From New Horizons to JUNO: A look at Io, 16 years later

On March 1, 2023, JUNO, the spacecraft born from a collaboration between NASA and ASI, flew over Io, the Solar System’s most volcanic moon, at its 49th perijove and observed new details of its surface.

*JUNO’s photo montage from 49th perijove. Credits: NASA/JPL–Caltech/SwRI/MSSS/Bjorn Jonsson*

At a distance of just 51,000 kilometers from the surface, the JUNO spacecraft was able to take a high-resolution image that allowed scientists to observe what was happening on the surface of the moon, a full 16 years after the last image was taken by the New Horizons spacecraft, which was on its way to Pluto.

JUNO: An eye on Jovian’ system

The JUNO spacecraft is part of NASA’s JUNO mission, which aims to study Jupiter’s magnetic field from a polar orbit.

JUNO left Earth aboard an Atlas 5 rocket from Cape Canaveral on 5 August 2011 and arrived at its destination about five years later. In 7 years, JUNO has contributed to a significant increase in knowledge of the Jovian system, in particular by measuring Jupiter’s composition, its gravitational field and, in particular, its magnetosphere.

Last but not least, it has contributed to finding clues about the formation of the planet itself, inferring whether or not it contains a rocky core in its interior, whether or not there is water in the clouds of the deep atmosphere, and how strong the winds are that blow below the upper atmosphere. These objectives were pursued thanks to the presence on the probe of no less than 29 sensors, including radiometers, magnetometers and spectrographs.

If all goes according to plan, the JUNO probe will remain operational until 2025, when it will disintegrate in the upper atmosphere of Jupiter.

*Artist rendition of JUNO orbiter at Jupiter. Credits: NASA/JPL*

Io: The most volcanic moon in the Solar System

Io is a natural satellite of Jupiter, and along with Europa, Ganymede and Callisto, belongs to a group called the “Medicean satellites” in honour of their discoverer, Galileo Galilei.

Io is the closest moon to the gas giant and holds the world record for the number of active surface volcanoes.

The intense volcanic activity on the moon is no mystery, but is mainly due to its proximity to Jupiter and its orbital resonance with Europa and Ganymede. During its orbit, Io “shapes” Jupiter’s magnetic field by acting as a veritable 3 million ampere electric generator, releasing ions that further amplify and strengthen the planet’s own magnetic field.

Resonance with nearby moons means that the heat flux generated by the interaction with Jupiter remains trapped within the moon, which has since experienced a circularisation of its orbit. Friction therefore causes Io’s inner core to heat up considerably, leading to radioactive decay and melting of the moon’s mantle, generating an intense heat flux that rises to the surface and escapes through volcanic cones.

*Io, the most volcanic moon of the Solar System with Jupiter in background. Credits: Nicholas Hylton*

From New Horizons to JUNO: What happened in these year on Io?

On January 19, 2006 New Horizons, NASA’s probe to study Pluto, lifted off from Cape Canaveral aboard an Atlas V rocket.

Before taking advantage of Jupiter’s gravitational assist, scientists took advantage of the situation to study the gas giant. Unfortunately, New Horizons was unable to study the four Medicean satellites in detail due to their awkward positions, but an image taken by the LORRI camera showed a plume of lava rising hundreds of kilometres from the surface. As the lava flow falls back to the surface, it expands to a diameter of about 1,000 kilometres, forming a beautiful red ring around the volcanic cone.

In images taken 16 years after New Horizons, volcanic cones, calderas and rivers of lava and ash can still be seen, albeit at a distance of 51,000 kilometres. There are no major surface changes, explained Jason Perry, a planetary imaging scientist, but three details deserve special attention.

The first detail concerns East Girru, a hotspot first observed by New Horizons, where a fairly extensive surface lava flow can be seen, like in the next image taken by Galileo’s probe.

An active volcanic eruption on Jupiter's moon Io, captured by NASA's Galileo spacecraft. Credits: NASA/JPL — *An active volcanic eruption on Jupiter’s moon Io, captured by NASA’s Galileo spacecraft. Credits: NASA/JPL*

The second detail concerns the Chors Patera volcanic crater, which is redder than ever. This marked change in colour is due to the presence of S3-S4, short-chain sulphur, which is regularly replenished by high-temperature volcanic phenomena.

Third, but no less important, is the discovery of no less than 23 never-before-seen hot spots, bringing the total number of known active cones to 242.

But the story does not end there! JUNO has already orbited Jupiter 49 times, and the flyby of Io a week ago was certainly not the last: in fact, 6 more flybys of the moon are planned, but the most important of all will be on February 3 2024, when JUNO will pass just 1,500 kilometers from the surface of the moon, surely revealing never-before-seen details and, it is hoped, even breathtaking images.