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Ganymede is Jupiter’s largest moon, and the ninth largest object in our solar system. This moon is bigger than the planet Mercury. The last time we had good images was from the Galileo mission to Jupiter in the late 1990s. Those images provided a detailed look at 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 is not specifically designed to study Jupiter’s moons. Juno’s images were not as high-resolution as Galileo’s in the 1990s, as they were taken from afar. And Juno images show the view in the infrared – the amount of heat radiated by Ganymede is shown at various points near its pole – instead the light images it shows reveal what our eyes see.
Nevertheless, the images provide a glimpse of this part of Ganymede which was hitherto unseen. The JIRAM instrument on the Juno spacecraft – aka Jovian Infrared Arloral Mapper – acquired images as part of the spacecraft’s mapping of the northern regions of the moon. JIRAM used high-contrast imaging and spectroscopy during December 26, 2019 to study Jupiter’s polar regions, Jupiter’s flyby. At the time, Ganymede was in sight of Juno. The spacecraft was programmed to turn its equipment toward Ganymede during this time.
At the closest approach to Ganymede – approximately 62,000 mi (100,000 km) – JIRAM collected 300 infrared images of the lunar surface with a spatial resolution of 14 mi (23 km) per pixel.
Now … what do the new images show?
We already knew that, like many moons in the outer solar system, Ganymede is largely composed of water ice. It is believed that there is a water ocean under an outer ice crust.
An important finding from these new Juno images is that the ice at the North Pole of Ganymede is not ancient. It is acted upon by plasma from Jupiter’s colon magnetosphere, which is about 20,000 times stronger than Earth’s magnetosphere. Ganymede has only a very thin atmosphere, so plasma from Jupiter can reach the surface unmatched, greatly affecting surface ice. Alessandro Mura, a Juno co-investigator at the National Institute for Astrophysics in Rome, commented:
The JIRAM figures show snow and the North Pole of Ganymede have been modified by plasma precipitation. This is an event that we are able to learn for the first time with Juno as we are able to see the North Pole in its entirety.
Researchers can also see that the snow north of Ganymede And Ganymede at the south poles has a different infrared signature than the ice at the equator. This polar ice is amorphous, meaning it lacks a certain shape or form. Ganymede being the only moon in the solar system is amorphous its own Magnetic Field. Charged particles follow the moon’s magnetic field lines to the poles, where they impact the ice and wreak havoc, preventing it from being an ordered (or crystalline) structure.
Scientists have found that frozen water molecules in these regions also have no order in their arrangement.
New data from Juno suggest how it can contribute to the study of Jupiter’s moons, even if its primary mission is to study Jupiter. According to Giuseppe Sindoni, Program Manager of JIRAM Equipment for the Italian Space Agency:
These data are another example of the great science Juno is capable of when observing Jupiter’s moons.
JIRAM’s primary function is to see the infrared light coming from within the deep atmosphere of Jupiter. It can probe 30 to 45 miles (50 to 70 km) below Jupiter’s cloud top. But, with Ganymede, the tool can also be used to study the moons Io, Europa and Callisto. Those four moons are known as 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 an auroras like the Earth, even though its atmosphere is extremely hard and almost non-existent. Ganymede’s ocean is estimated to be 60 miles (100 km) thick – 10 times deeper than the Earth’s oceans – and is mostly buried under 95-miles (150-km) of ice.
With a diameter of 3,273 mi (5,268 km), it is 26% larger than the planet Mercury by volume, although it is only 45% of the mass. Ganymede has an iron core, a rocky mantle surrounded by an ice mantle, a water ocean surrounded by a mantle and an ice crust at the top.
Juno can only see Ganymede from afar, but the European Space Agency (ESA) jupiter ICA Mons Explorer (JUIES), scheduled for launch in 2022, studies Ganymede, Callisto and Europa in detail after Jupiter’s arrival in 2029 Will do. It would be the first mission to do so after Galileo.
For now, Juno has provided scientists with at least a glimpse of Ganymede’s North Pole for study. In the not-too-distant future, missions like JUICE will reveal much about this tantric world.
Read more about NASA’s Juno mission.
Bottom line: NASA’s Juno spacecraft has taken first-hand images of Ganymede’s North Pole.
Jet Jet Propulsion Laboratory