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A New View of the Most Explosive Moon in the Solar System

Io, the third largest of Jupiter’s moons, is caught in a pressurized, explosive dance.

Orbiting near Ganymede and Europa, two of the other largest Jovian moons, and the planet itself, Io’s mineral composition is constantly pulled and pushed by gravity, creating frictional heat deep inside the moon. This makes it extremely volcanically active — there are hundreds of volcanoes and extensive networks of lava flows marking Io’s surface.

“It’s being squeezed like an anger ball,” said Jeff Morgenthaler, an astrophysicist at the Planetary Science Institute.

Despite a number of close-flying spacecrafts over the past few decades — including the Voyager 1 and Galileo missions — as well as constant observation from Earth, there are lasting mysteries about the kind of volcanic activity on Io and how the moon’s fiery energy interacts with Jupiter and other nearby bodies.

Last year, Dr. Morgenthaler, who studies gases Io emits and the cloud said gases create around Jupiter, picked up signs that a different kind of eruption — a more powerful or more persistent one — was occurring.

“It’s an exciting observation,” said Ashley Davies, a planetary scientist and volcanologist at NASA’s Jet Propulsion Laboratory who was not involved in Dr. Morgenthaler’s study. “It’s showing that Io is certainly one of the most energetic bodies in the solar system, and you have no idea how it’s going to appear when you turn your telescope on it.”

The observation could help to guide future study of Io, including preparations for NASA’s Juno space probe, which has been orbiting Jupiter since 2016 and is scheduled to fly only a couple hundred miles from the Jovian moon this December.

Because Io is far from the sun and has a very thin atmosphere, its surface, on average, sits at around minus 200 degrees Fahrenheit, and it is coated in a frosty layer of sulfuric compounds. Volcanic eruptions there, which come in many different forms and intensities, can reach temperatures up to 2,500 degrees Fahrenheit. When super hot meets super cold, molecules like sulfur dioxide and sodium can be shot into space. Some of the most explosive eruptions come from fissures in the surface and throw fountains of lava half a mile into space. The charged molecules create what is known as a “plasma torus” in Io’s wake: a doughnut-shaped cloud of ionized gas that collects in Jupiter’s magnetic field.

It is possible to look directly at Io’s volcanic hot spots with infrared telescopes. However, since 2017, Dr. Morgenthaler has taken a different approach, focusing on the moon’s plasma torus through the Planetary Science Institute’s Io Input/Output observatory (IoIO), in Arizona. Instead of using infrared light, Dr. Morgenthaler uses IoIO to block light from Jupiter and measure the gas around it.

Io seen by Juno from a distance of about 40,000 miles on Dec. 14.Credit…NASA/JPL-Caltech/SwRI/MSSS
An infrared image of Io’s southern hemisphere collected by the Juno spacecraft on Dec. 16, 2017, at a distance of about 290,000 miles. The brighter the color, the higher the temperature recorded.Credit…NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

Dr. Davies said that while infrared telescopes can tell us where volcanoes are erupting on Io and how powerful they may be, studying the plasma torus can tell us when an eruption is chemically rich — signaling that it may be more powerful, more persistent or just more peculiar. One eruption could push more ionized gas into the torus. Another could send out a lot of neutral gas. “It doesn’t happen every time, and it’s an interesting link,” Dr. Davies said.

Each year Dr. Morgenthaler has monitored volcanic activity through IoIO, he has noticed some kind of increased concentration, or brightening, of gases in the plasma torus. These changes correlate with volcanic outbursts, the intensities of which can be measured by the levels of sodium emitted from the moon. But, in September through December 2022, after a large volcanic outburst, he noticed that the torus contained much less sulfur dioxide than the size of the eruption would suggest. The torus wasn’t as bright as it should have been.

This could mean that the eruption had a different chemical composition from the others, or that different kinds of minerals had been disturbed. It would be like Mount St. Helens, a steep stratovolcano that can erupt explosively, sending dirt, rock and sodium into the atmosphere, erupting on Earth, rather than Mauna Loa, a gently-sloped shield volcano that erupts with liquid lava flows.Or it could mean that the torus was rapidly diffusing in response to the intense eruption.

More than anything, Dr. Morgenthaler said, it is a call for more research.

“I’m just raising the flag, and saying, ‘This has happened,’” Dr. Morgenthaler said after announcing the observation this month.

Studying the anomaly might draw out, in better detail, the different kinds of volcanoes on Io, as well as the interactions between the plasma torus and other massive moons around Jupiter. However, much more data will have to be gathered to put all the pieces together, including from other powerful telescopes on Earth, like the James Webb Space Telescope as well as from the Juno space probe.

For the moment, to study gases from Io, Dr. Morgenthaler said that his method, which is cheap and could be adapted by small research organizations and even some backyard astronomers, is often underutilized. But his work may open the door for similar and widespread research that could provide data to help understand the Jovian system.

Dr. Davies said that this kind of piecemeal research is integral to understanding Io. “You can think of it like looking at different parts of an elephant,” he said.

The fact that Dr. Morgenthaler’s most recent observation was made with largely accessible instruments opens the possibility of more studies, similar and different, in kind. “The more monitoring we can get, the better it will be,” said Dr. Davies.

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