Evrim Yazgin
Cosmos science journalist
An asteroid 20 times larger than the one that ended the Age of Dinosaurs smashed into Jupiter’s largest moon Ganymede 4 billion years ago. New research suggests the impact shifted the moon’s axis.
Ganymede is the solar system’s largest moon. With a diameter more than 5,260km, it is larger than the planet Mercury.
It is also the only moon in the solar system with a magnetic field and is believed to have a saltwater ocean 100km deep beneath a 150km-thick crust of ice. These features also make Ganymede a candidate in the search for extraterrestrial life in our solar system.
This possibility will be investigated by the European Space Agency’s JUICE space probe which was launched in April 2023 and due to reach Jupiter’s moons by 2034.
Like Earth’s moon, Ganymede is tidally locked to its host planet, meaning it always shows the same face to Jupiter.
It is one of the 4 “Galilean satellites” identified by Galileo Galilei using his telescope in 1610. These were the first solar system objects discovered since humans started tracking the classical planets visible to the naked eye.
“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 planetologist Hirata Naoyuki from Kobe University in Japan.
The furrows are the earliest recognised surface features of Ganymede.
They form concentric circles thousands of kilometres in size. Scientists in the 1980s concluded that they must have been formed by a major impact event.
“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,” Hirata says.
Hirata was the first to realise that the impact is almost precisely on the meridian (line between the north and south pole) farthest away from Jupiter. The researcher drew a link to Pluto which also shows evidence of an impact event. Data from the New Horizons space probe allowed scientists to determine that the Pluto impact caused the dwarf planet’s rotational axis to shift.
This suggested to Hirata that Ganymede’s axis had also shifted.
In research published Scientific Reports, Hirata shows that the asteroid which hit Ganymede was about 300km in diameter. Immediately after the impact, it would have created a crater 1,400 to 1,600km wide.
Hirata’s simulations showed that only such a large impact could have shifted the planet’s rotational axis.
“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,” explains Hirata.
