Large Scale Geography

The large scale geography of the Moon has two major components. The Highlands are the rugged, heavily-cratered, bright regions, which cover about 83% of the total surface. The dark, smooth Maria comprise the other 17%. Galileo saw these regions with his telescope. Thinking their smoothness was due to being made of water, he called them "seas" (i.e., maria in Italian).


The highlands are the oldest features, having been formed more than four billion years ago. The highlands are the oldest surviving part of the lunar crust. They are material that solidified on the crust of the cooling Moon within a hundred million years or so of its formation. Because they are so old, the highlands are extremely cratered. Craters are seen on top of craters.


The creation of the maria was a multi-step process. First, a very large projectile impacted the Moon and created an extremely deep and large basin. Second, over millions of years, magma from beneath the surface periodically filled in part of the basins. As this process continued, any features in the basin were covered.


a) The rather fluid magma was composed of basalt, which contains some iron. This is the reason for the dark color.

b) The deep basins were initially created around four billion years ago. Although the projectiles were much larger at that time than those that created many of the previous craters, these objects were the last of the major left-over debris in the Solar System. The maria are smooth because the infill process continued until about 3.3 billion years ago and there have only been a few impacts since then.

c) The maria are primarily found only on the near-side of the Moon. The best hypothesis is that the Moon's crust was slightly thinner on this side, so cracks from the initial impact were able to reach to the molten magma layers.


Types of Features

There are six specific types of lunar features.

1. Maria are the dark, smooth areas, and these features are unique to the Moon. The singular form of "maria" is "mare". The major ones are

Oceanus Procellarum (Ocean of Storms)

Mare Crisium (Sea of Crises)
Mare Imbrium (Sea of Showers)
Mare Nectaris (Sea of Nectar)
Mare Nubium (Sea of Clouds)
Mare Serenitatis (Sea of Serenity)
Mare Tranquillitatis (Sea of Tranquility)


2. Craters are impact sites from high-speed objects. The collision caused a large explosion that "dug out" a basin and vaporized the projectile. Often the debris created a rim that is slightly higher than the surrounding terrain. In several of the larger basins, a central mountain peak was created. Many of the large, prominent craters have been named after famous scientists (Copernicus, Tycho, and Kepler) or figures of ancient history, including Plato, Aristotle, and Pythagoras.


3. Mountains are all the result of impacts, such as the rugged borders of many of the maria. Lunar mountains have low, rounded profiles that resemble old, eroded mountains on the Earth. However, the mountains of the Moon have not been eroded, except for the effects of meteoritic impacts. They are rounded because there has been no water or ice to carve them into cliffs and sharp peaks. Many of the lunar mountains are named after terrestial ones: the Alps, Apennines, and Carpathians.


4. Valleys are broad, but shallow gorges that cut across mountains and craters. These tend to be rather straight.


5. Rilles are deep, narrow crevasses that were once lava-flow channels. The tend to be "wiggly" and to appear like dried river beds.


6. Rays are bright "fingers" of ejected material that radiate outward from large, relatively recent craters. Those around Copernicus and Tycho are especially easy to see.



James Sowell, 2013