Tiny but mighty 💪 Saturn ice moon Mimas wears its history on its surface. The tiny moon – only 247 miles (397 km) across – is covered in impact craters dating from the turbulent beginnings of our solar system. Its most distinguishing feature is a giant impact crater named Herschel (not shown here), after the English astronomer who discovered Mimas in 1789. Shock waves from the Herschel impact may have caused the fractures, also called chasmata, on the opposite side of Mimas. Puzzlingly, Mimas appears to be frozen solid. Mimas is closer to Saturn and has a much more elongated orbit than Enceladus, another of Saturn’s icy moons, so it should have more tidal heating than Enceladus. Yet Enceladus has geysers of water, which implies internal heat, while Mimas has one of the most heavily cratered surfaces in the solar system, which suggests a frozen surface that has lasted long enough to preserve all those craters. This paradox has prompted the "Mimas Test" – any theory that claims t...