Billions of years ago, Jupiter's planet-sized moon Ganymede got a helluva whack: Looking over data from probes that have visited the moon, scientists estimate it got hit by an object a staggering 300 kilometers wide.
That's…well I was going to say "cataclysmic" but even that underplays the nightmarish over-the-top violence of this event. Everything within a thousand kilometers of the impact point would've been melted, and the entire surface of the moon would've been severely affected. Even the interior suffered huge changes from this. An impact like this today on Earth would scour our planet clean of life. On a moon it must have been apocalyptic.
Ganymede is one of the four big moons of Jupiter, and is in fact the biggest moon in the solar system — bigger than Mercury. At 5,260 kilometers in diameter it's the ninth largest object we know of in the solar system.
It was discovered by Galileo in 1610, and first visited by spacecraft in the early 1970s, but it was the visits by Voyager 1 and 2 in 1979 that showed us the moon up close. It has two main kinds of surface features; what are called dark and light terrains. The dark ...