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When NASA's Mariner 10 flew by Mercury three times in the mid-1970s, it spotted what Blewett described as "certain odd, bright patches" within the impact craters. The patches showed up again during MESSENGER's flybys of the planet. It wasn't until 2011, when MESSENGER entered orbit and began to capture higher-resolution images, that the bright areas were revealed as shallow, irregularly shaped depressions on the surface. Hollows are relatively small landforms — shallow features on the surface that stretch at most 0.6 miles (1 kilometer). This small size implies a relatively young age, as cratering would have eroded them away over...

Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2003 February 16 Southwest Mercury Credit: Mariner 10, NASA Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more...