Jupiter’s sixty-seven moons are a constant source of study for astronomers. Many of these moons are tiny and travel on distant, irregular orbits, while four of the moons are closer to Jupiter and circle the gas giant in regular orbits. These four “Galilean moons,” named after the 17th century astronomer who discovered them, are among the largest objects in our solar system next to the Sun and eight planets (sorry, Pluto).
One of these Galilean moons, Io, has been studied extensively due to its incredibly high rate of volcanic activity; in fact, it’s believed that the rocky sulfur-covered moon is the most volcanically-active object in our solar system. Now, researchers at the Texas-based Southwest Research Institute (SwRI) have discovered a strange occurrence in Io’s atmosphere, and have published their findings in the Journal of Geophysical Research.
According to a SwRI press release, Io’s atmosphere of volcanic sulfur dioxide (SO2) gas undergoes a regular pattern of disappearing and reappearing. During the course of its orbit around Jupiter, Io is subjected to long periods of shade when Jupiter passes between Io and the Sun. During these periods, the atmosphere cools to the point where the sulfur dioxide freezes and crystallizes as ice on the moon’s surface. As Io comes out of its eclipse and back into the warmth of the Sun’s radiation, the frozen SO2 thaws and sublimates back into the atmosphere.
John Spencer, astronomer at SwRI, stated in SwRI’s press release that these newest findings confirm long-standing theories about Io’s atmosphere:
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. 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.
SwRI used the massive Gemini North telescope in Hawaii and the Texas Echelon Cross Echelle Spectrograph (TEXES) to make this discovery. Since Io spends long periods of time hidden behind Jupiter, conventional telescopes have been unable to observe these changes in Io’s atmosphere; luckily, the TEXES spectrograph enables astronomers to measure atmospheric changes using radiation rather than sunlight.
This announcement comes just weeks after NASA’s Juno spacecraft entered Jupiter's orbit on July 4; space-watchers should expect further Jupiter findings to be announced in the weeks to come.