In this global view the most obvious giveaway are the moon's "seas," which together form its familiar face. These are vast plains of once-liquid rock. Mercury, too, shows signs of lava flowing over its surface. In fact, many of its craters are completely buried in it. But Mercury still lacks those obvious oceans of stone.

One of these images was captured on the ground through a telescope, the other "in person" by a passing spacecraft. Both reveal something hidden: color. Mercury and the moon look mostly gray to the naked eye, but if you digitally exaggerate the natural color in the pictures, you suddenly see blues, yellows, and flecks of orange. This is another clue to the differences between the two worlds, since it turns out that those colors result from rocks with significantly different compositions.

Another distinction are the crater rays. The big craters on both the moon and Mercury formed in violent impacts. Surrounding the younger ones you can sometimes still see long streaks formed by the ejecta that was blasted out of the ground. The moon has several sets of brilliant rays, such as those that radiate from the crater Tycho. But Mercury's rays are often even brighter, and in places they are visible stretching across nearly an entire hemisphere.

Next, a look at some craters. Many craters on Mercury are at first indistinguishable from their lunar counterparts. But in some of them something strange is going on. For example, which one of these is a Mercurian crater?