Did Ganymede tip over? – 300-kilometer-wide asteroid could have shifted the crust of Jupiter's moon

Impact with serious consequences: The largest moon in the solar system could have tipped over around four million years ago – Ganymede's entire crust slipped. The trigger for this was the impact of a 300-kilometer-wide asteroid on Jupiter's moon, as a planetary researcher now reports in “Scientific Reports”. He sees evidence for this in the position of the concentric impact marks that are still visible on Ganymede today. The JUICE space probe could check on site in a few years whether his theory is correct.

Jupiter's moon Ganymede is unusual in several respects: it is the largest moon in the solar system and the only one with a magnetic field and a water vapor atmosphere. Its inner workings are as complex as they are mysterious: beneath the moon's icy crust lies an ocean of liquid water, and there may also be alternating layers of water and ice. Ganymede is therefore considered a promising candidate for extraterrestrial life in the solar system.

The secret of the furrows

But Ganymede has another special feature: its surface is riddled with ancient furrows thousands of kilometers long and up to 700 kilometers deep. Their orientation suggests that they date back to a gigantic impact in the early days of the moon around four billion years ago. “This furrow system is the largest impact structure in the outer solar system,” explains planetary researcher Naoyuki Hirata from Kobe University. “But how big the asteroid war was and what consequences its impact had is unclear.”

To gain more clarity, Hirata has now examined the position and extent of the furrows more closely. He noticed that the center of Ganymede's furrow system lies exactly on the tidal axis – opposite the side of the Jupiter moon that always faces Jupiter. The impact marks therefore form a counterpole to the gravitational pull of the gas giant.

Slipped crust

This may not be a coincidence, as Hirata explains. For moons in tact rotation – like Ganymede or the Earth's moon – a large excess of mass can lead to an imbalance at one point in the crust. At some point, the forces acting on it cause the crust to slip. “The positive gravity anomaly thus moves closer to the tidal axis,” explains the researcher. This gives the celestial body a more stable position.

The dwarf planet Pluto could also have experienced such a “tipping over” after an impact, as US researchers determined in 2016. The important thing for Ganymede, however, is that the slipping of the crust can provide information about how massive the triggering impact must have been. Hirata therefore used a special model simulation to investigate this for Ganymede. “Until now, it has been difficult to determine the size of the impactor because there is no clear crater rim on Ganymede,” he explains.

20 times bigger than the “Dino Killer”

The analyses showed that Jupiter's moon Ganymede could have been hit by a gigantic asteroid around 300 kilometers in diameter. This chunk was 20 times larger than the asteroid that wiped out the dinosaurs 66 million years ago and caused a global catastrophe. Its impact on Ganymede must have created a transition crater 700 to 800 kilometers in diameter before it rebounded, collapsed and created the furrow system, as Hirata explains.

Whether this scenario is correct could be revealed in the near future by the JUICE space probe, which is currently on its way to Jupiter's icy moons. Its main purpose is to map and examine Ganymede in detail for the first time. “Many regions of Ganymede have not yet been mapped with sufficiently high resolution,” writes Hirata. It is therefore currently difficult to find and evaluate possible further impact traces on Jupiter's moon.

“The thermal and structural consequences of the impact on the interior of the icy moon have not yet been investigated,” says the planetary researcher. However, the data collected by JUICE from 2031 onwards could change and provide new insights into the development of the largest moon in the solar system. (Scientific Reports, 2024; doi: 10.1038/s41598-024-69914-2)

Source: Kobe University

September 4, 2024 – Nadja Podbregar