An Ancient Asteroid Smashed Into Jupiter’s Moon Ganymede and Tipped It Over, Study Finds

The history of our solar system is a turbulent one. The infamous Chicxulub asteroid, which decimated the dinosaurs 66 million years ago, was just one of many space rocks that have pelted the planets and their moons since their formation.

Now, new research has dug up an even more calamitous collision in our solar system’s distant past: An asteroid 20 times the size of the Chicxulub impactor wreaked planetary-scale havoc four billion years ago. And the unfortunate recipient was Ganymede, Jupiter’s largest moon.

In the study published Tuesday in the journal Scientific Reports, author Naoyuki Hirata, a planetologist at Kobe University in Japan, details the topographical clues on Ganymede that illustrate how far-reaching the consequences of that collision were.

Ganymede’s face bears circular furrow marks that seem to emanate from a single point. Astronomers have long seen these as evidence that something had smashed into it. “We know that this feature was created by an asteroid impact about four billion years ago,” Hirata says in a statement. “But we were unsure how big this impact was and what effect it had on the moon.”

What caught Hirata’s attention, however, wasn’t the size or depth of the crater, but its position. The bullseye faces directly away from Jupiter. That uncanny alignment points to the possibility that the moon had shifted on its rotational axis in the past. Could the cataclysmic impact that sculpted Ganymede’s signature scars also be behind the satellite’s present configuration?

To understand the origins of the furrow system, Hirata simulated impacts on a computer to see which scenario could have created Ganymede’s present-day landscape. His models found that the asteroid and the material it ejected upon impact weighed down the moon at the site of the crater, enough to swivel the entire planetary body around. Ganymede is a tidally locked moon, which means it always shows the same face to Jupiter—and, as a result of the impact, the crater is permanently shielded from Jupiter’s view.

As revolutionary as it might seem, this isn’t the first known instance of an asteroid tipping over a celestial body. From images gathered by NASA’s New Horizons space probe, scientists inferred that a similar asteroid strike had altered the distribution of ice on Pluto and shifted its orientation in the distant past.

By reverse engineering Ganymede’s position, Hirata calculated that the moon’s impactor was 186 miles wide, or about 20 times the size of the Chicxulub asteroid. As it smacked into Ganymede, it marked the ground with a crater roughly equal in area to the state of Alaska, which was later filled in by falling debris. According to Hirata, only a crater of this scale could be impactful enough to realign Ganymede, regardless of where the asteroid made landfall.

Andrew Dombard, a planetary scientist at the University of Illinois Chicago who was not involved with the research, tells New Scientist’s Jacklin Kwan that the study’s model might not take into account all the components of Ganymede’s icy structure. “I think it is very good for establishing that this process could occur, but I don’t necessarily trust the numbers,” he tells the publication.

Ganymede is the largest moon in the solar system, and it’s slightly bigger than the planet Mercury. Even so, the monstrous asteroid that slammed into it would have rejiggered that world inside and out. Besides reorienting the moon, Hirata suggests the impact would also have blasted away Ganymede’s surface layer and reshaped its interior.

“Making sense of all those overlapping events on satellite surfaces is challenging,” Leigh Fletcher, a planetary scientist at the University of Leicester in England who wasn’t involved in the study, tells Ian Sample of the Guardian. “This is a neat attempt to rewind the clock via computer simulations, searching for an explanation for the distribution of scars across Ganymede.”

Ganymede’s lively history is just one of its many scientific allures. Additionally, the icy world may be hiding a subterranean sea of liquid water that’s roughly ten times deeper than Earth’s oceans. Researchers think this moon could potentially be habitable for life as we know it.

A spacecraft launched by the European Space Agency last year is already on its way to take a closer look. The Jupiter Icy Moons Explorer, or JUICE, will arrive in Jupiter’s orbit in 2034 after 11 years of spacefaring. Its mission is slated to last for eight months. Among the nearly 100 Jovian moons that encircle the gas giant, JUICE’s primary study subject is none other than Ganymede, a lunar behemoth that hosts its own magnetic field—the only moon with that distinction—and boasts a complex core. 

Adorned with high-resolution cameras, spectrographs and magnetometers, the JUICE mission should be more than equipped to shed new light on Ganymede’s past. And it will be able to get a close-up look at the moon’s topography, providing further data on the ancient asteroid impact.

“The Voyager 1 and 2 and Galileo spacecraft have observed Ganymede; however, many areas of Ganymede still have not been imaged with sufficient resolution,” Hirata tells Newsweek’s Tom Howarth. “I am really looking forward to the data from the JUICE.”

Shi En Kim | | READ MORE

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.