Scientists take a closer look at Io's collapsible atmosphere

The volcanic moon's atmosphere temporarily freezes and collapses as it passes through Jupiter’s shadow, a finding that ends a long-running debate.

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

While Earthlings await data from the Juno mission's historic first lap of Jupiter, researchers have offered up some intriguing information about the gas giant's volcanic moon, Io: It has a collapsible atmosphere.

The peculiar phenomenon occurs when Io's orbit takes the moon behind Jupiter, which eclipses the sun, causing Io's atmosphere to rapidly freeze, a team of researchers led by the Southwest Research Institute (SwRI) in San Antonio, Texas, report in a paper published Tuesday in the Journal of Geophysical Research. About two hours (or 1.7 Earth days) later, when it re-enters the sun’s view, the frozen sulfur dioxide sublimates, or immediately returns to its gaseous state.

Scientists have speculated before that such a process might occur on Io, but the idea had been fiercely debated until now, Alfred McEwen, a professor of planetary science at the University of Arizona, tells The Christian Science Monitor in a phone interview. The findings discussed in Tuesday's paper help clear the air, so to speak.

“It’s always good to have a direct observation of something rather than a lot of confusing observations,” says Dr. McEwen.

The majority of Io's thin atmosphere is comprised of sulfur dioxide given off by the moon's hundreds of volcanoes, which can emit plumes of gas up to 300 miles high. As Io passes into Jupiter's shadow, that SO2  condenses on the surface of the moon as frost, causing the atmosphere to collapse until sunlight can once again sublimate the frost back into a gas.

"These observations provide the first direct evidence that Io's primary molecular SO2  atmosphere collapses in eclipse, with SO2  condensing on the surface as SO2  frost," the researchers write.

The new research "confirms that Io’s atmosphere is in a constant state of collapse and repair," John Spencer, an SwRI scientist who participated in the study, said a press release.

"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," Dr. Spencer said.

The discovery is simple, but significant, Mikhail Zolotov, a research professor at the School of Earth and Space Exploration at Arizona State University, tells the Monitor in a phone interview.

Dr. Zolotov calls the process a balancing act between the composition of the atmosphere, the volume of gasses the moon's volcanoes emit, and the amount that the atmospheric temperature drops during eclipse. 

Another example of a space object that undergoes sublimation is Pluto, whose atmospheric pressure also depends on its illumination by the sun, because its elliptical orbit brings it closer to the sun at times and farther at others, Zolotov says.

Researchers were able to verify their suspicions by using new tools: the Texas Echelon Cross Echelle Spectrograph (TEXES), which measures the atmosphere using heat radiation instead of sunlight, and the Gemini telescope, “which was able to sense the faint heat signature of Io’s collapsing atmosphere,” according to the press release. Io’s atmosphere had previously been difficult to observe during the dark hours of eclipse.

Learning more about Io may illuminate more about Jupiter, scientists say. “Io spews out gases that eventually fill the Jupiter system, ultimately seeding some of the auroral features seen at Jupiter’s poles,” SwRI’s Constantine Tsang said in the press release. “Understanding how these emissions from Io are controlled will help paint a better picture of the Jupiter system.”

Observing planets like Jupiter and its moons could help scientists understand how gas giants formed and the solar system took shape, as well. Because Jupiter has a similar composition to the sun, mostly hydrogen and helium, leading scientists suggest it was born early in the development of our solar system.

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