Ganymede, the moon of Jupiter, would be almost covered with a huge crater

By analyzing the surface of Ganymede, one of Jupiter’s moons, researchers have discovered that many crevices on almost the entire star would be due to a single giant impact. It would be the largest such structure in the Solar System.

Almost all of Ganymede’s surface would in fact be covered by a huge crater. In any case, this is what a team of Japanese researchers thinks, who have carried out a detailed analysis of the relief of this moon of Jupiter and who publish their results, presented August 13, in the review Icarus.

It all started in 1979 when the Voyager 1 and 2 probes flew over this unknown world. Previous observations made by Pioneer weren’t precise enough to distinguish anything of interest, but here, for the first time, scientists discovered that Ganymede’s surface had some sort of rings throughout. Furrows which traversed the rugged terrain of this moon and which were sometimes interspersed with more recent craters, a sign that they were indeed the oldest geological marks left on the surface.

The biggest shock in the solar system

After that, images taken by the Galileo probe in 1995 showed that these grooves were also present in areas that Voyager had not been able to explore. But it is only more recently, with the evolution of data observation techniques, that these findings have been re-analyzed to establish that these circular marks did indeed cover the whole star, and the authors of the study in question were able to reconstruct a more precise structure at the global level. They used all the images of Ganymede at their disposal to find that all these furrows were well concentrated around a single point. ” It is therefore the largest impact structure ever identified in the Solar System. », Summarizes the main author Naoyuki Hirata with Numerama. Previous studies leaned more towards the theory that several impacts caused the appearance of the grooves, but no, it seems that everything is linked to one and the same event.

It remains to be seen where the point of impact is. As the marks were present on the whole surface, there were, to put it simply, two candidate poles, but it is the region of Marius (dark part of Ganymede named thus in honor of Simon Marius, the German astronomer who discovered the moon) which took it away. It turns out that on the other side, the Barnard region, the part without rings, the one around the point of impact, was too small.

A “meteorite” 150 kilometers in radius

As for the event itself and the size of the object that hit Ganymede, it’s a bit more complicated. Usually the craters form recognizable walls: we see it on another moon of Jupiter, Callisto, with the basins of Valhalla and Asgard (yes, Callisto is very much into Norse mythology) clearly identifiable, although the weather and the others impacts can erase the contours a little. But here, nothing of the kind, it was thus necessary to be cunning. First, the scientists made comparisons with the better-preserved Callisto craters in order to study their diameter and the different zones they formed around the impact. By applying these measurements to Ganymede, it was possible to estimate the size of the original crater although this remains unclear due to the many other impacts on the surface. They arrived at a result between 7000 and 8000 kilometers of radius.

Not very precise, but sufficient to have an idea, knowing that the largest craters of Callisto are less than 2000 kilometers and already represented a record. The measurements are still sufficient to estimate which impact size is sufficient to form concentric rings around the globe. The rest was played out with computer simulations taking into account the hardness of the crust, the size of the object and its speed. ” For speed, we’re pretty sure of ourselves, assure Naoyuki Hirata, on the other hand for the size it remains rather vague. According to the authors, we are on an object at least 50 kilometers in radius, which was going at 20 kilometers per second. But they specify that the radius can also be 150 kilometers, which leaves a hell of a range.

If this theory turns out to be correct, it would call into question some knowledge about Ganymede, in particular around the notable difference with his neighbor Callisto. The giant impact would have erased the previous craters, which would explain why Callisto is much more studded with marks. The particular rotation of Ganymede, which is synchronous with Jupiter (it always presents it the same face) could also be explained by a gigantic impact which would have upset even its inclination. ” Callisto and Ganymede are very different, adds Naoyuki Hirata, and the impact could have contributed to considerably modifying the internal structure of Ganymede, which would explain these differences. »

While waiting for JUICE

To find out more, there aren’t an infinite number of solutions: you have to produce new images and return to the place. This is precisely the challenge of ESA’s mission, JUICE, which plans to take off in 2022. The Jupiter Icy Moons Explorer satellite will take a look at Jupiter, Callisto, Europe, but its main target is well Ganymede and its possible underground ocean. Its magnetic field will also be studied since it is the only satellite to have one, included in that of Jupiter much larger. A phenomenon that occurs during the rotation of a possible ferrous core, past or still in progress. On board JUICE, many instruments, mostly French, will be able to scrutinize Ganymède in detail. ” Traveling took great photos, describes Carole Larigauderie, project manager of French contributions to JUICE at CNES. But here we’re going to go below the surface, probe the ice sheet, and study the underground ocean that’s below. »

The satellite which will fly over Ganymede up to an altitude of 500 kilometers at its lowest carries with it many instruments very useful for exploration. Among them is the RPWI which will study the electromagnetic field and the influence on the surface. But also MAJIS, a spectrometer responsible for examining the physical properties of the moon. The RIME radar is to probe underground layers up to 9 kilometers deep. So many tools that will provide valuable knowledge about the Jovian system and its history. ” Ganymede is very interesting with its ocean and its magnetosphere, and we want to know it better, adds Carole Larigauderie. And all this is directed towards one objective: to assess its habitability. The mission is expected to arrive on site from 2030.