The existence of our solar system's innermost planet has been common knowledge since ancient times, but that doesn't actually mean we've always know much about it. Mercury's proximity to the Sun has allowed it to jealously guard its secrets, and so this NASA video offers an unprecedentedly detailed view of the planet's surface.

This video is based on images taken by the MESSENGER probe, the first spacecraft to actually orbit Mercury. It has allowed astronomers to get the first up-close view at the planet's geology; until now, the sum total of our knowledge was limited to a single Mariner flyby in 1975. Over the past year, MESSENGER has taken over 80,000 images with plans to take 80,000 more. That's allowed us to assemble this complete visual representation of Mercury's surface, with each pixel representing about a square kilometer; the video expands on this previously published image.

It's perhaps not quite enough detail to plan out a day hike, but it's a gigantic leap in our understanding. After all, it wasn't so long ago that it was mistakenly thought that the planet was tidally locked around the Sun, meaning one half of the planet would be bathed in searing, eternal sunlight, while the other half would be a forever frozen ice cube. Since then, astronomers have learned that Mercury actually rotates three times for every two revolutions about the Sun.

Courtesy of the MESSENGER website, here's a breakdown of what the various colors represent in the video:

This view captures both compositional differences and differences in how long materials have been exposed at Mercury's surface. Young crater rays, arrayed radially around fresh impact craters, appear light blue or white. Medium- and dark-blue areas are a geologic unit of Mercury's crust known as the "low-reflectance material," thought to be rich in a dark, opaque mineral. Tan areas are plains formed by eruption of highly fluid lavas. The large circular area near the top center is the Caloris impact basin, whose interior is filled with smooth, somewhat younger volcanic plains. Small orangish spots are materials deposited by explosive volcanic eruptions.

MESSENGER via NASA's Astronomy Photo of the Day.