The north polar region of Jupiter’s volcanic moon Io was captured by NASA’s Juno probe during the spacecraft’s 57th flyby of the gas giant on December 30, 2023. Data from recent flybys is helping scientists understand the interior of Io. Credits: NASA/JPL-Caltech/SwRI/MSSS; Image processing by Gerald Eichstädt
Many worlds in our solar system have oceans, and not all of them are made of water. Io, Jupiter’s innermost moon, was assumed to have one made of basalt and bits of sulfur just beneath its surface, fueling its volcanic eruptions.
But according to a study published in Nature last week, a global magma ocean is not the key to explaining Io’s volcanic activity. Thanks to NASA’s Juno spacecraft and its flybys of the Moon, scientists have discovered that its mantle is mostly solid. Its volatile nature can instead be explained by the occurrence of tidal bending, or friction caused by tidal forces that generates internal heat.
What exists underneath
Io is close enough to Jupiter to be constantly pushed and pulled by tidal forces. These tidal forces push magma to the surface, and these forces were also believed to maintain a large, shallow reservoir of magma bubbling beneath its surface.
However, instead of an ocean, lead study author Ryan Park, a senior research scientist at NASA’s Jet Propulsion Laboratory, suggested that Io more likely has a porous mantle with chambers that allow magma to pass through. “The rocks are partly molten, like under Hawaii or in Iceland on Earth,” says Park. “It’s a bit like a muddy, solid material (ice) with melted water in the cracks in between. The melt eventually escapes to the surface and erupts.”
The study was based on Doppler data from two Juno flybys last year, as well as less sensitive data from one of its predecessors, NASA’s Galileo spacecraft. Essentially, when the spacecraft encountered Io, measurements were taken by gravitational instruments on board the spacecraft. The encounter affected the spacecraft’s acceleration on first approach, during the flyby, and as it left the system. This could be triangulated through measurements between Juno, Io and NASA’s Deep Space Network, a system of telescopes that downlink data from off-world spacecraft.
The Doppler data revealed that Io is not pushed and pulled by gravity in a way that would be consistent with a magma ocean.
Worlds beyond
These measurements represent a good basis for exploring other worlds in our Solar System. Europa, Io’s neighbor moon, is thought to most likely have a global water ocean. The Europa Clipper mission, launched in October and arriving in 2030, could determine whether there is a large body of water beneath the icy crust. “Our result shows that a similar approach/technique (i.e. flybys) could be used to measure Europa’s tidal deformation; therefore confirming that Clipper could answer whether Europa has a global ocean or not,” says Park.
The findings could influence our understanding of other worlds, including the asteroid Vesta. Measurements taken by NASA’s Dawn spacecraft indicated that this body may have once had volcanic activity and was initially believed to have a shallow magma ocean. This study questions this theory.
Juno has made its 66th flyby of Jupiter, but may still provide information about Io in the future.
“No further close flybys of Juno are planned,” Park says. “However, Juno will observe Io from afar, so there will be more information from visible and infrared data that could help better understand Io’s surface processes.”
