Free Astronomy Magazine January-February 2021

23 JANUARY-FEBRUARY 2021 ASTRO PUBLISHING nia, Berkeley. “Is it volcanic activity, or gas that has sublimated (transitioned from solid to gaseous state) from the icy surface when Io is in sunlight?“ To distin- guish between the differ- ent processes that give rise to Io’s atmosphere, a team of astronomers used ALMA to make snapshots of the moon when it passed in and out of Jupiter’s shadow (they call this an “eclipse”). “When Io passes into Jupiter’s shadow, and is out of direct sunlight, it is too cold for sulfur dioxide gas, and it condenses onto Io’s surface. Dur- ing that time we can only see vol- canically-sourced sulfur dioxide. We can therefore see exactly how much of the atmosphere is impacted by volcanic activity,” explained Statia Luszcz-Cook from Columbia Univer- sity, New York. Thanks to ALMA’s exquisite resolution and sensitivity, the astronomers could, for the first time, clearly see the plumes of sul- fur dioxide (SO 2 ) and sulfur monox- ide (SO) rise up from the volcanoes. Based on the snapshots, they calcu- lated that active volcanoes directly produce 30-50 percent of Io’s at- mosphere. The ALMA images also showed a third gas coming out of volcanoes: potassium chloride (KCl). “We see KCl in volcanic regions where we do not see SO 2 or SO,” said Luszcz- Cook. “This is strong evidence that the magma reservoirs are different under different volcanoes.” Io is volcanically active due to a process called tidal heating. Io or- bits Jupiter in an orbit that is not quite circular and, like our Moon al- ways faces the same side of Earth, so does the same side of Io always face Jupiter. The gravitational pull of Jupiter’s other moons Europa and Ganymede causes tremendous amounts of internal friction and heat, giving rise to volcanoes such as Loki Patera, which spans more than 200 kilometers (124 miles) across. “By studying Io’s atmos- phere and volcanic activity we learn more about not only the vol- canoes themselves, but also the tidal heating process and Io’s inte- rior,” added Luszcz-Cook. A big unknown remains the tem- perature in Io’s lower atmosphere. In future research, the astronomers hope to measure this with ALMA. “To measure the temperature of Io’s atmosphere, we need to obtain a higher resolution in our observa- tions, which requires that we ob- serve the moon for a longer period of time. We can only do this when Io is in sunlight since it does not spend much time in eclipse,” said de Pater. “During such an observa- tion, Io will rotate by tens of de- grees. We will need to apply soft- ware that helps us make un- smeared images. We have done this previously with radio images of Jupiter made with ALMA and the Very Large Array (VLA).” T his video shows images of Jupiter’s moon Io in radio (made with ALMA), and optical light (made with Voyager 1 and Galileo missions). The ALMA images were taken when Io passed into Jupiter’s shadow in March 2018 (eclipse), and from Jupiter’s shadow into the sunlight in September 2018. These radio images for the first time show plumes of sulfur dioxide (in yellow) rise up from the volcanoes on Io. [ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Dagnello; NASA] !