Plenty of worlds in our solar system have oceans — and not all of them are made of water. Io, Jupiter’s innermost moon, was purported to have one made of basalt and bits of sulfur just under its surface, powering 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 close flybys of the moon, scientists have found that its mantle is mostly solid. Its volatile nature can instead be explained by the occurrence of tidal flexing — friction from tidal forces that generates internal heat.
What exists below
Io is close enough to Jupiter that it is constantly pushed and pulled by tidal forces. These tidal forces pull magma up to the surface, and it was believed that these forces also kept a large, shallow reservoir of magma bubbling under its surface.
However, instead, of an ocean, the study’s lead author Ryan Park, a senior research scientist at NASA’s Jet Propulsion Laboratory, suggested Io more likely has a porous mantle with chambers that allows magma through. “The rocks are partly molten, similar to under Hawaii or Iceland on Earth,” Park says. “It’s kind of like a slushy — solid material (ice) with melt (water) in the cracks in between. The melt eventually escapes to the surface and erupts.”
The study relied 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, as the spacecraft encountered Io, measurements were made by gravitational instruments aboard the spacecraft. The encounter affected the acceleration of the spacecraft on first approach, during the flyby, and as it left the system. This could be triangulated by 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 are a good groundwork for exploring other worlds of our Solar System. Europa, Io’s neighboring moon, is considered very likely to have a global water ocean. The Europa Clipper mission, which launched in October and will arrive in 2030, could determine whether there is a large body of water under its icy crust.“Our result shows that a similar (i.e., flybys) approach/technique could be used to measure Europa’s tidal deformation; thus, confirming that Clipper could answer whether Europa has a global ocean or not,” Park says.
The findings could affect our understanding of other worlds, including the asteroid Vesta. Measurements from the NASA’s Dawn spacecraft indicated this body may have once had volcanism, and was initially believed to have had a shallow magma ocean. This study casts doubt on that theory.
Juno has moved on to perform its 66th flyby of Jupiter but it still might provide information about Io in the future.
“There are no additional close flybys planned form Juno,” Park says. “However, Juno will be observing Io from distance, so there will be more information from visible and infrared data that could help better understanding Io’s surface processes.”
