Ganymede, the largest moon in the solar system, is tidally locked. It always shows the same side to the planet it is orbiting and hence also has a far side. On large parts of its surface, the moon is covered by furrows that have captured scientists’ attention.
These furrows form concentric circles around one specific spot. Based on that, in 1980s, scientists concluded that they are results of a major impact event that happened around 4 billion years ago when an asteroid hits the Ganymade.
Now, a Kobe University researcher has realized that the axis of the Solar System’s biggest moon has shifted as a result of the impact. This confirms that the asteroid was around 20 times larger than the one that ended the age of the dinosaurs on Earth and caused one of the biggest impacts with clear traces in the Solar System.
The Kobe University planetologist HIRATA Naoyuki said, “The Jupiter moons Io, Europa, Ganymede, and Callisto all have interesting individual characteristics, but the one that caught my attention was these furrows on Ganymede. We know that this feature was created by an asteroid impact about 4 billion years ago, but we were unsure how big this impact was and its effect on the moon.”
Data from the remote object is limited, making research challenging. Hirata was the first to note that the impact site is nearly on the farthest meridian from Jupiter. He compared it to an impact on Pluto, which shifted Pluto’s rotation, as seen by the New Horizons probe. This suggested Ganymede might have experienced a similar shift.
He later used this insight to figure out what type of impact could have caused Ganymede’s change in orientation.
In this new study, Hirata reports that the asteroid probably had a diameter of around 300 kilometers and created a transient crater between 1,400 and 1,600 kilometers in diameter. His simulations suggest that only an impact of this size would make it likely that the mass distribution change could cause the moon’s rotational axis to shift into its current position. This result holds true irrespective of where, on the surface, the impact occurred.
“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 the interior of Ganymede have not yet been investigated at all. I believe that further research applying the internal evolution of ice moons could be carried out next,” explains Hirata.
Journal Reference:
- Hirata, N. Giant impact on early Ganymede and its subsequent reorientation. Sci Rep 14, 19982 (2024). DOI: 10.1038/s41598-024-69914-2