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These pictures of the moon were taken by the Galileo spacecraft in early December 1990. The "blemish" in the center of the left image is called the Orientale Basin. From Earth, this formation is difficult to see, as it's at the far left side of the moon as seen from Earth. But the view was no trouble at all for the Galileo spacecraft, which provided this profile view.
The picture on the right shows the dark Oceanus Procellarum (normally near the left "edge" of the moon as seen from Earth) in the upper center, with Mare Imbrium above it and the smaller circular Mare Humorum below. The Orientale Basin, with a small mare in its center, is on the lower left near the limb or edge. Between stretches the cratered highland terrain, with scattered bright young craters on highlands and maria alike.
"Maria" (MAH-ree-ah) is the Latin word for "seas." The name comes from when stargazers thought, hundreds of years ago, that the dark formations on the moon were bodies of water. Maria were actually formed -- more than a billion years ago -- when magma broke through the lunar surface and pooled in large impact craters.