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

IO, THE THIRD largest of Jupiter’s moons, is caught in a pressurised, 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, 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 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, 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, Morgenthaler uses IOIO to block light from Jupiter and measure the gas around it.

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 — signalling 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,” Davies said.