An Ancient Collision Fractured Ganymede and Caused it to Shift Off its Axis

Four billion years ago, an asteroid larger than the one that ended the age of dinosaurs may have struck the largest moon in our solar system, Ganymede, knocking it off its axis and causing it to crack like an egg.

Discovered by Galileo in 1610 while experimenting with his new telescope, Ganymede is not only Jupiter’s largest moon but the largest in the solar system, even surpassing the size of Mercury. Since Pioneer 10’s visit in December 1973, subsequent missions have revealed that Ganymede is the only moon with a magnetic field, has an outer layer of ice, and may harbor a vast ocean beneath its frozen surface that could potentially support life.

Mysterious Concentric Rings

One unique feature of Ganymede has puzzled scientists: its surface is marked by concentric rings, dating back to the moon’s early history, that span entire regions. These rings converge at a point located at 21° S 179° W, thought to be the site of a massive asteroid impact around four billion years ago.

The impact that shattered Ganymede didn’t leave a clear crater because it essentially splashed the surface, which then quickly filled in the resulting hole.

Here’s where it gets fascinating.

Ancient Impact Tilted Ganymede Off Its Axis

Planetologist Hirata Naoyuki from Kobe University in Japan explains that the impact didn’t just fracture Ganymede—it caused a gravitational imbalance that knocked the moon off-kilter, tilting it on its axis. Originally near the north pole, the impact site has since shifted to the southern midlatitudes due to this tilt.

Positioned South of Ganymede’s Tidal Axis

Even more intriguing, the impact site lies just south of Ganymede’s tidal axis. Like Earth’s Moon, Ganymede is tidally locked to Jupiter, always showing the same side to the planet. The location of the impact site, just south of the farthest point on the side facing away from Jupiter, suggests it acts as a kind of counterbalance.

Using computer simulations and comparing the Ganymede impact to similar ones on Pluto and Jupiter’s moon Callisto, Hirata estimated the object that struck Ganymede was about 300 km (190 miles) in diameter and 20 times heavier than the asteroid that wiped out the dinosaurs 65 million years ago. It also temporarily created a crater 1,400 to 1,600 km (870 to 990 miles) wide.

“I want to understand the origin and evolution of Ganymede and other Jupiter moons,” said Hirata. “This giant impact likely played a major role in Ganymede’s early development, but its thermal and structural effects on the moon’s interior haven’t been explored yet. I believe future research could delve into the internal evolution of icy moons.”

Read the original article on: New Atlas

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