.
"This research is the first time scientists have observed this remarkable phenomenon directly, improving our understanding of this geologically active moon," said Constantine Tsang, a scientist at the Southwest Research Institute in the US.
Io is the most volcanically active object in the solar system. The volcanoes are caused by tidal heating, the result of gravitational forces from Jupiter and other moons.
These forces result in geological activity, most notably volcanoes that emit umbrella-like plumes of sulphur dioxide gas that can extend up to 480 kilometres above Io and produce extensive basaltic lava fields.

The new study documents atmospheric changes on Io as the giant planet casts its shadow over the moon's surface during daily eclipses.
Io's thin atmosphere, which consists primarily of sulphur dioxide (SO2) gas emitted from volcanoes, collapses as the SO2 freezes onto the surface as ice when Io is shaded by Jupiter, then is restored when the ice warms and sublimes (transforms from solid back to gas) when the moon moves out of eclipse back into sunlight.

The study used the large eight-metre Gemini North telescope in Hawaii and an instrument called the Texas Echelon Cross Echelle Spectrograph (TEXES).
Data showed that Io's atmosphere begins to "deflate" when the temperatures drop from minus 148 degrees Celsius in sunlight to minus 167 degrees Celsius during eclipse.

Eclipse occurs two hours of every Io day (1.7 Earth days). In full eclipse, the atmosphere effectively collapses, as most of the sulphur dioxide gas settles as frost on the moon's surface. The atmosphere redevelops as the surface warms once the moon returns to full sunlight.
"This confirms that Io's atmosphere is in a constant state of collapse and repair, and shows that a large fraction of the atmosphere is supported by sublimation of SO2 ice," said John Spencer, also from the Southwest Research Institute.
"Though Io's hyperactive volcanoes are the ultimate source of the SO2, sunlight controls the atmospheric pressure on a daily basis by controlling the temperature of the ice on the surface," said Spencer.

Prior to the study, no direct observations of Io's atmosphere in eclipse had been possible because its atmosphere is difficult to observe in the darkness of Jupiter's shadow.
This breakthrough was possible because TEXES measures the atmosphere using heat radiation, not sunlight, and the giant Gemini telescope can sense the faint heat signature of Io's collapsing atmosphere, researchers said.
The study was published in the Journal of Geophysical Research.