On July 20, 2021, NASA’s Juno spacecraft performed a flyby of Jupiter’s (and solar system’s) largest moon, Ganymede. This narrow pass was conducted as part of the gas giant’s thirty-fourth orbit (Perijove 34), during which the probe penetrated to 50,109 km (31,136 miles) from the lunar surface. The mission team took this opportunity to use Juno’s Jovian Infrared Auroral Mapper (JIRAM) to capture images of Ganymedes.
These were combined with images captured during two previous flybys to create a new infrared map of the surface of Ganymede, released in honor of the mission’s tenth anniversary (launched from Earth on August 5, 2011). This map and the JIRAM instrument could provide new information about Ganymede’s icy shell and the composition of his inner ocean that could tell whether or not it could support life.
The JIRAM instrument is designed to detect infrared light emerging from inside Jupiter and characterizing the atmospheric dynamics at a depth of 50 to 70 km (30 to 45 miles) below the cloud peaks of Jupiter. However, the instrument can also be used to study Jupiter’s largest moons Io, Europa, Ganymede and Callisto – collectively known as the Galilean moons in honor of their discoverer (Galileo Galilee).
As Scott Bolton, Junos Principal Investigator at Southwest Research Institute (SwRI) stated in a NASA press release:
“Ganymede is larger than the planet Mercury, but pretty much everything we explore on this mission to Jupiter is on a monumental scale. The infrared and other data Juno collected during the flyby provide basic clues to understanding the evolution of Jupiter’s 79 moons from the time they were formed to the present day. “
The previous two flybys were on June 7, 2021 and December 26, 2019, when the orbiter reached 50,109 km (31,136 miles) and 1,046 km (650 miles), respectively. The observation geometries provided enabled the JIRAM instrument to see the northern polar region of Ganymede for the first time and to compare the diversity of composition between the low and high latitudes.
In particular, the Juno probe was able to visualize the dramatic effect of charged particles (plasma) from Jupiter’s magnetic field on Ganymede’s surface. Ganymede is unique among the moons of the solar system in that it is the only satellite that has its own magnetic field. On earth, the presence of a magnetic field causes charged particles from the sun to enter the earth’s atmosphere around the poles.
These particles then interact with gas molecules in the Earth’s upper atmosphere, resulting in aurora activity – Aurora Borealis in the northern hemisphere and Aurora Australis in the southern hemisphere. Since Ganymede does not have an atmosphere to obstruct the flow of charged particles, the surface around the poles is constantly bombarded with plasma generated by Jupiter’s huge magnetosphere.
Annotated map of Ganymede. Photo credit: NASA / JPL-Caltech / SwRI / ASI / INAF / JIRAM / USGS
Alessandro Mura, a Juno fellow researcher from the National Institute for Astrophysics (INAF) in Rome, said:
“We found Ganymede’s high latitudes, dominated by water ice, with fine grain that is the result of intense bombardment of charged particles. Conversely, low latitudes are shielded by the moon’s magnetic field and contain more of its original chemical composition, especially non-water ice components such as salts and organic substances. It is extremely important to characterize the unique properties of these icy regions in order to better understand the space weathering processes that the surface is subject to. “
The polar views and close-ups Juno was able to build on observations made on previous NASA missions to explore the Jupiter system. These include the Voyager 1 and 2 missions, which passed through the system in 1979 on their way to the outer solar system; the missions Cassini-Huygens and New Horizons, which flew by in 2000 and 2007 on their way to Saturn and Pluto, respectively.
This latest information also builds on observations from the Galileo spacecraft, which was the first dedicated mission to study Jupiter and its moons (Juno is the second) and the first mission to orbit an outer planet. In the near future, ESA and NASA will send the JUpiter ICy moons Explorer (JUICE) and the Europa Clipper to explore Jupiter’s moons more closely – with a focus on Ganymede and Europe, respectively.
Artist’s impression of Juno at Jupiter. Photo credit: NASA
The Juno mission started on August 5, 2011 from Cape Canaveral Air Force Station and traveled 2,800 million kilometers on July 4, 2016 to reach Jupiter (2018), but has been extended twice since then: first to July 2021 and finally to September 2025 Ed Hirst, the project manager of the Juno mission at NASA JPL:
“Since launch, Juno has executed over 2 million commands, orbited Jupiter 35 times, and collected about three terabits of scientific data. We are excited with our ongoing exploration of Jupiter, and much more is to come. We have started our expanded mission and look forward to 42 additional orbits to explore the Jupiter system. “
In the next phase of his expanded mission, Juno will narrow its orbit around Jupiter, which will allow it to make several narrow passes to observe Jupiter’s north polar cyclones. It will also do additional Ganymede flybys and close flybys of Europe and Io (and more of Ganymede). Perhaps most importantly, the orbiter will make the first exploration of the faint ring system that surrounds the planet, in which some of Jupiter’s smaller satellites orbit.
These observations will expand on the discoveries Juno has already made about Jupiter’s internal structure, internal magnetic field, atmosphere, and strong magnetosphere. Wrapped in a gaseous shell and ruled by such powerful forces, Jupiter is a proverbial onion, the many layers of which hide additional scientific discoveries. With its many orbits ahead, Juno will continue to peel off layers to learn more about the formation and evolution of Jupiter and other gas giants.
Further reading: NASA