Juno’s new images reveal Ganymede’s north pole

Grayish sphere with curved white latitude and longitude grid lines, on black background.
The north pole of Ganymede, as seen in the infrared by Juno on December 26, 2019. Image via NASA/ JPL-Caltech/ SwRI/ ASI/ INAF/ JIRAM.

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Ganymede is Jupiter’s largest moon, and the ninth-largest object in our solar system. This moon is even bigger than the planet Mercury. The last time we had good close-up images of it was from the Galileo mission to Jupiter in the late 1990s. Those images provided a detailed look at much of Ganymede’s surface, but Galileo never managed to capture Jupiter’s north pole. Now, thanks to NASA’s Juno spacecraft currently orbiting Jupiter, we have the first images of the north pole of this large Jovian moon.

Unlike the Galileo mission, Juno isn’t designed specifically to study Jupiter’s moons. Juno’s images aren’t as high-resolution as those of Galileo in the 1990s, in part because they were taken from farther away. And the Juno images show the view in infrared – showing the amount of heat radiated by Ganymede at various points near its pole – rather than being visible light images revealing what our eyes would see.

Still, the images provide a glimpse of this part of Ganymede that was unseen until now. The JIRAM instrument on the Juno spacecraft – aka the Jovian Infrared Auroral Mapper – acquired the images as part of the spacecraft’s mapping of the northern regions of the moon. JIRAM used high-contrast imaging and spectroscopy to study Jupiter’s polar regions during the December 26, 2019, flyby of Jupiter. At that time, Ganymede happened to be within view of Juno. The spacecraft was programmed to turn its instruments toward Ganymede during this time.

At closest approach to Ganymede – about 62,000 miles (100,000 km) – JIRAM collected 300 infrared images of the moon’s surface, with a spatial resolution of 14 miles (23 km) per pixel.

Now … what do the new images show?

Five greyish spheres on black background, phases from crescent to gibbous.
Various infrared views of Ganymede’s north polar, captured by the Juno spacecraft on December 26, 2019. Image via NASA/ JPL-Caltech/ SwRI/ ASI/ INAF/ JIRAM.

We knew already that, like many moons in the outer solar system, Ganymede is composed largely of water ice. It’s thought to have a water ocean beneath an outer ice crust.

A significant finding from these new Juno images is that the ice at Ganymede’s north pole isn’t pristine. It’s been acted upon by plasma from Jupiter’s colossal magnetosphere, which is some 20,000 times stronger than Earth’s magnetosphere. Ganymede has only an extremely thin atmosphere, so plasma from Jupiter can reach the surface unimpeded, greatly affecting the surface ice. Alessandro Mura, a Juno co-investigator at the National Institute for Astrophysics in Rome, commented:

The JIRAM data show the ice at and surrounding Ganymede’s north pole has been modified by the precipitation of plasma. It is a phenomenon that we have been able to learn about for the first time with Juno because we are able to see the north pole in its entirety.

The researchers can also see that the ice near Ganymede’s north and south poles has a different infrared signature than ice at Ganymede’s equator. This polar ice is amorphous, that is, lacking a definite shape or form. The amorphousness is due to Ganymede’s being the only moon in the solar system with its own magnetic field. Charged particles follow the moon’s magnetic field lines to the poles, where they impact and wreak havoc on the ice, preventing it from having an ordered (or crystalline) structure.

Scientists have found that even frozen water molecules in these regions have no order in their arrangement.

The new data from Juno show how it can contribute to the study of Jupiter’s moons, even though its primary mission is the study of Jupiter itself. According to Giuseppe Sindoni, program manager of the JIRAM instrument for the Italian Space Agency:

These data are another example of the great science Juno is capable of when observing the moons of Jupiter.

Colorful marble-like moon on black background.
Ganymede, as seen in enhanced color by the Galileo spacecraft in the late 1990s. Image via NASA/
Mottled, marble-like moon on black background.
Another, natural color view of Ganymede from Galileo in the late 1990s. Image via NASA.
Moon with blue auroras and Jupiter and stars in background.
Artist’s concept of auroras on Ganymede. This is made possible by the fact that Ganymede is the only moon known to have a magnetic field. Image via NASA/ ESA.
Concentric circles representing layers of a rocky and watery moon, with text annotations, on black background.
Cross-section of Ganymede’s interior showing a subsurface ocean of water. Image via NASA/ ESA/ A. Feild (STScI)/

JIRAM’s primary task is to look at infrared light coming from within the deep atmosphere of Jupiter itself. It can probe down to 30 to 45 miles (50 to 70 km) below Jupiter’s cloud tops. But, along with Ganymede, the instrument can also be used to study the moons Io, Europa and Callisto. Those four moons are known as the Galilean moons, named after the astronomer Galileo who discovered them in 1610. They are the four largest moons of Jupiter.

Thanks to its magnetic field, Ganymede also has auroras, like Earth, even though its atmosphere is extremely tenuous and almost non-existent. Ganymede’s ocean is estimated to be 60 miles (100 km) thick – 10 times deeper than Earth’s oceans – and is buried under a 95-mile (150-km) crust of mostly ice.

With a diameter of 3,273 miles (5,268 km), it is 26% larger than the planet Mercury by volume, although it is only 45% as massive. Ganymede has an iron core, a rocky mantle surrounded by an ice mantle, a water ocean ocean surrounding the mantles and an ice crust on top.

Smiling man in suit with trees behind him.
Giuseppe Sindoni, program manager of the JIRAM instrument for the Italian Space Agency. Image via ResearchGate.

Juno can see Ganymede only from a distance, but the European Space Agency’s (ESA’s) JUpiter ICy moons Explorer (JUICE), scheduled for launch in 2022, will study Ganymede, Callisto and Europa in detail after it arrives at Jupiter in 2029. This will be the first such mission to do so since Galileo.

For now, Juno has provided at least a glimpse of Ganymede’s north pole for scientists to study. In the not-too-distant future, missions like JUICE will reveal much more about this tantalizing world.

Read more about NASA’s Juno mission.

Bottom line: NASA’s Juno spacecraft has taken the first-ever images of Ganymede’s north pole.

Via Jet Propulsion Laboratory

July 31, 2020

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