Jupiter’s moons: a magma ocean inside Io

With the JIRAM infrared camera, the Jovian Infrared Auroral Mapper on board the Jupiter Juno probe, the volcanoes of the large innermost moon Io can be observed and analyzed, in addition to the northern lights on the gas giant. A team led by Ashley Gerard Davies of NASA’s Jet Propulsion Laboratory in Pasadena, California, has now used image data from several Juno flybys of Io to complete the map of active volcanoes in the area of ​​the moon’s polar regions, which previously it had hardly been photographed by previous space probes. The group published their results in the journal Nature Astronomy. As the analysis showed, there are currently 266 hot to hot spots on Io, which are more or less homogeneously distributed across the entire lunar surface. There are slightly fewer volcanoes at the poles, especially at the South Pole. This may be due to differences in the structure of the rocky crust. All volcanoes are believed to be fed by an ocean of magma just beneath the solid crust, about 40 kilometers thick.

This means that, in contrast to Earth, Io is molten, at least in the upper part of its rocky mantle. On Earth, the mantle is solid, but can deform plastically. Only in the boundary layer between the crust, more precisely the lithosphere, and the Earth’s mantle is there a maximum of one to two percent melting in a solid rock in the asthenosphere. On Io, however, the asthenosphere has largely melted.

For the investigations, the group used image data from the Juno probe, which orbits Jupiter from 2016 to 2022. The spatial resolution of the images is between 151 and 20 kilometers per pixel. The images were captured with JIRAM at a wavelength of 4.8 micrometers. The 266 hot to hot spots have temperatures between -70 and +1160 degrees Celsius. Io’s inactive surface has an average temperature of -140 degrees Celsius.

Using the image data, Davies’ team created a global map of Io, on which all active areas are recorded. The heat flow emanating from them can vary by a factor of 10,000, from the faintest, barely noticeable hot spots to highly active volcanoes. The coldest hot spots release about 0.01 gigawatts per micrometer of heat, while the hottest ones release up to 100 gigawatts per micrometer. In general, Io emits about ten times more heat from its interior than the much larger and more massive Earth.

This enormous heat flow can be attributed to tidal friction. Io Jupiter, like Earth’s moon, always faces the same side as our planet, so it rotates in a fixed way. This means that the duration of an orbit of around 42 hours corresponds exactly to the rotation around its own axis. However, this only applies to an undisturbed system. Due to the gravity of the neighboring moons Europa and Ganymede, Io is repeatedly deflected slightly from its resting position, so that the tidal mountains created by Jupiter’s strong gravity move back and forth a little. They are up to 100 meters high on solid rock. For comparison: the tidal mountains on solid Earth created by the Sun and Moon are about half a meter high. Because the orbital periods of Europa and Ganymede are in integer proportion to the orbital period of Io, this moon is literally cycled through during each orbit, which creates enormous amounts of heat through friction in the rock.

At the turn of the year and at the beginning of February 2024, the Juno space probe will come particularly close to the moon Io. During flybys, the probe ventures up to around 1,500 kilometers from the surface. JIRAM and the JunoCam camera will capture numerous sharp images, providing even more information about volcanic events on Io.