Bad Day to Be a Martian

One of my favorite science-fiction movie scenes is the opening sequence of Armageddon, which depicts the asteroid impact that marked the end of The Dinosaur Show. After the impact, hellfire rains down across the globe in deadly, but photogenic, fashion. But as impressive a visual as that scence is, it is small beans compared to what scientist think might have actually happened to Mars. After sifting through huge amounts of data sent back from NASA's Mars Reconnaissance Orbiter and the Mars Global Surveyor probes, scientists believe they are closer to an explanation of one of the great puzzles of the solar system: why the northern hemisphere of the planet is so different from the southern hemisphere. The southern hemisphere is a jangle of ancient and rough terrain. The surface of northern hemisphere is much younger, and one of the flatest places in the solar system. Suggested explanations include the notion that the northern hemisphere is the sea bed of long-vanished ocean, that lava flows from the interior smoothed out the surface, or that it's actually just a really big crater from a really, really big asteroid. A new analysis of the shape of the Northern plain that (and this was the hard part) took into account later volcanic action that distorted the outline over the eons has put considerable weight behind the crater theory. This would make Mars host to the largest crater in the solar system and give us new insight into just how dangerous the early solar system was—it's believed that an even bigger asteroid collided with the Earth, splitting the planet completely open and splashing off gobs of material that later formed our moon.