Ganymede, the Solar System's largest moon, is even bigger than Mercury and is notable for its subsurface liquid water oceans beneath a thick icy crust. Similar to Earth's moon, Ganymede is tidally locked, meaning the same side always faces Jupiter, resulting in a permanent far side. The moon's surface features furrows forming concentric circles around a specific region, which researchers in the 1980s attributed to a major impact event. "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," says Kobe University planetologist Hirata Naoyuki. He adds, "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 what effect it had on the moon."
Due to limited data from this remote object, research has been challenging. Hirata was the first to recognize that the suspected impact site is nearly precisely on the meridian farthest from Jupiter. Drawing parallels to a similar event on Pluto, which caused the dwarf planet's rotational axis to shift-a phenomenon observed through the New Horizons space probe-Hirata inferred that Ganymede likely experienced a similar reorientation. As a specialist in simulating impact events on moons and asteroids, Hirata used this insight to calculate the impact's magnitude necessary to cause this reorientation.
Published in Scientific Reports, Hirata's findings suggest the asteroid had a diameter of around 300 kilometers, making it about 20 times larger than the asteroid that struck Earth 65 million years ago. This impact created a transient crater between 1,400 and 1,600 kilometers in diameter. (Transient craters are the initial cavities formed immediately after impact, before material settles into and around the crater.) According to Hirata's simulations, only an impact of this scale could likely cause a shift in the moon's rotational axis to its current position. This conclusion remains valid regardless of the impact's exact location on Ganymede's surface.
"I want to understand the origin and evolution of Ganymede and other Jupiter moons. 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," Hirata explains.
Ganymede's subsurface oceans make it a prime target for exploration, and the moon is the ultimate destination for ESA's JUICE space probe. If all goes according to plan, the spacecraft will enter orbit around Ganymede in 2034, conducting a six-month observation mission that will return valuable data to help answer the questions Hirata and other researchers are exploring.
Research Report:Giant impact on early Ganymede and its subsequent reorientation
Related Links
Department of Planetology, Graduate School of Science, Kobe University
Asteroid and Comet Mission News, Science and Technology
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