Massive Asteroid Impact Made Mars Lopsided

They call it the Mars dichotomy, or say that the planet is "two-faced." Researchers have known for decades that the Red Planet is divided between smooth, low-lying plains in the north, and craggy, cratered highlands in the southern hemisphere. Mars orbiters have also confirmed that the planet's crust is thinner in the north. Now, a new study offers an explanation for this strange phenomenon: Around 4 billion years ago, an enormous asteroid smashed into Mars and changed the character of its northern half.