Io Outburst Shines in Plasma Torus

Between September and December 2022, a small telescope in the Arizona desert observed a brightening of the plasma torus caused by Io’s charged volcanic particles.

IMAGE: The volcano-laced surface of Jupiter’s moon Io was captured in infrared by the Juno spacecraft’s Jovian Infrared Auroral Mapper (JIRAM) imager as it flew by at a distance of about 80,000 kilometers on July 5, 2022. Brighter spots indicate higher temperatures in this image. CREDIT: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

Io is incredibly volcanic and is often referred to as the ‘most volcanic body in the solar system’, which may not be true and is still debated, but it sure sounds cool. Or hot. Something neat. No matter the answer to that debate, Io has a lot of volcanos — over 400 of them; however, they are not caused by plate tectonics like volcanos are on Earth. Due to the tidal heating caused by Jupiter as well as the other Galilean moons, Io’s interior stays hot enough to create magma and gets fractures by the constant cooling and heating for that magma to breach the surface. All those eruptions on Io send particles into space, and those particles have caused several different observed phenomena.

Back in 2021, researchers led by George Clark analyzed data from NASA’s Juno spacecraft and found that Io is, in fact, a particle accelerator for Jupiter. All those volcanic particles start out by forming an exceedingly thin atmosphere around the tiny moon; it’s so thin that it’s technically an exosphere and not an atmosphere! From there, the particles interact with charged particles surrounding Jupiter and end up charged themselves and then swept into the powerful magnetic field lines. Basically, an electric circuit is formed between Io and Jupiter, stretching over 400,000 kilometers.

IMAGE: A close-up image of Jupiter’s aurora, taken in ultraviolet light by the Hubble Space Telescope in 1998. Peripheral to Jupiter’s main aurora are the auroral spots or “footprints” where electric currents from Jupiter’s moons Io, Europa, and Ganymede rain electrons down onto the planet’s upper atmosphere. CREDIT: Modified from NASA/STSci/AURA

And where that circuit interacts with Jupiter’s atmosphere, the particles actually create an auroral footprint that has been seen in ultraviolet images taken by the Hubble Space Telescope in 1998. When Juno passed close to that region during its twelfth flyby (or perijove), an instrument called the Jupiter Energetic Particle Detector Instrument (JEDI) managed to detect protons that were zooming back from Jupiter to Io at about 50 million kilometers per hour. Clark noted in the press release: It’s the first time we’ve ever seen this type of event with a planet interacting with one of its active moons.

Here’s where it gets even more fun. In that same press release, co-author Jamey Szalay commented: These observations have revealed a whole new line of research on the complexity of Io’s interaction with Jupiter and the auroral features it creates. These protons are in a sense tracers that allow us to better understand how waves can interact with charged particles.

That’s exactly what happened. Enter the Planetary Science Institute's Io Input/Output observatory (IoIO), a 14” Celestron telescope with a coronagraph, located near Benson, Arizona, that has been monitoring Io since 2017. The coronograph dims the light from Jupiter, allowing the telescope to image faint gases around the giant planet. And between September and December of 2022, IoIO observed a brightening of two gases — sodium and ionized sulfur. The latter forms a ‘donut-like structure’ or torus that surrounds Jupiter. The Io plasma torus, however, did not brighten as much as in previous outbursts. Scientist Jeff Morgenthaler, who programs and operates the telescope, explains: This could be telling us something about the composition of the volcanic activity that produced the outburst or it could be telling us that the torus is more efficient at ridding itself of material when more material is thrown into it.

IMAGE: IoIO image of Jovian sodium nebula in outburst. CREDIT: Jeff Morgenthaler, PSI

Morgenthaler hopes that as the end of this outburst was coincidentally close to December’s flyby of Io by Juno, instruments onboard the spacecraft can provide us with more information about the composition of the particles in this outburst.

Additionally, these particles can cause changes to the overall plasma environment around Jupiter, and for sensitive instruments such as those on Juno as well as ESA’s upcoming JUICE mission, those changes could disrupt or even damage the instruments. In fact, Io is the only Galilean moon the JUICE mission isn’t going to visit because of the risk to its instrumentation.

That doesn’t mean observing Io is out of the question. In addition to last month’s flyby, Juno is scheduled to make another pass by the volcanic moon in December 2023. JWST will likely list Io among the upcoming targets at some point. The telescopes at the Keck Observatory have imaged Io’s volcanic eruptions from their mountain peak. And we’ll always have that little telescope in the Arizona desert, patiently watching and collecting data on my favorite moon.