Jupiter’s volcanically active satellite Io has a thin atmosphere that tends to fluctuate as the largest planet in the Solar System casts its shadow over the moon’s surface during daily eclipses, a new study, funded by NASA, has revealed.

The study, published Tuesday in the Journal of Geophysical Research, concluded that Io’s atmosphere collapses as sulfur dioxide — Io’ atmosphere consists primarily of this gas — emitted from the moon’s volcanoes freezes onto the surface as ice when Jupiter’s shadow falls on it. The atmosphere is restored when the moon moves out of its daily eclipse, causing the ice to warm and transform back to gas.

“This research is the first time scientists have observed this phenomenon directly, improving our understanding of this geologically active moon,” Constantine Tsang, a scientist at the Southwest Research Institute (SwRI) in Boulder, Colorado, said in a statement.

Jupiter-Io-Concept An artist’s rendering depicts the atmosphere on Io, Jupiter’s volcanic moon, as it collapses during daily eclipses. Photo: Southwest Research Institute

The volcanoes on Io, the most volcanically active object in the solar system, are caused by tidal heating caused by gravitational forces from Jupiter and other moons. The volcanoes on the moon emit umbrella-like plumes of sulfur dioxide gas that can extend up to 300 miles above Io, producing extensive basaltic lava fields that can flow for hundreds of miles, researchers said.

As part of the study, the researchers used the large eight-meter Gemini North telescope in Hawaii and an instrument called the Texas Echelon Cross Echelle Spectrograph (TEXES). The data collected from these two instruments showed that Io’s atmosphere begins to “deflate” once the temperatures drop from -235 degrees Fahrenheit in sunlight to -270 degrees Fahrenheit during eclipse, which occurs for two hours of every Io day (1.7 Earth days).

“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, a SwRI scientist, who also participated in the study. “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. We’ve long suspected this, but can finally watch it happen.”

The researchers observed Io over two nights in November 2013, when the moon was more than 420 million miles from Earth. On both occasions, Io moved in and out of Jupiter’s shadow for nearly 40 minutes before and after eclipse.