The largest moon in the solar system was hit by an ancient asteroid 20 times larger than the rock that crashed into Earth and ended the reign of the dinosaurs 66m years ago, research suggests.
The devastating impact occurred 4 billion years ago and caused Ganymede, one of nearly 100 known moons of Jupiterto rotate so that the impact crater points almost directly away from the gas giant.
According to computer models, the asteroid was most likely 185 miles across and struck at an angle of 60-90 degrees. The impact created an initial crater up to 1,000 miles wide that was partially filled in as rock and dust ejected by the impact fell back.
Dr Naoyuki Hirata, a planetary scientist at Kobe University in Japan, said distinctive furrows covering the surface of Ganymede had long been thought to be remnants of multiple concentric rings created by the giant asteroid impact. But it was unclear how big the impact was and what effect it had on the Jovian moon, he said.
Write in Scientific reportsHirata describes how the center of the protosystem points almost directly from Jupiter. This could happen if the asteroid that hit Ganymede added extra weight to the impact zone, destabilizing the moon and causing it to spin on its axis.
Hirata’s computer simulations confirm that this may have been the case, with an asteroid slamming into Ganymede, causing it to spin so that the impact site is always on the moon’s far side. Like Earth’s moon, Ganymede is tidally locked to Jupiter, meaning it constantly shows the same face to the gas giant.
The impact would have had dramatic implications for Ganymede, which is larger than Mercury at more than 3,000 miles across. That would have “completely removed the original surface”, Hirata said, and affected the moon’s interior, which scientists say contains a hidden saltwater ocean.
Hirata cannot rule out alternative explanations, but further evidence for or against his theory can come from the European Space Agency’s Juice investigationwhich last year set course for Jupiter and its moons. As part of its mission, the spacecraft will search for pockets of water and sources of energy essential for life on the Jovian moons.
Prof Leigh Fletcher, a planetary scientist at the University of Leicester, said the ancient sites on Jupiter’s moons testify to billions of years of bombardment, which can still be seen today. “Making sense of all those overlapping events on satellite surfaces is challenging,” he said. “This is a neat attempt to turn back the clock through computer simulations, looking for an explanation for the distribution of scars across Ganymede.”
The Juice mission was well equipped to further explore Ganymede, he added. “Not only will this produce the best images of the surface break ever obtained, but any remnants of this impact and reorientation, in Ganymede’s shape or gravity field, can be probed by Juice’s suite of instruments,” said Fletcher. “The Juice observations will provide excellent new constraints on this ‘impact and reorientation’ hypothesis.”
