Jupiter's moons are probably some of the most interesting worlds in the solar system. Two of the largest moons, Europa and Ganymede, likely contain subsurface oceans, providing tempting spots to go looking for life. But while potentially habitable waters may be especially intriguing to us earthlings, Jupiter's innermost moon Io is nothing to sneeze at, either.
It's a world practically bursting with volcanic activity that has unique interactions with its host planet. Scientists think that these eruptions, which are the result of the intense gravitational energy Jupiter exerts on its moon, might help create the extravagant auroral light displays detected on Jupiter. And a new study shows another unique connection between Io and Jupiter: Io's atmosphere collapses when it passes into Jupiter's shadow, and scientists saw it for the first time as it happened. They published their results in the Journal of Geophysical Research: Planets.
Scientists already knew that Io's grip on its atmosphere was tenuous at best, and they previously suspected that it might frequently be lost to space and be re-created by volcanic activity. The mostly sulfur-dioxide atmosphere has a surface pressure just one billionth of the one on Earth at sea level.
Every time Jupiter passes between Io and the sun, casting the moon into cold darkness -- from -235 degrees Fahrenheit in sunlight to -270 degrees in shadow, to be precise -- that sulfur-dioxide gas cools and drops down to the surface, freezing into a solid. That means the atmosphere is collapsed and trapped as surface ice for about two hours every day on Io, where the days are 42 hours long.
And after that two hours, the atmosphere is reborn: The relative warmth of the sun casts itself upon Io's surface once again, causing the newly formed frost to sublimate, or heat from a solid directly back into a gas.
"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," John Spencer of the Southwest Research Institute, one of the study's co-authors, said in a statement, using the scientific term for sulfur dioxide. "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."
NASA recently inserted its Juno probe into Jupiter's orbit, but the latest findings don't come from that spacecraft, which won't start sending data in earnest until later this month. Scientists used the 8-meter Gemini North telescope in Hawaii and the Texas Echelon Cross Echelle Spectrograph to confirm the atmosphere collapse.
The fact that the phenomenon occurs when Io is shrouded in darkness made it difficult to study with most telescopes, so scientists had to look for changes in the atmosphere's heat radiation signature instead.
A Section on 08/07/2016