Ganymede is a moon that bears the marks of a veteran’s resignation. Its wrinkled shell speaks of hard times, its hunchback a limp from an injury he would rather never speak of again. Kobe University planetary scientist Naoyuki Hirata continued to infer the effects of one of the largest impacts ever to leave a mark on the outer solar system by comparing its characteristics with known impacts on other planets.

At more than 5,000 kilometers (more than 3,000 miles) across, Ganymede rivals Mercury for the top spot as the largest natural satellite in the Solar System. The monolith has been a prominent landmark in the sky since the famous astronomer Galileo Galilei first spotted it through his telescope on a fateful evening in 1610.

For all its size and familiarity, distant Ganymede keeps its secrets well hidden. It wasn’t until the Voyager missions gave a close-up view of Jupiter’s moon system that astronomers were able to examine its craters and canyons for clues about the object’s turbulent past.

Of particular interest were a series of long grooves and ridges that spread across much of the lunar surface.

Given the age and concentric structure of the lines beneath layers of new rock, it was immediately assumed that they were the result of a collision that occurred while the solar system was still learning to regulate its emotions.

For Hirata, the rhythms were a calling.

“Jupiter’s moons Io, Europa, Ganymede, and Callisto all have interesting individual features, but what caught my attention were these stripes on Ganymede,” Hirata says. “We know that this feature was formed by an asteroid impact about 4 billion years ago, but we weren’t sure how big the impact was and what kind of impact it had on the moon.”

Several years ago, Hirata and his colleagues ran a series of simulations to estimate the size and speed of the meteoroid behind the impact. They determined that it was likely an asteroid about 150 kilometers across that hit the icy surface at about 20 kilometers per second, causing it to ripple and erupt.

Considering that the chunk of space rock that destroyed the Earth and killed the dinosaurs 66 million years ago was 10 to 15 kilometers in diameter, it’s easy to understand that this was no small crater.

A follow-up study using two different planetary models suggests that the redistribution of all this material due to the collision would be enough to change the orientation of the Moon’s axis. Such chaos in the formation of planets makes it unlikely that there is an object in the entire solar system that is not spinning, wobbling, or tilting diagonally, thanks to the vast achievements of physics.

Perhaps Uranus was dragged along by the runaway moon. Our Moon was probably the result of a merger that restarted Earth’s formation. Pluto’s one-sidedness is thought to be the result of a feedback loop between ice accumulation and the moon’s tidal forces.

Like Pluto and its moon Charon, Ganymede is tidally connected to its source and is forever forced by gravity to face the cloud tops below, due to the intense gravitational pull between its own mass and Jupiter’s, which slows its rotation.

Ganymede’s lines, perhaps a little too coincidentally, converge at a point roughly aligned with the moon’s face directly opposite Jupiter, as if it were an eye cursed to peer into space far beyond its host planet.

Hirata’s models provide compelling evidence that just a thousand years after a massive asteroid hits the moon’s surface, slight changes in its mass due to ejected rocks and raining ice cause the object to spin and lock into its new position.

Questions remain about how other factors might have affected the mass distribution and hence gravitational influence over time. Given that much of Ganymede has not yet been imaged at high resolution, new discussions are bound to arise about how the liquid oceans below might affect the thick crust above.

Yet this ancient break could not only provide insight into future formation patterns for Jupiter’s moons, but also how the various grooves and scars can be better interpreted to predict the distribution of material and energy throughout the Solar System’s history.

“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 Ganymede’s interior have not yet been fully investigated,” Hirata says.

“I believe that further studies using the internal evolution of icy moons can be done at a more advanced level.” This study was published on: Scientific Reports.