An ancient asteroid 20 times larger than the one that wiped out the dinosaurs likely struck Jupiter's moon Ganymede

(CNN) – Jupiter's moon Ganymede may have shifted on its axis when a massive asteroid struck it about 4 billion years ago, a new study suggests.

Ganymede, the biggest moon of the solar system, is even larger than Mercury and the dwarf planet Pluto. And previous research has found evidence suggesting that beneath its thick icy crust lies a salty ocean 10 times deeper than Earth's oceans.

But many questions remain about the moon, and scientists need more high-resolution images of its surface to solve the mysteries surrounding Ganymede's history and evolution.

Deep grooves cover large swaths of Ganymede's surface, forming a pattern of concentric circles around a point that has led some astronomers to believe the moon experienced a large impact in its past.

“Jupiter’s moons Io, Europa, Ganymede and Callisto all have interesting individual features, but what caught my attention most were these grooves on Ganymede,” Naoyuki Hirata, an assistant professor of planetology at Kobe University in Japan, said in a statement. “We know this feature was created by an asteroid impact about 4 billion years ago, but we weren’t sure how large this impact was or what effect it had on the moon.”

Hirata is the author of a new studypublished on Tuesday in the academic journal Scientific reportswhich explores what created Ganymede's groove system and the aftermath of the impact, a question that could be investigated more closely by the European Space Agency's Juice spacecraft, which is currently en route to study Jupiter and its moons.

Ganymede has long intrigued Hirata, who believes that discovering its evolution is “significant.” The moon’s surface is a study in contrasts, with bright regions of ridges next to grooves cutting through darker areas.

Hirata looked closely at Ganymede's system of grooves, which extend from a single point on the lunar surface like the concentric cracks that form when a rock hits a car windshield, he said. The groove's center point was along the moon's rotation axis, implying that something like a large impact caused a complete reorientation of the moon.

Previous research suggested that a large planetary body collided with Pluto early in its history, rearranging the distribution of ice on the dwarf planet and leading to the creation of a distinctive “heart” on the planet’s surface. Hirata said he thinks a similar scenario occurred on Ganymede, with its icy shell and subsurface ocean.

A sudden change in the way mass is distributed on a planet can shift the location of its axis, or the imaginary line around which planetary bodies rotate. When a large asteroid hits a planet, it creates a gravitational anomaly that changes the way the planet spins. So Hirata calculated what kind of impact could have created Ganymede's current orientation.

Their equations revealed that an asteroid about 300 kilometers (186 miles) wide initially created a crater between 1,400 and 1,600 kilometers (870 and 994 miles) in diameter.

The asteroid was 20 times larger than the one that collided with what is now the Yucatan Peninsula in Chicxulub, Mexico, and caused the decline of the dinosaurs on Earth 66 million years ago. According to the study, the crater left on Ganymede is 25% the size of Jupiter's moon.

It's not yet clear how much Ganymede's axis shifted, Hirata said. But future data collected by the Jupiter Icy Moons Explorer, or JUICE, mission could shed more light on the history of Ganymede and the impact event.

The spacecraft, launched in April 2023, completed a historic flyby of Earth and the moon on Aug. 21 that has put it on track to reach Jupiter and its moons in 2031.

It's difficult for researchers to know whether an ancient impact created the grooves on Ganymede without more data, which the Juice mission could provide, said Adeene Denton, a postdoctoral researcher at the University of Arizona's Lunar and Planetary Laboratory. She was not involved in Hirata's study.

“This work presents an interesting premise, with much to think about for the evolution of icy moons and ocean worlds,” said Denton, co-author of a April study about impacts on Pluto and its large basin, called Sputnik Planitia, which forms the left lobe of the heart feature observed by astronomers.

“It’s worth noting that some skepticism may be warranted when considering old, weathered geologic features on planetary bodies and how they affect a planet’s orientation,” he said. “With so little information on these large, old features, it’s difficult to be confident in identifying this feature as a basin, as well as a possible mass anomaly. Fortunately, unlike Pluto and (Sputnik Planitia), we will be returning to Ganymede soon and will be able to obtain the additional information needed to resolve this.”

Researchers believe Ganymede's interior may be like a sandwich, stacked with alternating layers of ice and ocean. Understanding how the impact altered the moon could reveal insights into its intriguing internal structure, Hirata said.

“I want to understand the origin and evolution of Ganymede and other moons of Jupiter,” he said. “The giant impact must have had significant repercussions 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. I think further research could be done by applying the internal evolution of icy moons.”