Four billion years ago, Ganymede literally got knocked sideways. A giant asteroid slammed into Jupiter’s largest moon, creating a crater so massive it reoriented Ganymede’s entire axis. Now new research from Kobe University reveals just how huge—and game-changing—that impact really was.

A massive asteroid impact on the solar system’s largest moon

Ganymede is a moon larger than Mercury and packed with features like icy oceans and crisscrossing furrows that have intrigued scientists for decades. The new study, published in Scientific Reports, has solved one of those mysteries: the strange furrows are the lasting scars from an enormous impact.

“The Jupiter moons Io, Europa, Ganymede and Callisto all have interesting individual characteristics, but the one that caught my attention was these furrows on Ganymede,” said Kobe University planetologist Naoyuki Hirata. “We know that this feature was created by an asteroid impact about four billion years ago, but we were unsure how big this impact was and what effect it had on the moon.”

The asteroid, estimated to be around 190 miles (300 kilometers) wide, hit with such force that it gouged out a crater between 870 and 994 miles (1,400 – 1,600 kilometers) in diameter. The asteroid is believed to have been 20 times larger than the one that wiped out the dinosaurs 65 million years ago.

The impact didn’t just leave a giant hole in Ganymede—it actually tipped the moon’s axis, changing the way it spins and points in space. This kind of event, known as true polar wander, is rare—and impressive.

Ganymede, like Earth’s moon, is tidally locked, which means the same side always faces Jupiter. The researchers now found that its furrows on Ganymede aren’t just random scars; they line up with the moon’s tidal axis. This alignment suggests that the impact’s ejecta—the debris ejected by the asteroid—created a gravitational anomaly strong enough to pull the moon into a new orientation.

Drawing parallels from Pluto’s famous Sputnik Planitia basin, which also shifted the dwarf planet’s axis after a major impact, Hirata used computer simulations to map out how Gannymede’s collision could have tilted the moon.

The researcher found the collision didn’t just reorient the moon; it likely also had a lasting effect on Ganymede’s internal structure. The heat generated by the crash could have influenced the development of the moon’s subsurface ocean, a body of water hidden beneath its icy shell. This ocean is thought to contain more water than all of Earth’s oceans combined, making Ganymede an intriguing target for future exploration.

“The giant impact must have had a significant impact on the early evolution of Ganymede, but the thermal and structural effects of the impact on the interior of Ganymede have not yet been investigated at all,” Hirata said.

In 2034, the European Space Agency’s JUICE (Jupiter Icy Moons Explorer) mission will arrive at Ganymede. It will spend six months orbiting the moon, collecting data on its surface, oceans and magnetic field. Scientists hope this mission will answer lingering questions about Ganymede’s past—and maybe even reveal if life could exist in its hidden waters.