Juno Visits Jupiter’s Large Moons Ganymede and Europa (and Io)

On June 7, 2021, NASA’s Juno orbiter encountered Jupiter’s largest moon Ganymede, the only moon with its own intrinsic internal magnetic field. The encounter at ~1046 kilometers (650 miles) altitude was part of Juno’s extended mission, during which the most distant planetary orbiter will continue its investigation of the solar system’s largest planet through September 2025 (or until the spacecraft’s end of life, whichever comes first).


In these images of Ganymede, JunoCam revealed 12 paterae — broad, shallow bowl-shaped features on a planetary body’s surface — only two of which are evident in the Voyager data. These features were likely formed by late-stage volcanic processes. The top two views compare how well Voyager and Juno resolved one of these paterae. Credit: NASA/JPL-Caltech/SwRI/MSSS.

Proposed in 2003 and launched in 2011, Juno went into orbit around Jupiter on July 4, 2016. The prime mission was completed in July 2021. The extended mission involves 42 additional orbits, including close passes of Jupiter’s north polar cyclones; flybys of Ganymede, Europa, and Io; as well as the first extensive exploration of the faint rings encircling the planet. Successful Ganymede and Europa encounters were achieved in June 2021 and September 2022 respectively and several visits to Io are in the planning stages over the next year.

“Since its first orbit in 2016, Juno has delivered one revelation after another about the inner workings of this massive gas giant,” said principal investigator Scott Bolton of the Southwest Research Institute in San Antonio. “With the extended mission, we will answer fundamental questions that arose during Juno’s prime mission while reaching beyond the planet to explore Jupiter’s ring system and Galilean satellites.”


This image of the dark side of the jovian moon Ganymede was obtained by the Stellar Reference Unit star camera onboard NASA’s Juno spacecraft during its June 7, 2021, flyby of the icy moon. Usually used to keep the spacecraft on course, the navigation camera was able to obtain an image of the moon’s dark side (the side opposite the Sun) because it was bathed in the dim light scattered off Jupiter; the camera operates exceptionally well in low-light conditions. Credit: NASA/JPL-Caltech/SwRI.

“By extending the science goals of this important orbiting observatory, the Juno team will start tackling a breadth of science historically required of flagships,” said Lori Glaze, planetary science division director at NASA Headquarters in Washington. “This represents an efficient and innovative advance for NASA’s solar system exploration strategy.”

Juno’s close satellite encounters were the first since the end of the Galileo mission in 2003 and will be the last before the arrival of the next generation of missions to the jovian system: NASA’s Europa Clipper and the European Space Agency’s (ESA) JUpiter ICy moons Explorer (JUICE) mission. [New Horizons made important observations of these moons but was further away when it passed through the jovian system in 2007 (and Cassini from even further away in 2000).] Juno mapped parts of these satellites that were poorly observed by Voyager and Galileo and included science instruments not on either spacecraft, such as the Microwave Radiometer (MWR). Hence scientists and engineers were eager to see what Juno revealed. Juno’s investigation of Jupiter’s volcanic moon Io will also address many science goals identified by the National Academy of Sciences for a future Io explorer mission.


This image of the jovian moon Ganymede was obtained by the JunoCam imager onboard NASA’s Juno spacecraft during its June 7, 2021, flyby of the icy moon. At the time of closest approach, Juno was within 1038 kilometers (645 miles) of its surface — closer to Jupiter’s largest moon than any other spacecraft has come in more than two decades. This image is a preliminary product — Ganymede as seen through JunoCam’s green filter. Credit: NASA/JPL-Caltech/SwRI/MSSS.

The extended mission’s science campaigns will expand on discoveries Juno has already made about Jupiter’s interior structure, internal magnetic field, atmosphere (including polar cyclones, deep atmosphere, and aurora), and magnetosphere, but the satellite observations are an unplanned science bonus given by the evolving nature of Juno’s orbit.

The natural evolution of Juno’s orbit around the gas giant provides a wealth of new science opportunities that the extended mission capitalizes on. Every science pass sends the spinning solar-powered spacecraft zooming low over Jupiter’s cloud tops, collecting data from a unique vantage point no other spacecraft has enjoyed.

The point during each orbit where Juno comes closest to the planet is called perijove (or PJ). Over the course of the mission, Juno’s perijoves have migrated northward, dramatically improving resolution over the northern hemisphere. This evolving geometry also brings the large moons into range on the inbound leg of Juno’s orbits, which now cross the orbits of the satellites as Juno’s orbit rotates. By adjusting the orbital trajectory Juno was able to be at their orbits when the moons were in the same position. The design of the extended mission takes advantage of the continued northward migration of these perijoves to sharpen its view of the multiple cyclones encircling the north pole while incorporating ring and Galilean moon flybys.

Magnetic field surrounding Ganymede

This animation (click image to start) illustrates how the magnetic field surrounding Jupiter’s moon Ganymede (represented by the blue lines) interacts with and disrupts the magnetic field surrounding Jupiter (represented by the orange lines). During the June 2021 close approach to Ganymede by NASA’s Juno spacecraft, the Magnetic Field (MAG) and Jovian Auroral Distributions Experiment (JADE) instruments onboard the spacecraft recorded data showing evidence of the breaking and reforming of magnetic field connections between Jupiter and Ganymede. Credit: NASA/JPL-Caltech/SwRI/Duling.

“The mission designers have done an amazing job crafting an extended mission that conserves the mission’s single most valuable onboard resource — fuel,” said Ed Hirst, the Juno project manager at the Jet Propulsion Laboratory (JPL) in Pasadena, California. “Gravity assists from multiple satellite flybys steer our spacecraft through the jovian system while providing a wealth of science opportunities.” The satellite flybys also reduce Juno’s orbital period, which increases the total number of science orbits that can be obtained.”

The satellite encounters began with a low-altitude flyby of Ganymede on June 7, 2021 (PJ34), which reduced the orbital period from about 53 days to 43 days. (Callisto was never in the right place for Juno to get a close look.) The Ganymede flyby set up a close flyby of Europa on September 29, 2022 (PJ45), reducing the orbital period further to 38 days. A pair of close Io flybys, scheduled for December 30, 2023 (PJ57), and February 3, 2024 (PJ58), will combine to reduce the orbital period to 33 days.

Juno will also fly through the Europa and Io tori — ring-shaped clouds of ions — on multiple occasions, characterizing the radiation environment near these satellites to better prepare the Europa Clipper and JUICE missions for optimizing observation strategies and planning, science priorities, and mission design. The extended mission also adds planetary geology and ring dynamics to Juno’s extensive list of science investigations. “With this extension, Juno becomes its own follow-on mission,” said Steve Levin, Juno project scientist at JPL. “Close-up observations of the pole, radio occultations [a remote sensing technique to measure properties of a planetary atmosphere or ring systems that uses the spacecraft radio waves], satellite flybys, and focused magnetic field studies combine to make a new mission, the next logical step in our exploration of the jovian system.”

Citizen scientists have been processing the JunoCam images of Ganymede since they were released on the JunoCam website shortly after the encounter. The science teams and their affiliates have also been processing and analyzing the data from all the instruments and the first science findings from the Ganymede encounter were published in the journal Geophysical Research Letters in December 2022 and several additional articles are to be published early this year.

False color map of Juno-UVS Ganymede flyby observations, overlaid on a USGS geologic map. Auroral emissions of atomic oxygen at wavelengths of 130.4 and 135.6 nanometers are colored blue and green, respectively, while longer far-ultraviolet wavelengths (mostly from reflected sunlight) are colored red. The start and end times on (on June 7, 2021) for each UVS swath are indicated (Juno spins once every 30 seconds), along with the terminator in orange. From Greathouse T. K. et al. (2022) Geophys. Res. Lett., 49, e2022GL099794, https://doi.org/10.1029/2022GL099794.

The close pass by the rotating spacecraft allowed for radio tracking that improved our understanding of the interior. The results suggest that Ganymede may be in hydrostatic equilibrium and appear to confirm that there are internal density variations of unknown origin. The Clipper and JUICE missions will be required to map out and resolve these variations in greater detail.

The MWR instrument performed the first resolved investigation of the subsurface ice for both Ganymede and Europa, with the ability to compare subsurface characteristics of the two very distinct bodies. With six frequencies, MWR was able to analyze six depths and discern lateral and vertical variations in the subsurface thermal properties among older dark and younger bright terrains and with bright crater deposits such as at Tros. This type of instrument has never been flown to the outer solar system before. The Jovian InfraRed Auroral Mapper (JIRAM) also provided some of the highest-resolution infrared spectroscopy of Ganymede.

The 90-kilometer (56-mile) Tros crater was well resolved in the JunoCam stereo images, which reveal that it is ~1 kilometer (0.6 miles) deep. A total of 11 new paterae (features that resemble volcanic caldera or depressions on other planetary bodies) were identified in these new images. Stereo overlap of the four JunoCam mosaics allow for mapping of major topographic features and the only unusual feature was the 3-kilometer-high (1.9-mile-high) dome observed from Voyager data at the subjovian point. The JunoCam elevation map shows it to be oval in shape. The Stellar Reference Unit, an imager intended for navigation, also provided one image each of Europa and Ganymede in terrains poorly resolved in previous imaging, and both of these images were taken in reflected Jupiter light. This type of imaging was very successful at Saturn during the Cassini mission and should be equally successful during the Europa Clipper and JUICE missions.

Ganymede has a complex interaction with the jovian magnetosphere and the space environment due to its own internal magnetosphere. Juno’s Ultraviolet Spectrograph (UVS) mapped out the location of Ganymede’s northern and southern aurorae bands in greater detail than ever before, mapping the locations of the open and closed field lines. Juno crossed into this magnetosphere and sampled the outflowing ion composition. Radio emissions were detected from the magnetosphere related to asymmetries and disturbances within this magnetosphere and in the charged particle environment around Ganymede. The extremely thin atmosphere around Ganymede is dominated by water products ejected from the surface by charged particles.

Jupiter's moon Io

The volcano-laced surface of Jupiter’s moon Io was captured in infrared by the Juno spacecraft’s Jovian InfraRed Auroral Mapper (JIRAM) imager as it flew by at a distance of about 80,000 kilometers (50,000 miles) on July 5, 2022. Brighter spots indicate higher temperatures in this image. Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM.

The data from the successful Europa encounter in September 2022 are also being analyzed and will be reported on at future science conferences, including the upcoming Lunar and Planetary Science Conference in March. While it is too early to report on those findings, the images reveal a landscape similar to that observed by Voyager and Galileo but with additional clusters of shallow depressions that might be associated with global features attributed to polar wander (of rotation) of Europa’s icy shell. They also fill a gap in the Galileo mapping coverage and improve our understanding of the distribution of geologic units.

Observations from the MWR also suggest that Europa may have a different (and possibly more uniform?) structure within its outermost icy layers than does Ganymede. Other instruments will be looking to improve our understanding of Ganymede’s surface composition, interactions with the jovian magnetosphere, and internal gravitational signature as it relates to internal density variations. Both Clipper and JUICE will be looking at these data to anticipate mission results and adjust mission plans as needed.

The Io flybys in 2023/2024 will look for major surface changes due to ongoing volcanism, the composition of that volcanism, the distribution of Io’s volcanoes, its internal structure, and the magnetospheric and particulate interactions with the surface. As Io is deep within the most intense radiation zones at Jupiter, scientists are hopeful that radiation-induced noise can be minimized. Regardless, Juno’s encounters with the large Galilean satellites have already been a boon to our understanding of these enigmatic and dynamic objects. They are also important preludes to the mapping missions to Europa and Ganymede to be launched in 2023 and 2024.

For an overview of the Ganymede science results, see Hansen C. et al. (2022) Juno’s close encounter with Ganymede — An overview. Geophys. Res. Lett., 49(23), e2022GL099285, https://doi.org/10.1029/2022GL099285.

Cover photo: On June 6, 2021, Juno encountered Jupiter’s largest moon Ganymede at a distance of 1046 kilometers (650 miles). This view simulates the perspective of an observer near Juno as it passes by, using imaging acquired by the spacecraft and processed by Kalleheike Kannisto. Credit: NASA/JPL-Caltech/SwRI/K. Kannisto.