Jupiter’s moons are central to NASA’s Juno spacecraft bonus science.

In 2016, while conducting routine tests on NASA’s Juno spacecraft, the mission team discovered that parts of the engine were not working as expected.

The spacecraft, which arrived at Jupiter in July 2016, orbited the giant planet every 53 days and, due to acceleration, reduced this period to 14 days. Given the engine issues, the Juno team decided not to risk the change, instead leaving the spacecraft on a longer and wider orbital period. And when the success of the mission yielded an additional 42 planet passes, the team had an unexpected opportunity to catch a glimpse of some of Jupiter’s moons.

“Luckily, when we moved on to our extended mission, we were able to make close flybys of … the moons,” said Scott Bolton, a planetary scientist at the Southwestern Research Institute in San Antonio and principal investigator for NASA’s Juno mission. at a press conference on Wednesday (December 14) as part of the American Geophysical Union’s fall meeting this week.

Related: NASA’s Juno spacecraft captures most detailed image of Jupiter’s icy moon Europa

Juno’s observations of Jupiter and its moons reveal new insights and will serve as the basis for future missions to the moons. One of those missions, the European Space Agency’s Jupiter Icy Moons Explorer (JUICE), has already used Juno’s images of the solar system’s largest moon, Ganymede, to create a detailed map of Ganymede’s surface. The map is based on data collected by NASA’s Voyager missions, which passed by the Moon in 1979, and the Galileo mission, which studied the Jupiter system in the 1990s and 2000s.

The resulting map shows a staggering variety of features on Ganymede’s icy surface. “It has dark terrain and it has this bright terrain,” Bolton said.

“It has linear features that look like they’re probably caused by tectonics, and it has large white patches where there are craters that produce fresh, clean ice,” he added. “It’s a very diverse place.”

Juno also peered under this interesting surface using the spacecraft’s microwave sensors, which show what might be going on under the surface of Ganymede and Jupiter’s other icy moon, Europa. The data shows not only that there is a patchwork of hotter and colder regions beneath Ganymede’s surface, likely due to different types of terrain, but also that lunar ice makes it highly reflective. Microwave readings showed that Ganymede appears to be even colder than previous measurements, indicating that the lunar ice reflects some of the heat reaching the Moon.

NASA’s Jupiter observing mission Juno has captured the closest image to date of the gas giant’s enigmatic ice-covered moon Europa, which is illuminated by Jupiter’s aurora. (Image credit: NASA/JPL-Caltech/SwRI)

According to Bolton, this reflective effect was even more extreme on Europa. Juno was also able to use one of its navigation cameras, which use detection of surrounding stars in low light for navigation, to photograph the night side of Europa.

“You are looking at the night side, illuminated by the glow of Jupiter,” Bolton said. “So it’s a very innovative way to look at Europe and use all of our sensors.” The glow of Jupiter, like the glow of the Earth on our Moon, occurs when Jupiter reflects sunlight and dimly projects it onto the surface of Europa.

Juno also took the opportunity to study a unique aspect of Ganymede, its magnetic field. Ganymede is the only moon in our solar system known to generate its own magnetic field. Juno has collected data showing that the magnetic fields of Jupiter and Ganymede are connecting and pulling apart, releasing ultraviolet radiation in the process.

Depiction of the interacting magnetic fields of Ganymede (blue) and Jupiter (orange). (Image credit: NASA/JPL-Caltech/SwRI/Duling)

“You can get a snapshot of all the geometries of Jupiter’s and Ganymede’s magnetic fields tied together by looking at UV observations,” Thomas Greathouse, a planetary scientist at the Southwestern Research Institute who has studied the ejecta, said during a press conference.

And, of course, the Juno’s main camera worked hard during those flybys. The stunning images taken by Juno show details of features “hidden in plain sight” from earlier images, such as images from the Voyager mission, Candice Hansen, a planetary scientist at the Planetary Science Institute, said at a press conference. Hansen is a co-investigator of the Juno mission and began her career working with the Voyager imaging team.

Extremely high-quality images taken by Juno include the patera on Ganymede, a feature resembling a volcanic crater. Juno was also able to capture new photographs of Europa’s surface, which is smoother and less cratered than the mottled surface of some of Jupiter’s other moons, meaning its surface is very young, Hansen said, consistent with previous data.

Of course, Juno still photographs Jupiter itself. Among the latest images of Juno are striking images of Jupiter’s turbulent clouds, including its northern cyclones, which look like swirling structures of green-gray and pale yellow against a blue background, straight out of a Van Gogh painting.

Jupiter’s volcanic moon Io as seen in infrared by the Juno probe in July 2022. (Image credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM)

The mission is also one of the first times scientists have been able to see the poles of Jupiter’s volcanic moon Io. Infra-red photographs of the Moon taken during the July 5 flyby show many glowing hotspots, with more of these hotspots found at the poles than at the Moon’s equator, Bolton said, surprising scientists. Juno flew past Io again on Thursday (December 15), taking the closest images of the moon to date.

Even with a mission extension, Juno won’t last forever. In the next few years, intense radiation around Jupiter and its moons could disable its equipment. Even if it doesn’t, Juno will eventually run out of fuel and be unable to turn towards Earth and send data back. While the team originally planned to intentionally crash the spacecraft into Jupiter to protect potentially habitable moons like Europa, its current trajectory will eventually cause the spacecraft to crash into Jupiter on its own, Bolton said, where it will burn up in a dense atmosphere. . Bolton added that this hands-off approach is still being officially approved, but it will probably be the easiest way to get rid of the spacecraft.

Future missions, such as NASA’s Europa Clipper, due to launch in October 2024, and JUICE, due to launch in April 2023, will build on what scientists have learned from Juno and the moon’s remaining mysteries. For example, Juno was unable to observe the mysterious watery plumes of Europa, which could allow scientists to look into the global ocean, which, according to scientists, is hidden under the ice shell.

According to Bolton, the JUICE team has already used Juno data, including to compile the map of Ganymede shown at the start of the press conference. The team is “trying to figure out what to look at with their cameras and sensors,” he said. “So they’re already looking at the regions we’ve identified in higher resolution than before and starting to make plans.”

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