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.
Scientists have known for years about the Borealis Basin — a region of lowlands that take up the northern half of Mars. Some thought a volcanic cataclysm caused the crater, while other speculated it could have been an impact. With the help of detailed geological readings from the Mars Reconnaissance Orbiter and Global Surveyor, they've been able to solve the mystery, concluding that something struck Mars with the force of 1 million billion Nagasaki-sized atomic bombs on a bad day four billion years ago.
Nature has devoted a special issue called "Cosmic Impacts" centered on the new findings, with a cool story on the 100th Anniversary of the Tunguska Event and a sweet photo gallery of the solar system's prettiest craters that are worth a look.