ScienceShot: World of Fire

Cratered Mercury (inset) won't win any beauty prizes, but it sure had a colorful past. Thanks to the Messenger spacecraft, which began orbiting the world in March 2011, planetary scientists have been able to count craters over Mercury's entire surface. The crater counts help gauge the age of different terrains, because older regions have suffered more impacts. In the main image, the most heavily cratered regions—such as the one surrounded by the black contour—are colored red. By extrapolating from the moon, where Apollo astronauts retrieved rocks that scientists dated, the researchers concluded in this week's Nature that lava flooded all of Mercury 4.0 to 4.1 billion years ago; the global volcanism ended 300 to 400 million years later. This period coincides with the Late Heavy Bombardment, a torrential rain of asteroids that pummeled the planets, suggesting the collisions may have triggered the widespread lava flows that marked Mercury's youth.

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