The last time we saw Jupiter’s largest moon Ganymede up close was over two decades ago. Now NASA has released new closeups of the moon, taken on June 7, 2021, by the Juno orbiter. The stunning images reveal Ganymede’s icy surface in striking detail. They show the moon’s craters, bright and dark terrain, and long linear features that may be similar to tectonic faults on Earth A mysterious world, that Juno is helping to bring back to the center of scientific attention after a period of relative silence. But before talking about the Juno flyby, it is worth explaining what extraordinary celestial body we are talking about. The scenario that is slowly composing has something incredible. Before the Voyager and Galileo missions, there were even those who thought that the Galilean satellites were worlds without particular characteristics of relief, probably united by a unique genesis and undifferentiated by surface characteristics and internal structure. Today we are instead in front of four real “planets”, each one having exceptional peculiarities, and it is not finished… because an enormous amount of new information is continuously forcing the experts to rewrite the books of astronomy. Volcanoes that erupt incandescent materials at hundreds of kilometers of height, where they freeze and fall back to cover the surface with a blanket thick but soft as cotton wool; hidden oceans that can potentially host possible life forms under immense sheets of ice; radioactive elements that keep alive geological systems otherwise dying; a complex tangle of magnetic fields that, while maintaining well-defined boundaries and limited autonomy, are incorporated in the powerful magnetic field of the giant Jupiter that controls everything. For some time now, Ganymede has been added to the list of possible worlds that could hide an ocean in their bowels, thanks to the recent discovery of a magnetic field that surrounds it. But let’s not get ahead of ourselves and first introduce Ganymede in terms of history and numbers. Ganymede was discovered by Galileo Galilei on 7 January 1610 along with the three other Jovian moons Europa, Io, and Callisto. Together they are referred to as the Galilean moons. The discovery of these four Galilean satellites is what eventually led to understand that the planets in the solar system were orbiting the Sun and that Earth was not the center of the solar system. The name Ganymede, along with those of the other three largest moons of the Jovian system was chosen by the German astronomer Simon Marius, who discovered the moons independently at the same time as Galileo: he named them at the suggestion of Johannes Kepler after lovers of the god Zeus. In the Greek story Zeus, a counterpart of Jupiter in Roman mythology, carried the prince Ganymede to Olympus, where he became a cupbearer for the Olympian gods. In order of distance from Jupiter, the moons are: Io, Europa, Ganymede, and Callisto Galileo steadfastly refused to use Marius’ names and invented the numbering scheme that is still used nowadays, in parallel with proper moon names. The numbers run from Jupiter outward, thus I, II, III, and IV for Io, Europa, Ganymede, and Callisto respectively. With a diameter of 5,268 km, Ganymede is larger than Mercury and is 2/3 the size of Mars. – 8% larger than Mercury. It has the highest mass of all planetary satellites and has more than twice the mass of the Earth’s Moon. The moon orbits a una distanza da Giove di poco più di un milione di chilometri and takes 7.15 Earth days to complete its orbit. Like most moons in the solar system, Ganymede is tidally locked to Jupiter and one side is always facing the planet. Interestingly, for every orbit of Ganymede, Europa orbits Jupiter twice and Io orbits four times. Ganymede is the only moon in the solar system known to have its own magnetic field. It is believed that the magnetosphere of Ganymede is likely to have been created through convection within the liquid iron core of the moon. The field is very small though and is barely noticeable because it is buried within Jupiter’s much larger magnetic field. Ganymede is composed of equal amounts of silicate rock and water ice and has several layers. It has a metallic iron and iron sulfide core which is surrounded by a rocky mantle and a very thick icy crust. 40% of Ganymede’s surface is covered with highly cratered dark regions. These dark regions are believed to be from heavy impact by asteroids or comets and date back to around 4 billion years ago. The lighter regions of the moon’s surface are not quite so old and cover the rest of Ganymede. The cause of the light terrain’s disrupted geology is thought to be the result of tectonic activity caused by tidal heating – a build up of friction, orbital and rotational energy that are dissipated as heat in the crust of the moons and planets involved. The moon has a very thin atmosphere and it does contain small amounts of oxygen but there is not enough to support any form of life. Ganymede is characterized substantially by dark, ancient, strongly cratered terrain, alternating with areas variously rippled, light-colored and different shades, believed to be of more recent origin, volcanic or tectonic. The presence of bright rays around the numerous impact craters suggested the hypothesis that, just below the surface crust, there was a thick mantle of water ice, from which the impacts would have extracted material, projecting it ballistically all around. On September 6, 1996, Galileo passed very near over the surfaces of the large moon, sending back a lot of new data such as the discovery of the magnetic field, the ocean beneath Ganymede’s surface, and the highest resolution close up images of the moon. The possibility that there may be liquid water on Ganymede, at least at depth, is supported, moreover, by other Galileo measurements. The magnetometer of the probe showed that the satellite has its own magnetosphere generated by a dipolar field “bar”, which traps in the majority currents of charged particles, ie those that escape the dense magnetic belts of the planet Jupiter. Although resulting a thousand times less intense than that of Earth, the magnetism of Ganymede indicates that, under its ice mantle, about 200 km deep, there is a circulation of fluid matter, electrically conductive, which experts attribute to the convective motions of a mass of saline water. The presence of this ocean has been confirmed by readings taken by orbiters and through studies of how Ganymede’s aurora behaves. In short, the moon’s auroras are affected by Ganymede’s magnetic field, which in turn is affected by the presence of a large, subsurface salt-water ocean. Evidence even suggests that Ganymede’s oceans might be the largest in the entire Solar System! “Hey, guys, just a moment before we continue… BE sure to join the Insane Curiosity Channel… Click on the bell, you will help us to make products of ever-higher quality!” Six spacecraft have visited Ganymede since 1973. The first spacecraft to approach close to Ganymede was Pioneer 10, which performed a flyby in 1973 as it passed through the Jupiter system at high speed. Pioneer 11 made a similar flyby in 1974. Data sent back by the two spacecraft was used to determine the moon’s physical characteristics and provided images of the surface with up to 400 km resolution. Pioneer 10’s closest approach was 446,250 km. Voyager 1 and Voyager 2 both studied Ganymede when passing through the Jupiter system in 1979. Data from those flybys were used to refine the size of Ganymede, revealing it was larger than Saturn’s moon Titan, which was previously thought to have been bigger. Images from the Voyagers provided the first views of the moon’s grooved surface terrain. The Pioneer and Voyager flybys were all at large distances and high speeds, as they flew on unbound trajectories through the Jupiter system. Better data can be obtained from a spacecraft which is orbiting Jupiter, as it can encounter Ganymede at a slower speed and adjust the orbit for a closer approach. In 1995, the Galileo spacecraft entered orbit around Jupiter and between 1996 and 2000 made seven close flybys of Ganymede. During the closest flyby, Galileo passed just 264 km from the surface of Ganymede which remains the closest approach by any spacecraft. During the first flyby in 1996, Galileo instruments detected Ganymede’s magnetic field. Data from the Galileo flybys was used to discover the sub-surface ocean, which was announced in 2001. High spatial resolution spectra of Ganymede taken by Galileo were used to identify several non-ice compounds on the surface. The New Horizons spacecraft also observed Ganymede, but from a much larger distance as it passed through the Jupiter system in 2007 (en route to Pluto). The data were used to perform topographic and compositional mapping of Ganymede. But we are now in the present day… Like the Galileo probe, the NASA spacecraft Juno also orbited around Jupiter. On 2019 December 25, Juno performed a flyby of Ganymede, at a distance of about 100,000 km. This flyby provided poor images of the moon’s polar regions. On June 7, 2021, however, Juno performed a second flyby, at a closer distance of 1,038 kilometers. This encounter was designed to provide a gravity assist to reduce Juno’s orbital period from 53 days to 43 days. This is the closest that any spacecraft has come to the giant moon since the Galileo spacecraft’s close flyby on May 20, 2000; so much that Scott Bolton, Juno Principal Investigator, said in a statement: “This is the closest any spacecraft has come to this mammoth moon in a generation. We are going to take our time before we draw any scientific conclusions, but until then we can simply marvel at this celestial wonder.” During his rapid flyover, Juno was able to capture almost an entire side of Ganymede with JunoCam, with a resolution of 1 km per pixel. Right now most of the images are in black-and-white, but other pending photos will be in color. But dozens of other shots, besides the ones we’re showing you, are waiting to be processed. The dark terrain, which comprises about one-third of the surface, is so-colored because the surface ice in these regions contains clays and organic materials. It has been theorized that these have been left behind by impactors, which accords with the fact that impact craters are far more extensive in the areas of dark terrain. Meanwhile, the grooved terrain is believed to be tectonic in nature; which could be due in part to cryovolcanism, but is thought to be mostly the result of tidal heating events. The tidal flexing could have heated the interior and strained the lithosphere, leading to the development of cracks, graben, and faults that erased the old, dark terrain on 70% of the surface. Though craters are more common in darker areas, they are seen all over the surface. Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon. If true, the vast majority of impacts happened in that epoch, whereas the cratering rate has been much smaller since. Craters on Ganymede are also flatter than those on the Moon and Mercury, which is probably due to the relatively weak nature of Ganymede’s icy crust. Very soon we will be able to know much more about the secrets of this fascinating satellite. It is in fact in preparation for a European mission called Jupiter Icy Moon Explorer, or JUICE. The mission will study three of Jupiter’s Galilean moons: Ganymede, Callisto, and Europa (excluding the more volcanically active Io) all of which are thought to have significant bodies of liquid water beneath their surfaces, making them potentially habitable environments. The spacecraft is set for launch in June 2022 and will reach Jupiter in October 2029 after five gravity assists and 88 months of travel. By September 2032, the spacecraft will enter orbit around Ganymede for its close up science mission, becoming the first spacecraft to orbit a moon other than the moon of Earth. A few more years and maybe we will be able to understand if indeed, as many scientists speculate, the deep and “warm” ocean of Ganymede can host life forms at least at the microbial level An analysis published in 2014, taking into account the realistic thermodynamics for water and effects of salt, suggests in fact that Ganymede might have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle below. This is important, since the layer closest to the rocky interior would be subject to heating due to tidal flexing in the mantle. This heat could be transferred into the water via hydrothermal vents, which could provide the necessary heat and energy to sustain life. Combined with oxygenated water, life forms could exist at the core-mantle boundary in the form of extremophiles, in a way that is similar to what is found in Earth’s oceans. Be patient guys…it’s a matter of waiting a few more years!