Jupiter’s moon Ganymede is covered in cracks that may be evidence of a huge collision, making them the largest known impact structure in the solar system.
Ganymede is the biggest moon in the solar system, and the ninth biggest object – larger even than the planet Mercury – measuring more than 5000 kilometres across. It has been visited by multiple spacecraft, including NASA’s Voyager 1 and Voyager 2 probes in 1979, and the Galileo spacecraft from 1996 to 2000.
Images from these spacecraft revealed cracks or furrows on the surface of the moon, each up to several kilometres wide, which appeared to be in concentric rings. Researchers thought they may have been caused by an impact that rocked half the moon, but the true extent of the collision or its location wasn’t clear.
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Now Naoyuki Hirata from Kobe University in Japan and his colleagues have re-examined the images, finding the size of the impact structure had been underestimated. They believe it is nearly 16,000 kilometres in diameter, meaning it wraps around almost the entire moon, just narrowly avoiding meeting up on the other side, and was caused by an impactor 300 kilometres across.
This dwarfs the next biggest known impact structure in the solar system, the South Pole-Aitken basin on Earth’s moon, which is 2500 kilometres across and was caused by an impactor under 200 kilometres across.
“This large impact should have affected the geology of Ganymede significantly,” says Hirata. It is thought to have occurred about 4 billion years ago in a region of Ganymede called Marius Regio. Since then, the moon’s surface has been restructured significantly by volcanic and tectonic activity, erasing some of the history of the impact. But the furrows remain on older, dark terrain, which covers about a third of the surface.
The size of the impact structure had been difficult to constrain until now, says Paul Schenk at the Lunar and Planetary Institute in Houston, Texas, because it didn’t form in a normal way. “The icy shell of Ganymede was too thin to form a classical rim like you see on other large craters elsewhere,” he says. “It created ripple fractures surrounding the [impact] basin, but we don’t actually have the basin itself preserved.”
Images from future missions, most notably the European Space Agency’s JUICE mission scheduled to orbit Ganymede in 2032, should tell us even more. “It will improve our knowledge of the interior structure, especially over the site of the impact,” says Schenk.
Icarus
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