A very large high resolution version of this image is available for download.
Note: As mentioned in the first post, these posts will not list the Lunar 100 features in order because the features are not always visible at different times. Item 2, moonshine, will be posted when viewing conditions allow.
The third target on the Lunar 100 list is the mare/highland dichotomy. When you look at the surface of the moon, you'll notice two distinctive surface types. Smoother dark areas, the maria, are so named because they were mistaken for actual seas by early astronomers. The cropped photo above highlights the Mare Crisium. It is approximately 550 km wide. The maria are relatively smooth plains of basaltic rock that formed from cooling lava produced by volcanic eruptions between 3 and 4 billion years ago. It is believe that deep impressions formed by impacts were filled in with magma, and then hardened to form the maria. Higher concentrations of titanium and iron make this rock significantly darker than the rest of the surface of the moon. They are the youngest lunar surfaces, and show significantly less crater activity from impacts than the lunar highlands.
The darker, smoother mare shows a lower density of impact craters than the surrounding lighter highlands
The lighter areas of the lunar surface are significantly older. They are believed to have formed between 4 and 4.5 billions years ago when the surface of the moon was still molten. They are composed primarily of anorthosite, which is an igneous rock. Anorthosite forms when molten rock cools more slowly than in the formation of basalts. This indicates that the highlands solidified under different conditions than the maria. The highlands formed very early in the formation of the solar system, which is itself estimated to be 4.6 billion years old.
Notably, the rocks from the lunar highlands are older than the oldest Earth rocks found this far. On earth, the igneous rocks formed at the beginning of the Earth's life have been predominately covered by tectonic activity and sedimentary rock formation. The moon has cooled sufficiently that it has no significant tectonic activity, and the lack of water and atmosphere make sedimentary rock formation impossible. The oldest rocks on the moon are still exposed.