On 26 December, 2019, the Juno spacecraft’s orbit around Jupiter brought it near the north pole of the ninth-largest object in the solar system, the moon Ganymede. The spacecraft’s Jovian Infrared Auroral Mapper (JIRAM) instrument took the first infrared images of the massive moon’s north pole.
Ganymede only moon in the solar system that is larger than the planet Mercury. It’s mostly water ice. It is also the only moon in the solar system with its own magnetic field. On Earth, the magnetic field provides a pathway for plasma (charged particles from the Sun) to enter our atmosphere and create aurora. Ganymede has no atmosphere to impede the progress of those charged particle, so the surface at its poles is constantly being bombarded by plasma from Jupiter’s gigantic magnetosphere. The bombardment has a dramatic effect on Ganymede’s ice.
The ice near both poles of the moon is amorphous. This is cause by the impact of the plasma on the surface. That pounding prevents the ice from having a crystalline structure.
This video shows a map of Ganymede based on images from the Galileo orbiter. The U. S. Geological Survey has classified the surface of Ganymede into various types of terrain. The regions shown in brown are those that are heavily cratered and much older than the light shaded regions that are smoother with few craters. The lighter shaded regions were likely formed by flooding of the surface with water coming from faults or cryo-volcanos that has occurred over billions of years. Tectonic processes may be at work with some crustal ice sheets being forced downward by the emergence of newer icy material. The best models of Ganymede from the Galileo data suggest a deep ocean under a thick ice crust.
Not really. The trick is that the planet seems to be looking back at us because Hubble happened to catch the shadow of Ganymede, Jupiter’s largest moon, as it moved across the Giant Red Spot. Hubble was monitoring changes in that huge storm last April when the moon’s shadow moved across the center of the storm. For a moment, Jupiter became Cyclops.
This rotating animation of Jupiter’s moon Ganymede begins as a global color mosaic image of the moon assembled from data from Voyager 1, Voyager 2, and the Galileo spacecraft and then fades in a newly developed geologic map.
Video Credit: USGS Astrogeology Science Ctr / Wheaton / ASU / NASA / JPL-Caltech
One common observation made by amateur astronomers is watching Jupiter and it’s four large moons, the Galilean satellites Ganymede, Io, Europa, and Callisto. The satellites move quite noticeably over a one night’s viewing, and all that is required is a decent pair of binoculars (although the moons simply appear as points of light with low magnification).
Of course, if you have access to a good telescope, you can see more detail. This picture of Jupiter and Ganymede was made using the Hubble Space Telescope just as the moon was being eclipsed. Even some surface details of the moon can be seen.
This image of Ganymede was taken by NASA’s Galileo mission. Ganymede is the largest moon in the Solar System. It’s diameter is 8% greater than the planet Mercury’s, although it’s mass is 45 % of the inner planet’s. If it had found an orbit around the Sun rather than Jupiter, it could have been one of the major planets.