The asteroid collision that wiped out the dinosaurs 65 million years ago pales in comparison to a larger impact that occurred approximately four billion years ago, involving an incoming asteroid and Jupiter’s moon Ganymede. A study published in *Scientific Reports* suggests that this ancient bombardment completely tilted the distant world on its axis.
Ganymede is one of the most complex of Jupiter’s moons—and indeed, one of the most intriguing worlds in the entire solar system. It is the only known moon with its own magnetic field and is believed to possess a salty ocean 60 miles deep, beneath a 95-mile thick crust. This makes it a prime candidate for the potential emergence of life. With a diameter of nearly 3,300 miles, Ganymede is also the solar system’s largest moon—even bigger than the 3,030-mile wide planet Mercury. However, this doesn’t mean it’s immune to impacts.
Similar to our moon, Ganymede is tidally locked, meaning it always presents the same face to its parent planet. In the 1980s, astronomers discovered a radiating system of ripple-like furrows exceeding 1,000 miles in diameter at the center of the moon’s far side. Such a scar could only have been caused by an impact, but the severity of the event remained unclear. The scale of the collision could have implications for the moon’s internal structure and temperature, which in turn could impact the potential for life to emerge. Now, planetologist Hirata Naoyuki from Kobe University in Japan believes he has a better understanding of the impact’s force and the size of the impactor.
Naoyuki ran a computer model and estimated that the incoming object responsible for the ripples would have to have been at least 185 miles across—absolutely massive compared to the dinosaur-killing rock, especially considering the relative sizes of Ganymede and Earth. The asteroid that struck our planet measured no more than 10 miles across, or 0.112% of the size of the 7,917-mile diameter Earth. Ganymede’s asteroid was a full 5% of its diameter.
Such a blow not only left a scar but also knocked the world askew. According to Naoyuki’s calculations, the far side of Ganymede, where the ripples are, was once located in the moon’s north polar region. However, the force of the impact and the added weight of the asteroid tilted the moon on its side. As discovered in 2015, a similar collision led to a similar tilting at Pluto, a dwarf planet that rotates effectively on its side, at a 57-degree angle relative to its revolution around the sun. That impact in creating Pluto’s litter of five moons—.
Ganymede will be studied more closely in 2034 when the European Space Agency’s launched in 2023, arrives at the moon and goes into orbit around it. Hirata hopes this mission will further expand astronomers’ knowledge of Ganymede and similar moons.
“I want to understand the origin and evolution of Ganymede and other Jupiter moons,” he said in a statement accompanying his study. “I believe that further research applying [to] the internal evolution of ice moons could be carried out next.”