by Io, Jupiter’s third largest moon, is caught in an explosive, pressurized dance.
Orbiting close to Ganymede and Europa, two of Jovian’s other largest moons, and the planet itself, Io’s mineral makeup is constantly pulled and pushed by gravity, creating frictional heat deep within the moon. This makes it extremely volcanically active—hundreds of volcanoes and extensive networks of lava flows dot the surface of Io.
“It squeezes like an exercise ball,” said Jeff Morgenthaler, an astrophysicist at the Planetary Science Institute.
Despite a flurry of spacecraft in recent decades—including the Voyager 1 and Galileo missions—as well as continued observation of Earth, there are long-standing mysteries about the type of volcanic activity on Io and how the moon’s fiery energy interacts with Jupiter and other nearby bodies.
Last year, Morgenthaler, who studies the gases emitted by Io and the gas cloud around Jupiter, noticed signs that a different kind of explosion was happening — more powerful, or more persistent.
“It’s a fascinating observation,” said Ashley Davies, a planetary scientist and volcanologist at NASA’s Jet Propulsion Laboratory, who was not involved in Morgenthaler’s study. “It shows that Io is definitely one of the most energetic bodies in the solar system, and we have no idea what it will look like when we point the telescope at it.”
The observation could help guide future study of Io, including preparations for NASA’s Juno spacecraft, which has been orbiting Jupiter since 2016 and is scheduled to fly within a few hundred kilometers of the moon Jovian in December.
As Io is far from the Sun and has a very thin atmosphere, its surface averages minus 130 °C and is coated with an icy layer of sulfates. Volcanic eruptions there, which come in many different forms and intensities, can reach temperatures of nearly 1,400°C.
When superheat meets supercold, molecules like sulfur dioxide and sodium can be ejected into space. Some of the most explosive eruptions come from fissures in the surface and shoot fountains of lava half a mile into space. The charged molecules create what’s known as a “plasma torus” in Io’s wake: a tire-shaped cloud of ionized gas that gathers in Jupiter’s magnetic field.
It is possible to see Io’s volcanic hot spots directly with infrared telescopes. However, since 2017, Morgenthaler has taken a different approach, focusing on the moon’s plasma through the Io Input/Output Observatory (IoIO) at the Planetary Science Institute in Arizona. Instead of using infrared light, Morgenthaler uses IoIO to block Jupiter’s light and measure the gas around it.
Davis said that while infrared telescopes can tell us where volcanoes are erupting on Io and how powerful they might be, studying the plasma can tell us when an eruption is chemically rich — signaling that it might be more powerful, more persistent. or weirder. An explosion could push more ionized gas into the torus. Someone else could send a lot of neutral gas. “It doesn’t always happen, and it’s an interesting link,” Davies said.
Every year that Morgenthaler monitored volcanic activity using IoIO, he noticed some sort of increase in the concentration, or brightness, of gases in the plasma torus. These changes are associated with volcanic eruptions, the intensity of which can be measured by the levels of sodium emitted by the moon. But from September to December 2022, after a huge volcanic eruption, he noticed that the torus contained much less sulfur dioxide than the size of the eruption. The torus was not as bright as it should have been.
This could mean that the eruption had a different chemical composition than the others, or that different types of minerals were disturbed. It would look like Washington State’s Mount St. Helena, a steep stratovolcano that can erupt explosively, sending earth, rock and sodium into the atmosphere, rather than Hawaii’s Mauna Loa, a shield volcano that erupts with flows of liquid lava. Or it could mean that the torus spread rapidly in response to the intense explosion.
It’s primarily a call for more research, Morgenthaler said. “I’m just waving the flag and saying, ‘This happened,'” Morgenthaler said after the sighting was announced this month.
Studying the anomaly could reveal, in more detail, the different types of volcanoes on Io, as well as the interactions between the plasma torus and other large moons around Jupiter. However, much more data will need to be collected to connect all the pieces together, including from other powerful telescopes on Earth, such as the James Webb Space Telescope, as well as the Juno space probe.
For now, to study Io’s gases, Morgenthaler said his method, which is cheap and can be adapted by small research organizations and even some amateur astronomers, is often underutilized. But their work could open the door to similar, broader research that could provide data to help understand the Jovian system.
Davies said this kind of piecemeal research is essential to understanding Io. “You can think of it like looking at different parts of an elephant,” he said.
The fact that Morgenthaler’s most recent observation was made with widely available instruments opens up the possibility of further studies, both similar and different. “The more tracking we can get the better,” Davies said.
Translated by Luiz Roberto M. Gonçalves
