Those of you who know me know that I have a favorite small body in our solar system: Jupiter’s moon Io. It is covered in volcanoes. Its surface is constantly being re-covered by lava. And plate tectonics is not the reason.

Jupiter heats up its moon by pulling on Io gravitationally, the same way the Sun pulls on the Earth, making our home planet more of an oblate spheroid than a perfect sphere. However, Jupiter has some help in the process: 1) Io is tidally locked, so the same side always faces Jupiter, and 2) the outer Galilean moons – particularly Europa and Ganymede – pull on Io in the opposite direction, and they do it regularly because of resonant orbits.

Orbital resonance occurs when two or more bodies orbit a larger body at cycles that regularly overlap. In this case, for every one time Io orbits Jupiter, Europa orbits twice and Ganymede orbits four times. All the time. Every time. So there is a regular back and forth gravitational pull between the outer moons and Jupiter, and this pulling causes Io to expand and contract. That expansion/contraction cycle causes friction, and friction causes tidal heating.

All that heat allows Io to have a layer of liquid magma, and all the expansion and contraction make lots of easy places for volcanic eruptions. And voila, the most geologically active body in our solar system is a moon!

Of course, with volcanoes comes sulfur dioxide gas, and the condensate of that gas is what makes Io’s surface so very yellow and orange and, frankly, looking like pizza. Keeping all this bright condensate on the surface is Io’s thin atmosphere, which is, as expected, full of sulfuric gas. And scientists have wondered how much of that gas came directly from the volcanoes and how much came from sublimation due to sunlight.

All of which leads me to today’s first news story. (Did I mention I love Io?) In a new paper, published in The Planetary Science Journal with lead author Imke de Pater, researchers used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to take pictures of Io while it was both eclipsed by Jupiter from the Sun and when it was back in direct sunlight. As Statia Luszcz-Cook explained: 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. During that time we can only see volcanically-sourced sulfur dioxide. We can therefore see exactly how much of the atmosphere is impacted by volcanic activity.

Based on the images, which can be seen in a video on Daily Space, the team calculated that the active volcanoes directly produce 30-50 percent of Io’s atmosphere. That’s a lot. All this science leads up to solving what makes the tidal heating process tick, so to speak. As Luszcz-Cook said: By studying Io’s atmosphere and volcanic activity we learn more about not only the volcanoes themselves, but also the tidal heating process and Io’s interior.

Next up, the team hopes to use ALMA to get even more images of Io’s surface in an attempt to measure the temperature of the moon’s lower atmosphere. When that work is done and published, you know I will bring the story to you.

More Information

ALMA press release 

UC Berkeley press release 

“ALMA Observations of Io Going Into and Coming Out of Eclipse,” Imke de Pater, Statia Luszcz-Cook et al., 2020 Oct. 21, Planetary Science Journal (preprint on arxiv.org)

“High Spatial and Spectral Resolution Observations of the Forbidden 1.707-Micron Rovibronic SO Emissions on Io: Evidence for Widespread Stealth Volcanism,” Imke de Pater et al., 2020 July 20, Planetary Science Journal