For a small planet, Mars sure knows how to go big. It's about half as large as Earth, but it's got the hugest volcano in the solar system in the Arizona-sized Olympus Mons and the grandest of all canyons in the 7 kilometer-deep Vallis Marineris. Now it can add its coolest, most-braggable title: the Biggest Impact Crater in the Solar System. In a new study out in Nature, scientists have shown that Mars was probably hit by an asteroid the size of the Moon sometime in its early history, which left a crater the size of the planet's entire northern hemisphere.