Approximately 4 billion years ago, a significant asteroid collided with Ganymede, one of Jupiter’s moons. Researchers have now discovered that this impact has caused a shift in the moon’s axis. This finding indicates that the asteroid was about 20 times larger than the one responsible for the extinction of the dinosaurs on Earth, marking it as one of the most notable impact events with clear evidence in the Solar System.
Ganymede is the largest moon in the Solar System, even larger than the planet Mercury. It is particularly interesting because it harbors liquid water oceans beneath its icy exterior. Similar to Earth’s moon, Ganymede is tidally locked, which means it consistently shows the same side to Jupiter and has a hidden far side. Much of Ganymede’s surface features furrows that create concentric circles around a central point. Researchers in the 1980s believed these were the result of a significant impact. “Among Jupiter’s moons—Io, Europa, Ganymede, and Callisto—each has unique traits, but I was particularly drawn to the furrows on Ganymede,” says planetologist HIRATA Naoyuki from Kobe University. He adds, “We know that this feature resulted from an asteroid impact around 4 billion years ago, yet we were uncertain about the scale of this event and its effects on the moon.”
Researching Ganymede is challenging due to the limited data available from this distant object. Hirata was the first to determine that the possible impact site is nearly on the meridian that is the farthest from Jupiter. Drawing parallels from a similar impact event on Pluto, which altered the dwarf planet’s rotational axis and was studied by the New Horizons probe, he inferred that Ganymede likely experienced a similar shift. With expertise in simulating impact events on moons and asteroids, Hirata was able to calculate the type of impact that might have led to this realignment.
In a study published in the journal Scientific Reports, Hirata revealed that the asteroid likely measured about 300 kilometers in diameter—approximately 20 times larger than the one that struck Earth 65 million years ago, leading to the demise of the dinosaurs. This colossal impact created a temporary crater between 1,400 and 1,600 kilometers across. (These transient craters are the cavities formed immediately after the crater’s creation before any settling of material occurs.) His simulations suggest that only a collision of this magnitude could plausibly result in a shift in the moon’s mass distribution, leading to the current positioning of its rotational axis, regardless of the specific impact site.
“My goal is to deepen our understanding of the formation and development of Ganymede and other moons around Jupiter. The massive impact must have played a crucial role in Ganymede’s early evolution, yet the thermal and structural consequences of the impact on its interior have yet to be examined. I believe future inquiries should focus on the internal evolution of ice moons,” Hirata explains.
With its intriguing subsurface oceans, Ganymede is the target of ESA’s JUICE space mission. If everything proceeds as planned, the spacecraft is expected to enter orbit around the moon in 2034 and will conduct a six-month observational study, providing a wealth of information that may help to address Hirata’s research inquiries.
This research received funding from the Japan Society for the Promotion of Science (grants 20K14538 and 20H04614) along with support from the Hyogo Science and Technology Association.
